Page 1
Journal of Asian Concrete Federation
Vol. 6, No. 1, pp. 1-9, June 2020
ISSN 2465-7964 / eISSN 2465-7972
https://doi.org/10.18702/acf.2020.6.6.1
1
Technical Paper
Analysis of stress block parameters for high strength con-
crete
Brijesh Singh, Vikas Patel*, P N Ojha, VV Arora
(Received January 27, 2020; Revised June 10, 2020; Accepted June 21, 2020; Published June 30, 2020)
Abstract: In Indian Standard code IS: 456-2000 for concrete of grades higher than M55, the design
parameters given may not be applicable as structural behaviour of concrete changes as strength of con-
crete increases. Different international standards give different stress block parameters which can be re-
duced to these two basic factors. In this paper, stress block parameters K (strength reduction factor) and
k2 (factor for the depth of resultant compressive force) were calculated from experimental strain values
considering the stress-strain curve as parabolic for lower grades and as linear for higher grades. Also, the
method and approach for the calculation of stress block parameters have been worked out. The method
so proposed is compared with the model proposed in European design standard EC: 02-2004 by trans-
forming the European stress block parameters to the basic parameters used. Also, the effect of the shape
of the stress-strain curve and value of ultimate strain in concrete on stress block parameters and moment
capacity of the members was analyzed by working on the representative section. The method or approach
so proposed will be useful to understand and compare flexural design philosophies used in different in-
ternational standards by reducing the stress block parameters to two basic factors.
Keywords: high strength concrete; stress block; strength reduction factor; Eurocode; stress-strain
1. Introduction
The availability and advancement of material
technology and the acceptance has led to the
production of higher grades of concrete. High
strength concrete offers superior engineering
properties i.e. compressive strength, tensile strength,
durability, modulus of elasticity and overall better
performance when compared to the conventional
concrete [1,2]. However, high-strength concrete is
more brittle in nature because cracks in this material
do not always follow the aggregate-hardened cement
paste interfaces due to the improved interfacial bond
strength of high-strength concrete but may cut right
through the hardened cement paste and even the
aggregate particles leading to rapid propagation of
the cracks and sudden or sometimes explosive failure
of the concrete. Because of this problem, many
structural engineers hesitate in using high-strength
concrete, despite its obvious advantages. Research
on the behavior of HSC beams with concrete
strength higher than 55 MPa has been carried out in
the past and is still continuing, to understand the
behavior of HSC beams in flexure. Whilst there are
many publications proposing stress block models for
HSC beams, a universally accepted stress block
model is yet to be developed. In most design
standards, the conventional rectangular stress block
developed for Normal Strength Concrete is still
being used for design of HSC beams. Rectangular
stress block is generally used to calculate the
ultimate moment capacity of reinforced concrete
beams. The stress-strain curves for high strength
concrete are more linear than parabolic and hence it
was reasonable to infer that the rectangular stress
block parameters could be different.
The idea of using the equivalent rectangular
stress distribution was first proposed by Emperger [3]
and then modified by Whitney [4] for application to
ultimate strength design and later experimentally
verified by Hognestad et al. [5] and Mattock et al. [6].
To obtain accurate as well as well-controlled data on
flexure compression-loaded members, a test
procedure for a series of experiments on C-shaped
Brijesh Singh is a Manager at the Centre for Construction De-
velopment and Research National Council for Cement and
Building Materials
Corresponding author Vikas Patel is a Project Engineer at the
Centre for Construction Development and Research National
Council for Cement and Building Materials
P N Ojha is a Joint director at National Council for Cement
and Building Materials (NCB).
VV Arora is a joint director at the Centre for Construction De-
velopment and Research National Council for Cement and
Building Materials.
Page 2
Journal of Asian Concrete Federation, Vol. 6, No. 1, June 2020
2
concrete specimens subjected to axial load and
bending moment was proposed by Hognestad et al.
and later was used by several researchers. The
position of neutral axis depth was kept fixed by
continuously monitoring strains on one surface of the
C-shaped specimen and adjusting the eccentricity of
the applied force so that the strains on the neutral
surface remain zero.
The rectangular stress block model was first in-
troduced by Hognestad et al. (1955) from experi-
mental work involving normal strength concrete.
Ashour [7] has shown that the flexural rigidity in-
creases as concrete compressive strength increases.
From the experimental study by Oztekin et al. [8], it
was observed that the rectangular stress block pa-
rameters used in ordinary concrete members cannot
be used safely for high strength concrete members.
Attard and Stewart (1998) [9] examined the applica-
bility of ACI 318-95 rectangular stress block param-
eters to high-strength concretes. They have shown
that for a ductile singly-reinforced rectangular sec-
tion, the ultimate moment capacity is relatively in-
sensitive to the stress block model. An experimental
study on the evolution of depth of neutral axis at fail-
ure with the ductility at bending on HSC beams was
carried out by Bernardo & Lopes (2004) [10].
The equivalent rectangular concrete stress block
adopted by various current RC design codes [11,12]
(European Committee for Standardization 2004;
Standards New Zealand 2006; ACI Committee 318-
2008) are depending only on the concrete strength.
However, from the comparison conducted by the Ho
et al. [13,19] using previous experimental test results
done by other researchers, the theoretical flexural
strengths predicted by RC design codes are signifi-
cantly smaller than the actually tested flexural
strengths. And from the results obtained by previous
researchers [14-17], it was found that the stress block
parameters were fairly scattered even though the
concrete strength is the same. Therefore, the assump-
tion of stress block parameters should depend on
other factors apart from concrete strength only.
It was found that the theoretical formulations
based on the use of the rectangular block diagram for
the concrete to compute the depth of neutral axis at
failure gave substantially smaller values as com-
pared to the experimental values. As such, it was
concluded that the rectangular stress block diagram
proposed by ACI 318-1989 was not adequate for
HSC beams. Cetin and Carrasquillo (1998) [18] re-
ported that no single equation of various codes and
research done in past seems to represent the flexural
strength of HSC with sufficient accuracy and, there-
fore, measured values should be used instead of
predicated ones.
Ultimate concrete compressive strength is an-
other important variable in the ultimate strength de-
sign. Although the ultimate flexural strength of rein-
forced concrete sections does not depend on this var-
iable, it can noticeably affect the ultimate curvature
of reinforced cross sections. Mattock et al. [6] con-
cluded that the value of 0.003 is a reasonably con-
servative value for ultimate strain of concrete. This
value has been accepted by many design codes (NZS
3101 2006; ACI 318-08 2008; AS 3600 2009). Kahn
et al. [20] reported that the ultimate value of 0.003 is
valid for concrete up to 102MPa and provided the
best prediction of the ultimate moment. According to
Mansur et al. study (1997), the maximum of 0.003
for concrete in compression may be extended to high
strength concrete. Ibrahim and MacGregor (1996)
results for ultimate concrete strain were considerably
higher than the limiting value of 0.003. However,
they concluded that based on the reported values in
previous tests of C-shaped specimens, the value of
0.003 used by the ACI code, seems appropriate as a
conservative lower bound of experimental data.
The paper focuses on calculation of stress block
parameters K (strength reduction factor) and k2 (fac-
tor for depth of resultant compressive force) from ex-
perimental strain values. In this study, the model pro-
posed in European design standard EC: 02-2004
have been analyzed and stress block parameters are
transformed to the parameters used in IS code design
procedure. Thereafter, comparison with the design
parameters are done with experimental results.
2. Concrete Ingredients
Crushed aggregate with a maximum nominal
size of 20 mm was used as coarse aggregate and nat-
ural riverbed sand confirming to Zone II as per IS:
383 was used as fine aggregate. Their physical prop-
erties are given in Table 1. The petrographic studies
conducted on coarse aggregate indicated that the ag-
gregate sample is medium grained with a crystalline
texture and partially weathered sample of granite.
The major mineral constituents were quartz, biotite,
plagioclase-feldspar and orthoclase-feldspar. Acces-
sory minerals are calcite, muscovite, tourmaline and
iron oxide. The petrographic studies of fine aggre-
gate indicated that the minerals present in order of
abundance are quartz, orthoclase-feldspar, horn-
blende, biotite, muscovite, microcline-feldspar, gar-
net, plagioclase-feldspar, tourmaline, calcite and
iron oxide. For both the coarse aggregate and fine
aggregate sample the strained quartz percentage and
their Undulatory Extinction Angle (UEA) are within
permissible limits. Feldspar grains are partially frac-
tured and shattered. The quality of both coarse and
fine aggregate is fair. The silt content in fine aggre-
gate as per wet sieving method is 0.70 percent.
Page 3
Journal of Asian Concrete Federation, Vol. 6, No. 1, June 2020
3
Table 1 Properties of aggregates
Property Granite Fine Aggregate
20 mm 10 mm
Specific gravity 2.83 2.83 2.64
Water absorption (%) 0.3 0.3 0.8
Sieve
Analysis
Cumulative per-
centage
passing (%)
20 mm 98 100 100
10 mm 1 68 100
4.75 mm 0 2 95
2.36 mm 0 0 87
1.18 mm 0 0 68
600 µ 0 0 38
300 µ 0 0 10
150 µ 0 0 2
Pan 0 0 0
Abrasion, Impact & Crushing Value 19, 13, 19 - -
Flakiness % & Elongation % 29, 25 - -
One brand of Ordinary Portland cement (OPC
53 Grade) with fly ash and silica fume are used in
this study. The chemical and physical compositions
of cement OPC 53 Grade, Properties of fly ash and
silica fume are given in Table 2. Polycarboxylic
group based superplasticizer for w/c ratio 0.20, 0.27,
0.30 and 0.36 and Naphthalene based for w/c ratio
0.47 complying with requirements of Indian Stand-
ard: 9103 is used throughout the investigation. Water
complying with requirements of IS: 456-2000 for
construction purpose was used. The 3 days, 7 days
and 28 days compressive strength of cement OPC 53
Grade were 36.00 N/mm2, 45.50 N/mm2 and 57.50
N/mm2 respectively. The 28 days compressive
strength of controlled sample and sample cast with
flyash was 38.53 N/mm2 and 31.64 N/mm2 respec-
tively, when testing was done in accordance with IS:
1727. The 07 days compressive strength of con-
trolled sample and sample cast with silica fume was
12.76N/mm2 and 14.46 N/mm2 respectively, when
testing was done in accordance with IS: 1727.
3. Mix Design Details
In this study, the four different mixes ranging
from w/c ratio 0.47 to 0.20 using granite aggregate
were studied for determining short term mechanical
properties of High Strength Concrete. For each type
of aggregate, three separate batches were prepared.
Table 2 Physical, chemical and strength characteristics of cement
Characteristics OPC -53 Grade Silica Fume Fly Ash
Physical Tests:
Fineness (m2/kg) 320.00 22000 403
Soundness Autoclave (%) 00.05 - -
Soundness Le Chatelier (mm) 1.00 - -
Setting Time Initial (min.) &
(max.)
170.00 & 220.00 - -
Specific gravity 3.16 2.24 2.2
Chemical Tests:
Loss of Ignition (LOI) (%) 1.50 1.16 -
Silica (SiO2) (%) 20.38 95.02 -
Iron Oxide (Fe2O3) (%) 3.96 0.80 -
Aluminium Oxide (Al2O3) (%) 4.95 - -
Calcium Oxide (CaO) (%) 60.73 - -
Magnesium Oxide (MgO) (%) 4.78 - -
Sulphate (SO3) (%) 2.07 - -
Alkalies (%) Na2O & K2O 0.57 & 0.59 -
Chloride (Cl) (%) 0.04 - -
IR (%) 1.20 - -
Moisture (%) - 0.43 -
Page 4
Journal of Asian Concrete Federation, Vol. 6, No. 1, June 2020
4
The slump of the fresh concrete was kept in the range
of 75 - 100 mm. A pre-study was carried out to de-
termine the optimum superplasticizer dosage for
achieving the desired workability based on the slump
cone test as per Indian Standard. The mix design de-
tails of specimens are given in Table 3. Adjustment
was made in mixing water as a correction for aggre-
gate water absorption. For conducting studies, the
concrete mixes were prepared in pan type concrete
mixer. Before use, the moulds were properly painted
with mineral oil, casting was done in three different
layers and each layer was compacted on vibration ta-
ble to minimize air bubbles and voids. After 24
hours, the specimens were demoulded from their re-
spective moulds. The laboratory conditions of tem-
perature and relative humidity were monitored dur-
ing the different ages at 27±2°C and relative humid-
ity 65% or more. The specimens were taken out from
the tank and allowed for surface drying and then
tested in saturated surface dried condition.
4. Stress Strain Study on High Strength
Concrete and Normal Strength Concrete
For stress strain characteristics of the high
strength concrete, concrete specimens were tested in
a closed-loop servo hydraulic compression testing
machine of 3000 kN capacity. Two extensometers at
the middle half of the height were used to get strain
and two strains were averaged. To obtain a full
stress-strain curve, a slow rate of loading in the range
of 1300 to 1500 N/sec was adopted for a whole com-
pression test. In general, the normal strength con-
crete gradually fails after reaching its peak load, but
the high strength concrete suddenly explodes at peak
load. Strain at peak stress and ultimate strain were
recorded for further analysis (Table 4).
4.1 Determination of stress block parameters
from experimentally obtained strain values
The total compressive force Cu and its location
below the top fibre can be expressed in terms of
stress block factors k1, k2 and k3.
k1= shape factor = ratio of the area of stress block
ABCD to area of rectangle AFCD
k2= ratio of depth of resultant compressive force to
depth of neutral axis(X)
r1 = 𝐴𝐸
𝐴𝐷=
𝜀𝑐
𝜀𝑐𝑢 and r2 =
𝐸𝐷
𝐴𝐷=
𝜀𝑐𝑢−𝜀𝑐
𝜀𝑐𝑢,
𝜀𝑐 = strain after which concrete yields at constant
stress of (αcc×S1) fck
𝜀𝑐𝑢 = ultimate strain in concrete
αcc = factor for consideration of long term effects
including the way load is applied = 0.85
S1 = factor for conversion of cube to cylinder
strength
Here, k1 is calculated by calculating the area of the
shaded portion and dividing by the area of the rec-
tangle, thus represents the ratio of the area of stress
Table 3 Concrete mix design details for study done
w/c
Total Cementitious Content
[Cement C + Fly ash (FA) +
Silica Fume (SF)] (Kg/m3)
Water
Content
(kg/m3)
Admixture
% by
weight of
cement
Fine aggregate as
% of total aggre-
gate by weight
28-Days
strength of con-
crete (N/mm2)
0.47 362 (290+72+0) 170 1.00 35 45.72
0.36 417 (334+83+0) 150 0.45 39 68.57
0.27 525 (400+75+50) 140 0.70 39 88.60
0.20 750 (548+112+90) 150 1.75 35 97.76
Table 4 Strain at peak stress and ultimate strain recorded
Cyd str,
N/mm2 Cube/Cyd
Cube Str,
N/mm2
Ec,
micro-strain
Ecu,
micro-Strain
24.00 1.28 30.72 2284 3711
23.50 1.28 30.08 1972 3789
34.40 1.28 44.03 2175 3063
33.80 1.28 43.26 1877 3220
48.60 1.24 60.26 2151 3324
46.09 1.24 57.15 2341 3323
76.83 1.16 89.12 2702 2931
76.18 1.16 88.37 2539 2729
103.90 1.14 118.45 2774 2774
106.00 1.14 120.84 2799 2799
Page 5
Journal of Asian Concrete Federation, Vol. 6, No. 1, June 2020
5
For Concrete up to M55 For Concrete above M55 to M90
For calculation of area ABE parabola is
considered
For calculation of area ABE, Triangle is considered as for
grade between M55 and M90 in actual stress diagram the
shape of area ABE is somewhere between linear and parabola
k1= 2
3 r1 + r2
k2 = [2
3 r1 (r2 +
3
8 r1) + r2 (
r2
2)]/k1
k3 = αcc×S1
k1= 1
2 r1 + r2
k2 = [1
2 r1 (r2 +
1
3 r1) + r2 (
r2
2)] /k1
k3 = αcc×S1
block ABCD to the area of rectangle AFCD. k2 is
calculated by taking moment of shaded areas about
axis CD and equating it to the moment of resultant
force Cu about axis CD. K3 is the stress reduction
factor calculated by considering αcc = 0.85 and con-
version factor S1.
Cu = b × area ABCD = b × k1 × area AFCD
Cu = b × k1 × k3 × fck × X
Partial factor of safety, γmc = 1.5 for concrete & γs =
1.15 for steel
Cu = k1 × b× Xu × k3∗fck
γmc
Cu = K× fck × b × Xu (1)
where, K = k1 ∗k3
γmc (2)
Xu
𝑑=
fyγs
∗ Ast
𝐾 ∗ fck ∗ b ∗ d
Mu = Cu × (d-k2 × X) (Compression) (3)
Mu = Tu × (d-k2 × X) where Tu = fy
γs × Ast (Ten-
sion) (4)
The two basic factors K and k2 for flexural de-
sign are worked out from the strain values recorded
experimentally for different strength of concrete
based on the above method (Table 5).
Transformation of equation of Euro-code into IS
code format (basic flexural design factors)
Euro-code uses different philosophy for deter-
mination of compressive force in a section and there-
fore has different factors. These factors were clubbed
together to form the representative equation similar
to IS code equation for calculation of compressive
force to compare the reduction factor K and factor to
calculate lever arm k2.
Cu = λ × ƞ × fcd × b × Xu
Table 5 Calculation of K and k2 as per the IS code approach with experimentally obtained strain values
Cyd str Cube/Cyd Cube Str Ec Ecu r1 r2 k1 S1 k3 Ymc K(IS cur) k2
24.00 1.28 30.72 2284 3711 0.62 0.38 0.79 0.78 0.66 1.5 0.35 0.41
23.50 1.28 30.08 1972 3789 0.52 0.48 0.83 0.78 0.66 1.5 0.37 0.42
34.40 1.28 44.03 2175 3063 0.71 0.29 0.76 0.78 0.66 1.5 0.34 0.40
33.80 1.28 43.26 1877 3220 0.58 0.42 0.81 0.78 0.66 1.5 0.36 0.41
48.60 1.24 60.26 2151 3324 0.65 0.35 0.68 0.81 0.69 1.5 0.31 0.36
46.09 1.24 57.15 2341 3323 0.70 0.30 0.65 0.81 0.69 1.5 0.30 0.36
76.83 1.16 89.12 2702 2931 0.92 0.08 0.54 0.86 0.73 1.5 0.26 0.34
76.18 1.16 88.37 2539 2729 0.93 0.07 0.53 0.86 0.73 1.5 0.26 0.33
103.90 1.14 118.45 2774 2774 1.00 0.00 0.50 0.88 0.75 1.5 0.25 0.33
106.00 1.14 120.84 2799 2799 1.00 0.00 0.50 0.88 0.75 1.5 0.25 0.33
Page 6
Journal of Asian Concrete Federation, Vol. 6, No. 1, June 2020
6
Euro-code equation for compressive force
fcd = {αcc × S1 / γcc} × fck
Cu = λ × ƞ × {αcc × S1 / γcc} × fck × b × Xu (5)
Compare equation (i) and (iii) and assume, K’ = λ ×
ƞ × αcc × S1 / γcc (6)
Cu = K’ × fck × b × Xu
Euro-code equation in IS code format
k2 = λ/2
The factor K and k2 are directly related to the
bandwidth between strain at peak stress and ultimate
strain of the concrete as well as the shape of the stress
strain curve of the concrete. The strength reduction
factor K reduces as the bandwidth between strain at
peak stress and ultimate strain decreases with in-
crease in strength and also the stress strain curve be-
comes steeper or linear for higher grades of concrete.
4.2 Calculation of moment capacity for balanced
section and comparison with that obtained as per
Euro-Code
To understand the effect of ultimate strain val-
ues on the moment capacities of the flexural mem-
bers moment capacities were calculated for a bal-
anced section from stress block parameters derived
from experimentally obtained strain values and were
also compared with the moment capacities calcu-
lated as per Euro-code. A representative section with
dimension (b=200 mm and D=400 mm with a clear
cover of 25 mm) was used for calculation of moment
capacity.
IS method:
Xu(max)
𝑑=
∈𝑐
∈𝑐+ ∈𝑠𝑢
where, Ɛc is ultimate strain in top most compression
fiber; Ɛsu is ultimate strain in steel = 0.002 +
0.87fy/Es
Mu (KN-M) = Cu × (d - k2 × X)
Mu(max) = K × fck × b × Xu(max) × (d - k2 × Xu(max)) for
balanced section
Euro-code method:
K4 = 1.25 × (0.6 + (0.0014 × 1000000 / Ɛcu))
Xu(max)
𝑑 = ((1 - 0.44) / k4) for cylindrical strength less
than 50 MPa
Xu(max)
𝑑 = ((1 - 0.54) / k4) for cylindrical strength
more than 50 MPa
Mu = Cu × (d - (λ/2) × X)
Mu(max) = K’ × fck × b × Xu × Xu(max) × (d - (λ/2) ×
Xu(max)) for balanced section
The ultimate strain of concrete has direct impact
on depth of neutral axis for balanced section which
is directly related to the maximum capacity of the
member. The ultimate strain values decreased as the
strength of concrete increases and becomes nearly
constant after 90 N/mm2 as seen from experimental
values and also as per many international standards.
Current IS code gives a constant value of ultimate
strain of 0.0035 up to concrete grade M50. This
Table 6 Calculation of K and k2 as per the Euro code transformed in IS code parameters
Cyd str
N/mm2 Cube/Cyd
Cube Str,
N/mm2
Ec,
micro-
strain
Ecu,
micro-
strain
λ ƞ αcc S1 Ycc K'(EC) k2
24.00 1.28 30.72 2000.00 3500.00 0.80 1.00 0.85 0.78 1.5 0.35 0.40
23.50 1.28 30.08 2000.00 3500.00 0.80 1.00 0.85 0.78 1.5 0.35 0.40
34.40 1.28 44.03 2000.00 3500.00 0.80 1.00 0.85 0.78 1.5 0.35 0.40
33.80 1.28 43.26 2000.00 3500.00 0.80 1.00 0.85 0.78 1.5 0.35 0.40
48.60 1.24 60.26 2000.00 3500.00 0.80 1.01 0.85 0.81 1.5 0.37 0.40
46.09 1.24 57.15 2000.00 3500.00 0.81 1.02 0.85 0.81 1.5 0.38 0.40
76.83 1.16 89.12 2485.95 2610.53 0.73 0.87 0.85 0.86 1.5 0.31 0.37
76.18 1.16 88.37 2479.67 2612.77 0.73 0.87 0.85 0.86 1.5 0.31 0.37
103.90 1.14 118.45 2600.00 2600.00 0.67 0.73 0.85 0.88 1.5 0.24 0.33
106.00 1.14 120.84 2600.00 2600.00 0.66 0.72 0.85 0.88 1.5 0.24 0.33
Page 7
Journal of Asian Concrete Federation, Vol. 6, No. 1, June 2020
7
value of 0.0035 is valid upto M35 grade of concrete
only as per the experimental results. Based on trend
shown by experimental results the ultimate strain
value given in Euro Code EC-02 seems to be more
realistic for higher grade concrete whereas constant
value of ultimate strain for concrete grade between
M25 to M50 (cylindrical strength) does not seems to
be realistic. Therefore, the value of ultimate strain in
concrete should be restricted to a lower value for
higher grades of concrete.
5. Conclusions
The method or approach worked out in the pa-
per will be useful in understanding the basic flexural
design philosophy. The stress-strain characteristics
or the effect of stress-strain parameters on the flex-
ural design equations are the fundamental of stress
block parameters and same needs to be derived from
the strain at peak stress and ultimate strain. The fac-
tor K (strength reduction factor) and k2 (factor for
the depth of resultant compressive force) are directly
related to the bandwidth between strain at peak stress
and ultimate strain of the concrete as well as the
shape of the stress-strain curve of the concrete. The
strength reduction factor K reduces as the bandwidth
between strain at peak stress and ultimate strain de-
creases with an increase in strength and also the
stress-strain curve becomes steeper or linear for
higher grades of concrete. The ultimate strain of con-
crete has a direct impact on the depth of the neutral
axis for a balanced section which is directly related
to the maximum capacity of the member.
The ultimate strain values decrease as the strength of
concrete increases and become nearly constant after
90 N/mm2 as seen from experimental values and also
Table 7 Calculation of moment capacity as per the IS code approach with experimentally obtained strain val-
ues for balanced section
Cyd str.
N/mm2 Cube/Cyd
Cube
Str
N/mm2
Ɛc,
micro-
strain
Ɛcu,
micro-
strain
Ɛsu,
micro-
strain
K
(IS cur) k2 Xu(max)/d
Mo-
ment
(kN-M)
24.00 1.28 30.72 2284.00 3711.00 4175 0.35 0.41 0.471 109.35
23.50 1.28 30.08 1972.00 3789.00 4175 0.37 0.42 0.476 111.49
34.40 1.28 44.03 2175.00 3063.00 4175 0.34 0.40 0.423 139.38
33.80 1.28 43.26 1877.00 3220.00 4175 0.36 0.41 0.435 146.72
48.60 1.24 60.26 2151.00 3324.00 4175 0.31 0.36 0.443 184.52
46.09 1.24 57.15 2341.00 3323.00 4175 0.30 0.36 0.443 168.28
76.83 1.16 89.12 2702.00 2931.00 4175 0.26 0.34 0.412 222.27
76.18 1.16 88.37 2539.00 2729.00 4175 0.26 0.33 0.395 210.97
103.90 1.14 118.45 2774.00 2774.00 4175 0.25 0.33 0.399 271.46
106.00 1.14 120.84 2799.00 2799.00 4175 0.25 0.33 0.401 278.21
103.90 1.14 118.45 2600.00 2600.00 4175 0.25 0.33 0.384 262.51
106.00 1.14 120.84 2600.00 2600.00 4175 0.25 0.33 0.384 267.82
Table 8 Calculation of moment capacity as per the Euro code (transformed in IS code parameters) for bal-
anced section
Cyd str.
N/mm2 Cube/Cyd
Cube
Str,
N/mm2
Ɛc,
micro-
strain
Ɛcu,
micro-
strain
k4 K'(EC) λ/2 Xu(max)/d
Mo-
ment
(kN-M)
24.00 1.28 30.72 2000.00 3500.00 1.25 0.35 0.40 0.448 106.60
23.50 1.28 30.08 2000.00 3500.00 1.25 0.35 0.40 0.448 104.38
34.40 1.28 44.03 2000.00 3500.00 1.25 0.35 0.40 0.448 152.79
33.80 1.28 43.26 2000.00 3500.00 1.25 0.35 0.40 0.448 150.13
48.60 1.24 60.26 2000.00 3500.00 1.25 0.37 0.40 0.368 186.19
46.09 1.24 57.15 2000.00 3500.00 1.25 0.38 0.40 0.368 179.93
76.83 1.16 89.12 2485.95 2610.53 1.42 0.31 0.37 0.324 210.12
76.18 1.16 88.37 2479.67 2612.77 1.42 0.31 0.37 0.324 209.60
103.90 1.14 118.45 2600.00 2600.00 1.42 0.28 0.35 0.323 251.85
106.00 1.14 120.84 2600.00 2600.00 1.42 0.28 0.35 0.323 256.94
Page 8
Journal of Asian Concrete Federation, Vol. 6, No. 1, June 2020
8
as per many international standards.
The study indicates that it is better to give ide-
alized stress strain curve for different concrete
grades in Design Standards which will also highlight
the decrease in the bandwidth of strain values with
increase in the grade of concrete and will take into
effect the steepness of curve also. Therefore, it is im-
portant to include the influence of stress-strain char-
acteristics i.e., the shape of the stress-strain curve,
the strain at peak stress and ultimate strain in the
flexural design for safe and efficient design of struc-
tural members using high strength concrete.
References
1. V.V. Arora; Brijesh Singh; and Shubham Jain
(2017) “Effect of indigenous aggregate type on
mechanical properties of High Strength
Concrete”, Indian Concrete Journal, 91(1):34-
44.
2. V.V. Arora; Brijesh Singh; and Shubham Jain
(2016) “Experimental studies on short term
mechanical properties of High Strength
Concrete”, Indian Concrete Journal, 90(10):65-
75
3. Emperger, F. (1904) "Ein graphischer Nachweis
der Tragfahigkeit und aller in einem Tragwerke
aus Eisenbeton auftretenden Spannungen",
Beton und Eisen 4(5):306-320
4. Whitney, C. S. (1937) “Design of reinforced
concrete members under flexure or combined
flexure and direct compression”, ACI Journal
Proceedings, ACI
5. Hognestad, E., N. W. Hanson, et al. (1955)
“Concrete stress distribution in ultimate strength
design”, ACI Journal Proceedings, ACI
6. Mattock, A. H., L. B. Kriz, et al. (1961)
“Rectangular concrete stress distribution in
ultimate strength design”, ACI Journal
Proceedings, ACI
7. Ashour, S. A. (2000) “Effect of compressive
strength and tensile reinforcement ratio on
flexural behavior of High-Strength Concrete
Beams”, Engineering Structures, Vol.22, p.413-
423
8. Oztekin E.; Pul S.; Husem, M. (2003)
“Determination of rectangular stress block
parameters for High Performance Concrete”,
Engineering Structures, 25:371-376
9. Attard M.M.; Stewart M.G. (1998) “Two
parameter stress block for High-Strength
Concrete”, ACI Structural Journal 95(3):305–
317
10. Bernardo, L.F.A.; Lopes S.M.R. (2004)
“Neutral axis depth versus flexural ductility in
High-Strength Concrete Beams”, J. Struct. Eng.,
130(3):452-459
11. ACI Committee 318 (2008) “Building Code
Requirements for Reinforced Concrete and
Commentary ACI 318M-08”, Manual of
Concrete Practice. American Concrete Institute.
Michigan. pp 465. USA
12. Standards New Zealand, NZS 3101 (2006),
“Concrete Structures Standard, Part 1 - The
Design of Concrete Structures”, Wellington,
New Zealand
13. Ho JCM; Pam HJ; Peng J; Wong YL (2010)
“Maximum concrete stress developed in
flexural RC members”, Computers and
Concrete
14. Swartz SE; Nikaeen A; Narayan BHD;
Periyakaruppan N; Refai TME (1985)
“Structural bending properties of high strength
concrete”, ACI Special Publication, 87(9):
p.147-178
15. Tan TH; Nguyen NB (2005) “Flexural behavior
of confined high-strength concrete columns”,
ACI Structural Journal. 102(2):198-205
16. Mansur MA; Chin MS; Wee TH (1997)
“Flexural behavior of high-strength concrete
beams”, ACI Structural Journal. 94(6):663-673
17. Kaar PH; Hanson NW; Capell HT (1978)
“Stress-strain characteristics of high strength
concrete”, ACI Special Publication. Douglas
McHenry International Symposium on Concrete
and Concrete Structures. ACI. Detroit. pp.161-
185
18. Cetin, A.; Carrasquillo, R. L. (1998) “High-
Performance Concrete: Influence of Coarse
Aggregates on Mechanical Properties”, Journal
of ACI Materials, 95:252-261
19. Peng Ho; Pam Wong (2012) “Equivalent stress
block for normal-strength concrete
incorporating strain gradient effect”, Magazine
of Concrete Research Volume 64 Issue
20. Kahn, L. F.; K. F. Meyer (1995) "Rectangular
Page 9
Journal of Asian Concrete Federation, Vol. 6, No. 1, June 2020
9
stress block for nonrectangular compression
zone", ACI Structural Journal 92(3)
21. Ibrahim, H. H. H.; J. G. MacGregor (1996)
"Flexural behavior of laterally reinforced high-
strength concrete sections", ACI Structural
Journal 93(6)
22. Eurocode 2-2004: Design of concrete structures,
European Committee for Standardization
23. IS: 456-2000, “Code of Practice for Plain and
Reinforced Concrete”, Bureau of Indian
Standards, New Delhi
24. IS: 383-2016, “Coarse and fine aggregate for
Concrete-Specification”, Bureau of Indian
Standards, New Delhi
25. CPWD-2009, Central Public Works Department
Manual, Government of India
26. IS: 1727-1967, “Methods of Test for Pozzolanic
Materials”, Bureau of Indian Standards, New
Delhi
27. IS: 516-2018 "Hardened concrete-method of
tests", Bureau of Indian Standards, New Delhi
Page 10
Journal of Asian Concrete Federation
Vol. 6, No. 1, pp. 10-25, June 2020
ISSN 2465-7964 / eISSN 2465-7972
https://doi.org/10.18702/acf.2020.6.6.10
10
Technical Paper
Effect of clay as deleterious material on properties of nor-
mal-strength concrete
Harish R. Choudhary, Saha Dauji*, Arham Siddiqui
(Received February 12, 2020; Revised July 2, 2020; Accepted July 3, 2020; Published July 3, 2020)
Abstract: Sustainability concerns prompted use of crushed aggregates in concrete, wherein deleterious
materials might get inadvertently included. Some deleterious materials are allowed up to limiting values
by most standards, which, however, are silent about the quantification of their effects on properties of
concrete – which would be site specific. For an important Indian infrastructure, this study quantifies
effects of clay fines as deleterious material in concrete, on workability (slump) and strength (cube com-
pression; split tensile; flexural tensile tests) around the limit (1% of fine aggregates by weight) stipulated
by the Indian standard. The novelty of the study is that, contrary to the literature in this domain, the
effects are studied without alteration of the mix proportions – a different practical scenario. The limit of
clay fines in concrete allowed by Indian standard was found to be adequate considering strength param-
eters, but for maintaining target workability, the limit would be revised to 0.5% of the fine aggregates.
Generally, the variations of concrete properties with the increasing clay fines were: (1) the workability
and split tensile strength reduced monotonically, in non-linear fashion; (2) compressive strength (beyond
7 days) and the flexural tensile strength (modulus of rupture) reduced monotonically in linear manner.
Keywords: concrete; clay; workability; compressive strength; modulus of rupture; split tensile strength
1. Introduction
During last four to five decades, concrete has
grown in popularity as a building material
particularly due to its flexibility in the geometry, its
resistance to the environment and fire as compared
to steel, and cost effectiveness. The concrete
generally consists of the ingredients: coarse
aggregate, fine aggregate, cement and other binding
materials, water, and admixtures, as applicable. The
design of the concrete mix is performed with the
national building codes [1,2] in order to achieve the
desired characteristic strength. The mix design,
being conducted in the laboratory, happens under
strict quality control. However, the actual concrete
being mixed either at site (more uncertainty) or at the
batching plant (less uncertainty and better control
than site mix), there always exist the possibilities of
deleterious materials being inadvertently mixed in
concrete. In fact, Indian and International codes [3,4]
permit certain limiting amount of deleterious
material being present in concrete, and still it can be
acceptable. However, the standards are silent on the
effects of the deleterious material, if present, on the
properties of green or hardened concrete.
The deleterious material is defined in literature
as materials which might affect the concrete in the
following ways:
1) Materials which might interfere with the
process of hydration of cement [5];
2) Coatings which would prevent development
of good bond between aggregate and cement paste
[5];
3) Weak or unsound materials which would
affect the strength of concrete [5].
The classification of deleterious materials was
conducted as early as 1950-s [6]. Broadly classified,
the deleterious materials would fall under the follow-
ing categories:
1) Organic impurities: they interfere with the
hydration process of cement. They are generally pre-
sent in the form of humus or organic loam and mostly
found in the fine aggregates [5,6].
2) Clay and other fine materials: Clay is usually
present in the form of coating on the aggregates and
interferes with bond between aggregates and cement
matrix. Silt and other fine materials might be present
as coatings, when the bond is affected; or as loose
Harish R. Choudhary is a Scientific Officer at Nuclear Fuel
Complex, Kota, India.
Corresponding author Saha Dauji is a Scientific Officer at
Bhabha Atomic Research Centre, Mumbai, India and a lecturer
at Homi Bhabha National Institute, Mumbai, India
Arham Siddiqui is a is a Scientific Officer at Nuclear Fuel
Complex, Kota, India.
Page 11
Journal of Asian Concrete Federation, Vol. 6, No. 1, June 2020
11
particles which are not bonded and reduces the
strength and durability of concrete [5,6].
3) Salt contamination: mostly occurs when the
fine aggregate (sand) originates from sea and is a ma-
jor durability concern as it could effect chloride in-
duced corrosion of reinforcement [5,6].
4) Unsound particles: these are particles which
might lose their integrity after casting of concrete or
might undergo disruptive expansion in contact with
water or in case of freezing. Clay lumps, if present in
concrete, would be considered as the unsound parti-
cles [5,6].
In developing countries, the issue of utilization
of locally available aggregates for concrete has been
examined [7]. The effect of presence of clay in con-
crete has been investigated by many researchers
across the world [8-21], and techniques have been
proposed to evaluate the possible harmful influences
of the clay in concrete [21].
There has been a growing concern of the eco-
logical and environmental impacts of the large scale
quarrying or dredging conducted for the supply of
aggregate as raw materials for concrete. These in-
clude the destruction of habitat, disruption of food
chains, disturbance to the local ecology, instability
of the courses of rivers, increased erosion, scour, or
depositions on the banks of rivers or the coasts due
to sand dredging, and changes in the river regimes or
coastal hydrodynamics [22]. Therefore, researchers
all over the world are exploring alternate sources of
aggregate such as recycled concrete, building mate-
rials, glass, tyres or ceramic tiles [22-25]. The dis-
posal of the excavated materials from major con-
struction projects pose another concern which often
becomes extremely expensive because of the large
lead distances involved between the construction
sites and the disposal site. As a practical solution to
either of the aforementioned problems, in many pro-
jects, the practice of using crushed aggregates from
the excavated rock from the same site is presently
getting preference from sustainability considerations
[26-28].
Till recently, use of crushed fine aggregates was
less popular in Indian construction projects, particu-
larly due to the lack of experience and data. But ex-
perience from the earlier works and stipulations of
the national codes [3] have fostered use of up to
100% crushed coarse and fine aggregate manufac-
tured from the locally excavated rock, provided they
met the quality standards [3]. In a major infrastruc-
ture project in India, the same scheme was adopted
and the excavated rock (Fig. 1a) from the site was
crushed in the local crusher plants (Fig. 1b) for sub-
sequent use as aggregates, both coarse and fine, in
normal-strength concrete. The geology of the site
showed that the major rock type was sandstone with
clay matrix. This could lead to contamination of
crushed aggregates with clay. Clay being an unsound
particle would act like weak intrusion in concrete
mix. The standard practice of preparation of aggre-
gates includes washing procedure, precisely for re-
moval of such impurities. However, the possibility
of clay in the concrete made with the crushed aggre-
gates from the excavated rock could not be ruled out.
It has been mentioned earlier that the national code
allows a certain amount of clay in the concrete (1%
for India [3]), but does not specify the effect on the
workability or strength of concrete. There are vari-
ous limits of clay in concrete based on its mineral
composition available in literature [20]. This is un-
derstandable as the deleterious materials, in this case
– clay, would vary in properties from site to site and
consequently, the effect on concrete properties could
also be different. The deleterious materials being site
specific, the study on the effect of the same on con-
crete would also be site specific. As the project was
of national importance, the possible effects of inad-
vertent inclusion of clay on the properties of fresh
and hardened concrete were deemed necessary to be
critically examined.
(a) (b)
Fig 1: (a) excavated rock at site; (b) crusher plant at site
Page 12
Journal of Asian Concrete Federation, Vol. 6, No. 1, June 2020
12
Schuster [8] presented a detailed review of the
studies pertaining to the effects of various deleteri-
ous materials present in concrete aggregates on the
properties of concrete. The effect of very fine aggre-
gate was reported as increase in the 91-day strength,
at constant workability [9]. Though some researchers
[29] indicated that very large amount of fines could
be incorporated into concrete without much detri-
mental effect on its properties. Abou-Zeid and
Fakhry [10] reported that addition of clay fines to
concrete in even small amount reduced the workabil-
ity to a significant extent, though small proportions
of clay increased the compressive strength by a cer-
tain extent. Gullerd [11] examined the effects of var-
ious aggregate coatings on the performance of Port-
land cement concrete, including durability (drying
shrinkage, freeze-thaw, chloride vulnerability) and
strength (compressive) and concluded that the clay
coatings were more detrimental compared to the dust
or carbonates. Munoz et al. [12] conducted detailed
experimentation on the effect of clay coatings on ag-
gregates and reported that fraction of clay getting de-
tached and entering water phase would depend on
the original clay content as well as the properties of
clay, which could vary from site to site. The clay in
cement paste affected the rate of hydration, and this
effect would also be dependent on the nature of clay,
they indicated. Katz and Baum [13] explored the
higher levels of fines on the strength properties of
concrete with detailed experimental program, but us-
ing concrete of constant workability. They con-
cluded that as long as workability was controlled to
the target values by addition of suitable admixtures,
the strength of concrete increased up to 30% with in-
creases in volume change of fresh and hardened con-
crete – with addition of small amounts of fines.
Munoz et al. [14] examined the effects of vari-
ous aggregate coatings and films on the concrete per-
formance, and concluded that the clay coatings were
more harmful than other microfine mineralogy such
as dust or carbonates, the observations being similar
to earlier literature [11]. Cramer [30] indicated that
the stipulations of the codes might not be adequate in
assuring the desired properties of concrete when mi-
crofines were concerned, due to their typical miner-
alogical composition. They investigated the effect of
microfines on strength and workability of concrete,
and concluded that dolomitic microfines did not af-
fect workability much, but clay microfines definitely
affected the workability adversely with a little im-
provement in strength properties. Abib et al. [15] ex-
plored the properties of self-compacting concrete
with fine clay from waste crushed brick, and con-
cluded that the strength improved with up to 5% of
the fine clay addition.
The earlier studies examined the strength prop-
erties of concrete at constant workability – a condi-
tion which might not actually exist at a construction
site. Hence, in this study, the concrete mix is kept
unaltered, and the effect of clay on both workability
and the strength of concrete are studied for different
proportions of clay in concrete. To the knowledge of
the authors, this approach has not been earlier re-
ported in literature. The initial results of the experi-
ments were presented by the authors in a conference
earlier [16] and the analysis of the complete set of
results is presented in this article.
This article presents the investigation conducted
with slump test for the workability of fresh concrete
and a series of cube and cylinder tests for the strength
properties of hardened concrete at different ages
from 7 days to 91 days. The effect of clay as delete-
rious material in concrete was investigated around
the acceptable limit stipulated by the Indian code [3].
The results of this study would help to reaffirm the
limit stipulated in the code as acceptable for the site,
or would help to fix site specific limit on the clay in
aggregates for attaining the desired strength and
workability of the resulting concrete.
2. Methodology
2.1 Standard mix design
The grade of concrete for the facility was M25,
that is, the 95 percent confidence value of 28-day
cube compressive strength was 25 MPa. The target
slump value was 120 mm, measured according to the
Indian standard [31]. The design of the concrete mix
was carried out at the site according to the Indian
standard [2], and the final proportions for the M25
mix is reproduced in Table 1 for ready reference. The
target strength of the mix thus prepared would be
31.6 MPa.
Table 1 Standard design mix (M25)
Grade of
concrete W/C ratio
Quantities of materials per cubic meter of concrete
Slump
(mm) Cement
(kg)
Water
(L)
Plasticizer
(kg)
Sand
(kg)
Coarse aggregate
(kg)
20 mm 10 mm
M25 0.48 334 160 2 806 624 416 120
Page 13
Journal of Asian Concrete Federation, Vol. 6, No. 1, June 2020
13
The crushed aggregates used in the mix design,
namely, 20 mm, 10 mm, and crushed sand are de-
picted in Fig. 2(a), Fig 2(b) and Fig. 2(c) respec-
tively.
2.2 Properties of deleterious material: clay
The origin of the clay at the site can be traced
back to the Vindhyan range of sedimentary layer of
reddish black soil [32]. The clay fines have a liquid
limit of 42% and plasticity index of 27%, which clas-
sifies the fines as clay of medium to high expan-
sion/compression properties, according to Indian
standard [33]. The clay fines have a free swell index
of 53% lying in the range of very high degree of ex-
pansiveness. Particularly, this property might cause
porosity in concrete and this might eventually lead to
strength and durability issues [11,12,14,30].
2.3 Design of experiment: concrete mix with clay
The IS code [3] stipulation is 1% as a maximum
for clay to be present in concrete, as a percentage of
the crushed fine aggregate by weight. The standard
mix (SM) contains no clay, and two mixes are taken
between standard mix (SM) and the IS limit. Two
other mixes are taken higher than the IS code stipu-
lation, namely, 1.5 times and 2 times the limiting
value. All the details of the mixes are reproduced in
the Table 2. The workability (slump) was tested for
all the mixes before the casting of the cubes and cyl-
inders. The strength tests were conducted at different
ages of concrete from 7 days to 91 days and the spe-
cific details are listed in Table 3. The cube strength
was tested at 7 days, 14 days, 28 days, 63 days and
91 days whereas the cylinder and flexural tests were
conducted at 28 days only.
The IS code [3] stipulation is 1% as a maximum
for clay to be present in concrete, as a percentage of
the crushed fine aggregate by weight. The standard
mix (SM) contains no clay, and two mixes are taken
between standard mix (SM) and the IS limit. Two
other mixes are taken higher than the IS code stipu-
lation, namely, 1.5 times and 2 times the limiting
value. All the details of the mixes are reproduced in
the Table 2. The workability (slump) was tested for
all the mixes before the casting of the cubes and cyl-
inders. The strength tests were conducted at different
ages of concrete from 7 days to 91 days and the spe-
cific details are listed in Table 3. The cube strength
was tested at 7 days, 14 days, 28 days, 63 days and
91 days whereas the cylinder and flexural tests were
conducted at 28 days only.
(a) (b)
(c) (d)
Fig 2: Aggregates (a) 20 mm; (b) 10 mm; (c) crushed sand; (d) deleterious material: prepared clay before be-
ing pulverized
Page 14
Journal of Asian Concrete Federation, Vol. 6, No. 1, June 2020
14
2.4 Standard procedure for mixing, casting and
curing of concrete specimens
The experiments are targeted towards capturing
the effects of deleterious materials which comes to
around a thousandth by weight. Hence, it was essen-
tial to adopt a carefully formulated Standard Proce-
dure for conducting the experiments, which is briefly
discussed in the following sub-sections. These are
conforming to the stipulations laid down by the In-
dian standards [1,34,35].
2.4.1 Preparation of materials
All materials are brought to room temperature,
that is, within 27ºC ± 3º C, before commencing with
the mixing. The cement samples, on arrival at the la-
boratory, were thoroughly mixed dry by hand so as
to ensure the greatest possible blending and uni-
formity in the material. The cement was stored in a
dry place, in air-tight metal containers. Aggregates
for each batch of concrete were air dried before using
in mix. Water was potable water and was at room
temperature. The preparation of the clay, to be added
as deleterious material deserves particular mention.
The soil was sieved through 75 micron sieve in wet
condition (slurry) to collect the clay fines in the pan.
This clay slurry is oven-dried and then pulverized to
obtain the clay for mixing with concrete during the
preparation of various concrete mixes.
2.4.2 Proportioning of materials
The proportioning of the materials was done by
weight, per cubic meter of concrete according to the
mix design listed in the Table 2 for the particular
mix. Weigh batching was used for proportioning.
The weighing of the cement and each size of aggre-
gate is done to an accuracy of 0.1 percent of the total
weight of each batch.
2.4.3 Mixing
A small mixing machine was used for mixing of
materials. Hand loading was adopted for loading the
materials on the mixer. The mixing drum was loaded
with about one-half of the coarse aggregates, fol-
lowed by the fine aggregates, the deleterious mate-
rial: clay, the cement and finally with the remaining
coarse aggregate on top, in that order. The water, in
which the admixture was already mixed, was added
to the mixing drum just before starting the mixing
machinery. The period of mixing was kept not less
than 2 minutes and was continued till the resulting
concrete was uniform in appearance.
2.4.4 Casting of cubes, beams and cylindrical
specimens
The size of the cube test specimens was 150 mm
× 150 mm × 150 mm, confirming to Indian standard
[36]. The average strength of three cubes was re-
ported as the compressive strength at a particular age
for the mix, except for 28 days' strength – whence
the number of cubes was 13. For the six different
mixes (CM, and C1 to C5) and five different ages (7,
14, 28, 63 and 91 days) for test of compressive
strength, the total number of cube specimens tested
was 150 (Table 3). The cylindrical moulds were of
150 mm diameter and 300 mm length, and they con-
formed to [36]. The beam moulds were of 150 mm
square cross-section and 700 mm length, confirming
to [36]. The total number of cylinder specimens as
Table 2 Mix design with clay as deleterious material
Mix designa-
tion
Percent-
age of
clay with
respect to
sand
Quantities of materials per cubic meter of concrete
Re-
marks Ce-
ment
(kg)
Wa-
ter
(L)
Plasticiz-
ers.
(kg)
Sand
(kg)
Deleteri-
ous mate-
rial
(kg)
Coarse aggre-
gate (kg)
20mm 10mm
CM Nil 334 160 2 806 Nil. 624 416 Control
Mix
C1 0.25 334 160 2 804 2 624 416 -
C2 0.50 334 160 2 802 4 624 416 -
C3 1.00 334 160 2 798 8 624 416 IS Code
Limit
C4 1.50 334 160 2 794 12 624 416 -
C5 2.00 334 160 2 790 16 624 416 -
Page 15
Journal of Asian Concrete Federation, Vol. 6, No. 1, June 2020
15
well as the total number of beam specimens was 18
(Table 3). All the joints between the various sections
of moulds were thinly coated with mould oil in order
to ensure that no water escapes during the casting
procedure as well as to prevent adhesion of the con-
crete to the moulds.
2.4.5 Compaction
The concrete was filled into the mould in layers
approximately 5 cm deep. In placing each scoopful
of concrete, the scoop is moved around the top edge
of the mould. A total of 35 strokes were given with
tamping rod for each layer thus laid. The strokes
were given in a manner such that the tamping rod
penetrated all the underlying layers. The sides of
mould were also tapped to close the voids, which
might have been left by tamping rod. Thus, dense
compact concrete samples were casted for later test-
ing.
2.4.6 Specimen identification
After the initial setting, all the specimens (cu-
bes, cylinders and beams) were labelled with perma-
nent marker with mix designation as show in Table
3 along with the date of casting.
2.4.7 Curing
The test specimens, in their respective moulds,
were stored in a place free from vibration and away
from direct sunlight for 24 ± 1/2 hours from the time
of mixing. After this period, the specimens were de-
moulded, marked and submerged in clean, fresh wa-
ter for the purpose of curing. The water in which the
specimens were submerged was periodically re-
newed, every seven days.
2.5 Standard procedure for tests on concrete
specimens
The samples after casting are shown in Fig. 3(a)
for cube, Fig. 3(b) for cylinder, and Fig. 3(c) for
beam. At the designated date of testing, the samples
were taken out of the curing pond and tested accord-
ing to the standard procedures, discussed in subse-
quent sub-sections.
2.5.1 Slump test for workability
Slump cone method of testing was used for
evaluation of workability according to the Indian na-
tional standard [31]. The internal surface of the
mould was cleaned and made free from superfluous
moisture and any set concrete before commencing
the test. The mould was placed on a smooth, horizon-
tal, rigid and non-absorbent surface, in the form of a
carefully levelled metal plate. The mould was firmly
held in place while it was filled. The mould was
filled in four layers, each approximately one-quarter
of the height of the mould. Each layer was com-
pacted with 25 strokes of the rounded end of the
tamping rod, distributed in a uniform manner over
the cross-section of the mould, which for the second
and subsequent layers penetrated into the underlying
layer/s. The bottom layer was tamped throughout its
depth. After completely filling the mould, the con-
crete was struck off level at the top with a trowel, so
that the mould is exact1y filled. Any mortar which
might have leaked out between the mould and the
base plate was cleaned away. Then, the mould was
removed from the concrete by raising it slowly and
carefully in a vertical direction. This allowed the
concrete to subside and the slump was measured im-
mediately thereafter by determining the difference
between the height of the mould and that of the high-
est point of the particular specimen. The slump
measured was recorded in terms of millimetres of
subsidence of the specimen during the test.
Table 0 Test matrix
Mix Designation
Number of cubes casted for compressive strength
test
Nos. of cylinders
casted for split
tensile test
Nos. of beams
casted for flex-
ural strength
test
7 days 14 days 28 days 63 days 91 days 28 days 28 days
CM 3 3 13 3 3 3 3
C1 3 3 13 3 3 3 3
C2 3 3 13 3 3 3 3
C3 3 3 13 3 3 3 3
C4 3 3 13 3 3 3 3
C5 3 3 13 3 3 3 3
Page 16
Journal of Asian Concrete Federation, Vol. 6, No. 1, June 2020
16
2.5.2 Cube test of compressive strength
Specimens were tested immediately on removal
from the water and while they were still in the wet
condition [34]. The bearing surfaces of the testing
machine was wiped clean and any loose sand or other
material were removed from the surfaces of the spec-
imen which would be in contact with the compres-
sion plates. The specimen was placed in the machine
such that the load would be applied to opposite sides
of the cubes as cast, that is, not to the top or bottom.
The axis of the specimen was carefully aligned with
the centre of thrust of the seating plates. The load
was applied without shock and increased continu-
ously at a rate of approximately 140 kg/cm2/min un-
til the resistance of the specimen to the increasing
load breaks down and no greater load can be sus-
tained [34]. The maximum load applied to the speci-
men was then recorded. The measured compressive
strength of the specimen was calculated by dividing
the maximum load applied to the specimen during
the test by the cross-sectional area, calculated from
the mean dimensions of the section. In general, In-
dian standard stipulates that an average of three spec-
imen values should be taken as the representative of
the sample, provided the individual variation is not
more than ± 15 percent of the average [1]. For 28
days' strength, 13 cubes were tested and the mean of
the 13 was taken as the representative compressive
strength at 28 days. For all other ages, the average of
three samples was adopted as the representative
compressive strength.
2.5.3 Split tensile test
The split tensile strength test on cylindrical
specimens was performed according to the guide-
lines of Indian national standard [35]. The test spec-
imen was tested in wet condition soon after its re-
moval from the curing tank. The surface in contact
with the loading plate was made smooth and free
from protruded aggregates or any kind of undula-
tions. The centre line was marked across length of
the cylindrical specimen on the opposite sides and
the dimensions of the specimen were measured, to
the nearest 0.2 mm. The weight of the specimen was
measured and recorded. The specimen was placed in
the loading machine between the edges of two angles
ISA 65x65x6. The specimens were so placed that the
edge of the angles coincides with the centre line
made on the opposite faces of the cylindrical speci-
men. The jig was then placed in the machine so that
the specimen was located centrally. Care was taken
so that the loading through the packing strip was
truly axial. The load was applied without shock and
increased continuously at a nominal rate within the
range 1.2 N/mm2/min to 2.4 N/mm2/min [35]. The
maximum load at which the specimen failed was rec-
orded and the split tensile strength fst, of the speci-
men was calculated to the nearest 0.05 MPa using
Eq. (1) [35]:
𝑓𝑠𝑡 =2𝑃
𝜋𝑙𝑑 (1)
where,
fst = Split tensile strength of cylindrical specimen
(MPa)
P = maximum load applied to the specimen (N),
l = Length of the specimen (mm).
d = Cross sectional dimension of the specimen (mm).
2.5.4 Flexural test
The flexural strength test on beam specimens
was performed along the guidelines of Indian na-
tional standard [35]. As for the earlier samples, the
beam specimen was tested in wet condition, soon af-
ter its removal from the curing tank and the test was
conducted according to the Indian standard [34]. The
dimensions and weight of the specimens were rec-
orded before the testing. In this case, surface prepa-
ration was not required before placing the concrete
beam on the testing machine. The axis of the speci-
men was carefully aligned with the axis of the load-
ing device. The test setup was done on normal com-
pressive strength testing machine. A channel of
depth 150 mm was placed on the bottom loading
plate. Over it two angles were placed and welded
such that their vertex edge lied towards the top. The
distance between the two angles was maintained
such that their vertex was 600 mm apart. The beam
specimen was placed on these two angles without
disturbing the bottom channel and angle setup, with
50 mm overhang on either side of the vertices of the
angles. Another channel and angle setup, as de-
scribed above, was made for the top. In this case, the
distance between the vertices of the angles was kept
as 200 mm, and these would be the two-point loading
locations. When this was placed above the beam cen-
trally, it was thus possible to divide the beam into
three equal parts of 200 mm each. After fixing the
setup the load application was started, without shock
and increasing continuously at a rate such that the
extreme fibre stress increased at approximately 7
kg/cm2/min, that is, at a rate of loading of 400 kg/min
for the 15·0 cm specimen [34]. The load was in-
creased until the specimen failed, and the maximum
load was recorded. The appearance of the fractured
faces of concrete and any unusual features in the type
of failure was also noted. The flexural strength of the
specimen was expressed as the modulus of rupture
fcr, which, if ‘a’ equals the distance between the line
of fracture and the nearer support, measured on the
centreline of the tensile side of the specimen, in cm,
shall be calculated to the nearest 0.5 kg/sq.cm using
Eq. (2) when ‘a’ is greater than 20.0 cm for 15.0 cm
Page 17
Journal of Asian Concrete Federation, Vol. 6, No. 1, June 2020
17
specimen, or Eq. (3) when ‘a’ is less than 20.0 cm
but greater than 17.0 cm for 15.0 cm specimen, ac-
cording to the failure condition:
𝑓𝑐𝑟 =𝑝𝑙
𝑏𝑑2 (2)
𝑓𝑐𝑟 =3𝑝𝑎
𝑏𝑑2 (3)
Where
fcr = flexural modulus of concrete (MPa)
b = measured width of the specimen (mm),
d = measured depth in mm of the specimen at the
point of failure (mm),
l = length in mm of the span on which the specimen
was supported (mm), and
p = maximum load applied to the specimen (N).
3. Results and discussion
In this section, the results of the experiments on
workability of green concrete and the strength of the
hardened concrete are presented and the salient ob-
servations are discussed.
3.1 Effect on fresh concrete: workability using
slump test
The results of slump value for concrete mix with
clay fines as deleterious material are presented in Ta-
ble 4. The effect observed on the slump values at the
explored concentrations (0.25% to 2%) of clay as
deleterious material in fine aggregate is reduction
ranging from 7% to 22%. The clay present in the
concrete mix absorbs the water and reduces the free
water thereby reducing the workability. This might
also affect the strength properties due to possible
lack of water for the purposes of complete hydration
of cement. The prevalent practice of evaluation of
strength at constant workability (by suitable altera-
tion of water-cement ratio or by addition of suitable
admixtures), as reported in literature, would be lim-
ited in examining this aspect. This study targets to
identify such possible effects on the strength proper-
ties of concrete by examining the cases with the de-
sign water-cement ratio and the admixture quantity.
The variations in the slump has been presented
in pictorial form in Fig. 4, where it can be clearly
noted that even at the IS code limit of 1% (C3) the
slump is lower than the target slump of 120 mm and
hence the IS code [3] limit of clay of 1% is unac-
ceptable for this mix. Thus, for the present case, the
limit of clay in concrete is recommended to be re-
vised to 0.5% according to the findings of this study.
In literature [30], it had earlier been indicated that the
stipulations in codes regarding the microfines such
as clay might prove inadequate depending upon their
(a) (b) (c)
Fig 3: Samples (a) cube; (b) cylinder; (c) beam;
Table 4 Results of slump test on green concrete with clay as deleterious material
Mix designation
Deleterious
material (clay)
(%)
Slump (mm) Percent variation Remarks
CM Nil 135 - Control Mix
C1 0.25 125 7 Acceptable slump
C2 0.50 120 11 Target slump
C3 1.00 110 19 IS code limit - Unac-
ceptable slump
C4 1.50 110 19 Unacceptable slump
C5 2.00 105 22 Unacceptable slump
Page 18
Journal of Asian Concrete Federation, Vol. 6, No. 1, June 2020
18
mineralogical compositions and this was proved
true in this case. Further, that the presence of small
amount of clay in concrete might reduce workability
to a significant extent, as inferred from this study,
was earlier reported in literature [10].
3.2 Effect on hardened concrete: compressive
strength
The summary of the average compressive
strength of cubes for the different ages and the vari-
ous percentages of clay (deleterious material) are
presented in Table 5 (detailed results are provided in
the Appendix A). The general trend is observed that
for all ages, the higher percentages of deleterious
material (clay) result in lower strength. The standard
deviation obtained from the 13 samples (as elabo-
rated in Appendix A) for 28 days compressive
strength are included in Table 5. The small values of
standard deviation (between 1.04 MPa and 1.57
MPa) indicate that the quality control implemented
for the experiment was good. However, it is to be
noted that the standard deviation estimated from
small samples might be prone to errors, and should
be used with caution.
The compressive strength as a function of in-
creasing clay percentages in concrete are plotted in
Fig. 5a for the various ages. It was observed that at
the IS code [3] limit (1%), the compressive strength
obtained with the design mix attains the target com-
pressive strength of 31.6 MPa. In fact, the target
compressive strength could be achieved with inclu-
sion of up to around 1.6% clay. Hence from com-
pressive strength considerations, the IS code limit
may be concluded to be conservative. It may be
noted that at the IS code limit of 1% (C3), the com-
pressive strength reduced by around 14%, compared
to the control mix (CM), whereas the reduction in-
creased to around 32% for 2% clay (C5). The
strength reduction with increasing percentage of clay
appears to be linear in this case for all ages, except
the 7-day tests.
Fig 4: Variation of workability: slump
Table 5 Average compressive strength (MPa) of hardened concrete with clay as deleterious material
Mix designation
Age of concrete (days) Percent
variation
of 28
days
Strength
Standard
deviation of
28-days
Strength
7 days 14 days 28 days 63 days 91 days
Remarks:
28-day
strength
CM 31.08 38.27 41.78 47.45 48.71 - 1.57 Acceptable
C1 32.07 36.77 39.73 43.94 46.39 - 5 1.25 Acceptable
C2 31.88 35.96 37.28 42.34 44.15 - 11 1.04 Acceptable
C3 30.07 32.95 35.80 39.69 41.35 - 14 1.07 Acceptable
C4 27.14 30.79 33.30 36.34 38.25 - 20 1.26 Acceptable
C5 23.13 25.91 28.34 31.30 33.04 - 32 1.30 Unaccepta-
ble
Page 19
Journal of Asian Concrete Federation, Vol. 6, No. 1, June 2020
19
The rate of gain of strength appeared to be af-
fected, particularly in the initial period of hydration.
This aspect is examined in the Fig. 5b where the
strength development is plotted against the age, for
the different mixes. It can be seen in Fig. 5b that the
slope of strength gain between age 7 and 14 days is
steeper for Mix CM than the mixes with clay as del-
eterious material, which indicates that the presence
of clay reduces the rate of strength gain in concrete.
From 7 days to 28 days, the strength gain for Mix
CM is 34.4%, for Mix C1 it is 24% and for remaining
mixes the gain is around 20%–with reference to the
strength at 7 days. This shows that rate of strength
gain or the rate hydration is affected due to presence
of clay fines. This may be attributed to the high water
absorbing property of clay minerals. From 7 days to
28 days, the rate of gain for all mixes appears to be
similar and a considerable strength gain takes place.
Increase in average compressive strength con-
tinued beyond 28 days, and was observed to be
around 15% from 28 to 91 days. Use of fly-ash-based
cement, as in this case, improves the concrete
strength at later ages due to the pozzolanic reaction
[5]. As noted earlier the final compressive strength
at any age for concretes with higher percentage of
clay fines is lower [17]. This would be possibly
(a)
(b)
Fig 5: Variation of compressive strength: (a) with clay percentage (b) with age
Page 20
Journal of Asian Concrete Federation, Vol. 6, No. 1, June 2020
20
because the presence of clay fines in cement paste
matrix. Clay fines are soft, have high affinity to wa-
ter and exhibit high swelling properties [17,18]. This
swelled clay particles would dry up with progress of
hydration of cement and time, leaving several voids
within concrete–which could lead to reduced
strength. The presence of clay in the interfacial tran-
sition zone may further contribute towards the re-
duced mechanical properties of concrete [18].
3.3 Effect on hardened concrete: tensile strength
using split tensile test
The results of the split tensile test of concrete
mix with clay fines as deleterious materials are pre-
sented in Fig. 6 and the details are tabulated in Table
6. As mentioned earlier, the split tensile strength test
was conducted with concrete cylinders at 28 days
age of concrete, for all the mixes. It is generally ob-
served that the tensile strength evaluated from split
tensile test reduced with the increasing clay percent-
ages in concrete, and the variation was non-linear in
the range investigated.
From Table 6, it may be noted that at the IS code
limit of 1% (C3), the tensile strength reduced by
around 17%, compared to the control mix (CM),
whereas the reduction increased to around 30% for
2% clay (C5). The percent reduction in tensile
strength at 28 days, evaluated from split tensile test
(Table 6, last column), for various percentages of
clay as deleterious materials are similar to the varia-
tions observed in compressive strength at 28 days
(Table 5, last column). The Indian standard [1] stip-
ulates the tensile strength should be determined from
the tests conducted on the specimens, and provide a
guideline for the flexural tensile strength (= 0.7 × √
fck MPa), but there is no suggested guideline for split
tensile strength. Hence, using a conversion factor of
0.8 from cube to cylinder compressive strength
[5,34], the correlation between the average cube
compressive strength (fcube) and the split tensile
strength was taken from literature as: 0.2585 ×
(fcube)(2/3) [37] and 0.1711 × (fcube)
(0.7) [38].
The split tensile strength of concrete as a func-
tion of the average cube compressive strength
worked out from these relationships are compared to
the experimental split tensile strength in Table 7. In
Fig 6: Variation of split tensile strength
Table 6 Split tensile strength of concrete with clay as deleterious materials
Mix desig-
nation
Date of
casting
Date of test-
ing
Split tensile strength (MPa) Average split
tensile strength
(MPa)
Percent
variation 1 2 3
CM 04-04-2019 02-05-2019 3.777 3.579 3.650 3.669 -
C1 08-05-2019 05-06-2019 3.282 3.438 3.310 3.343 -9
C2 09-05-2019 06-06-2019 3.084 3.155 3.268 3.169 -14
C3 16-05-2019 13-06-2019 2.943 2.985 3.169 3.032 -17
C4 20-05-2019 17-06-2019 2.900 2.759 2.985 2.881 -21
C5 20-05-2019 17-06-2019 2.504 2.631 2.603 2.579 -30
Page 21
Journal of Asian Concrete Federation, Vol. 6, No. 1, June 2020
21
Table 7 Comparison of experimental split tensile strength of concrete with literature
Mix designa-
tion
Average cube
compressive
strength (MPa)
Split tensile strength from literature (MPa) Average split tensile
strength (MPa) [38] [37]
CM 41.78 2.33 3.11 3.669
C1 39.73 2.25 3.01 3.343
C2 37.28 2.15 2.89 3.169
C3 35.8 2.09 2.81 3.032
C4 33.3 1.99 2.68 2.881
C5 28.34 1.78 2.40 2.579
this case, it would appear that the relationships from
literature would yield conservative tensile strength
value, even up to 2% of deleterious materials (clay)
in concrete mix. However, determination of concrete
properties for a mix from actual tests should be fa-
vored over the use of values stipulated in literature,
to get more accurate estimates of the tensile strength.
3.4 Effect on hardened concrete: tensile strength
using flexural test
The results of the flexural test of concrete beams
with clay fines as deleterious materials are presented
in Fig. 7 and the details are tabulated in Table 8. As
mentioned earlier, the flexural test was conducted
with concrete beams at 28 days age of concrete, for
all the mixes. It is generally observed that the tensile
strength evaluated from flexural tests reduced with
the increasing clay percentages in concrete in an al-
most linear manner.
From Table 8, it may be noted that at the IS code
limit of 1% (C3), the tensile strength reduced by
around 12%, compared to the control mix (CM),
whereas the reduction increased to around 22% for
2% clay (C5). The percent reduction in tensile
strength at 28 days, evaluated from flexural test (Ta-
ble 8, last column), for various percentages of clay
as deleterious materials are lower than the variations
observed in both compressive strength at 28 days
(Table 5, last column) and tensile strength at 28 days
from split tensile test (Table 6, last column), though
the general reducing trend is common to all. As men-
tioned earlier, the Indian standard [1] stipulates the
tensile strength should be determined from the tests
conducted on the specimens, and provides a guide-
line for the flexural tensile strength (= 0.7 × √ fck
MPa) from the characteristic strength of concrete
(fck). The suggested values in international stand-
ards range from 0.623 × √ fc MPa to 0.6268 × √ fc
MPa [39,40], where the average cylinder strength
(fc) is used. The comparison is presented with the IS
code [1] in this study.
The flexural tensile strength of concrete as a
function of the characteristic compressive strength
(25 MPa) works out to be 3.5 MPa [1]. In this case,
it would appear that the IS guideline [1] would yield
conservative flexural tensile strength value for con-
crete, up to 2% of deleterious material (clay) in con-
crete mix. However, determination of concrete prop-
erties for a mix from actual tests should be favored
over the use of values stipulated in literature, to get
more accurate estimates of the tensile strength.
4. Summary and conclusions
Concrete is extensively used in infrastructure
development in modern times. Various replacement
options for aggregates for production of sustainable
concrete are being attempted and practiced, with a
thrust on use of locally available materials for the re-
placement. The use of 100% crushed fine aggregates
in concrete mix has been recently approved by Indian
standards, and for wider acceptance, particularly in
government sector, evaluation of the various proper-
ties of green and hardened concrete using crushed
aggregates has become relevant. Particularly, as the
crushed aggregates are prone to contain various del-
eterious materials such as crushed fines (or crusher
dust) and clay (from clay matrix in the rock from
which the aggregates are produced), evaluation of
their effects on properties of concrete needs detail in-
vestigation. In fact the codes and standards [3] allow
a certain limiting amount of deleterious materials in
concrete mix but are silent over the quantification of
their effects on the properties of concrete. In litera-
ture, various studies are reported wherein the effects
of deleterious materials were investigated, but in al-
most all the cases, the workability was kept constant
by addition of extra water or admixtures – which
might not be true in actual practice. The present ex-
perimental study targeted quantification of the effect
of clay fines in concrete mix, at and around the lim-
iting amount of clay fines according to the Indian
standard [3,4]. The novelty of the study over the ma-
jority reported in literature is that here the original
concrete mix proportions were maintained while
adding various percentages of clay as deleterious
Page 22
Journal of Asian Concrete Federation, Vol. 6, No. 1, June 2020
22
Table 8 Tensile strength of concrete from flexural test with clay as deleterious materials
Mix desig-
nation
Date of
casting
Date of test-
ing
Split tensile strength (MPa) Average split
tensile strength
(MPa)
Percent
variation 1 2 3
CM 04-04-2019 02-05-2019 4.764 5.031 4.907 4.901 -
C1 08-05-2019 05-06-2019 4.782 4.711 4.889 4.794 -2
C2 09-05-2019 06-06-2019 4.640 4.676 4.569 4.628 -6
C3 16-05-2019 13-06-2019 4.409 4.320 4.249 4.326 -12
C4 20-05-2019 17-06-2019 3.947 4.142 4.213 4.101 -16
C5 20-05-2019 17-06-2019 3.929 3.822 3.787 3.846 -22
material in concrete mixes. The experiments in-
cluded the slump test for workability; compressive
test, split tensile test and flexural test for strength
properties of concrete mix with deleterious materi-
als. The conclusions from the study are as follows:
1) The workability of concrete measured by
slump test reduced with increase in percentage of
clay from 7% reduction at 0.25% to 22% reduction
at 2%. This would be due to the higher water absorp-
tion by the clay particles, thereby reducing the free
water and resulting in harshness of mix. The ob-
served slump equaled the target slump at 0.5% of
clay and was less than target slump for all higher clay
percentages in concrete. Hence, the limiting clay per-
centage is suggested to be fixed at 0.5%, as against
the IS code [3] stipulation of 1% of fine aggregates.
2) The 7-day compressive strength increased
slightly for clay up to 0.5%, and reduced thereafter.
For all other ages, the compressive strength reduced
with increased clay percentages monotonically. The
reduction varied from 5% for 0.25% to 32% for 2%
clay in concrete mix. This could be attributed to the
swelling of clay with absorption of water [19] and
creation of voids due to drying of the same, thereby
reducing strength [17, 19]. Another possible reason
could be weakening of the interfacial transition
zones due to presence of clay [17]. For 2% clay fines
the average 28 day compressive strength was below
the target strength and thus would be unacceptable.
However, the IS code [3] stipulated value of 1% clay
fines would result in acceptable target strength.
3) The split tensile strength reduced with in-
creasing clay percentages monotonically, but in a
non-linear fashion. The reduction varied from 9% for
0.25% to 30% for 2% clay in concrete mix. There is
no guideline in IS code for the split tensile strength
and so the experimental results were compared to
values suggested in literature [37,38]. The split ten-
sile strength obtained for the concrete mixes with up
to 2% of clay as deleterious material was higher than
the values stipulated in literature [37,38].
4) The flexural tensile strength or modulus of
rupture reduced with increasing clay percentages
monotonically, apparently in a linear fashion. The re-
duction varied from 2% for 0.25% to 22% for 2%
clay in concrete mix. The flexural tensile strength
obtained for the concrete mixes with up to 2% of clay
as deleterious material was higher than the values
stipulated in IS code [1].
The major findings of this study regarding re-
duction of workability of concrete due to presence of
small amounts of clay are similar to [10], and that the
limits on microfines such as clay stipulated by the
codes might fall short of desired properties and
would have to be revised was indicated by [30]. The
findings that there was no strength improvement
(none except 7 days compressive strength up to
around 0.75% of clay), only reduction, was reported
in the same study earlier [30]. Increase in strength of
concrete due to addition of clay as reported exten-
sively in literature [9,10,13,15,29] was not observed
in this case, for any of the strength parameters exam-
ined.
Further studies are under progress for evalua-
tion and quantification of the effects of crusher fines
for the concrete mix for this site. It is recommended
that when crushed aggregates are being used for con-
crete, the effects of the possible deleterious materials
on the properties of green (workability) and hard-
ened (compressive and tensile strength; durability)
concrete should be carefully investigated, without
addition of extra water or admixtures for maintaining
constant workability. As the deleterious materials
would be site specific, separate investigation for
each source of the aggregates would be desirable.
Future studies might be towards investigating the ef-
fects of other deleterious materials or even various
combinations of the same on the workability,
strength and durability properties of concrete.
Acknowledgements
The first author sincerely acknowledges the en-
couragement and gracious support received from Dr.
M. Rajashaker (EIC, NFC, Kota) and Shri. P. A.
Page 23
Journal of Asian Concrete Federation, Vol. 6, No. 1, June 2020
23
Pratap (Project Director, NFC, Kota) during this re-
search work. The excellent contributions from the
Tata Projects Ltd. Team for providing the laboratory
infrastructure and generous help in conducting the
experiments are gratefully appreciated.
References
1. IS 456: 2000 (Reaffirmed 2005), Plain and
Reinforced Concrete - Code of Practice, Bureau
of Indian Standard, India.
2. IS 10262: 2009, Concrete Mix Proportioning -
Guidelines, Bureau of Indian Standard, India.
3. IS 383: 2016, Coarse and Fine Aggregate for
Concrete - Specifications, Bureau of Indian
Standard, India.
4. ASTM C33/C33M – 18 (2018), Standard
Specification for Concrete Aggregates, ASTM
International, West Conshohocken, PA, United
States.
5. Neville, A.M. (2011) Properties of Concrete,
Pearson, Malaysia.
6. Swenson, E.G.; and Chaly, V. (1956) "Basis for
Classifying Deleterious Characteristics of
Concrete Aggregate Materials," ACI Journal,
52(5), pp. 987~1002.
7. Fernandes, V. A., Purnell, P., Still, G. T., Thomas,
T. H. (2007), The effect of clay content in sands
used for cementitious materials in developing
countries, Cement and Concrete Research, 37,
751~758. doi:10.1016/j.cemconres.2006.10.016
8. Schuster, R.L. (1957) A Review of Research on
Deleterious Substances in Concrete Aggregate,
Publication FHWA/IN/JHRP-57/37. Indiana
Department of Transportation and Purdue
University, West Lafayette, Indiana, USA.
https://doi.org/10.5703/1288284313540
9. Kronlof, A. (1994) "Effect of very fine
aggregate on concrete strength," Materials and
Structures, 27, pp. 15~25.
10. Abou-Zeid, M.; and Fakhry, M.M. (2003)
“Short-Term Impact of High Aggregate Fines
Content on Concrete Incorporating Water-
Reducing Admixture,” ACI Materials Journal,
100(4), pp. 280~285.
11. Gullerd K.J. (2002) The Effects of Aggregate
Coatings on the Performance of Portland
Cement Concrete, M.Sc. Thesis, University of
Wisconsin – Madison, Wisconsin ,USA.
12. Munoz, J.F.; Tejedor, I.; Anderson, M.A.; and
Cramer, S.M. (2005) Effects of Coarse
Aggregate Clay-Coatings on Concrete
Performance, Report IPRF-01-G-002-01-4.2,
University of Wisconsin-Madison, Wisconsin ,
USA.
13. Katz, A.; and Baum, H. (2006) "Effect of High
Levels of Fines Content on Concrete
Properties," ACI Materials Journal, 103(M53),
pp. 474~482.
14. Munoz, J.F.; Tejedor, I.; Anderson, M.A.; and
Cramer, S.M. (2007) Expanded Study on the
Effects of Aggregate Coating and Films on
Concrete Performance, Report # 0092-04-12
submitted to The Wisconsin Department of
Transportation, University of Wisconsin-
Madison, Wisconsin , USA.
15. Abib, Z.E.; Gaher-Abib, H.: Kharchi, F. (2013)
"Effect of Clay Fines on the Behavior of Self-
Compacting Concrete," Engineering, 5, pp.
213~218. doi:10.4236/eng.2013.52031
16. Choudhary, H.; Siddique, A.M..; and Saha, D.
(2019) "Effect of clay fines on strength and
workability of concrete," Proceedings of
International Conference on Architecture, Civil
and Environmental Engineering (ACEE2019),
Jawaharlal Nehru University, New Delhi, India.
17. Kirthika, S. K., Surya, M., and Singh, S. K.
(2019), Effect of clay in alternative fine
aggregates on performance of concrete,
Construction and Building Materials, 228,
116811. doi:
10.1016/j.conbuildmat.2019.116811
18. Munoz, J. F., Gullerud, K. J., Cramer, S. M.,
Tejedor, M. I., and Anderson, M. A. (2010),
Effects of coarse aggregate coatings on concrete
performance, ASCE Journal of Materials in
Civil Engineering, 22(1), 96~103. doi:
10.1061/(ASCE)0899-1561(2010)22:1(96)
19. Norvell, J. K., Stewart, J. G., Juenger, M. C. G.,
and Fowler, D. W. (2007), Influence of clay and
clay-sized particles on concrete performance,
Journal of Materials in Civil Engineering,
19(12), 1053~1059. doi: 10.1061/(ASCE)0899-
1561(2007)19:12(1053)
20. Nehdi, M. L. (2014), Clay in cement-based
materials: Critical overview of state-of-the-art,
Page 24
Journal of Asian Concrete Federation, Vol. 6, No. 1, June 2020
24
Construction and Building Materials, 51,
372~382. doi:
10.1016/j.conbuildmat.2013.10.059
21. Yool, A. I. G., Lees, T. P., and Fried, A. (1998),
Improvements to the Methylene Blue Dye test
for harmful clay in aggregates for concrete and
mortar, Cement and Concrete Research, 28(10),
1417~1428.
22. Bisht, K.; and Ramana, P.V. (2018) "Sustainable
production of concrete containing discarded
beverage glass as fine aggregate," Construction
and Building Materials, 177, pp. 116~124.
https://doi.org/10.1016/j.conbuildmat.2018.05.
119
23. Singh, N; and Singh, S.P. (2018) "Evaluating the
performance of self compacting concretes made
with recycled coarse and fine aggregates using
non destructive testing techniques,"
Construction and Building Materials, 181, pp.
73~84.
https://doi.org/10.1016/j.conbuildmat.2018.06.
039
24. Siddique, S.; Shrivastava, S.; Chaudhary, S.
(2018) "Durability properties of bone china
ceramic fine aggregate concrete," Construction
and Building Materials, 173, pp. 323~331.
https://doi.org/10.1016/j.conbuildmat.2018.03.
262
25. Bicini, H.; and Aksogan, O. (2018) "Durability
of concrete made with natural granular granite,
silica sand and powders of waste marble and
basalt as fine aggregate," Journal of Building
Engineering (Accepted Manuscript).
https://doi.org/10.1016/j.jobe.2018.04.022
26. Arulrajah, A.; Ali, M.M.Y.; Piratheepan, J.; and
Bo, M.W. (2012) "Geotechnical Properties of
Waste Excavation Rock in Pavement Subbase
Applications," Journal of Materials in Civil
Engineering, ASCE, 24(7), pp. 924~932. ).
https://doi.org/10.1061/(ASCE)MT.1943-
5533.0000419
27. VanWyk P.R.; and Croucamp, L. (2014)
"Evaluation of rock types for concrete aggregate
suitability for the construction of a gravimeter
vault and access road at the Matjiesfontein
Geodesy Observatory site near Matjiesfontein,
South Africa," Journal of South African
Institution of Civil Engineering, 56(2), 30~36.
28. Magnusson, S.; Lundberg, K.; Svedberg, B.; and
Knutsson, S. (2015) "Sustainable management
of excavated soil and rock in urban areas - A
literature review," Journal of Cleaner
Production, 93, 18~25.
29. Okamura, H.; and Ouchi, M. (2003) “Self
Compacting Concrete,” Journal of Advanced
Concrete Technology, 1(1), pp. 5~15.
30. Cramer, S. (2011) Defining the Impact of
Aggregate Fine Particles on Concrete Pavement
Performance, Wisconsin Highway Research
Program Report no. 0092-07-02, Department of
Transportation, Wisconsin, USA.
31. IS 1199: 1959 (Reaffirmed 2004), Method of
Sampling and Analysis of Concrete, Bureau of
Indian Standard, India.
32. Bhardwaj, B.D. (1970) Upper Vindhyan
Sedimentation in the Kota-Rawatbhata Area,
Rajasthan, PhD Thesis. Aligarh Muslim
University, Aligarh, India.
33. IS 1498: 1970 (Reaffirmed 2007), Classification
and Identification of soils for General
Engineering Purposes, Bureau of Indian
Standard, India.
34. IS 516: 1959 (Reaffirmed 2004), Methods of
Tests for Strength of Concrete, Bureau of Indian
Standard, India.
35. IS 5816: 1999 (Reaffirmed 2004), Splitting
Tensile Strength of Concrete – Method of Test,
Bureau of Indian Standard, India.
36. IS 10086: 1982 (Reaffirmed 2008),
Specification for Moulds for Use in Tests of
Cement and Concrete, Bureau of Indian
Standard, India.
37. Raphael, J.M. (1984) "Tensile Strength of
Concrete," ACI Materials Journal, 81(2), pp.
158~165.
38. Oluokun, F.A. (1991) "Prediction of Concrete
Tensile Strength from Compressive Strength:
Evaluation of Existing Relations for Normal
Weight Concrete," ACI Materials Journal, 88(3),
pp. 302~309.
39. ACI 435R-95 (1995: Reapproved 2000),
Control of Deflection in Concrete Structures,
Report by ACI Committee 435, American
Concrete Institute, USA.
Page 25
Journal of Asian Concrete Federation, Vol. 6, No. 1, June 2020
25
40. ACI 318R-14 (2014) Building Code
Requirements for Structural Concrete, Report
by ACI Committee 318, American Concrete
Institute, USA.
Page 26
Journal of Asian Concrete Federation
Vol. 6, No. 1, pp. 26-36, June 2020
ISSN 2465-7964 / eISSN 2465-7972
https://doi.org/10.18702/acf.2020.6.6.26
26
Technical Paper
Optimization and evaluation of ultra high-performance con-
crete
P. N. Ojha, Piyush Mittal*, Abhishek Singh, Brijesh Singh, V. V. Arora
(Received February 17, 2020; Revised June 9, 2020; Accepted June 9, 2020; Published June 30, 2020)
Abstract: Ultra-High Performance Concrete (UHPC) has been defined as a cementitious based compo-
site material with compressive strength above 150 MPa and enhanced durability via its discontinuous
pore structure. The microstructure of UHPC is denser and more homogeneous in comparison to conven-
tional concrete. UHPC has several advantages over conventional concrete but the use of it is limited due
to the high cost and limited design codes. Methodology for production and development of UHPC needs
to be established. The paper covers both optimization and evaluation of Ultra-High-Performance Con-
crete along with highlighting the importance of packing density, mixing procedure and curing regimes
containing a high volume of mineral admixture and ultrafine materials. Cementitious content of all the
mixes in the study was kept in the range of 1000 kg/m3 and water to binder ratio was kept as 0.17. This
study focuses on the methodology to be adopted for optimizing the packing density of UHPC, the chal-
lenges associated with it and their influence on compressive strength.
Keywords UHPC; Packing Density; Compressive strength; Distribution Modulus; Curing Regime.
1. Introduction Over the last two decades, remarkable
advances have taken place in the research and
application of Ultra-High Performance Concrete
(UHPC) [1]. UHPC is the ‘future’ material with the
potential to be a viable solution for improving the
sustainability of buildings and other infrastructure
components. Ultra-high-performance concrete
(UHPC) with more than 150 MPa compressive
strength [2] generally incorporates relatively high
dosages of silica fume and superplasticizer, with a
relatively low concentration of aggregates of small
size. Some distinguishing features of UHPC include
an optimized gradation of the granular matter for
achieving high packing density, and water to
cementitious materials ratio of less than 0.25 [3]. The
hydrated paste in UHPC has a dense microstructure
which provides a distinct balance of strength, imper-
meability and durability [4, 5]. The superior mechan-
ical and durability characteristics of UHPC have led
to its use in the rehabilitation of concrete structures
[6, 7]. Recent developments in this field have em-
phasized the broadening of the raw materials selec-
tions and the use of common concrete production
methods to facilitate commercial applications of
UHPC [7, 8]. The basic principles for the develop-
ment of UHPC are as follows [9, 10]:
• Minimizing composite porosity by optimizing
the granular mixture through a wide distribution
of powder size classes and reducing the wa-
ter/binder ratio.
• Enhancement of the microstructure by the post
set heat treatment to speed up the pozzolanic re-
action of Silica Fume and other ultrafine ce-
mentitious materials to improve the mechanical
properties.
• The optimal usage of superplasticizer to reduce
water/binder ratio and improve workability.
• Improvement of homogeneity by eliminating the
coarse aggregate resulting in a decrease in me-
chanical effects of heterogeneity.
It has been well recognized by many researchers
that increasing the packing density of the granular
mix could lead to a better performance of the con-
crete. Increasing the packing density of the granular
mix would decrease the volume of paste needed to
P. N. Ojha is a Joint director at National Council for Cement
and Building Materials (NCB).
Corresponding author Piyush Mittal is a Project Engineer at
National Council for Cement and Building Materials (NCB).
Abhishek Singh is a Project Engineer National Council for Ce-
ment and Building Materials (NCB).
Brijesh Singh is a Manager at National Council for Cement
and Building Materials (NCB).
V. V. Arora is a Joint Director at National Council for Cement
and Building Materials (NCB)
Page 27
Journal of Asian Concrete Federation, Vol. 6, No. 1, June 2020
27
fill up the voids and increase the amount of ‘‘ex-
cess’’ paste that could be utilized to improve the
workability of the concrete. Drawing analogy to the
case of a paste, increasing the packing density of the
cementitious materials would decrease the volume of
water needed to fill up the voids and increase the
amount of ‘‘excess’’ water that could be utilized to
improve the flowability of the paste. Hence, the key
to the production of UHPC, which demands both a
low water/binder (w/b) ratio to be used and high
workability to be attained, is the maximization of the
packing density of the granular skeleton of the con-
crete [11,12]. The effectiveness of supplementary
cementitious material in filling up the voids or in im-
proving the packing of the cementitious materials is
dependent on the fineness of the supplementary ce-
mentitious material. In general, a broader range of
particle size distribution would yield a higher pack-
ing density. This is because, with a broader range of
particle size distribution, the medium particles would
fill up the voids between the large particles, the fine
particles would fill up the voids between the medium
particles and the very fine particles would fill up the
voids between the fine particles and so on, leading to
the removal of more voids by the successive filling
effect. The addition of a supplementary cementitious
material finer than Ordinary Portland Cement (OPC)
would broaden the range of particle size distribution
and thus increase the packing density. A supplemen-
tary cementitious material with a higher fineness is
more effective because it would produce a broader
range of particle size distribution. [13, 14, 15]. Parti-
cle packing density can be defined as the solid vol-
ume of particles in unit volume. It has been reported
that when water/cementitious materials ratio was re-
duced to as low as 0.14 by weight, concrete having
the strength of 165–236 MPa was produced [16]. By
maximizing the packing of all granular materials in
the concrete mix using the same packing model and
also applying other advanced production techniques,
concrete having strengths in the order of 200–800
MPa was developed [17]. In 1996, packing theory
was applied for the design of self-consolidating con-
crete and based on the successful outcome it was
concluded that the performance optimization of con-
crete is mainly a matter of improving the packing
density of its granular skeleton [18]. Particle optimi-
zation methods can be divided into three groups (fig-
ure 1):
Fig 1: Mix design approaches for optimizing packing density [13]
• Optimization curves: Groups of particles, with a
specific particle size distribution, are combined
in such a way that the total particle size distribu-
tion of the mixture is closest to an optimum
curve.
• Particle packing models: These models are ana-
lytical models that calculate the overall packing
density of a mixture based on the geometry of the
combined particle groups.
• Discrete element models: With numerical mod-
els, a ‘virtual’ particle structure from given par-
ticle size distribution is generated.
In the present study, approach of the ideal curve
has been adopted for the optimization of concrete
Page 28
Journal of Asian Concrete Federation, Vol. 6, No. 1, June 2020
28
mix to attain the maximum possible packing density.
The fundamental work of Fuller and Thomsen
showed that the packing of concrete aggregates is af-
fecting the properties of the produced concrete [19].
They concluded that a geometric continuous grading
of the aggregates in the composed concrete mixture
can help to improve the concrete properties. Based
on the investigation of Fuller and Thompson [19, 20]
& Andreasen and Andersen, a minimal porosity can
be theoretically achieved by an optimal particle size
distribution (PSD) of all the applied particle materi-
als in the mix as per empirical equation 1 below:
P(D) = ( D
Dmax)
q…………… (1)
Where P (D) is a fraction of the total solids be-
ing smaller than the particle size D (μm), Dmax is the
maximum particle size (μm) and q is the distribution
modulus. However, in the above empirical equation,
the minimum particle size is not incorporated, while
in reality there must be a finite lower size limit. In
continuation of this study, Funk and Dinger pro-
posed a modified model based on the Andreasen and
Andersen Equation. In this study, all the concrete
mixtures are designed based on modified Andreasen
and Andersen model, which is as follows [20,21]:
𝑃(𝐷) =𝑑𝑞−𝑑𝑚𝑖𝑛
𝑞
𝑑𝑚𝑎𝑥𝑞
−𝑑𝑚𝑖𝑛𝑞 ….……… (2)
Where Dmin is the minimum particle size (μm).
Using the above particle packing model, UHPC
mixes were optimised and compressive strength of
about 150 MPa at 28 days with low binder content
was achieved. The modified Andreasen and Ander-
sen packing model has already been successfully em-
ployed in optimization algorithms for the design of
normal density concrete and Lightweight concrete
[9, 10].
Different types of concrete mixes can be de-
signed using above equation 2 by applying different
values of the distribution modulus q, as the value of
q influences the ratio between coarse and fine parti-
cles. Higher values of distribution modulus (q>0.5)
lead to coarser mixtures whereas smaller values
(q<0.25) results in mixes that will be rich in fine par-
ticles [20]. A very high value of q above 0.50 leads
to higher aggregates to paste volumetric ratio and
therefore, the lesser paste will be available to lubri-
cate the fine aggregate particles, which in turn will
have decreased workability. Few trials were con-
ducted with a value of q below 0.37 and in those tri-
als as the value of q was lower, the concrete mix was
found to be stiff and required high-efficiency water
reducers. Hence, to obtain a workable mix with
available water reducing admixture and well-graded
particle size distribution, the value of distribution
modulus q has been taken as 0.37 in this study, keep-
ing in view the findings of the studies done by the
past researchers [20, 22]. Elrahman et al. also men-
tioned the work of Andreasen et al, wherein it was
suggested to use the exponent q in the range of 0.35
to 0.50 because fine particles are not able to pack
similar to bigger particles [22].
Table 1 Physical properties of materials
S. No. Properties Cement G.G.B.S Flyash UFGGBS Silica Fume
1. Fineness(m2/kg) 323 400 310 2026 16701
2. Specific Gravity 3.15 2.93 2.28 2.88 2.28
Table 2 Chemical properties of materials
S.No. Properties Cement G.G.B.S Flyash UFGGBS Silica
Fume
1 Loss of Ignition (LOI), % 2.3 0.33 0.4 0.17 2.73
2 Silica (SiO2), % 20.71 34.41 60.95 33.05 85.03
3 Iron oxide (Fe2O3), % 4.08 1.18 5.7 0.58 -
4 Aluminium oxide (Al2O3) , % 5.15 18.45 26.67 20.40 -
5 Calcium oxide (CaO), % 59.96 36.46 2.08 33.14 -
6 Magnesium oxide (MgO), % 4.57 7.00 0.69 7.62 -
7 Sulphate (SO3), % 1.84 0.097 0.29 0.19 -
8 Na2O, % 0.42 0.30 0.06 0.19 0.73
9 K2O, % 0.56 0.37 1.46 0.58 2.96
10 Chloride, % 0.012 0.022 0.009 0.016 -
11 Insoluble Residue, % 1.25 0.40 - 0.86 -
2. Materials
Properties of UHPC are highly dependent on the
type of material used in its production. In this study
cementitious material were selected in such a way
that particle size distribution has a wide range that
leads to the higher packing density of the concrete.
Cementitious Materials used in the study are OPC
53G, GGBS, ultrafine GGBS and Silica fume, Nano-
silica. Similarly, to increase the packing density of
Page 29
Journal of Asian Concrete Federation, Vol. 6, No. 1, June 2020
29
aggregates, three different types of aggregates
namely Fine Quartz sand, Ground Quartz and Coarse
Quartz sand were used.
2.1 Cementitious materials
Cement: Ordinary Portland Cement (OPC) of
53 grade complying with IS 269: 2015 [23] was used
throughout the study. The physical properties of ce-
ment are tabulated in Table 1. Chemical properties
of cement have been listed in Table 2. Particles of
OPC 53 were in the range of 1.375 to 175 microns.
Particle size distribution has been shown in figure 5.
Ground granulated blast furnace slag
(GGBS): GGBS complying with IS 16714:2015
[24] was used in this study. Physical and chemical
properties have been listed in Table 1 and Table 2
respectively. Particles of GGBS were in the range of
1.15 to 250 microns. Particle size distribution has
been shown in figure 5.
Ultra-Fine Ground granulated blast furnace
slag (UFGGBS): Ultrafine ground powder of GGBS
used in this study has ultra-fineness which shows im-
provement in the properties of regular GGBS like
high surface area. UFGGBS is a specially processed
product based on high glass content with high reac-
tivity obtained through the process of controlled
granulation. For the production of UFGGBS, the
granulated slag with value-added material is ground
in a ball mill attached with a high-efficiency classi-
fier, which classifies the material and ensures that
only the required micro size particle enters the final
product. The entire process is operated by an auto-
matic process controller. Ultrafine GGBS commer-
cially available as Alccofine-1203 is a low calcium
silicate-based mineral additive which is generally
used as a replacement of silica fume in high-perfor-
mance concrete. Its latent hydraulic property and
pozzolanic reactivity results in the enhanced hydra-
tion process. UGGBS complying with IS 16715:
2015 [25] was used in this study. Physical and chem-
ical properties have been listed in Table 1 and Table
2 respectively. Particles of Ultra-Fine Ground gran-
ulated blast furnace slag are very fine and were found
to be in the range of 1 to 18 microns. Particle size
distribution has been shown in figure 5.
Silica Fume: Silica fume complying with IS
15388 [26] was used in this study. Physical and
chemical properties have been listed in Table 1 and
Table 2 respectively. Majority of the particles of Sil-
ica fume were found to be in the range of 0 to 10
microns. Particle size distribution has been shown in
fig.5.
Fly ash: Flyash complying with IS 3812 [27]
was used in this study. Physical and chemical prop-
erties have been listed in Table 1 and Table 2 respec-
tively. Particles of Flyash are slightly coarser than
the particle size of OPC and are in range of 1 to 300
microns. Particle size distribution has been shown in
figure 5.
Nano Silica: The mechanism of influence of
Nano silica on the cement hydration are available in
the literature. The studies indicated that the hydra-
tion heat of Ordinary Portland Cement blended with
Nano silica in the main period increases significantly
with an increased surface area of silica and the hy-
dration of tri-calcium silicate (C3S) gets accelerated
by the addition of Nano-scaled silica or CSH parti-
cles. A Nano silica slurry is selected as a pozzolanic
material to be used in this study. The solid content
and Brunauer–Emmett–Teller (BET) fineness are 20
(% w/w) and 24 m2/g. The specific density of Nano
silica was found to be 2.21 g/cm3.
2.2 Fine aggregates
Ground Quartz: Particle size of Ground
Quartz used in this project is on the coarser side of
the particle size of cement particles. It is used as a
micro filler to optimize the packing density of the
powder mix. Its particle size ranges from 0.5 to 140
microns. Particle size distribution has been shown in
figure 5. The microstructure of ground quartz by Op-
tical microscopy (Fig. 2) suggests that the minerals
present in order of abundance are quartz, orthoclase-
feldspar and iron oxide. Subhedral to anhedral quartz
grains with sharp grain margins are well-graded and
homogenously distributed. The majority of quartz
grains are in the size range of 20µm to 30µm. The
strained quartz percentage is about 9% and their un-
dulatory extinction angle (UEA) varies from 100 to
120. Subhedral orthoclase grains with smooth grain
margins are fresh in nature. Subhedral iron oxide
grains with sharp grain margins are randomly distrib-
uted in the sample.
Fig 2: Distribution of minerals grains in Ground
Quartz. (5x, X-Nicols)
Fine Quartz Sand: UHPC mixes were pro-
duced using quartz sand having a particle size rang-
Page 30
Journal of Asian Concrete Federation, Vol. 6, No. 1, June 2020
30
ing from 150 to 996 microns. Particle size distribu-
tion has been shown in figure 5. The microstructure
of ground quartz sand by optical microscopy (Fig. 3)
suggests that it has a subhedral to anhedral quartz
grains with sharp grain margins which are well-
graded and homogenously distributed. The majority
of quartz grains are in the size range of 300µm to
600µm.
Fig 3: Distribution of minerals grains in fine Quartz
sand (11.25x)
Coarse Quartz sand: Coarse quartz sand used
in this study have a particle size ranging from 1mm
to 3mm. The microstructure of coarse quartz sand by
optical microscopy (Fig. 4) suggests that it has a sub-
hedral to anhedral quartz grains with sharp grain
margins which are well-graded and homogenously
distributed.
Fig 4: Distribution of mineral grains in Coarse
Quartz sand sample (11.25x)
Fig 5: Particle size distribution of materials
3. Experimental
3.1 Packing density
In the present research, the ideal curve method-
ology has been adopted. In this method, materials are
combined in such fractions that there combined grad-
ing lies close to a certain optimum curve given by
Modified Andreasen and Andersen equation as men-
tioned above in equation 2. The proportions of each
material in the mix are adjusted until an optimum fit
between the composed mix and the target curve is
reached, using an optimization algorithm based on
the Least Squares Method (LSM), as presented in
equation 3. When the deviation between the target
curve and the composed mix expressed by the sum
of the squares of the residuals (RSS) at defined par-
ticle sizes, is minimized, the composition of the con-
crete is treated as the most optimum one.
RSS = ∑ (𝑃𝑚𝑖𝑥(𝐷𝑖𝑖+1) − 𝑃𝑡𝑎𝑟(𝐷𝑖
𝑖+1))2𝑛
𝑖=1… (3)
where, Pmix is the composed mix and Ptar is the target
grading.
Around 40 mixes with cementitious materials of
OPC-53, GGBS, UFGGBS & Silica fume, Flyash
and aggregates including Fine Quartz Sand, Quartz
powder and Coarse Quartz sand were theoretically
optimized for optimum particle packing with the
help of above mentioned Modified Andreasen and
Andersen equation. The value of q adopted in this
research is 0.37 based on the literature. The mixes
Page 31
Journal of Asian Concrete Federation, Vol. 6, No. 1, June 2020
31
with the least value of RSS were selected for experi-
mental study. Mixing of UHPC requires equip-
ment that provides more energy and shear than reg-
ular concrete mixers due to the low water content and
high powder content. In general, the expected perfor-
mance (including fresh and hard-solid properties) of
the selected mixture cannot be achieved when low-
energy mixers are used to mix UHPC. Moreover, the
use of a low-energy mixer will also increase the turn-
over time of the mixture, causing the temperature of
the mix to rise, which is detrimental to the UHPC
mixing process [28, 29]. Therefore, for homogene-
ous mixing of UHPC authors designed and devel-
oped planetary mixer of 60 litre capacity with high
mixing efficiency that can be operated at variable
speed with maximum speed up to 325 RPM. Selected
mixes were cast using the developed planetary
mixer. Compressive strength was measured using a
cube with a specimen size of 70.6 mm. Wet packing
density was also determined using the below-men-
tioned equation given by Kwan [30, 31, 32]. If a
UHPC mix consists of several different materials de-
noted by α, β, γ and so forth, ρ and R are solid density
and volumetric ratio of the respective material. uw is
the ratio of water to solid content. Then, the solid
volume of the cementitious materials Vc and wet
packing density ɸ may be worked out from equations
4 and 5 mentioned below:
𝑉𝑐 = 𝑀
ρ 𝑤u 𝑤+ρ αR α+ρ βR β+ρ γR γ …......... (4)
ɸ = 𝑉𝑐
𝑉................................ (5)
As per numerous researchers, the packing den-
sity of concrete is in the range as given below in table
3.
Table 3 Packing density of different types of concrete [27]
Type of Concrete Packing Density range
Normal strength Concrete 0.65 - 0.72
High Strength Concrete 0.72 – 0.8
Ultra high-performance concrete More than 0.8
3.2 Curing regimes
Heat treatment of concrete plays a significant
impact on the rate of strength development and also
has a strong influence on mechanical properties as
well as micro-texture of UHPC. Apart from the ef-
fect of accelerating the curing process and increasing
early strength, heat treatment could be useful for ce-
mentitious systems containing supplementary ce-
mentitious materials such as silica fume, granulated
blast furnace slag, fly ash by influencing the reaction
rate of these mineral additions compared to the same
system cured at ambient conditions. Compared to the
UHPC cured under ambient conditions, heat-treated
specimens of UHPC show generally a denser micro-
structure that can lead to an increase in compressive
strength and thus can improve overall mechanical
properties of UHPC [33, 34]. In the present study,
three different curing regimes are used as given be-
low:
a) Autoclave curing at 2.1 MPa and 215ºC for 8
hours followed by Standard curing till the age
of testing (up to 07 days)
b) Steam curing at 90ºC and 100% RH for 24
hours followed by Standard curing till the age
of testing (up to 28 days)
c) Standard water curing until the age of testing
(28 days).
3.3 Mix design details
In the present study, the proportion of individ-
ual cementitious material was decided using the
Modified Andreasen and Andersen equation. Out of
40 mixes that were theoretically optimized, three fi-
nal mixes were cast in the laboratory for a study on
compressive strength. The details of the composition
of the cementitious material of these three optimized
mixes are discussed later. The total cementitious
content in concrete mixes is kept around 1000 kg/m3.
To attain better particle packing density, a combina-
tion of three fine aggregates were used i.e. Ground
quartz, Fine quartz sand and Coarse quartz sand in
the proportion of 30:40:30 respectively. The nano-
silica was used as a 3% replacement to OPC content.
Dosage of PC based superplasticizer was kept as 2%
by weight of cementitious material.
3.4 Mix methodology
UHPC has been produced using a wide variety
of mixers, ranging from laboratory-sized pan mixers
to revolving drum truck mixers. In general, mixing
UHPC is a somewhat different process than mixing
conventional concrete. UHPC typically includes a
limited amount of water and no coarse aggregate. As
such, the UHPC requires the input of extra mixing
energy both to disperse the water and to overcome
the low internal mixing action from the lack of
coarse aggregate. Different researchers have adopted
various mixing protocols to achieve homogenization
of the UHPC mixture in a shorter span. Although
Page 32
Journal of Asian Concrete Federation, Vol. 6, No. 1, June 2020
32
specific details of the overall mixing process dif-
fered, all researchers were unified in that UHPC
components had to be dry mixed before adding water
and superplasticizer. Dry mixing intends to ensure
higher bulk density and lower moisture require-
ments. A typical mixing process involves first charg-
ing the mixer with the dry components and ensuring
that the mix is blended homogenously. Thereafter,
water and chemical admixtures are added and dis-
persed. Mixing continues, sometimes for an ex-
tended period depending on the mixer energy input,
until the UHPC changes from a dry powder into a
fluid mixture. The mixing methodology adopted in
this study is given in Table 4.
Table 4 Mixing methodology to prepare concrete mixes in this study
Step No. Mixing methodology Duration
Step 1 Dry mixing of all cementitious material and aggregates at low speed of 125
rpm 3 minutes
Step 2 Adding 100% water & 75% superplasticizer and mix at medium speed of 250
rpm 5 minutes
Step 3 Add remaining superplasticizer and mix the constituents at high speed of 325
rpm 5 minutes
Step 4 Mixing continues until concrete achieve the required flow -
Table 5 (a) Details of proportion (% of total cementitious content) of several combinations of cementitious
materials
Mixes OPC 53 (%) Silica Fume (%) GGBS (%) UFGGBS (%) Flyash (%) RSS
M1 40 10 20 30 - 424.8
M2 40 10 10 40 - 465.3
M3 35 - 35 30 - 454.3
M4 20 15 35 30 - 434.0
M5 30 10 40 20 - 439.4
M6 70 10 - 10 10 442.2
M7 60 20 - 10 10 424.0
PM 1 60 10 20 10 - 418.0
PM 2 80 20 - - - 417.0
PM 3 60 10 - 10 20 380.0
Table 5 (b) Details of proportion (% of total fine aggregates) of several combinations of fine aggregates
Mixes Fine Quartz sand (%) Coarse Quartz sand (%) Ground Quartz (%) RSS
C1 70 10 20 437.3
C2 50 30 20 275.5
C3 30 50 20 291.7
C4 10 70 20 486.1
C5 50 10 40 340.9
C6 30 30 40 295.5
C7 10 50 40 428.1
C 8 60 10 30 367.5
C9 40 30 30 263.8
C10 20 50 30 338.2
4. Results & discussion
4.1 Optimization of mix
As mentioned earlier, in the present study the
ideal curve methodology was used for the optimiza-
tion of particle packing of concrete mix. For optimi-
zation of concrete mix, proportions of cementitious
materials and fine aggregates were optimized sepa-
rately. Several mixes were theoretically analyzed
with help of Modified Andreasen and Andersen
equation and mix with least RSS values were used
for laboratory study. Out of several sets of combina-
tions having different proportions, 10 sets of combi-
nations for cementitious materials and 10 sets of
combinations for fine aggregates with least RSS val-
ues have been tabulated below in table 5(a) and 5(b).
Dmin and Dmax (for optimizing the proportion of ce-
mentitious materials) used in modified Andersen and
Page 33
Journal of Asian Concrete Federation, Vol. 6, No. 1, June 2020
33
Andreasen equation for the ideal curve are 0.112 mi-
crons and 1408 micron. Whereas, Dmin and Dmax (for
optimizing the proportion of inert fine aggregates)
used in modified Andersen and Andreasen equation
for the ideal curve are 0.5 microns and 2360 micron
respectively. Out of all the cementitious combina-
tions, PM1, PM2 and PM3 were selected to prepare
concrete. For fine aggregates, combination C9 was
selected to be used for all the three mixes (having
optimized cementitious combinations of PM1, PM2
and PM3) as it has the least RSS value in comparison
to other combinations. The combined particles size
distribution of selected cementitious combination
along with ideal particle size distribution calculated
using Modified Andreasen and Andersen equation is
given in Fig 6. The details of the composition of se-
lected concrete mixes are given in Table 6.
Table 6: Details of mix design
Fig 6: Grading of ideal curve and designed mixes
4.2 Compressive strength
As discussed earlier in section 4.1, cementitious
combinations, PM1, PM2 and PM3 with combina-
tion C9 of fine aggregates were selected to prepare
concrete. Concrete specimen of all the three mixes
were subjected to three curing regimes (mentioned in
3.2) and were evaluated for compressive strength.
For each mix, nine cubes were cast and an average
compressive strength of three cubes for each curing
regime has been plotted for these three selected
mixes (Fig 7). Apart from the interfacial transition
zone (ITZ) between aggregate and cement paste, the
role of the particle packing density is a major factor
governing the compressive strength of the UHPC.
For all three curing regimes studied, PM3 has a
highest compressive strength in comparison to PM1
and PM3. Wet packing density was determined as
per equation 4 and 5. It was observed that for all the
three mixes, value of wet packing density (ɸ) is more
than 0.8. Such a high value of wet packing density
suggests that mixes optimized using ideal curve
methodology have a dense microstructure that helps
in achieving ultra-high compressive strength. Com-
pressive strength in the case of the Autoclave curing
regime is significantly higher in comparison to the
other two curing regimes for all the three optimized
mixes. The maximum compressive strength
achieved is 186.5 MPa for mix PM3 in the case of an
autoclave curing regime. The increment in compres-
sive strength in case of steam curing varied from
12% to 47.3% and in the case of autoclave curing it
varied from 19.11% to 81.9%. Under the conditions
of high temperature and pressure, the chemistry of
hydration is substantially altered. CSH forms but is
converted to a crystalline product α -calcium silicate
hydrate (α-C2S) which causes an increase in porosity
and reduction in strength. However, in the presence
of silica α -C2S converts to tobermorite (C5S6H5) on
continued heating thus high strength can be obtained.
On the other hand, prolonged autoclaving may cause
the formation of other crystalline calcium silicate hy-
drates with a strength reduction. It is believed that
Mix Cement
kg/m3
Silica fume (SF) + GGBS (BS) +
UFGGBS (UFBS) + Flyash (FA)
+ Nano Silica (NS) kg/m3 W/B
Fine Quartz Sand (FQ)
+ Ground Quartz Sand
(GQ) + Coarse Quartz
Sand (CQ) kg/m3
Super
Plasticizer
kg/m3 SF BS UFBS FA NS
FQ GQ CQ
PM1 582 100 200 100 0 18 0.17 498.4 292.6 371.0 20
PM2 776 200 0 0 0 24 0.17 501.0 294.1 372.9 20
PM3 582.0 100 0 100 200 18 0.17 489.9 287.6 364.7 20
Page 34
Journal of Asian Concrete Federation, Vol. 6, No. 1, June 2020
34
the complete conversion to tobermorite is not desir-
able and that there is an optimum ratio of amorphous
to crystalline material for maximum strength [35,
36]. The strength level after autoclaving generally
cannot be reached even with 24-hour steam curing
for all three mixes. This can be explained by the dif-
ferent hydration mechanisms due to these curing
methods. While steam curing increases the reactivity
of ingredients, autoclaving leads to the development
of different phases and incorporation of ultrafine ma-
terial is essential to achieve ultra-high compressive
strength.
Fig 7 Compressive strength for three different curing regimes
5. Conclusions
(1) In order to obtain a workable and satisfactory
mix with available water reducing admixture and
well-graded particle size distribution, the value
of distribution modulus q was adopted as 0.37.
Ideal curve methodology has been used for the
optimization of particle packing of concrete mix.
For the preparation of concrete mix, proportions
of cementitious materials and fine aggregates
were optimized separately. Several mixes were
theoretically analyzed and optimized with help
of Modified Andreasen and Andersen equation
and mixes with least RSS values were used for
laboratory study. Mixes optimized and prepared
using lower values of q may result in higher
compressive strength. However, such mixes
were found to be very stiff and required very
high-efficiency water reducers
(2) Wet packing density was determined as per the
method given by Kwan. It was observed that the
value of wet packing density (ɸ) for all the three
mixes is more than 0.8. Such a high value of wet
packing density suggests that mixes optimized
using ideal curve methodology have a dense mi-
crostructure that helps in achieving ultra-high
compressive strength.
(3) Cementitious content (fine particles) in a UHPC
mix is quite high in comparison to a normal
strength concrete mix. Mixing methodology and
type of mixer used for the preparation of UHPC
have a large influence on the mixing efficiency
and uniformity of UHPC which eventually af-
fects its properties in a fresh and hardened state.
The mixing methodology adopted in this study
and preparation of concrete mix using planetary
mixer developed for this study ensured homoge-
neous mixing without any lump formation.
(4) The curing regime plays a vital role in the devel-
opment of hardened properties of the UHPC
mix, which contains a high amount of mineral
admixtures. Post set heat treatment enhances the
microstructure by speeding up the pozzolanic re-
action of Silica fume and other ultrafine cementi-
tious materials to enhance the mechanical prop-
erties. From results, it can be observed that the
maximum compressive strength achieved was
186.5 MPa in case of autoclave curing for mix
PM3. The percentage improvement in compres-
sive strength in case of steam curing varied from
12% to 47.3% and in the case of autoclave curing
it varied from 19.11% to 81.9% in comparison to
the compressive strength observed in case of
standard curing.
Page 35
Journal of Asian Concrete Federation, Vol. 6, No. 1, June 2020
35
Reference
1. Norzaireen Mohd Azmee; and Nasir Shafiq
(2018) “Ultra-High Performance Concrete:
From Fundamental to Applications”, Case
Studies in Construction Materials
2. Wille K.; Naaman A. E.; and Parra-Montesinos
G. J. (2011) “Ultra-high performance concrete
with compressive strength exceeding 150 MPa:
A simpler way”, ACI Materials Journal,
108(1):46–54
3. Schießl P; Mazanec O; and Lowke D (2007)
“SCC and UHPC - Effect of mixing technology
on fresh concrete properties”, Advances in
construction materials 513–522, Heidelberg,
Germany, Springer
4. Graybeal B. A. (2006) “Material property
characterization of ultra-high performance
concrete”, Report No: FHWA-HRT-06-103
p.188
5. Russell H. G.; and Graybeal B. A. (2013) “Ultra-
high performance concrete: A state-of-the-art
report for the bridge community”, Federal
Highway Administration, McLean
6. Richard P.; and Cheyrezy M. (1995)
“Composition of reactive powder concretes”,
Cement and Concrete Research, p. 1501–1511
7. Sbia L. A.; Peyvandi A.; Lu J.; Abideen S.;
Weerasiri R. R.; Balachandra A. M.; and
Soroushian P. (2017) “Production methods for
reliable construction of ultra-high performance
concrete (UHPC) structures”, Materials and
Structures
8. Yang Chen; Faris Matalkah; Parviz Soroushian;
Rankothge Weerasiri; and Anagi Balachandra
(2019) “Optimization of ultra-high performance
concrete, quantification of characteristic
features”, Cogent Engineering
9. Senthil Kumar V.; and Manu Santhanam (2004)
“Use of a particle packing model to produce
HPC at optimum cement content”, The Indian
Concrete Journal
10. A.S. Dili; and Manu Santhanam. (2004)
“Investigation on reactive powder concrete: A
developing Ultra high-strength technology”,
The Indian Concrete Journal
11. Ghoddousi P.; Javid A. A. S.; and Sobhani J.
(2014) “Effects of particle packing density on
the stability and rheology of self-consolidating
concrete containing mineral admixtures”,
Construction and Building Materials, p. 102–
109
12. “Ultra-High-Performance Concrete: An
Emerging Technology Report” (2018), Reported
by ACI Committee 239
13. Fennis S.; and Walraven J. (2012) “Using
particle packing technology for sustainable
concrete mixture design”, Delft University of
Technology, Netherlands
14. Caijun Shi; Zemei Wu; Jianfan Xiao; Dehui
Wang; Zhengyu Huang; and Zhi Fang (2015) “A
review on Ultra high performance concrete: Part
I. Raw materials and mixture design”,
Construction and Building Materials, p. 741–
751
15. A.M. Said; M.S. Zeidan; M.T. Bassuoni; and Y.
Tian (2012) “Properties of concrete
incorporating nano-silica”, Construction and
Building Materials, p. 838–844
16. De Larrard F.; and Sedran T. (1994)
“Optimization of ultra-high performance
concrete by the use of a packing model”,
Cement and Concrete Research, p. 997–1009
17. Richard P.; and Cheyrezy M. (1995)
“Composition of reactive powder concretes”,
Cement and Concrete Research, p. 1501–1511
18. Sedran T; De Larrard F; Hourst F; and
Contamines C (1996) “Mix design of self-
compacting concrete”, Proceedings of the
international RILEM conference on production
methods and workability of concrete, Paisley,
Scotland, p. 439 - 450
19. Fuller W. B.; and Thompson S. E. (1907) “The
laws of proportioning concrete”, Transactions of
the American Society of Civil Engineers, p. 222
- 298.
20. Hüsken G.; and Brouwers H. J. H. (2008) “A
new mix design concept for earth-moist
concrete: A theoretical and experimental study”,
Cement and Concrete Research, p. 1246 - 1259
21. Yu Q. L.; Spiesz P.; and Brouwers H. J. H. (2013)
“Development of cement-based lightweight
composites - Part 1: Mix design methodology
and hardened properties”, Cement and Concrete
Composites, p. 17 - 29
22. Mohamed Abd Elrahman; and Bernd Hillemeier
(2014) “Combined effect of fine fly ash and
Page 36
Journal of Asian Concrete Federation, Vol. 6, No. 1, June 2020
36
packing density on the properties of high
performance concrete: an experimental
approach” Construction and Building Materials
23. IS: 269-2015 "Ordinary Portland Cement -
Specification", Bureau of Indian Standards,
New Delhi
24. IS: 16714-2015 " Ground Granulated Blast
Furnace Slag for Use in Cement, Mortar and
Concrete", Bureau of Indian Standards, New
Delhi
25. IS: 16715-2015 " Ultrafine Ground Granulated
Blast Furnace Slag — Specification", Bureau of
Indian Standards, New Delhi
26. IS: 15388-2003 “Silica Fume Specification”,
Bureau of Indian Standards, New Delhi
27. IS 3812 (Part 2) : 2013, “Pulverised Fuel Ash-
Specification, Part 2 For Use as Admixture in
Cement Mortar and Concrete”, Bureau of Indian
Standards, New Delhi
28. Xinpeng Wanga; Rui Yua; Qiulei Songa;
Zhonghe Shuia; Zhen Liua; Shuo Wua; and
Dongshuai Houd (2019) “Optimized design of
Ultra high performance concrete (UHPC) with a
high wet packing density”, Cement and
Concrete Research
29. Parameshwar N. Hiremath; and Subhash C.
Yaragal (2017) “Influence of mixing method,
speed and duration on the fresh and hardened
properties of Reactive Powder Concrete”,
Construction and Building Materials, p. 271 -
288
30. Li L. G.; and Kwan A. K. H. (2014) “Packing
density of concrete mix under dry and wet
conditions”, Powder Technology, p. 514 - 521
31. Wong H. H. C.; and Kwan A. K. H. (2008)
“Packing density of cementitious materials: part
1 - measurement using a wet packing method”,
Materials and Structures, p. 689 - 701
32. Kwan A. K. H.; and Wong H. H. C. (2008)
“Packing density of cementitious materials: part
2 - packing and flow of OPC + PFA + CSF”,
Materials and Structures, p. 773 - 784
33. Libya Ahmed Sbia (2014) “Contribution and
Mechanism of action of graphite nano materials
in Ultra High Performance Concrete ”, Ph.D.
thesis, Michigan State University
34. Halit Yazıcı (2007) “The effect of curing
conditions on compressive strength of Ultra
high strength concrete with high volume mineral
admixtures”, Building and Environment, p.
2083 - 2089
35. Glasser F. P.; and Hong S. Y. (2003) “Thermal
treatment of C–S–H gel at 1 Bar H2O pressure
up to 200 C”, Cement and Concrete Research
36. Quanbing Yang; Shuqing Zhang; Shiyuan
Huang; and Yaohui He (2000) “Effect of ground
quartz sand on properties of high-strength
concrete in the steam-autoclaved curing”,
Cement and Concrete Research
Page 37
Journal of Asian Concrete Federation
Vol. 6, No. 1, pp. 37-51, June 2020
ISSN 2465-7964 / eISSN 2465-7972
https://doi.org/10.18702/acf.2020.6.6.37
37
Technical Paper
Bond and durability investigation of basalt fiber and PEN fi-
ber reinforced composites for concrete applications
Donguk Choi, Youngho Kim*, Batzaya Baasankuu, Gombosuren Chinzorigt
(Received September 23, 2019; Revised February 24, 2020; Accepted April 26, 2020; Published June 30, 2020)
Abstract: Bond and durability characteristics of basalt fiber reinforced polymer (BFRP) and polyeth-
ylene naphthalate (PEN) fiber/PEN FRP were investigated. Magnitude and distribution of the bond stress
between BFRP/PEN FRP and concrete were investigated by double lap shear test. Four different types
of durability test were performed: (1) Beam bond test following accelerated conditioning protocols by
ACI 440.9R using plain concrete beams strengthened with BFRP or PEN FRP; (2) tensile test of PEN
fiber/PEN FRP after immersion in 1N NaOH, 3% NaCl solutions, and water up to 6 months; (3) tensile
test of PEN fiber/PEN FRP after immersion in 5% and 10% diluted solutions of HCl; and (4) exposure
to natural outdoor environment. Bond test results indicated high bond stress developing over relatively
short distance for BFRP that has high elastic modulus (EBF = 68.4 GPa) while relatively low bond stress
developing over longer length for PEN FRP that has low elastic modulus (EPEN = 17.4 GPa). In the beam
bond test, very good behavior was shown by PEN FRP after 4 month’s exposure to wet and alkaline
conditions while moderate behavior was shown by BFRP. Overall, the performance of PEN fiber/FRP
was satisfactory in all durability tests conducted in this study.
Keywords: fiber reinforced polymer; double lap shear test; durability; beam bond test; basalt fiber;
PEN fiber.
1. Introduction
Various fibers, including carbon fiber (CF), ar-
amid fiber (AF), glass fiber (GF), and basalt fiber
(BF), are used for the purpose of external strength-
ening of RC structures and members. The mechani-
cal characteristics common to CF, AF, GF, and BF
include linearly-elastic stress-strain relationship,
high tensile strength, and high elastic modulus while
they have relatively small rupture strain (< 3%). On
the other hand, a new class of fiber including poly-
ethylene terephthalate (PET) fiber and polyethylene
naphthalate (PEN) fiber has non-linear stress-strain
relationship, good strength in tension, low elastic
modulus, and large strain capacity in tension (4%-
15%): i.e. PET/PEN fibers are often said to have
LRS (large rupture strain) capacity.
In this study, bond and durability characteristics
of two different fibers were investigated: BF and
PEN fiber. BF is an inorganic fiber produced from
natural basalt rocks by melting and extrusion process
[1, 2]. BF, which has been used in Civil Engineering
discipline only recently, is more economical than CF
or AF. Basalt fiber has excellent thermal resistance
such that it can be used as insulating material replac-
ing asbestos which poses health hazards by damag-
ing respiratory systems [1]. PEN fiber is a synthetic
fiber and is a product of petrochemical industry. PEN
fiber has good strength (over 800 MPa) and low elas-
tic modulus (about 1/4th of BF), exhibits non-linear
stress-strain behavior, and has large rupture strain in
tension (7%-9%).
Thin BFRP sheets are often used for external
strengthening in flexure and/or shear of RC beams
[3, 4]. BFRP sheet or the basalt fiber rope can also
be utilized for seismic strengthening of RC columns
in the form of external wrapping [5-7]. Bond charac-
teristics of BF bonded to concrete using adhesive
have been investigated [8-10]. Shen et al. studied
bond behavior of 21 test specimens by double lap
shear test subjected to different strain rate. Effective
bond length of BFRP ranged between 56-72 mm
while the effective bond length decreased with the
Donguk Choi is a Professor of School of Architecture and De-
sign Convergence of Hankyong National University, Korea.
Corresponding author Youngho Kim is a Professor of Depart-
ment of Chemical Engineering of Hankyong National Univer-
sity, Korea.
Batzaya Baasankuu is an Assistant Lecturer of Department of
Civil Engineering and Architecture of Mongolian University of
Science and Technology, Mongolia.
Gombosuren Chinzorigt is an Engineer at Gurvan Tahilgat
Orgil LLC, Mongolia.
Page 38
Journal of Asian Concrete Federation, Vol. 6, No. 1, June 2020
38
increasing strain rate which varied between 70
mm/sec to 0.07 mm/sec [8]. Xie et al. studied the
bond behavior BFRP-concrete interface subjected to
wet-dry cycling in a marine environment using 26
small beams specimens [10]. The concrete beams
were externally strengthened using BFRP sheet on
the tension side. The test specimens were immersed
in salt water with concentration of 3.5% and sub-
jected to wet-dry cycles up to 360 days (one cycle
per day). All beam specimens were tested by 4-point
bending. For the control specimens, the failure mode
was debonding of the adhesive layer before exposure
while the failure mode changed to BFRP fracture in
tension after exposure to marine environment. The
fatigue strength of the BFRP/concrete interface after
the wet-dry cycling was approximately 60% of the
ultimate load capacity (of the control specimen). Ex-
isting durability test results of BFRP indicate that the
BF may also degrade under wet and alkaline condi-
tions [11]. The durability performance of BFRP can
be improved by coating the BF with zirconium diox-
ide or titanium dioxide [12-14].
Recent research explored possible application
of the LRS PET FRP and PEN FRP composites
mainly for the external strengthening of the RC col-
umns by confinement utilizing its large rupture strain
capacity [15-20]. Baasankhuu et al. compared the be-
haviour of concrete cylinders confined by BFRP and
PEN FRP [18]. The strength of confined concrete
wrapped by PEN FRP was achieved at much higher
axial/lateral strain of the concrete than the BFRP
wrapping: PEN FRP wrapped concrete deformed
more laterally to develop axial strength equivalent to
BFRP wrapped concrete. Park et al. reported results
of a rare study on the flexural strengthening of RC
beams using PET FRP [21]. Despite very low elastic
modulus (about 1/20th that of steel) of PET FRP, the
external strengthening by 6-mm-thick multi-layer
PET FRP sheet was effective to significantly im-
prove the flexural strength and ductility of the RC
beams. PET FRP sheet did not debond from the con-
crete substrate at ultimate.
At present, there are few studies that focused on
the bond behavior between LRS PEN FRP and con-
crete. In addition, few studies can be found in the ex-
isting literature either on the durability characteris-
tics of PEN FRP clearly indicating a research need.
One objective of this study was to investigate and
compare the bond behavior of BFRP-to-concrete and
PEN FRP-to-concrete interfaces in terms of magni-
tude and distribution of the bond stress. Bond behav-
ior was investigated by double lap shear test in this
study. The other objective was to investigate the du-
rability characteristics of BFRP, PEN fiber/PEN
FRP. Four different durability tests were performed:
(1) beam bond test by accelerated conditioning pro-
tocols (ACP) by ACI 440.9R for both BFRP and
PEN FRP; (2) durability test of PET fiber/FRP under
wet, saline, alkaline conditions up to 6 months; (3)
durability test of PET fiber/FRP under acidic condi-
tions; and (4) exposure to natural outdoor conditions.
The durability test program especially concentrated
on the PEN fiber/FRP.
Table 1 Mechanical properties of fiber roving and adhesive
Type Stress
(MPa)
Strain
(%)
E1
(GPa)
E2
(GPa)
Area
(mm2)
Thick-
ness
(mm)
Density
(g/mm3)
Basalt fiber roving 1226 1.95 68.4 n/a 0.45 0.113 0.0027
PEN fiber roving 822 8.03 17.4 8.30 2.00 0.840 0.0014
PEN fiber sheet 842 9.01 17.5 8.33 -- -- --
PEN FRP 912 9.13 21.4 8.59 -- -- --
Adhesive 40.9 2.58 1.59 -- -- -- --
NOTE: 1. Results show mean value of 12 tests for fibers and 5 tests for adhesive; 2. PEN fiber shows bilinear
response and so the slope of the first and the second line is shown as E1 and E2, respectively (see Fig. 1(b); 3.
Adhesive mechanical properties were tested 7 days after hardening.
2. Material properties and test method
2.1 Material properties of fibers and adhesive
Tensile properties of BF roving, PEN fiber rov-
ing, PEN fiber sheet, and PEN FRP (i.e. PEN fiber
sheet embedded by two-part epoxy) were tested fol-
lowing ISO 10406-2 with results summarized in Ta-
ble 1 [22]. Twelve tensile tests were completed for
each fiber/FRP while the mean values are shown in
Table 1. Figure 1(a) shows the stress-strain relation-
ship of BF and PEN fiber determined in this study.
In Figure 1(a), the stress-strain relationship is linear
for BF while it is non-linear for PEN fiber. The non-
linear stress-strain relationship of PEN fiber can be
modelled using a bilinear relationship with each line
having a slope of E1 (slope of the first line connecting
the origin and the stress corresponding to 1% strain)
and E2 (slope of the second line), respectively, as
shown in Table 1 and Fig. 1(b). For the double lap
Page 39
Journal of Asian Concrete Federation, Vol. 6, No. 1, June 2020
39
shear test, beam bond test, and tensile test of the
BFRP and PEN FRP, a two-part epoxy (adhesive)
was used. The tensile properties of the adhesive were
determined following ASTM D638 [23]. As shown
in Table 1, tensile strength, ultimate strain in tension,
and elastic modulus of the adhesive are 40.9 MPa,
2.58%, and 1.59 GPa, respectively. A universal test-
ing machine (UTM) of 50-kN capacity was used for
tensile tests of fibers/FRP and adhesive.
(a) Stress-strain relationship
(b) Bi-linear model of PEN fiber
Fig. 1 Stress-strain relationship of BF and PEN
fibers
2.2 Double lap shear test
As the LRS PEN fiber has low elastic modulus,
a significant thickness of the PEN FRP is needed
when strengthening RC members such as RC beams,
columns, etc. [18]. Bond stress that develops at the
interface between the thick PEN FRP layer and the
concrete substrate can be high because a thicker FRP
may induce higher bond stress, which in turn may
lead to a premature debonding failure at the FRP-
concrete interface [24, 25]. Bond investigation by
double lap shear test was planned both for the PEN
FRP- and the BFRP-strengthened concrete speci-
mens to investigate magnitude and distribution of the
bond stress that develops between the FRP and the
concrete.
Modified double lap shear test setup was used
following recommendations of CSA S 806 Annex P
in general [26]. Figure 2 shows a double lap shear
test specimen used in this study. Size of the test spec-
imen was 140 (b) x 150 (h) x 550 mm (L). Concrete
with 30-MPa target strength was designed, cast in the
laboratory, and wet cured for 28 days. The 28-day
compressive strength was 36.0 MPa. A thin steel
plate placed at center of specimen at time of casting
practically disconnected a specimen into two con-
crete blocks of equal length. A 16-mm diameter
grooved steel rod was installed in the middle of each
concrete block in the axial direction with about 300-
mm length protruding outside the concrete block (for
use by testing grip of UTM). 28 days after casting,
two opposing side faces of the concrete blocks were
lightly roughened using a hand grinder. Two layers
of BF rovings or single layer of PEN fiber sheet was
applied (bFRP = 100 mm, LFRP = 500 mm), respec-
tively, on the roughened faces using the adhesive
with amount of 200% by vol. for BFRP and 150% by
vol. for PEN FRP (The same amount of adhesive was
used for BFRP and PEN FRP, respectively, through-
out this study). Seven days after application of the
adhesive, multiple 5-mm-long electronic strain
gauges were installed on the surface of BFRP and
PEN FRP starting from center of the specimen at 21-
mm spacing on center as shown in Fig. 2.
Table 2 Test variables of double lap shear test
Type No. of
layers
No. of
test
fck
(MPa)
Fiber thk.
(mm)
Adhesive thk.
(mm)
BFRP 2 2 36.0 0.226 0.452
PEN FRP 1 2 36.0 0.840 1.260
The double lap shear test specimen was sub-
jected to tensile force during test using 1,200-kN-ca-
pacity UTM operated in displacement control (ramp
rate = 1 mm/min). The applied load was measured
using load cell of the UTM. Signals from the strain
gauges and the load cell were electronically recorded
by a data logger. Two replicate specimens were
tested for each FRP type. Table 2 summarizes test
variables of the double lap shear test.
Page 40
Journal of Asian Concrete Federation, Vol. 6, No. 1, June 2020
40
Fig. 2 Double lap shear test specimen [21]
2.3 Beam bond test
100 (b) x 100 (h) x 400 (L) mm plain concrete
beam specimens were used for the beam bond test
following guide to accelerated conditioning proto-
cols (ACP) of durability assessment by ACI 440.9R
[27]. Concrete with 60-MPa compressive strength
was cast in the laboratory and wet cured for 28 days.
A 50 x 100 mm acrylic plate (thickness = 2 mm) was
inserted in the beam mid-span on the tension side up
to beam half-height to create a notch as shown in Fig.
4(a). After 28 days of wet cure, two layers of BF
rovings or single PEN fiber sheet was externally
bonded on the bottom surface of the beam using ad-
hesive. Concrete surface was lightly roughened be-
fore the FRP application. Both BFRP and PEN FRP
were 80 mm wide and 200 mm long (See Fig. 4(a)).
A week after the FRP application, the specimens
were subjected to three different environmental con-
ditions: Room condition (T = 23°C +/- 3°C) and im-
mersion in water and 1N NaOH solutions, respec-
tively (Temperature of water and NaOH solution =
60°C +/- 3°C). After immersion for 3,000 hours (4-
months) in water and in the alkaline solution, the
beams were retrieved and stored in the room condi-
tion for two days. Flexure test by three-point loading
following recommendations of ACI 440.9R was car-
ried out using a 1,200-kN UTM at ramp rate of 0.6
mm/min as shown in Fig. 4 [27]. Five replicate
beams were tested for the BFRP- and the PEN FRP-
strengthened beams, respectively.
A small strip of BFRP or the PEN FRP was ad-
hered using adhesive to one end of all beam bond
specimens as shown in Fig. 3(c). After completion of
the beam bond tests, the BFRP or PEN FRP strip at-
tached at the end were subjected to pull-off test using
a pull-off testing device equipped with a 50 x 50 mm
square steel end plate. The steel end plate was ad-
hered to the BFRP or PEN FRP strip using two-part
epoxy after the FRPs were cut to fit 50 mm x 50 mm
steel end plate (See Fig. 5). Seven days after the ad-
hesive application, the pull-off test was performed as
shown in Fig. 5(d). Average bond stress in tension
was determined by dividing the maximum pull-off
load (Pmax) by the contact area (A = 2,500 mm2). The
pull-off test results of the FRPs retrieved from the
specimens immersed in water and 1N NaOH solution
and those stored in the room condition were com-
pared.
(a) Water (b) 1N NaOH (c) Room condition
Fig. 3 Beam bond test specimens immersed in water/alkaline solutions and stored in room condition
Page 41
Journal of Asian Concrete Federation, Vol. 6, No. 1, June 2020
41
(a) A beam bond test specimen (b) Beam bond test in progress
Fig. 4 Beam bond test setup
(a) PEN FRP (b) BFRP (c) Steel end plate (d) Pull-off test
Fig. 5 Pull-off test
2.4 Tensile test of PEN fiber/FRP before/after
exposure to NaOH/NaCl solutions and wa-
ter
In this phase of study, durability characteristics
of PEN fiber/FRP (i.e. Uniaxial PEN fiber sheet and
PEN FRP) were investigated by observing weight
change and change in the tensile properties before
and after exposure to different environmental condi-
tions up to 6 months. Multiple PEN fiber/FRP tensile
specimens were prepared in length of 400 mm (PEN
fiber sheet) or 300 mm (PEN FRP). Test specimens
were immersed in three different environments: i.e.
Water (i.e. 100% R.H. at 40°C) and 3% NaCl and 1N
NaOH solutions at 20°C, respectively. The above en-
vironmental conditions simulated completely wet
condition, seawater, and sound concrete with high al-
kalinity, respectively.
The weight measurement and the tensile test
were carried out before the durability test began and
after immersion for 1, 3, and 6 months to determine
the weight loss and the strength/stiffness loss in ten-
sion, if any. After each planned immersion period,
test specimens were taken out of the environmental
chamber and dried for two days in a container about
quarter full of silica gel. After the specimens were
dried, they were weighed using a high precision scale
(accuracy = 0.0001 g) and then the tensile test was
performed following ISO 10406-2 (ten tests each).
Scanning electron microscopy (SEM) photos were
taken before/after the planned immersion period.
Figure 6 shows PEN fiber sheet and PEN FRP spec-
imens immersed in NaOH, NaCl solutions, or water.
(a) PEN fiber sheet (b) PEN FRP
Fig. 6 PEN fiber/FRP specimens immersed in NaOH/NaCl solutions or water
Page 42
Journal of Asian Concrete Federation, Vol. 6, No. 1, June 2020
42
2.5 PEN fiber/FRP immersion in diluted acidic
solution
This specific durability test by tensile testing af-
ter immersion in acidic solution was carried out only
for PEN fiber/FRP. Tensile test coupons consisted of
PEN fiber sheet and PEN FRP, which were im-
mersed in diluted acidic solutions of 5% HCl or 10%
HCl for 3, 7, and 14 days. After the planned immer-
sion period, the specimens were retrieved, dried for
2 days and subjected to tensile test. Tensile strength
after immersion in diluted acidic solution was com-
pared to that of the control specimen. Three replicate
specimens were tested.
2.6 Exposure in natural environment for PEN
fiber/FRP
This test was carried out for PEN fiber/FRP
only. PEN fiber sheet and PEN FRP tensile test spec-
imens were prepared which were exposed in the out-
door conditions in South Korea starting from late
winter (T = 0°C ~ -10°C, R.H. = 30%-40% typ.) to
early summer (T = 20°C-30°C, R.H. = 40%-60%
typ.). After six-months’ exposure in the natural envi-
ronment including direct sun ray and U.V., the spec-
imens were retrieved and subjected to tensile test.
Change of tensile strength before/after exposure was
examined using ten replicate specimens of PEN fi-
ber/FRP, respectively.
3. Test results
3.1 Double lap shear test results
The double lap shear test was performed using
two different FRP composites adhered to concrete:
BFRP (2 layers) and PEN FRP (1 layer). Figure 7
schematically shows the location of the strain gauges
attached on the surface of FRP and the force acting
on FRP at center and in between the strain gauge lo-
cations. As soon as a test specimen shown in Fig. 2
is subjected to tensile force P, the concrete is sepa-
rated into two parts due to a thin steel plate placed at
center and, as a result, each FRP sheet bonded on the
side face of the concrete block is subject to a tensile
force of P/2 at center (loading end). Bond stress (u)
acting at FRP-concrete interface between ith and
(i+1)th strain gauges in Fig. 7 can be determined from
Eqs. (1) through (4), where the stress is determined
from strain readings of the strain gauge:
𝑇𝑖 = 𝜎𝑥𝑖(𝑡𝑦). ……………… (1)
𝑇𝑖+1 = 𝜎𝑥𝑖+1(𝑡𝑦)…………..… (2)
∆𝑇 = 𝑇𝑖 − 𝑇𝑖+1……………….(3)
𝑢 =∆𝑇
𝑥𝑦……………………...(4)
Where Ti = tensile force acting on FRP at ith
strain gauge, σxi = axial stress of FRP at ith strain
gauge location, t = thickness of FRP, x = distance
between two adjacent strain gauges, y = width of
FRP.
Table 3 summarizes the maximum axial strain
observed by the measured strain (εx-frp), correspond-
ing axial stress (σx-frp), tensile force acting on the FRP
layer (Pfrp), and the maximum bond stress (umax) de-
termined by Eqs. (1) through (4). Figures 8 and 9
show distribution of axial strains and bond stresses
for 126-mm distance starting from center for BFRP
and PEN FRP, respectively. It is observed from
Table 3 and Fig. 8 that the maximum axial strain,
axial stress, in Table 3 and Fig. 9, the maximum axial
strain, axial stress, FRP tensile force, and bond stress
are 0.7%, 149 MPa, 15.1 kN, and 3.0 MPa, respecti-
vely, for PEN FRP-1 while they are 0.82%, 172
MPa, 17.4 kN, and 4.3 MPa for PEN FRP-2. The dis-
tance the bond stress develops (Le) is 63-84 mm for
BFRP while it is 105 mm for PEN FRP. The distance
the bond stress develops as shown in Figs. 8 and 9
and Table 3 is known as the effective bond length Le
in literature: i.e. even if the bond length of the FRP
is larger than Le, the bond stress still develops within
Le. It can be seen that the effective bond length of
BFRP (63-84 mm) determined in this study agrees
well with that suggested by Shen et al., where the ef-
fective bond length varied between 56-72mm depen-
ding on the strain rate [8]. The bond stress between
BFRP and concrete at the interface is relatively high
and narrowly distributed as shown in Fig. 8 and
Table 3. On the other hand, the bond stress is rather
low and distributed over a larger distance for PEN
FRP in Fig. 9 and Table 3. In all tests, the failure
mode was debonding at the FRP-concrete interface
as shown in Fig. 10 and Table 3.
Fig. 7 Force (P/2) acting at center and away from center [21]
Page 43
Journal of Asian Concrete Federation, Vol. 6, No. 1, June 2020
43
Table 3 Summary of test results at maximum fiber strain (εx-frp)
Fiber type εx-frp
σx-frp
(MPa)
Afrp
(mm2)
Pfrp
(kN)
Le
(mm)
umax
(MPa)
umean
(MPa)
umax /
umean
Failure mode
BFRP-1 0.0141 963 22.6 21.8 84 7.4 2.16 3.4 debonding
BFRP-2 0.0119 812 22.6 18.4 63 8.4 3.50 2.4 debonding
PEN FRP-1 0.0070 149 84.3 15.1 105 3.0 1.20 2.5 debonding
PEN FRP-2 0.0082 172 84.3 17.4 105 4.3 1.38 3.1 debonding
(a) BFRP-1 (a) BFRP-2
(b) BFRP-1 (b) BFRP-2
(c) BFRP-1 (c) BFRP-2
Fig. 8 Distribution of axial strain, axial stress, and bond stress at varying level of BFRP tensile force
Page 44
Journal of Asian Concrete Federation, Vol. 6, No. 1, June 2020
44
(a) PEN FRP-1 (a) PEN FRP-2
(b) PEN FRP-1 (b) PEN FRP-2
(c) PEN FRP-1 (c) PEN FRP-2
Fig. 9 Distribution of axial strain, axial stress, and bond stress at varying level of PEN FRP tensile force
3.2 Beam-bond test results of BFRP and PEN
FRP
The beam bond test specimens were retrieved
after 4-months’ immersion in water or alkaline solu-
tion (1N NaOH) as well as the control specimens
(stored in room condition for 4 months) and tested
by flexure test by three-point loading following ACI
440.9R [27]. As the applied load increased, a tensile
crack quickly appeared and developed upward star-
ting from top of the notch (See Fig. 4). At peak load,
BFRP- and PEN FRP-strengthened beams failed ei-
ther by debonding at the FRP-concrete interface or
FRP fracture in tension. Average peak load of C-
BFRP (Control), RH-BFRP (R.H. 100%), and
NaOH-BFP (1N NaOH) was 15.5 kN, 15.7 kN, and
7.94 kN, respectively, as shown in Table 4. Average
peak load for C-PEN FRP, RH-PEN FRP, and
Page 45
Journal of Asian Concrete Federation, Vol. 6, No. 1, June 2020
45
NaOH-PEN FRP was 19.7 kN, 17.4 kN, and 17.4
kN, respectively. Equation (5) was used to determine
beam bond retention (or residual mechanical pro-
perty) as suggested by ACI 440.9R [27].
𝑅𝑒𝑏 =𝑃𝑏2
𝑃𝑏1× 100 (%)………… (5)
Where, Pb2 is peak load of water/alkali condi-
tioned specimen, Pb1 is peak load of Control speci-
men and Reb is beam is beam bond retention.
Table 4 Results of beam bond and pull-off tests
Index Beam bond test Pull-off
Test
(MPa)
Index Beam bond test Pull-off
Test
(MPa) Load
(kN)
Reb
(%)
Load
(kN)
Reb
(%)
C-BFRP 15.5 -- 5.23 C-PEN FRP 19.7 -- 4.47
RH-BFRP 15.7 100 3.97 RH-PEN FRP 17.4 88 3.56
NaOH-BFRP 7.94 51.2 3.55 NaOH-PEN FRP 17.4 88 1.90
NOTE: Results shown are average of 5 tests.
(a) PEN FRP (b) BFRP (c) Debonding failure
Fig. 10 Double lap shear test in progress and failure mode
Table 4 summarizes the beam bond test results in
terms of peak load and beam bond retention. The
beam bond retention is 88% for PEN FRP both for
wet condition and in alkaline condition. On the other
hand, the beam bond retention is 100% and 51% in
wet condition and in alkaline condition, respectively,
for BFRP. Test results indicate that BFRP may be
vulnerable for alkaline exposure. Figures 11 and 12
show the beam bond test specimens after test. In Fig-
ure 11 which shows the control specimens, the fail-
ure occurred at the FRP-concrete interface both for
BFRP and PEN FRP. C-PEN FRP shows some con-
crete attached to the debonded FRP surface while C-
BFRP shows almost complete and clean interface
failure. Figure 12 shows top surfaces of C-BFRP,
RH-BFRP, and NaOH-BFRP after beam bond test.
C-BFRP retains original dark brown color of the
BFRP in Fig. 12(a). On the other hand, yellowish
discoloration of BFRP is shown for RH-BFRP in
Fig. 12(b) while the discoloration is more severe for
NaOH-BFRP in Fig. 12(c). It has been reported that
the basalt fiber can be degraded under soaked, saline,
and alkaline environment while the degradation of
the basalt fiber can be significantly slowed down by
oxide coating such as zirconium deoxide and tita-
nium deoxide coating [12-14]. Since the basalt fibers
used in this study were not coated, further investiga-
tion on the durability properties of the basalt fiber
was not pursued.
As described in Clause 2.3, the pull-off test of
BFRP or PEN FRP bonded to the end of the beam
bond test specimens was also performed after the
completion of the beam bond test. Table 4 and Fig-
ures 13 and 14 show the results of pull-off test. Pull-
off test results indicate that both immersion in water
and alkaline solution at an elevated temperature tend
to degrade the bond between concrete and
BFRP/PEN FRP. C-BFRP (Control), RH-BFRP, and
NaOH-BFRP have average pull-off bond strength of
5.23 MPa, 3.97 MPa, and 3.55 MPa, respectively, in
Table 4, while average bond strength of C-PEN FRP
(Control), RH-PEN FRP, and NaOH-PEN FRP is
4.47 MPa, 3.56 MPa, and 1.90 MPa, respectively. C-
BFRP specimens failed mostly in concrete substrate
while NaOH-BFRP specimens failed partially in
concrete substrate. RH-BFRP specimens failed
mostly at interface. PEN FRP pull-off specimens
show partial failure at interface and concrete sub-
strate for C-PEN FRP, and clean interface failure for
Page 46
Journal of Asian Concrete Federation, Vol. 6, No. 1, June 2020
46
RH-PEN FRP and NaOH-PEN FRP specimens indi-
cating damage at the interface by 4-months’ immer-
sion in water and alkaline solutions.
3.3 Test results of PEN fiber/FRP before/after
exposure to NaOH, NaCl solutions and wa-
ter
Table 5 compares the weights/weight losses be-
fore and after 30, 90, and 180 days of exposure for
PEN fiber sheet and PEN FRP under three different
conditions of completely wet (Wet, 40°C) and 3%
NaCl and 1N NaOH solutions (20°C), respectively.
In Table 5, PEN fiber sheet shows 0.33%, 0.18%,
and 10.5% weight loss after exposure to Wet, NaCl,
and NaOH conditions, respectively, while the loss is
0.80%, 0.76%, and 4.34% for PEN FRP, respec-
tively, after 180 days. PEN fiber shows some nega-
tive effect under alkaline environment. However, the
possible negative effect of alkaline environment on
PEN fiber is significantly reduced in case of PEN
FRP as the PEN fiber is embedded in the adhesive
matrix.
(a) C-BFRP (b) C-PEN FRP
Fig. 11 Control specimens after beam bond test
(a) C-BFRP (b) RH-BFRP (c) NaOH-BFRP
Fig. 12 BFRP specimens after beam bond test
Table 6 summarizes the tensile test results for
PEN fiber sheet and PEN FRP in terms of tensile
strength and elastic modulus before and after 30, 90,
and 180 days. In Table 6, the tensile strength of PEN
fiber sheet after 180 days is 114%, 115%, and 97%
of control when exposed to Wet, 3% NaCl, and 1N
NaOH conditions, respectively. The tensile strength
for PEN FRP after 180 days is 98%, 96%, 93% for
Wet, 3% NaCl, and 1N NaOH conditions, respec-
tively. Elastic modulus is also shown in Table 6. The
elastic modulus (E1) after 180 days ranges 98%-
103% for PEN fiber sheet and 100%-104% for PEN
FRP, respectively. It can be concluded that the PEN
fiber/FRP has satisfactory resistance after exposure
to all three environmental conditions of Wet, 3%
NaCl, and 1N NaOH after 180 days.
Figure 15 shows SEM images before and after
180 days of exposure to different environmental con-
ditions, where no significant change is visible be-
fore/after exposure. In Figure 15(a), the longitudinal
surface of PEN fiber filament is very smooth and
clean while the diameter is 30 μm before exposure.
In Figure 15(b)-(d), after 180 days’ exposure to wet,
saline, and alkaline conditions, the surfaces do not
show any significant trace of chemical etching or
bruises in all conditions.
Page 47
Journal of Asian Concrete Federation, Vol. 6, No. 1, June 2020
47
3.4 Influence of acidic condition
Figure 16 compares the test results of PEN fi-
ber/FRP before and after immersion in 5% and 10%
diluted solutions of HCl for 3, 7, and 14 days: i.e.
Tensile strengths after immersion are normalized in
terms of tensile strength before immersion in Fig. 16.
It is seen in Fig. 16(a) that the tensile strength of PEN
fiber sheet after immersion tends to be affected a lit-
tle, while the tensile strengths are almost unaffected
in Fig. 16(b) for PEN FRP specimens.
3.5 Exposure to natural environment
As described earlier, PEN fiber sheet and PEN
FRP tensile test specimens were prepared and ex-
posed in natural outdoor conditions for six months.
Tensile test results are summarized in Table 7 after
six months’ exposure in the natural environment in-
cluding direct sun ray and U.V. Degradation is
clearly noticed for PEN fiber sheet while the degra-
dation is significantly reduced for PEN FRP in Table
7.
Fig. 13 Pull-off test results
(a) C-BFRP (b) RH-BFRP (c) NaOH-BFRP
(d) C-PEN FRP (e) RH-PEN FRP (f) NaOH-PEN FRP
Fig. 14 Failure mode of pull-off test specimens
Page 48
Journal of Asian Concrete Federation, Vol. 6, No. 1, June 2020
48
Table 5 Summary of weight measured before/after exposure to wet, 3% NaCl and 1N NaOH conditions
Fiber type Environ-
ment
Temp.
(̊C)
Weight
before
(g)
Weight af-
ter 180d
(g)
Weight loss after (%)
30d 90d 180d
PEN fiber
sheet
Wet
NaCl
NaOH
40
20
20
7.7040
7.6722
7.6815
7.6784
7.6583
6.8789
0.37
0.02
1.64
0.22
0.02
5.51
0.33
0.18
10.5
PEN FRP
Wet
NaCl
NaOH
40
20
20
11.177
11.007
11.083
11.088
10.924
10.602
0.44
0.16
0.73
--
0.05
1.70
0.80
0.76
4.34
NOTE: PEN uniaxial fiber sheet consists of 6 PEN fiber rovings; 2. Specimen length is 400 mm for PEN fi-
ber roving, and PEN fiber sheet and 300 mm for PEN FRP; 3. Adhesive amount is 150% of fiber by vol. for
PEN FRP; 4. Average of twelve measurements
Table 6 Summary of tensile test results before/after exposure to wet, 3% NaCl and 1N NaOH conditions
Fiber
type
Environ-
ment
Temp.
(°C)
Tensile strength before/after
(MPa)
Elastic modulus before/after
(GPa)
before 30d 90d 180d before 30d 90d 180d
PEN fi-
ber sheet
Wet
NaCl
NaOH
40
20
20
842
786
923
861
932
935
851
961
966
816
17.5
15.9
17.2
17.0
17.3
16.9
17.7
18.0
17.9
17.2
PEN FRP
Wet
NaCl
NaOH
40
20
20
912
893
952
857
902
935
900
897
880
844
21.4
20.7
20.3
21.9
19.7
20.0
20.3
21.3
21.4
22.2
NOTE: Average of twelve tests
(a) Control (b) Wet
(c) 3% NaCl (d) 1N NaOH
Fig. 15 SEM photos of PEN after exposure for180 days to different environmental conditions
Page 49
Journal of Asian Concrete Federation, Vol. 6, No. 1, June 2020
49
(a) PEN fiber sheet (b) PEN FRP
Fig. 16 Results of tensile test of PEN fiber/FRP after immersion in acidic solutions
Table 7 Summary of tensile strengths for PEN FRP after exposure in natural conditions for up to 6 months
Fiber type Before
(MPa)
After 6 months
(MPa)
After/Before
(%)
PEN fiber sheet 842 594 70.5
PEN FRP 912 814 89.3
NOTE: Average of 10 tests
4. Conclusions
Bond between BFRP-to-concrete and PEN
FRP-to-concrete interface was studied in terms of
distribution and magnitude of bond stress by double
lap shear test. In addition, durability characteristics
of BFRP and PEN fiber/FRP have been studied by
(1) beam bond test following accelerated condition-
ing protocols for durability assessment by ACI
440.9R, (2) fiber tensile test before/after immersion
in 1N NaOH and 3% NaCl solutions, and water, (3)
fiber tensile test before/after immersion in diluted
acidic solutions, and (4) exposure to natural outdoor
environment. The durability investigation by beam
bond test included both BFRP and PEN FRP. Other
durability investigations concentrated on PEN fi-
ber/FRP. The following conclusions can be drawn
from the current experimental study.
(1) Double lap shear test results show that the bond
stress that develops at the interface between
BFRP and concrete is relatively high and nar-
rowly distributed: Maximum bond stress was 8.4
MPa and distance the bond stress distributed was
63-84 mm for BFRP. On the other hand, the
bond stress was relatively low and distributed
over a larger distance for PEN FRP: Maximum
bond stress was 4.3 MPa while the distance the
bond stress distributed was 105 mm for PEN
FRP. Test results show that the effective bond
length Le is 84 mm for BFRP and 105 mm for
PEN FRP.
(2) Beam bond test results show that the behavior of
PEN FRP under wet and alkaline conditions is
relatively good with beam bond retention of 88%
after immersion in water and alkaline solutions
for 4 months. Behavior of BFRP was not as good
under alkaline environment with the beam bond
retention of 51.5% after immersion for 4 months.
(3) Results of weight measurement and tensile test
of PEN fiber/FRP before and after wet, alkaline,
and saline conditions indicated possible degrad-
ing of PEN fiber under alkaline condition. For
the PEN FRP, however, the weight reduction
(about 4%) and reduction of the tensile strength
was small (7.5%) and there was no change of the
elastic modulus before/after immersion for 6
months.
(4) Tensile strength of PEN FRP immersed in 5%
and 10% diluted acidic solutions of HCl was
very good after 14 days: no reduction in 5% so-
lution and less than 3% reduction in 10% solu-
tion. Tensile strength of PEN FRP after exposure
for 6 months in natural environment showed
about 10% reduction.
(5) In overall, the performance of PEN fiber/FRP
was satisfactory in all durability tests conducted
in this study.
Acknowledgements
This study was supported by Grant from Korea
Re Research Foundation, Grant number [NRF2018-
R1D1A1B07049635]. Authors gratefully acknowle-
dge the generous support
Page 50
Journal of Asian Concrete Federation, Vol. 6, No. 1, June 2020
50
Reference
1. Singha K. (2012) “A Short Review on Basalt
Fiber”, International Journal of Textile Science,
1(4), pp. 19-28.
2. Prasad V.V.; Talupula S. (2018) “A Review on
Reinforcement of Basalt and Aramid (Kevlar
129)", Materials Today, proceedings, pp. 5993-
5998, available online at
www.aciencedirect.com.
3. Chen W.; Pham T.M.; Sichembe H.; Chen L.;
and Hao H. (2018) “Experimental study of
flexural behaviour of RC beams strengthened by
longitudinal and U-shaped basalt FRP sheet”,
Composites Part B, 134, pp. 114-126.
4. Choobbor S.S.; Hawileh R.A.; Abu-Obeidah A.;
and Abdalla J.A. (2019) “Performance of hybrid
carbon and basalt FRP sheets in strengthening
concrete beams in flexure”, Composite
Structures, 227, 111337.
5. Jiang S.-F.; Zeng X.; Shen S.; and Xu X. (2016)
“Experimental studies on the seismic behavior
of earthquake-damaged circular bridge columns
repaired by using combination of near-surface-
mounted BFRP bars with external BFRP sheets
jacketing”, Engineering Structures, 106, pp.
317-331.
6. Ouyang L.-J.; Gao W.-Y.; Zhen B.; and Lu Z.-D.
(2017) “Seismic retrofit of square reinforced
concrete columns using basalt and carbon fiber-
reinforced polymer sheets: A comparative
study”, Composite Structures, 162, pp. 294-307.
7. Rousakis T.C.; Panagiotakis G.D.; Achiontaki
E.E.; and Kostopoulos A.K. (2019) “Prismatic
RC columns externally confined with FRP
sheets and pretensioned basalt fiber ropes under
cyclic load”, Composites Part B, 163, pp. 96-
106.
8. Shen D.; Shi H.; Ji Y.; and Yin F. (2015) “Strain
rate effect on effective bond length of basalt
FRP sheet bonded to concrete”, Construction
and Building Materials, 82, pp. 206-218.
9. Nerilli F.; and Vairo G. (2018) ”Experimental
investigation on the debonding failure mode of
basalt-based FRP sheets from concrete”,
Composites Part B, 153, pp. 205-216.
10. Xie J.-H.; Wei M.-W.; Huang P.-Y.; Zhang H.;
and Chen P.-S. (2019) ”Fatigue behaviour of the
basalt fiber-reinforced polymer/concrete
interface under wet-dry cycling in a marine
environment”, Construction and Building
Materials, 228, 117065.
11. Baasankhuu B. (2019) ”Basic Study for Seismic
Strengthening of Reinforced Concrete (RC)
Structural Members Using Basalt Fiber (BF)
and Ductile Polyethylene Naphthalate (PEN) ”,
M.S. Thesis, Hankyong National University,
Anseong, South Korea.
12. Fiore V.; Bella G.D.; Valenza A. (2011) ”Glass-
basalt/epoxy hybrid composites for marine
applications”, Materials and Design, 32, pp.
2091-2099.
13. Rybin V.A.; Utkin A.V.; and Baklanova N.I.
(2013) ”Alkali resistance, microstructural and
mechanical performance of zirconia-coated
basalt fibers”, Cement and Concrete Research,
53, pp. 1-8.
14. Rybin V.A.; Utkin A.V.; and Baklanova N.I.
(2016) ”Corrosion of uncoated and oxide-coated
basalt fibre in different alkaline media”,
Corrosion Science, 102, pp. 503-509.
15. Anggawidjaja D.; Ueda T.; Dai J.; and Nakai H.
(2006) ”Deformation capacity of RC piers
wrapped by new fiber-reinforced polymer with
large fracture strain”, Cement & Concrete
Composites, 28, pp. 914-927.
16. Choi D.; Vachirapanyakun S.; Kim S.-Y.; and
Ha. S.-S. (2015) ”Ductile fiber wrapping for
seismic retrofit of reinforced concrete columns”,
Journal of Asian Concrete Federation, 1(1), pp.
37-46.
17. Dai, J.-G.; Bai, Y.-L.; and Teng, J. G.
(2011) ”Behavior and modelling of concrete
confined with FRP composites of large
deformability”, ASCE Journal of Composites
for Construction, 15(6), pp. 963–973.
18. Baasankhuu B.; Choi D.; and Ha S.S.
(2020) ”Behavior of Small-Scale Concrete
Cylinders in Compression Laterally Confined
by Basalt Fiber and PEN Fiber Reinforced
Polymer Composites”, IJCSM, 14(8), pp. 1-19.
19. Saleem S.; Pamanmas A.; and Rattanapitikon W.
(2018) ”Lateral response of PET FRP-confined
concrete”, Construction and Building Materials,
159, pp. 390-407.
Page 51
Journal of Asian Concrete Federation, Vol. 6, No. 1, June 2020
51
20. Suon S.; Saleem S.; and Pimanmas A.
(2019) ”Compressive behavior of basalt FRP-
confined circular and non-circular concrete
specimens”, Construction and Building
Materials, 195, pp. 85-103.
21. Park H.S.; Kim S.Y.; Lim M.K.; and Choi D.
(2016) ”Bond and Flexural behaviour of RC
Beams Strengthened Using Ductile PET,
Journal of the Korea Institute for Structural
Maintenance and Inspection”, 20(6), pp. 30-39.
(in Korean)
22. ISO 10406-2 (2015) ”Fiber reinforced polymer
(FRP) reinforcement of concrete – Test methods
- Part 2: FRP sheets”, International Organization
for Standardization, Geneva, Switzerland.
23. ASTM D 638-08 (2008) ”Standard Test Method
for Tensile Properties of Plastics”, American
Society of Testing and Materials, West
Conshohocken, PA.
24. Teng J.G.; Chen J.F.; Smith S.T.; and Lam
L. ”FRP Strengthened RC Structures”, Wiley,
2002.
25. Choi, D.-U.; Fowler, D.W.; and Wheat, D.L.
(1996) ”Thermal Stresses in Polymer Concrete
Overlays”, American Concrete Institute, Special
Publication SP-166, Properties and Uses of
Polymers in Concrete, pp. 93-122.
26. CSA S806-02 (2002) ”Design and Construction
of Building Components with Fibre-Reinforced
Polymers”, Canadian Standards Association,
Toronto, Ontario.
27. ACI 440.9R-15 (2015) ”Guide to Accelerated
Conditioning Protocols for Durability
Assessment of Internal and External Fiber
Reinforced Polymer (FRP) Reinforcement”,
ACI Committee 440, American Concrete
Institute, Detroit, Michigan.