Mechanical properties of steel fibre reinforced lightweight concrete with pumice stone or expanded clay aggregates

Materials and Structures/Mat~riaux et Constructions, Vol. 34, May 2001, pp 201-210

Mechanical properties of steel fibre reinforced lightweight concrete with pumice stone or expanded clay aggregates

G. Campione, N. Miraglia and M. Papia Dipartimento di Ingegneria Strutturale e Geotecnica, Universita di Palermo, Viale delle Scienze, I - 90128, Italy.

Paper received: March 16, 2000; Paper accepted:January 3, 200i

A B S T R A C T R I~ S U M I~

This paper presents basic information on the mechanical properties of steel fibre-reinforced lightweight concrete, manufactured using pumice stone or expanded clay aggregates. Results are presented for standard compressive tests and indirect tensile tests (splitting tests on cylinder specimens and flexure tests on prismatic beams using a three-point loading arrangement) under monotonically increasing or cyclically varying loads. The influence of steel fibres and aggregate types on modulus of elasticity, compressive and tensile strength and post-peak behaviour is evaluated. Test results show that compressive strength does not change for pumice stone aggregates, while an increase is observed for expanded clay; tensile strength and fracture toughness are significantly improved for both pumice stone and expanded clay. The results also show that with both expanded clay and pumice stone lightweight aggregates a suitable content of fibres allows one to obtain performances comparable with those expected from normal weight concrete, the important advantage of lower structural weight being maintained.

Cet article pr&ente des informations de base sur le comportement m&anique des b~tons 18gers fabriqu& avec des granulats d' argile expans8 et de ponce renforc& de fibres d' acie~: En particulier, on pr&ente les r&ultats d'essais de compression et de traction indirecte (essais par fendage et essais deflexion sur de petites poutres prismatiques appuy~es aux extrOmit& et ckarge'es dans la section mMiane). Les essais ont ~t~ r~alis& en agissant sons contr& des dSformations et en imposant des histoires de d~forma- tions monotoniques et cycliques. L'&ude a montr8 l'influence des divers pourcentages de fibres et du type de granulat lSger sur le comportement me'canique du b~ton, en particulier sur le module

~1 , , I I , d elastlate en compression et sur la resistance maximum en compression et en traction. Les r&ultats des essais out montr{ que l'ajout des fibres au bSton comportant des granulats de ponce ne produit pas de variation de la r&istance maximum a la compression. En revanche, l'introduction des fibres clans le b{ton avec granulats d'argile expansO entra~ne une augmentation sign~cative de la re'sistance maximale a la compression. Pour les deux bdtons, on constate que l' ajout des fibres augmente la r&istance a la traction et la t&acit~ en flexion. L'introduction des fibres d'acier clans tous les bOtons l~gers test& donne an mat&iau des prestations &v&s certainement comparables a celles des b~tons normaux, mais avec les avantages &idents li& a un poids inf&ieur.

1. I N T R O D U C T I O N

For some time lightweight concrete, besides being utilised for its non-structural properties (as lagging or sound-proofing material), has also been employed to make structural elements, in particular in the field ofpre- cast concrete structures. More recently the execution of high-rise buildings and large-span concrete structures has required concrete with higher strength and low weight, and this encourages the use of lightweight concrete, espe-

cially for constructions in seismic areas, where the saving in dead load is more appreciable. Maintaining the same strength level, lightweight concrete, with respect to normal weight concrete, exhibits a more brittle behaviour related to the low tensile/compressive strength ratio and to low fracture toughness and residual tensile strength. The brittle nature of lightweight concrete greatly depends on the aggregates used, and in particular on their density: in general, greater aggregate density improves the strength of the material to the detriment of the non-

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Materials and Structures/Mat6riaux et Constructions, Vol. 34, May 2001

structural properties mentioned above. This disadvantage can be overcome by increasing the ordinary confinement transverse reinforcement and/or by adding reinforcing fibres to the concrete matrix, as has been shown in sev- eral recent studies [1-3]. It has been observed that the presence of fibres reduces material decay in the field of the strains exceeding that corresponding to the peak value of strength. The experimental study presented here provides further information about this favourable effect when pumice stone or expanded clay aggregates are used. The results described confi rm that the presence of hooked steel fibres differently influences the compressive strength of the two types of lightweight concrete considered, but in both cases improves the behaviour in compression and in tension in the post-peak strain field. Depending on the volume fraction of fibres, greater strength and ductility can be reached in the case of a concrete matrix prepared with expanded clay aggregate under both monotonic and cyclic actions.

2. EXPERIMENTAL PROCEDURE

2.1 Characteristics of materials

The constituent materials and proportions considered in this investigation are shown in Table 1, where Lp denotes a lightweight concrete matrix with pumice stone coarse aggregate, L c a lightweight concrete matrix with expanded clay coarse aggregate, and N r a normal weight concrete matrix, the latter being manufactured with con- ventional aggregates of about the same size as lightweight aggregates, in order to make the different performances comparable.

Pumice is a porous, frothlike, volcanic rock. In this investigation it was taken from an outcrop on the island of Lipari, and it is light grey in colour, non-friable and abrasive. Pumice stone aggregate with a size of 3 to 7 mm (mean size) and 7 to 10 mm (great size) was used. The pumice stone fine portion (< 3 mm) was removed and substituted by sand of medium size in order to improve the mechanical properties of the concrete. The pumice stone aggregates had an apparent weight density of 750 kg/m 3.

The expanded clay aggregates, almost round but irregular in shape with a maximum diameter of 17 ram, had an apparent weight density of 650 kg/m 3. Medium size aggregate with diameter from 3 to 7 mm and large size aggregate with diameter from 7 to 17 m m were used. In the mixture with expanded clay the share of fine material was also removed. All matrices were prepared using type 425 Portland cement. The density 3' of hardened concrete without reinforcing fibres proved to be 1800, 1640 and 2520 kg/m 3 for lightweight concrete with pumice stone, lightweight concrete with expanded clay and normal weight concre te , respectively. Considering the ACI [4] recommendations for steel fibre reinforced concrete, the maximum size of aggregates was related to the fibre length. Dramix type hooked steel fibres having length Lf = 30 mm and diam-

eter r = 0.5 mm (with aspect ratio Lf/O o = 60), randomly distributed in the fresh concrete mixture, were utilised. The volume fractions of fibres were 0.5, 1 and 2%, corresponding to fibre contents of 40, 80 and 160 kg/m 3, respectively. Good workability of the mixture was obtained by adding 1.5% of superplasticizer by cement weight. The minimum nominal tensile strength of the fibres was equal to 1115 N/mm 2, Before mixing aggregates were placed in a tank full of water for half an hour; then they were mixed with the other components . Firstly, sand and one-third part of water was pre-mixed for about 2 minutes, and then cement was added and mixed for 3 more minutes. Finally, fibres and superplasticizer were added and mixed for a further 2 minutes. The concrete specimens were cast in plastic molds and compacted on a vibration table. After demolding, the specimens were kept in a tank with water until testing time. All tests were carried out at 28 days.

2.2 Test methods

The mechanical response of the concrete matrices considered to compressive loads was tested by means of cylindrical specimens having a diameter of 100 m m and a height of 200 mm, in agreement with the UNI [5] requirements, using a servo-controlled open loop testing machine with a deformation rate of 0.2 ram/rain (Fig. la). The stress value was the ratio between the load value provided by the load cell and the nominal value of the cross-section of the specimen.

Strains were evaluated by means of three displace- ment transducers located in plan in such a way as to form an angle of 120 ~ with one another. The gauge length was assumed to be equal to 140 mm, making it possible to avoid disturbance from the regions near the bonded ends of the specimens. The strains considered in drawing the stress-strain curves are the mean strains derived from the values measured by the three transducers. Adopting this equipment the secant modulus of elasticity was evaluated according to Eurocode 2 [6] as the ratio between the

Table 1 - Constituent materials (kg/m 3)

Component

Cement (Pt1425)

Effective water

Absorbed water

Pumice aggregate (3+7 mm)

Pumice aggregate (7+10 ram)

Expanded clay aggregate (3-:7 mm)

Expanded clay aggregate (7+17 mm)

Coarse aggregate (< 10 mm)

Natural sand

Concrete density, 7

Compressive strength, fc' (Mea)

Lp L c N r

350 350 300

135 135 180

I 70 70 / 320 / /

170 / /

/ 320 /

/ 170 /

/ / 1050

782 782 850

1800 1640 2520

20.60 21.83 22.00

202

Campione, Miraglia, Papia

a)

b)

Pf•j•ot a ble spherical

I I f ~ ~ - s t e e l olate

l ~ m e t a l ring

DTs

P | movable steel plate ~pher ica l joint

I }

~=~ ~ 1 0 0 x 2 0 0 mm

////////X~/////40x40x300 mm

- 40x40x300 mm

~ - 100x200 mm

C) P ~,~X, spherical joint cylindrical ~ 100x100x350 mm

; ' / / / / / / / / / / / / / J Fig. 1 - Test set-ups: a) compression test, b) split tension test, c) flexure test.

stress corresponding to 40% of the peak load and the corresponding strain value.

To characterise the behaviour in tension of brittle materials the most appropriate test is the direct tension test if correctly carried out. Nevertheless, because of the

difficulties involved in executing stable tests in direct tension, indirect tensile tests are more commonly utilised. On the other hand indirect tensile tests are also proposed as the standard method in the most recent codes, and they give suitable values to utilise for practical design.

The indirect tensile tests (splitting tests) were carried out in agreement with UNI requirements by means of cylindrical specimens having diameter D = 100 mm and length 1 = 200 ram, as in the previous tests, by applying the compressive load along a diametrical plane up to failure by means of the testing machine described above (Fig. lb). A 40 x 40 x 300 m m steel bar and a stratified wooden bar 5 m m thick were interposed between the loading plate of the press and the specimen to be tested, in order to ensure that the load was really acting along a diametrical plane and the failure was localised along the directly loaded region [7]. The width of the wooden bar that was in contact with the specimen along its whole length was equal to 10 mm, as suggested by the UNI requirements.

The flexure tests were carried out on 100 x 100 x 350 m m standard prismatic beams [5] supported at the ends on a net span of 300 mm and loaded at the mid- span (Fig. lc). Cylindrical hinges were interposed between end supports and specimen and between the loading plate of the testing machine and the loaded region, in order to make the load and reaction transmis- sions more uniform.

The results were derived in the form of load-deflection diagrams for each typology investigated. Since suitable devices like the Japanese yoke [8] or correction techniques [9], able to take into account local plasticiza- tions in the support regions and rigid motion effects, were not used, the results which will be shown later on make it possible to calculate the indirect tensile strength in agreement with the UNI 6130 requirement, but can- not be used to derive the toughness indices defined in ASTM [10] or in the JSCE [11] requirements. However, the results can be utilised to highlight the reinforcing effect of fibres on material behaviour.

3. EXPERIMENTAL RESULTS

3.1 Results in compression

Fig. 2 shows the monotonic and cyclic responses of the cylindrical specimens in compression, for the three kinds of concrete matrix considered (with pumice stone, expanded clay, normal-weight coarse) without reinforcing fibres. It is interesting to observe that the compressive strength of lightweight concrete with pumice stone is substantially the same as that of lightweight concrete with expanded clay, but the compressive strength of pumice stone aggregates is lower than that of expanded clay aggregates.

The curve showing the mono ton ic response is obtained from the average values referring to the three specimens tested; the cyclic response refers to a single specimen.

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Materials and Structures/Mat6riaux et Constructions, Vol. 34, May 2001

25 ] .. Monotonic response " 2o4

/ 0 / \ ", - - - N, 15 j / ! ~ ,, I.p ( l igthweight)

~'~ ] I i ~, ~ . . . . . I.c ( l igthweight~

,ol:! "-. Ill \ ' , " .

;Y . . . . .

25" Cyclic response

~ '20 . , ' , ; A ' ' , _ l e ] 1

l n . l l l ;l L \ ~"

- | i l l ;; % : r ,': J " . ,

5 ''~ " , k. ~..

! I |

0.000 0.005 0.010 0.015 s

F i g . 2 - Compressive behaviour of normal and lightweight c o n -

c r e t e .

The shapes of the curves show the more brittle behaviour of lightweight concrete with respect to normal weight concrete having about the same strength, as already observed in [3]. This is due to the use of aggregates having, with respect to those utilised in normal weight concrete, less strength and lower elasticity modulus. For lightweight concrete more than for normal weight concrete, after the maximum load is reached, the stiffness and post-peak stress decrease significantly, especially under cyclic excursions. This is due to the fact that in lightweight concrete, as opposed to normal weight concrete, the weakest component of the cementitious matrix is the aggregate rather than the hardened paste [12]. Lightweight concretes, compared to normal weight concrete having the same strength, are also characterised by the use of better quality mortar.

Figs. 3 and 4 show the favourable effect of the reinforcing fibres on the monotonic and cyclic responses in compression of lightweight concrete matrices with pumice stone (Fig. 3) and with expanded clay (Fig. 4).

In the case of expanded clay aggregate, the incorpo- ration of steel fibres into the matrix produces increases by up to 30% in strength with suitable increases in the volume percentages of fibres, while in the case of pumice stone the variation in strength is negligible.

As observed in [13], in lightweight fiber reinforced concrete the adding of fibres produces an increase in maximum compressive strength. This increase can be explicated with the following considerations: - the fiber- matrix bond mechanisms in concrete, as observed in [14], depend principally on the quality of cement mortar

and on the fiber properties (aspect ratio, volume percentages, etc.); - better fiber-matrix interfaces, generally related to the use of concrete having higher strength [14], determine better performance in fibrous composites, especially in the case of lightweight concrete compared to normal weight concrete with the same percent- age of fibres. Further studies on fibrous concretes [15] have also shown that fibers are more effective in composites when they are free from aggregate interference.

In the case of the experimental research mentioned here, due to the size and due to the shape of the aggregates (round and regular in shape in the case of expanded clay and irregular in shape in the case of pumice stone), the surface of the aggregates in contact with fibers decreases in the case of expanded clay compared to that of pumice stone and consequently an increase in maximum strength was observed for expanded clay.

With an increase in the volume fraction of fibres, higher post-peak stress and absorbed energy values are achieved. Under uniaxial compressive vertical loads, compressive strains and transverse tensile strains occur in the concrete until the ultimate tensile strength in lightweight aggregate is achieved. Then a further increase in the load produces an extension and propagation of cracks into the cement paste, until the maximum load is reached. After this stage further deformations produce a very brittle softening response due to the absence of interlock forces in lightweight aggregates. If fibres are present in the concrete, further deformations without significant loss of bearing capacity can also be achieved in the softening branch, due to the resulting growth of cracks arrested by the bond of steel fibre and cement paste [16].

Figs. 5 a) and 5 b) provide the characteristic values of strength in compression f'c (after 28 days), and the corresponding strain values %, varying with the volume percentages of fibres.

In all cases examined the increase in volume fraction of the fibers produced an increase in the strain %, as is shown in Fig. 5 b).

Fig. 6 shows the variation in the secant elastic modulus (Ec) of steel fibre reinforced lightweight concrete with variation in the volume fraction of fibres Vf.

The secant elastic modulus of concrete mainly depends on the stiffness and the volume of the components; lightweight aggregates, as ment ioned, have a lower elasticity modulus with respect to normal weight aggregates, due to their higher porosity, and this accounts for the fact that the secant modulus of lightweight concrete proves to be lower with respect to normal weight concrete.

In the case of expanded clay, when the volume fraction of fibers increases from 0% to 2% the average secant elastic modulus increases from 14450 MPa to 17010 MPa, while in the case of pumice stone it decreases from 15210 MPa to 13360 MPa.

In concrete wi th expanded clay aggregates the increase in the secant elastic modulus in the presence of fibres can be related to the decrease in the number of the original shrinkage cracks. By contrast, in concrete with pumice stone aggregates the addition of fibres produces a

2 0 4

Campione, Mlraglia, Papia

3 5 ' ' ' �9 . . . . . . . . . . . . . .

30 1.1, (pumice stone) V~ = 0 % %, 25 i I

~" 2o

0 �9 "- ",..- : ,. ..... :--

35 . . . . . . . . . . . . . . . . . . . . 30 Vi = 0.5 %

~'25- g 2 0 -

1

0 '1 . . . . , . . . . , . . . . . .

35 , , , , . . . . . . . . . . . , , l

t Vf= 1.0 % ,3O

15

1

' 'i, o = " i

15

1

i

0,000 0.005 0.010 g: 0.015

Fig. 3 - Compressive behaviour o f FR.C pumice stone varying w i t h V e

3 5 ' ' "

. - - . 30 k (expanded clay) Vf = 0 % 0

" b 2 0 -

15 l ~R,, /t431 t22J ,o-I / / ' ~ + +

' / / ~ Plain FRO

. . - , 30~ Vf = 0.5 %

':"~ # ".%. 1

! . . . . . . . . . . . . . . . .

,--. 30 Vf = 1.0 % 0

b~~ i ~ , , . \ ~

1 5 - [ , ~ r = .

"6" 3~

,:, ,01/ /

::7 1 7 i . . . . . . . .

0.000 0.005 0.010 s 0.015

Fig. 4 - C o m p r e s s i v e b e h a v i o u r o f F R C e x p a n d e d clay varying w i t h Vf.

reduction in the modulus, which is perhaps due to the reduced compacting of the composite occurring because the size and the shape of the aggregate are not appropriate with respect to the size of the fibres used. Fig. 7 shows the values of the ductility factor (Ix) with variation in V c. The ductility factor bt is defined as the ratio between the energy absorbed by the lightweight con-

crete matrices with fibres and the corresponding energy relative to normal concrete matrices with fibres.

In both cases this energy is measured from the area subtended by the stress-strain curve, and it is convention- ally evaluated up to the strain value of 1.5~ tn all cases examined an increase in the ductility factors was observed with an increase in the volume fraction of fibres.

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Materials and Structures/Mat~riaux et Constructions, Vol. 34, May 2001

I, .~ 35 A o ~t Pumice stone a)

30 - A Expanded clay

2 5 - A.,...- ~" . . . . "~ . . . . . . . . . " " A

20 - "It . . . . -~ . . . . . ~ . . . . . . . . . . -~

15 . . . . .

3 . 5 ; Pumice stone b~ ~o 3 0 * Expanded clay .~

1.5 . . . . . I 0.0 0.5 1.0 1.5 2.0

- - -

v, (%) '~,.., - variation in compressive strength and corresponding

strain with Vf.

20

LJ

18-

16-

14"

12~

10

r Pumice stone A Expanded clay

w i �9 i |

0.0 0.5 1.0 1.5 2.0

v,(%) Fig. 6 - Variation in elasticity modulus with Vf.

g 3.0

2.5

2.0

1.5

1.0

0.5

.,& Pumice stone ..-"

" Expanded clay ..-" # . . . . . ~.'"

r /

I I i I ]

0.O 0.5 1.0 1.5 2.0

v,(%)

Fig. 7 - Variation in ductility factor with Vf.

140 Z~ 120 i.~ (pumice stone)

~ '100 P,'

8O

otA 4O F

20

V f = 0 %

Plain !RC

v,=o.5 %

Vf= 1.0%

0 n ,,,

140 ~ ' 1 2 0

" - " 1 0 0 0_

Vf= 2.0 %

60

40

20 0 0.0 0.5 1.0 1.5 2.0 2.5 3.0

(5 (mm)

Fig. 8 - Sprit tests for FRC with pumice stone varying with Vf.

3.2 Results in split tension

Figs. 8 and 9 show results o f indirect tensile tests (splitting tests), under both monotonic and cyclic loads, with variation in the volume fraction of fibres in light- we igh t conc re t e mat r ices wi th p u m i c e s tone and expanded clay. It can be observed that the increases in the tensile strength are more significant in lightweight

2 0 6


140 l,OJ 1~176 1 n 8O

40

20

0

Lc (expanded clay) V~ = 0 %

T 1 Plain FRC

|

II I | |

140

~" 120 ,,3r

' - ' 100 n

8O

6 0 -

4-0-

20"

O' 140

V f = 0.5 %

i i ! i

Z'v 120 = " [

~ " 100

O_ 80

60

40

20

i i ! �9 i

140

~" 120

~'~ 100 n 80

60

40

20

0.0 0.5 1.0 1.5 2.0 2.5 3.0 (mm)

Fig. 9 - Split tests for FRC expanded clay varying with Vf.

c o n c r e t e with expanded clay than in that with pumice stone, analogously to what occurs for compression tests.

The adding of higher percentages of fibres produces an increase in post-peak strength; moreover, a more progressive failure is observed when the maximum load is reached. If very high percentages of fibres (1% and 2%) are used, composite materials exhibit strain hardening, and consequently an increase in the maximum strain corresponding to the failure of matrices is observed.

In the case of lightweight expanded clay aggregates,

140

~-" 100 r f r O-

flO 2 3 Vf=2 % 60- 1 ~

o / 20 mice ston

i i i i i

o.o o.s 1.o 1.s 2.0 2.s a.o 6 (mm)

~" 12o-I

Q_ 80 "t ~ 1 2 3 4

1 % ; o % _ , ,_ _, ,

O / . I Expanded clay

0.0 0.5 1.0 1.5 2.0 2.5 3.0 (~ (mm)

Fig. 10 - Splitting tensile failure mechanisms for FRC varying with Vf.

5.0 ' 2 -P

"-" ft = l t . D . L 4.0 .~

r 1' A ' - - '~"

2.0 ~, - - -'A' - - o ~r Pumice stone A Expanded clay

1.0 . . . . . 0 .0 0 .5 1.0 1.5 2 .0

v, (o,~)

Fig. 11 - Splitting tensile strength values for FRC varying with Vf.

multiple cracking before reaching the peak load was observed. Even when the principal crack is localised, the crack opening is limited by the presence of fibers; there- fore, finer cracks open progressively, with a more effi- cient dissipative mechanism with respect to the case of pumice stone lightweight concrete (Fig. 10).

After failure, fibres ensure high levels of deformation without a significant reduction in the bearing capacity. Under cyclic action it is also possible to reach very high load levels after repeated cycles of unloading and reload- ing with an increase in the maximum strain.

Fig. 11 shows the variation in tensile spli t t ing strength values f't with the volume percentages of fibres

207

Materials and Structures/Mat~riaux et Constructions, Vol. 34, May 2001

Vf. It is clear that with both pumice stone and expanded clay aggregates the variation in the splitting tensile strength is a quasi-linear function of Vf. In the case of expanded clay, when the fibre volume fraction increases from 0% to 2% the average splitting tensile strength increases from 2.12 MPa to 4.10 MPa, with a rate of variation of splitting tensile strength from 13 to 93%, depending on the fibre volume fraction. For pumice stone it increases from 1.93 MPa to 2.27 MPa, and the tensile strength increases from 3 to 18%, depending on the fibre volume fraction. It is interesting to observe that for a lower fibre volume fraction any improvement in the splitting maximum tensile strength of concrete is hardly effective, but the post-peak stress improves significantly in all cases and the behaviour in the softening branch is very ductile.

3.3 Results in flexure

Figs. 12 and 13 show monotonic and cyclic load-deflection curves observed from flexure tests on prismatic beams in the case of lightweight concrete with pumice stone and expanded clay aggregate. In each graph the abscissae and ordinates are dimensionless with respect to the maximum load and to the corresponding deflection of plain lightweight concrete beams (Vf = 0%). When the maximum load is reached in plain concrete beams, brittle failure and no post-peak stress were observed; when fibres were added the fracture process of steel fibre reinforced concrete con- sisted of progressive &bonding of fibre, during which slow crack propagation occurred and high levels of post-peak stress were observed. Under cyclic actions fibres efficiently bridge the cracks and no significant loss of bearing capacity is observed even when high levels of deflection are reached. Fig. 14 shows modulus of rupture (MOP,) values with variation in the volume fraction of the fibres. A quasi-linear variation in M O R values was observed with an increase in Vf. In the case of expanded clay, when the fibre volume fraction increased from 0% to 2% the average M O R values increased from 3.69 MPa to 8.04 MPa, with a rate of variation in flexural strength from 27 to 117%, depending on the fibre volume fraction. In the case of pumice stone it increased from 4.07 MPa to 5.82 MPa, with a rate of variation in flexural strength from 7 to 42%, depending on the fibre volume fraction. Figs. 15 and 16 show the mode of failure of specimens tested in compression and in splitting tension in the case of expanded clay with variation in the volume fraction of the fibres. Fig. 17 shows the mode of failure of specimens tested in flexure in the case of expanded clay with 2% of fibres.

4. C O N C L U D I N G REMARKS

Standard tests carried out considering monotonically increasing or cyclically varying imposed displacements highlight some significant aspects of the behaviour of steel fibre reinforced lightweight concrete with pumice stone and expanded clay aggregates:

2.5 (pumice stone) Vf = 0 %

0 9

rt

2.0

1.5

1.0

0.5

0.0

2.5

P,(S

P12 I' Tp,2

i I I I

0.9 2.0

n 1.5

Vf= 0.5 %

l

0.5

0.0

2.5

o.9 2.0

0_ 1.5

Vf= 1 .0%

l

0.5

0.0

2.5

0.9 2.0

n 1.5

V f=2 .0 %

0.5

0.0 0.0 1.0 2.0 3.0 4.0 5.0

Fig. 12 - Bending test on FRC pumice stone varying with Vf.

1. Lightweight concrete exhibits very brittle behaviour with respect to normal weight concrete having the same compressive strength.

2. The addition of fibres with an increase in the volume fraction corrects the brittle behaviour of lightweight concrete, ensuring higher post-peak stress and absorbed energy values.

3. In the case of expanded clay aggregates the maximum compressive strength can increase up to 30%, while no variation was observed for pumice stone aggregates.

2 0 8


O9 n

2.5

2.0

1.5

1.0 , ~

0.5

0 .0

2 .5

(expanded clay) V f = 0 %

,L P,5

P/2 T Tp/2

Vt= 0.5 % O92.O r 1.5

l

0.5

0.0

2.5 "

Vr= 1.0% 0.9 2.0 �9

~" 1.5

1.0 . ~ 0.5

0.0 ~

2.5

0.92.0

CL 1.5 1.0

0.5

0.0 i

0.0 1.0

�9 - - ~

i ! I

2.0 3.0 4.0 5.0

Fig. 13 - Bending test on FRC expanded clay varying with V r

4. Increases in splitting and flexural maximum and post-peak stress were observed for both expanded clay and pumice stone when fibers were added.

Exper imenta l results have shown higher per for - mances o f FRC with expanded clay than pumice stone, but pumice stone performs well, making it suitable for use even in seismic areas, especially in regions where its cost is low.

Fig. 14 - MOR values for FRC varying with V r

Fig. 15 - Mode of failure of specimens tested in compression.

Fig. 16 - Mode of failure of specimens tested in split tension.

Fig. 17 - Mode of failure of specimens tested in flexure.

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Materials and Structures/Mat&iaux et Constructions, Vol..34, May 2001

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