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Engineering properties of oil palm shell lightweight concrete containing fly ash

Payam Shafigh ⇑, U. Johnson Alengaram, Hilmi Bin Mahmud, Mohd Zamin Jumaat Department of Civil Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia

a r t i c l e i n f o

Article history:Received 15 November 2012 Accepted 2 February 2013 Available online 16 February 2013

Keywords:Oil palm shell Lightweight concrete Mechanical properties ShrinkageCuring

a b s t r a c t

The effect of cement replacement with type F fly ash at 0%, 10%, 30% and 50% on some engineering properties of an oil palm shell (OPS) high strength lightweight concrete was investigated. The properties stud- ied include workability, density, compressive strength, splitting tensile strength, flexural strength, water absorption and drying shrinkage. The effect of initial water curing periods of 2, 4 and 6 days after demoulding and air drying environmen t on the 28-day comp ressive strength was also investigated. The test results showed that even with 50% substitution of cement with fly ash, a low cost grade 30 OPS lightweight concrete can be produced. Although the inclus ion of fly ash in OPS concrete makes it more sensitive to curing, even 2 days moist curing after demoulding significantly reduces this sensitivity. The results of the tensile strengths of OPS concretes imply that OPS concretes containing up to 50% fly ash are suitable for use as structural concrete elements. The use of 10% fly ash in OPS concrete did not affect the drying shrinkage of OPS high strength concrete.

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1. Introduction

Structural lightweight concrete is defined by RILEM/CEB as having a density in the range of 1600–2000 kg/m 3 and a compressive strength of more than 15 MPa [1]. The minimum strengths for structural lightweight concrete, as identified by several codes are shown in Table 1 [2]. The usual method for producing structural lightweight concrete is by incorporating lightweig ht aggregate,which can be substituted either wholly or partially with conventional aggregates. In most cases, this is achieved using a lightweight coarse aggregate and a normal weight sand for the fineaggregate [3]. Lightweight aggregat es have a density significantlylower than conventional aggregat es, ranging from 560 to 1120 kg/m 3 [4]. Generally, the water absorption of lightweight aggregates varies from 5% to 30% in 24 h [5]. Although there are many types of lightweight aggregate, they can be broadly classifiedinto two groups; natural, such as pumice, scoria, diatomite, volca- nic cinders, sawdust, bottom ash and oil palm shell; and, artificial,such as perlite, sintered PFA, expanded clay, shall, slag, slate and vermiculite [6,7]. Artificial lightweight aggregates are obtained by heating suitable natural raw materials in a rotary kiln under temperature of 1000–1200 �C [8]. Therefore, the cost of production of these types of lightweig ht aggregat e is high [9], not only for countries that have such production technology and suitable natural materials for making it but also the cost may be significantlymore for those countries in which these materials are not available locally.

Oil palm shell (OPS) is an agricultural solid waste lightweight aggregat e for making lightweight concrete, the use of which has been recognized in Malaysia for more than two decades. The use of OPS as an aggregat e for making lightweight concrete was re- searched as early as 1984 by Abdullah [10]. He developed OPS concrete with a 28-day compressive strength in the range of 5–20 MPa with an air dry density in the range of 1725–2050 kg/m 3 [11]. Man- nan and Ganapathy [12] suggested mix design for OPS concrete having 28-day compress ive strength of about 24–29 MPa with and without calcium chloride (as accelerator) with demoulded density in the range of 1890–1905 kg/m 3. Mannan et al. [13] exam-ined several pre-treatment methods for improving the quality of OPS aggregates for achieving better strength for OPS concrete .Their methods were similar to preservative treatment to wood.Using these methods, they successfully produced OPS concrete with a 28-day compressive strength of about 33 MPa, which was about 39% higher than the reference concrete (OPS concrete without pre-treated OPS aggregates). Alengara m et al. [14] reported the highest 28-day compressive strength in their studies of about 38 MPa with a saturated density of about 1960 kg/m 3.

Zhang and Gjorv [15] stated that not all types of lightweight aggregat e are suitable for producing high strength lightweight concrete. However , recent studies [16,17] showed that OPS can be used as a lightweight aggregate for producing high strength lightweight concrete with 28-day compressive strength of up to 53 MPa with oven dry density of 1830–1920 kg/m 3.

The world consumptio n of Portland cement has risen from less than 2 million tonnes in 1880 to 1.3 billion tonnes in 1996 [18].Today, it is 1.5 billion tonnes and it has been estimate d that it will reach 2.5 billion tonnes by 2020 [19]. Huge amounts of raw

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⇑ Corresponding author. Tel.: +60 172437383; fax: +60 379675318.E-mail addresses: [email protected], [email protected] (P. Shafigh).

Materials and Design 49 (2013) 613–621

Contents lists available at SciVerse ScienceDi rect

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materials are needed to produce cement each year. Cement production is very energy intensive and the cement industry has been acknowledged as being responsib le for about 7% of all CO 2 emis-sions [20]. To reduce the environmental impact of cement, the use of supplementary cementitious materials such as fly ash,ground granulated blast furnace slag (GGBFS), silica fume, rice- husk ash, natural pozzolans and other similar pozzolanic materials ,can provide a good solution. The use of such supplem entary cement materials not only reduces the use of manufactur ed Portland cement, but, in addition, the concrete produced by such blended cement is more durable [21]. Fly ash and blast furnace slag are the two types of waste most used in blended cement [22]. Class F fly ash, which is a by-product of electricity coal-burning power plants is available at low cost in Malaysia. There is little information concerning the use of fly ash in OPS concrete . Mannan and Ganapathy [23] examined the effect of 10% and 15% fly ash as apartial cement replacemen t in OPS concrete with a 28-day compressive strength of about 24 MPa and cement content of 480 kg/ m3, as well as in a normal weight concrete with the same mix proportions. The test results of their study showed that when cement was substitut ed by fly ash at 10% and 15%, the density decreased by 2% and 3%, respectively. In addition, the 28-day compress ive strength decrease d by 7% and 19%, respectively . As can be seen from these results, the reduction in compressive strength for 15%fly ash concrete is significant. Alengara m et al. [24] reported experimental test results of several OPS concretes with cement content of 440–530 kg/m 3. They used 5% fly ash as cement replacemen t as well as 10% silica fume as additional cementitious material for all mixes. The effect of each cementiti ous material on the OPS concrete was not investiga ted individually. The highest 28-day compressive strength of 36 MPa was reported in their study. Because of the lack of information concerning the use of fly ash in OPS concrete, particularly at high percentage replacement levels, this study focused on the use of this supplementar y cement material in OPS concrete.

2. Significance of research

It has been reported that the concrete industry will consume 8–12 billion tonnes of natural aggregates annually after the year 2010 [25]. OPS is a renewab le resource with significant potential in the construction industry in countries such as Malaysia where oil palm is in abundan ce. The use of OPS as an aggregat e in concrete mini- mizes the demand on finite resources such as stone and gravel [26]. Nevertheles s, previous studies [12–14,24,27] showed that even for achieving normal strength of 20–35 MPa for OPS lightweight concrete, the cement consump tion is usually high (>400 kg/m 3). Therefore, successful production of a structural lightweight concrete using two waste materials (OPS as coarse aggregate and fly ash as supplementary cementitious material)not only reduces the cost of concrete production but also makes

it an environmental ly friendly material. In addition, this concrete is lightweig ht; the advantages of using lightweight concrete have been well recognized by many researche rs.

3. Experimen tal programme

3.1. Materials

Ordinary Portland cement (OPC) and class F fly ash were used as binder. The cement used was obtained from a local cement company and had 7- and 28-day compressive strength of about 30 and 39 MPa, respectivel y. The specific gravity, Blain surface area,initial and final setting time of the cement were 3.14 g/cm 3,3510 cm 2/g, 65 min and 140 min, respectively . The fly ash used was obtained from a local power station and was classified as class F according to ASTM: C618. The Blaine surface area of the fly ash was 7290 cm 2/g, which is about twice that of cement. In addition,a superpla sticizer (SP) based on polycarboxy lic ether (PCE) was used in all mixes. The chemical compositi ons of OPC and fly ash are shown in Table 2.

Local mining sand with a maximum nominal size of 4.75 mm,specific gravity of 2.65 and fineness modulus of 2.70 was used as fine aggregat e. Old OPSs were collected from a local crude palm oil producing mill. The OPSs were washed and then crushed with a stone crusher machine in the laborator y. They were then sieved with a 5 mm sieve. The OPS aggregate retained in the sieve had amaximum nominal size of 8 mm and were used as coarse lightweight aggregat e in all mixes. The 24-h water absorption of OPS was about 20%. In addition, crushed granite as normal coarse aggregat e with a maximum nominal size of 12.5 mm was used in this study.

3.2. Mix proportions

Four different OPS concrete mixtures were produced with the main difference being in the fly ash and cement content. The mix proportio n of all mixes is shown in Table 3. Mix F is an OPS concrete with high strength, which was designed with a low water/ce -ment ratio according to the method previously introduced by Shafigh et al. [17]. Mix F10 is similar to mix F except for the binder.In this mix, 10% of cement (by mass) was substituted by fly ash. For mix F30, 30% of the cement content was substituted by fly ash.When all the materials were mixed together (similar to the mixes F and F10), it was observed that for mixes F30 and F50 the materials tend to agglomer ated together (Fig. 1). This may be due to the fly ash being finer than cement and the mixes have low water to binder ratio. Further mixing time could not solve this problem.Therefore, the water content was adjusted in the F30 and F50 mixes in order to prevent agglomeration. Consequentl y, the F30 and F50 mixes had a higher water/binder ratio of 0.305 and 0.320, respectively , compared to mix F concrete of 0.288.

Table 1Specification of minimum compressive strength (N/mm2) requirement for structural lightweight conc rete [2].

Code Reinforced Prestressed

BS 8110 20 30 a and 40 b

BS 5400 25 Not permitted ACI 318 30 30 ENV 1992-1-4 12 25 a and 30 b

AS 3600 25 25 NS 3473 25 35 JASS 5 25 25

a For post-tensioned.b For pretensioned.

Table 2Chemical composition of cement and fly ash (%).

Oxide composition OPC Fly ash

SiO 2 19.80 64.60 Fe 2O3 3.10 4.00 CaO 63.40 1.00 MgO 2.50 0.66 Al 2O3 5.10 20.9 SO 3 2.40 0.30 K2O 1.00 1.20 Na 2O 0.19 0.32 LOI 1.80 5.10

614 P. Shafigh et al. / Materials and Design 49 (2013) 613–621


3.3. Test methods

As for the mixing procedure, all aggregates were mixed in amixer for 2 min. Then the binder was added to the aggregate and mixing continue d for another 3 min. Then the superplasticizer and the mixing water were gradually added and mixed for 2 min.Then mixing was continued for another 3 min. In the case of F30 and F50 mixes, in which agglomerati on occurred , after adding more water to resolve the agglomer ation problem, mixing was continued for a further 2 min. Then the slump test was performed.The concrete specimens were cast in 100-mm cubes, cylinders of 100-mm diameter and 200-mm height, prisms of 100 � 100 � 500 mm3 and prisms of 100 � 100 � 300 mm3 steelmoulds for measuring compress ive strength, splitting tensile strength, flexural strength and drying shrinkage, respectivel y. All specimens were compacted using a vibrating table. The specimens were demoulded 24 h after casting. At least three specimens were prepared for obtaining the average value for mechanical properties and two specimens were used for the drying shrinkage test. The drying shrinkage test, under laboratory environment condition,was conducted immediately after demoulding. The shrinkage value for each age is the average of six readings.

3.4. Curing regimes

For determining the effect of curing environment on the 28-day compressive strength of OPS concrete with and without fly ash, the specimens were cured under five curing regimes, as follows:

FW: specimens were immersed in water at 23 ± 2 �C until the age of testing.

3 W, 5 W and 7 W: curing in water for 2, 4 and 6 days, respectively, after demoulding and then air drying in a laboratory environment with RH of 67–82% and temperature of 29 ± 3 �C.

AC: specimens were kept in a laboratory environm ent after demoulding.

4. Results and discussion

4.1. Workabil ity and density

Table 4 shows the test results for the workability and density of all mixes. The workabili ty of the concrete was measure d using the slump test. Mix F had a slump value of 145 mm. Although the water/binder ratio of F and F10 mixes is the same, the slump value of mix F10 is comparable with mix F. This shows that replacing 10%cement with fly ash does not affect the workability of OPS concrete.F30 and F50 mixes had high slump values; however, it was visually observed that these mixes tend to be sticky. No floating of OPS aggregat e was observed in all mixes.

The density of the OPS concrete decreased when fly ash was incorporate d in the mixture. Mix F10, on average, has a 1.5% lower density than the reference concrete mix F. It was shown in previous studies [23,28] that substitution of cement by supplementar ycementing materials in structural lightweig ht aggregate concrete reduces the density of concrete. This is due to the lower density of the supplementar y cementitious material, such as fly ash and blast furnace slag, compared to Portland cement. Therefore, further reduction in the density of lightweight concrete is another advan- tage of using supplementar y cementing materials instead of Port- land cement. From the results of this study, it was observed that by substitut ing cement with high volume fly ash in OPS concrete with low water/ceme nt ratio, the reduction in density is more significant than at low replacement level. On average, F30 and F50 mixes had 3.5% and 5.3% lower density than the reference mix F,respectively .

Mannan and Ganapathy [23] reported that by incorporating 10%and 15% fly ash as cement replacemen t in OPS concrete , the density reduced by 2% and 3%, respectively . They also reported that reduc- tions in normal weight concrete with the same mix proportio ns were 1% and 2%, respectivel y.

4.2. Compressi ve strength

4.2.1. Continuous moist curing Table 5 shows the compressive strength of all mixes up to

56 days under continuo us moist curing. As can be seen in this table, the compressive strength of the OPS concrete containing flyash is lower than the OPS concrete without fly ash at early ages.At 28, and 56 days, mix F10 has slightly higher compress ive strength than mix F. The difference between the compress ive strength of these mixes at early and late ages are not significant.It is worth noting that to eliminate increases in creep and

Table 3Mix proportions of conc rete (kg/m3).

Mix no. Cement Fly ash Water Super plasticizer Sand Crushed granite Oil palm shell

F 550 0 158.4 5.5 900 178 326 F10 495 55 158.4 5.5 900 178 326 F30 385 165 167.8 5.5 900 78 326 F50 275 275 176.0 5.5 900 178 326

Fig. 1. Agglomeration behaviour of F30 and F50 mixes.

Table 4Workability and density.

Mix no. Slump (mm) Density (kg/m3)

Demoulded Air dry Oven dry

F 145 2046 2011 1898 F10 140 2021 1984 1862 F30 235 1985 1961 1804 F50 240 1948 1886 1805

P. Shafigh et al. / Materials and Design 49 (2013) 613–621 615


shrinkage, heat of hydration and cracking, it is recommend ed that the cement content of lightweight aggregate concrete does not exceed 500 kg/m 3 [29,30]. In this study, the reference mix of OPS high strength concrete (mix F), contain cement content of 550 kg/m 3,which was higher than the recommendati ons. However, mix F10 has a cement content of 495 kg/m 3, which followed the ACI criteria.As can be seen in Fig. 2, the compressive strength of mix F10 is comparable with mix F. On the other hand, it has been reported that the use of fly ash in concrete has advantages, such as positive effect on drying shrinkage and also better durabilit y [21,30,31].Therefore, for OPS high strength concrete and for better durability performanc e, it is strongly recomme nded that fly ash be used in the mixture at a certain amount (say 10%) such that the total cement content does not exceed 500 kg/m 3.

Mix F30 has a lower compressive strength than mix F at all ages.This mix has a 1, 3, 7, 28 and 56 days compressive strength of about 33%, 29%, 25%, 14% and 9% lower, respectivel y, than mix Fat the same age. As can be seen, the difference between the strengths of the two mixes gradually decreases over time. Reduc- tion in difference in strengths is more significant at 28 and 56 days due to pozzolanic reaction of fly ash at 28 days and beyond. Com- pared to mix F30, mix F50 has significantly lower compressive strength than mix F, even at later ages. The compressive strength of mix F50 is about 72%, 55%, 48%, 32% and 29% lower than mix Fat 1, 3, 7, 28 and 56 days, respectively. When the compositions of OPS concrete containing 30% and 50% fly ash in this study are compared with previous studies [13,14,33 ,34] , it can be seen that because of the lower cement content used in the present investigatio n, they are therefore more economical. For instance,the cement content of mix F50 was 275 kg/m 3. This concrete was of grade 30 at 28- and 56-days. According to the test results of arecent study [14], grade 30 OPS concrete was produced by using cement content in the range of 504–564 kg/m 3 with 5% fly ash and 10% silica fume as additional cementitious materials. The cement content of mix F50 is about 45–51% lower than the cement used in that study. Furthermore, mix F50 does not contain silica fume which is a very expensive material. Therefore, it is evident

that mix F50 was produced at a significantly cheaper cost than the previous OPS concrete of grade 30 [14]. Therefore, mix F50,which contains a high volume of waste materials (OPS as coarse aggregat e and fly ash as cementitious material) can be termed as an environmental ly friendly lightweig ht aggregat e concrete that can be used for making structural concrete members. Recently,the properties of a green structura l lightweight concrete containing low cement content was reported by Pelisser et al. [35]. They produced a lightweight concrete in which recycled tyre rubber was substituted for sand. This concrete had a compressive strength of 20 MPa and included metakaolin and a cement content of 260 kg/m 3.

In the case of mix F30, if the 56-day compressive strength of this concrete is considered , it can be seen that this OPS concrete is ahigh strength lightweight concrete containing 385 kg/m 3 cement.Compare d to previous studies [16,17] for the same compress ive strength of OPS concrete, the cement content required was 480–550 kg/m 3. This shows a saving in cement usage of about 20–30%in mix F30.

Table 6 shows several equation s regarding the prediction of the compress ive strength of OPS concrete containing fly ash. It is inter- esting to note that all equations have strong correlation. It is com- monly held that it is difficult to predict the 28-day strengths from the early strengths for high strength lightweight concrete because of the influence of the lightweight aggregat es [36]. However, as can be seen in Table 6, in the case of OPS lightweight concrete the prediction of compressive strength at later ages from early ages is pos- sible for both high and normal strengths.

Generally, lightweight concretes have higher or similar early strength as ordinary concrete of the same strength class, and 80–95% of the 28-day strength can be attained at 7 days [30]. Previous studies [16,17] have shown that the 7- to 28-day compress ive strength ratio for OPS concretes having 28-day compress ive strength of 34–53 MPa, is in the range of 86–96%. As can be seen in Table 5 this ratio for OPS concrete without fly ash (mix F) is 95%. However, substitut ion of cement by fly ash reduces this ratio.A higher fly ash dosage results in a smaller ratio. However, it should be noted that all OPS concrete s containing fly ash showed a 28- to 56-day compressive strength ratio of 91–98%, which is compara ble to the 7- to 28-day compressive strength ratio of OPS concrete without fly ash.

4.2.2. Air drying Fig. 3 shows the 28-day compressive strength of OPS concrete

with and without water curing. As can be seen, the specimens in air drying condition have lower compress ive strength. The rate of reduction in compressive strength is more for the OPS concrete containing fly ash. Mix F showed 14% lower compress ive strength in air drying while the reduction rate in the F10, F30 and F50 mixes was 17%, 23% and 27%, respectivel y. This means that the compressive strength of OPS concrete containing higher fly ash dosage is more sensitive to lack of curing. It should be noted that such sensitivity should be considered in respect of three phenomena . First,the high humidity of the laboratory environment, which is usual in tropical regimes, causes lower evaporation of concrete specimens

Table 5Compressive strength of concretes under continuous moist curing.

Mix no. Compressive strength (MPa)

1 day 3 days 7 days 28 days 56 days

F 21.09 (47%) 36.81 (83%) 42.29 (95%) 44.49 46.43 (104%)F10 19.73 (43%) 34.27 (74%) 39.23 (85%) 46.08 46.85 (102%)F30 14.03 (37%) 26.23 (69%) 31.67 (83%) 38.12 42.09 (110%)F50 5.94 (20%) 16.47 (55%) 22.10 (74%) 30.04 33.16 (110%)

a The data in parentheses are percentages of 28-day compressive strength.

15

20

25

30

35

40

45

50

55

0 20 40 60 80 100Age (days)

Com

pres

sive

str

engt

h (M

Pa)

F F10

Fig. 2. Compressive strength development of mixes F and F10.

Table 6Equations for predicting compressive strength of OPS concrete at different ages.

Equation Correlation (R2) Equation no.

f28 ¼ �0:41f 21 þ 5:04f 1 0.96 (1)

f28 ¼ �0:03f 23 þ 2:21f 3 1.00 (2)

f28 ¼ 1:1539f 7 0.97 (3)f56 ¼ �0:02f 2

7 þ 1:95f 7 1.00 (4)

f56 ¼ 1:0568f 28 0.99 (5)



compared to a drier environment. Second, the relatively high temperature of the lab environment, which is usual in tropical regimes,helps to promote the pozzolanic reaction of fly ash. Third, the effect of internal curing of water inside the OPS grains help to improve the hydration of the cement, and, consequentl y, promote the pozzolanic reaction of fly ash. As can be observed , accordin g to the three helpful paramete rs, a significant reduction in the compressive strength was observed in concrete containing fly ash.

Reports show that curing for concrete containing supplementary cementitious materials such as fly ash, silica fume and slag,is more important than for concrete without such materials [32,37,38]. Newman and Choo [1] reported that since fly ash concrete has slower hydration rates therefore lack of adequate curing will affect the final product. However, it should be noted that lab concrete specimens or thin concrete sections appear to be more vulnerable to the lack of curing than thick concrete sections [1,39]. This is because in large concrete elements, the heat of hydration will promote the pozzolanic reaction of a pozzolanic material [1].

Fig. 4 shows a linear relationship with a strong correlation between the compress ive strength of OPS concrete in a dry and wet curing environments. The following equation is suggested for OPS concretes:

fcðdryÞ ¼ 0:81f cðwetÞ R2 ¼ 0:93 ð6Þ

For OPS concrete containin g GGBFS up to 50% replacem ent of cement the following equation was reported [40]:


Atis et al. [38] proposed the following equation for normal weight concre te containing silica fume (up to 20% replaceme nt of cement):


4.2.3. Initial water curing The curing of concrete is essential for the reliable performanc e

of concrete structures [41]. It was reported that the requirements for curing lightweight concrete are not different than those for normal weight concrete [42]. ACI-318 [43] recomme nds that concrete (other than high-early strength) shall be maintain ed above 10 �Cand in a moist condition for at least the first 7 days after place- ment. In addition, it should be noted that in practice, structural concrete elements are rarely moist cured for more than 7 days [44].

In this study, the effect of three types of initial water curing,such as 3 W, 5 W and 7 W on the 28-day compressive strength OPS concrete s was investigated. From Table 7, it can be seen that the 28-day compress ive strength of all OPS concretes under initial water curing is lower than that when cured in continuous moist curing. However , under 5 W and 7 W curing conditions, except for mix F10, the results are compara ble to that under FW curing.A noticeable finding is that although OPS concrete containing higher amount of fly ash is more sensitive to lack of curing, even with 2 days initial water curing regime (3 W) significantly compensates for the reduction of compress ive strength of specimens under air drying (AC). For instance, mix F50, without any curing, showed astrength loss of about 27% (which was the highest strength loss under AC among all mixes) while the same concrete under 3 W curing condition only showed a 10% reduction in compressive strength.Therefore, it can be concluded that for OPS concrete containing high volume fly ash even with a very short time water curing can be very effective on the strength gain at 28 days. Teo et al.[45] investigated the durability performanc e of OPS concrete under different curing conditions. The OPS concrete had a 28-day compressive strength, air-dry density and air content of 28 MPa,1965 kg/m 3 and 5%, respectively . They reported that proper curing is required for OPS concrete to achieve better durability. They recommend ed that the minimum duration of moist curing should be at least 7 days.

4.3. Splitting tensile strength

The splitting tensile strength of all the mixes is shown in Table 8.All OPS concretes containing fly ash showed lower splitting tensile strength than the reference concrete. A noticeable point is that although mix F10 has a 3.6% higher 28-day compressive strength than mix F, this mix has a 19.7% lower splitting tensile strength.This may be due to the quality of the interfacia l transition zone (ITZ), which has a significant role in the tensile strength of concrete. Therefore, it can be concluded that the quality of ITZ in mix F10 is lower than for mix F.

In the literature, the following equation was suggested to predict the splitting tensile strength from the compressive strength of OPS concretes made from just OPC [46]:

05

101520253035404550

F F10 F30 F50Mix no.

28-d

ay c

ompr

essi

ve s

tren

gth

(MPa

)

FW AC

Fig. 3. Compressive strength of concretes under water curing and air drying conditions.

fc(dry) = 0.81fc(wet) R2 = 0.93

2022242628303234363840

25 30 35 40 45 50

28-day compressive strength under wet curing (MPa)

28-d

ay c

ompr

essi

ve s

tren

gth

unde

r dry

cur

ing

(MPa

)

Fig. 4. Relationship between 28-day compressive strength of OPS concrete under air drying and water curing conditions.

Table 728-day compressive strength of OPS concretes under different curing conditions.

Mix no. 28-day compressive strength (MPa)

FW AC 3 W 5 W 7 W

F 44.49 38.31 39.06 42.54 43.45 F10 46.08 38.14 40.28 41.17 41.45 F30 38.12 29.45 34.55 35.59 37.25 F50 30.04 21.97 27.01 28.66 28.03



ft ¼ 0:49ffiffiffiffiffifcu

pð9Þ

where ft is the splitting tensile strength and fcu is the cube compressive strength, both in MPa.

Table 8 shows the prediction of ft by using Eq. (9). It can be seen that this equation is for OPS concrete without fly ash and is not suitable for OPS concrete containing fly ash. If OPS concrete containing fly ash only is considered , the following equation with strong correlation is proposed:

ft ¼ 0:23f 0:64cu R2 ¼ 0:91 ð10Þ

For a structur al lightweigh t aggreg ate concre te made of an artificial lightweig ht aggreg ate, namely, cold-bon ded fly ash, the following equation is suggested to predict the splitting tensile strength from the compres sive strength (compressive strength in the range of 21–47 MPa) [47]:

ft ¼ 0:27f 0:67cu ð11Þ

Fig. 5 shows the relationship between the 28-day splitting tensile and compressive strengths of OPS concrete without fly ash (according to Eq. (9)) and with fly ash (according to Eq. (10)). It is clear that at the same compress ive strength, the splitting tensile strength of OPS concrete containing fly ash is lower than that without fly ash. The differenc e is about 20%. In addition, the predicted splitting tensile strength according to Eq. (11) (Table 8) shows that this equation is also suitable for predicting the splitting tensile strength of OPS concrete without fly ash. However, the equation overestimat es the values for OPS concrete containing fly ash.

It is worth noting that according to ASTM: C330, a minimum splitting tensile strength of 2.0 MPa is a requirement for structural grade lightweight aggregat e concrete. It can be seen that accordin gto this criteria, all OPS concrete containing fly ash, even mix F50,can be used for making structural concrete elements.

4.4. Flexural strength

It was reported that for concrete having a compress ive strength of more than 25 MPa, the flexural strength is 8–11% of the compressive strength [48]. As can be seen in Table 9, the ratio for all the mixes is within this range. According to Holm and Bremmer [49], the flexural strength of high strength lightweight aggregate

concrete under continuous moist curing, is generally in the range of 9–11% of the compressive strength. The F and F10 mixes are in the high strength grade with a flexural/compressive ratio of 11.1% and 9.7%, respectively, which are in the usual range for high strength lightweig ht concrete.

Fig. 6 shows the relationship between the flexural and compressive strengths of OPS concrete containing fly ash. There is a linear relationshi p with a strong correlation (R2 = 0.98) between these strength properties. This relation shows that in OPS concrete containing fly ash, the flexural strength is about 9% of the compress ive strength.

In this investiga tion, the flexural/splitting tensile strength ratio for F, F10, F30 and F50 mixes was found to be 1.53, 1.72, 1.47 and 1.37, respectively. Alengaram et al. [50] reported that the ratio for OPS concrete with normal compress ive strength is between 1.4 and 1.7. In general, as reported by Zheng et al. [51], the flexuralstrength of concrete is 35% higher than the splitting tensile strength.

The OPS concrete containing higher fly ash content showed lower flexural/splitting tensile ratio. The downwa rd trend of this ratio in high volume fly ash-OPS concrete represents greater nega- tive effect of inclusion of fly ash on the flexural strength compare dto the splitting tensile strength. For instance, when substitution of cement by fly ash in OPS concrete increased from 10% (mix F10) to 30% (mix F30) the flexural strength of concrete decrease d by about 18% while the splitting tensile strength decrease d by about 4%.

Table 8Splitting tensile strength of OPS concretes (MPa).

Mix no. Splitting tensile strength (MPa)

Measured (a) Predicted by Eq. (9) (b) ba

Predicted by Eq. (11) (c) ca

F 3.22 3.26 1.01 3.43 1.07 F10 2.59 3.32 1.28 3.52 1.36 F30 2.48 3.02 1.22 3.10 1.25 F50 1.98 2.68 1.35 2.64 1.33

Fig. 5. Relationship between splitting tensile strength and compressive strength of OPS concretes with and without fly ash.

Table 9Flexural strength of OPS concretes (MPa).

Mix no. fcu fr frfcu

frft

F 44.49 4.946 11.1 1.53 F10 46.08 4.452 9.7 1.72 F30 38.12 3.647 9.6 1.47 F50 30.04 2.702 9 1.37

fcu, fr and ft are 28-day compressive, flexural and splitting tensile strengths (MPa),respectively.

y = 0.09xR2 = 0.98

2

2.5

3

3.5

4

4.5

5

25 30 35 40 45 50Compressive strength (MPa)

Flex

ural

str

engt

h (M

Pa)

Fig. 6. Relationship between 28-day flexural and compressive strengths of OPS concrete containing fly ash.



4.5. Water absorption

The water absorption of all mixes is shown in Fig. 7. At 28 days,OPS concrete without fly ash (mix F) has the lowest water absorption of all the mixes. Mix F10 has 20% higher water absorption than mix F. This shows that substitution of cement by fly ash, even at alow percentage level, increases the water absorption of OPS concrete. This may be due to the fact that the pozzolanic effect of flyash occurs after 28 days. Gencel et al. [52] reported that when cement was replaced with fly ash at 0%, 10%, 20% and 30%, the water absorption of concrete s was 3.6%, 4%, 4.5% and 5%, respectively. In addition, a report [53] showed that by incorporating fly ash in foamed concrete, water absorption increased. Chindaprasi rt and Rattanasak [31] used fly ash as a partial replacement (15% and 30%) of cement and sand in foam lightweight concrete. They reported that foam concrete containing fly ash showed higher water absorption than the control concrete due to increase in the volume of paste matrix and capillary pores.

As expected, F30 and F50 mixes showed greater water absorption of about 27% and 60%, respectivel y, than mix F. Neville [8] re-ported that most good concrete has a water absorption of below 10% by mass. In this study, it is evident that if OPS high strength lightweight concrete with low water/ce ment ratio cement be substituted by fly ash up to 50%, the water absorption will be lower than 10%, and, in this respect, can be classified as good concrete.Teo et al. [27] reported that the OPS concrete containing 510 kg/ m3 Portland cement and with 28-day compressive strength of 28 MPa showed 11.2% and 10.6% water absorption, respectively ,under air drying and full water curing conditions. Furthermore,for OPS concrete with 28-day compressive strength in the range of 43–48 MPa, water absorption of about 3–6% was also reported [16].

4.6. Drying shrinkage

Drying shrinkage is an important property of concrete and is potentially deleterious when it is restrained [54]. It may also cause stress loss in prestressed members and failure of joints [54]. The drying shrinkage of insulating or moderate-strengt h lightweight concrete is not usually critical when it is used for insulation or fill,however, in structural use, shrinkage should be considered [4].Drying shrinkage depends on the type of cement and its contents ,water/ceme nt ratio, the degree of hydration, aggregat es and their elastic modulus, the characteristics and amount of admixtures,the time and the relative humidity of exposure, the size and shape of the concrete mass and the amount and distribution of internal reinforceme nt [11,55]. In the case of lightweight aggregat e concrete, it was reported [8,11] that the drying shrinkage is greater than in normal weight concrete and is mostly affected by the properties of lightweight aggregate and the aggregate content. In some

reports [56,57] it was specified that lightweight concrete has lower shrinkage than normal weight concrete . However for general design work, it was suggested that the shrinkage of lightweight aggregat e concrete is between 1.4 and 2 times that of normal weight concrete [2]. Fig. 8 shows the drying shrinkage strain for OPS concretes. As can be seen from the figure, the F and F10 mixes have similar shrinkage at all ages. Up to 28 days, mix F30 showed greater shrinkage than the other mixes. However, the shrinkage values of mix F50 are comparable with mixes F and F10 up to 28 days. After 28 days, mix F50 and F30 showed similar shrinkage.However , both have greater shrinkage than mixes F and F10. In general, it can be seen that at a specific age, the difference between the shrinkage values for all the mixes is not significant.

There are several reports concerning the effect of mineral admixtur es, such as fly ash on the drying shrinkage of concrete.Kostmatk a et al. [4] reported that when mineral admixtur es, such as fly ash, ground granulated blast furnace slag and silica fume are used in low to moderate amounts, their effect on drying shrinkage is generally small and of little practical significance. Lamond and Pielert [55] stated that fly ash and ground granulated blast furnace slag in the concrete mixture increase the drying shrinkage as well as to have minimal effect. In addition, it was reported that up to 20% replacemen t of cement with fly ash does not have a significanteffect on the drying shrinkage of concrete, however, fly ash concrete containing 30% and 50% fly ash exhibits more shrinkage than the control concrete [2].

From Fig. 8, it can be seen that the shrinkage rate decreased after about 80 days. At 120 days, the shrinkage of F and F10 mixes was about 435 microstrain . For F30 and F50 mixes, it was about 510 and 470 microstrain , respectively, which was about 17% and 8% greater than the reference concrete . One reason for having greater shrinkage is because F30 and F50 mixes have a higher water/binder ratio than mix F. Therefore, it can be concluded, that,in general, fly ash does not have a significant effect on the shrinkage results of OPS concrete.

There is a little information in the literature concerning the shrinkage of OPS concrete. Abdullah [58] reported that the shrinkage of OPS lightweight concrete was about five times that of normal weight concrete . Alengara m [59] measured the drying shrinkage of several types of OPS lightweight concrete without any initial moist curing. The specimens were exposed to uncon- trolled laborator y conditions (humidity of 75% and temperature of 30 �C) immediately after demoulding. The cement content, total binder content (cement + fly ash + silica fume), water/binder ratio and 28-day compressive strength varied between 500–560 kg/m 3,560–595 kg/m 3, 0.30–0.35 and 22–38 MPa, respectively. He reported that drying shrinkage of OPS concrete at 28, 56 and 90 days is in the range of 160–520, 300–990 and 540–1300 microstra in,respectively . He stated that high shrinkage of OPS concrete s is due to high cement and OPS (as coarse aggregat e) contents .

0123456789

10

F F10 F30 F50Mix no.

Wat

er a

bsor

ptio

n (%

)

Fig. 7. Water absorption of OPS concretes.

0

100

200

300

400

500

600

0 20 40 60 80 100 120 140 160

Age (days)

Dry

ing

shrin

kage

(mic

rost

rain

)

F F10F30 F50

Fig. 8. Drying shrinkage of OPS concretes.



It was reported that drying shrinkage values for structural lightweight concrete may vary between 0.04% and 0.15% [55]. For expanded clay and expanded shale lightweight concrete with strengths of 30–50 MPa, stored at 20 �C and at a relative humidity of 65%, a final shrinkage of 400–600 microstra in was reported [30].Kayali et al. [60] reported a drying shrinkage of about 1000 microstrain for sintered fly ash (Lytag) lightweight concrete with a total cementitious material content of 785 kg/m 3 after 400 days. They demonstrat ed that this value of shrinkage was about twice that of normal weight concrete containing a cementiti ous material content of 485 kg/m 3. Al-Khaiyat and Hague [61] reported a moderate high drying shrinkage of 640 microstrain for Lytag structural lightweight concrete after the age of 3 months.

It should be noted that realistic values of drying shrinkage for full-scale structures subject to external and internal exposure are 200–300 and 300–500 microstrain, respectivel y and where there is no other information a value of 350 microstrain could be as- sumed [2].

5. Conclusions

Based on the experimental results of this study, it can be concluded that:

(1) In an OPS high strength lightweight concrete with low water to cement ratio, when cement was substituted with fly ash by 30% or more, agglomer ation occurred in the mixture.Therefore, further water needs to be added to have a work- able concrete.

(2) The substitution of cement by fly ash (by mass) in OPS concrete reduced its density. A reduction in density of about 1.5%, 3.5% and 5.3% in fly ash content of 10%, 30% and 50%,respectivel y, was observed in OPS concrete.

(3) In continuous moist curing environment, the developmen tof compressive strength of OPS concrete containing 10% flyash up to 90 days was comparable with the reference concrete. At 30% and 50% fly ash replacemen t levels, the compressive strength was lower than the reference concrete at all ages.

(4) By incorporating fly ash in OPS concrete at 30% and 50%,grades 35 and 30 lightweight concrete with lower cement content than previous studies were successfully produced.

(5) When fly ash is used in OPS concrete, its sensitivity to lack of curing increases. However, initial moist curing (i.e. 2 days moist curing after demouldi ng) significantly reduces this sensitivit y.

(6) OPS concrete containing 30% and 50% fly ash, as well as without fly ash under 5 W and 7 W initial water curing,showed comparable compress ive strength compare d to FW curing at 28-day age.

(7) OPS concrete containing fly ash showed 20% lower splitting tensile strength than OPS concrete without fly ash. However ,the lowest measure d 28-day splitting tensile strength of about 2 MPa for mix F50, showed that OPS concrete containing fly ash (up to 50% replacemen t) can be used in making structura l concrete members.

(8) The flexural strength to compressive strength as well as to splitting tensile strength ratios of OPS concrete with and without fly ash is in the range of normal structural concrete .

(9) OPS concrete containing a higher amount of fly ash has greater water absorption. However , even OPS concrete incorporati ng 50% fly ash is in the range of good concrete.

(10) The use of 10% fly ash in OPS concrete did not affect the drying shrinkage of OPS high strength concrete. However,

generally, for higher percentage replacemen t levels (30%and 50%), the drying shrinkage increased but was not significant.

Acknowled gment

This research work is funded by University of Malaya under Vice-Chance llor’s High Impact Research (HIR) Grant no UM.C/ 625/1/HIR /093 (Synthesis of novel geopolymer oil palm shell lightweight concrete ). The authors would like to thank Mr. Mansor Hi- tam for his assistance in conducting some of the tests reported in this paper.

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Author's personal copy - University of Malayarepository.um.edu.my/29871/1/PAYAM_HIRVC-1_JMAD5138.pdf · into two groups; natural, such as pumice, scoria ... aggregat e are suitable

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