Benefits of using blended waste coarse lightweight aggregates in structural lightweight aggregate concrete

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Journal of Cleaner Production xxx (2016) 1e10

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Journal of Cleaner Production

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Benefits of using blended waste coarse lightweight aggregates instructural lightweight aggregate concrete

Muhammad Aslam a, Payam Shafigh b, Mohd Zamin Jumaat a, *, Mohamed Lachemi c

a Department of Civil Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysiab Department of Building Surveying, Faculty of Built Environment, University of Malaya, 50603 Kuala Lumpur, Malaysiac Department of Civil Engineering, Ryerson University, 350 Victoria Street, Toronto, Ontario M5B 2K3, Canada

a r t i c l e i n f o

Article history:Received 23 September 2015Received in revised form16 January 2016Accepted 25 January 2016Available online xxx

Keywords:Oil palm shellOil-palm-boiler clinkerLightweight aggregate concreteHigh strength lightweight concreteCuringMechanical properties

* Corresponding author. Tel.: þ60 379675203; fax:E-mail addresses: [email protected] (M.

(P. Shafigh), [email protected] (M.Z. Jumaat), mlache

http://dx.doi.org/10.1016/j.jclepro.2016.01.0710959-6526/© 2016 Elsevier Ltd. All rights reserved.

Please cite this article in press as: Aslam, Maggregate concrete, Journal of Cleaner Produ

a b s t r a c t

The use of industrial waste as construction material to build environmentally sustainable structures hasseveral practical and economic advantages. Oil palm shell (OPS) is a solid waste material from the palmoil industry that has been successfully used to produce high strength durable lightweight concrete.However, this concrete is very sensitive to a poor curing environment. Therefore, to produce a cleanerand greener concrete, this study used two waste materials from the palm oil industry as coarse aggre-gate; OPS aggregates were partially replaced with oil-palm-boiler clinker (OPBC) aggregates from 0 to50% in OPS lightweight aggregate concrete. Properties including workability, density, compressivestrength under eight different curing conditions, splitting tensile and flexural strengths, modulus ofelasticity and water absorption of green lightweight concrete were measured and discussed. The resultsshow that it is possible to produce environmentally-friendly and high strength structural lightweightaggregate concrete by incorporating high volume waste lightweight aggregates from the palm oilindustry.

© 2016 Elsevier Ltd. All rights reserved.

1. Introduction

Concrete is the most widely used construction material in civilengineering structures. It has an excellent resistance to water andcan be formed into a variety of shapes and sizes (Shafigh et al.,2010). Nowadays, the concrete industry consumes enormousamounts of natural resources and raw materials for production(Shafigh et al., 2013b). Because of the huge amount of concreteproduced daily, even a small reduction in the use of raw materialsin concrete mixtures will result in considerable benefits to theenvironment (Altwair and Kabir, 2010). The best way to achievesustainability in the concrete industry is to utilize by-products andwaste materials (Mannan and Neglo, 2010).

The use of lightweight concrete (LWC) in a structure reduces itsoverall dead load which can be considerable (Bremner and Eng,2001). Structural lightweight concrete is usually made with light-weight aggregate (LWA). Inmost cases, the LWAused in lightweightaggregate concrete (LWAC) is coarse. One LWA that is abundantly

þ60 379675318.Aslam), [email protected]@ryerson.ca (M. Lachemi).

., et al., Benefits of using blection (2016), http://dx.doi.or

available in most tropical countries is oil palm shell (OPS), a solidwaste from the palm oil industry. The density of OPS is within therange of most typical lightweight aggregates, with a specific gravityin the range of 1.1e1.4 (Shafigh et al., 2012b). However, reports(Alengaram et al., 2013; Teo et al., 2006) have shown that it has highwater absorption in the range of 14e33%.

In the last two decades, OPS has been used as an LWA for pro-ducing structural LWAC with a density 20e25% lower than NWC(Shafigh et al., 2010, 2012b). Ali et al., 1984; Salam et al. (1987)introduced the use of OPS as an LWA, achieving a compressivestrength of 20 MPa with a water to cement ratio of 0.4. Okafor(1988) reported a compressive strength of 25e35 MPa for OPSconcrete, which is in the range of typical compressive strength forstructural lightweight concrete (Kosmatka et al., 2002). Mannanand Ganapathy (2001b) reported that depending on the curingcondition, the 28-day compressive strength of OPS concrete rangedbetween 20 and 24 MPa with a water to cement ratio of 0.41. Later,they stated that OPS aggregate could be treated by using a 20% polyvinyl alcohol solution, which significantly reduces water absorp-tion and provides a better interlock with cement paste. They re-ported that lightweight concrete containing treated OPS has a 40%higher compressive strength than the control OPS concrete(Mannan et al., 2006). Yew et al. (2014) also examined the heating

nded waste coarse lightweight aggregates in structural lightweightg/10.1016/j.jclepro.2016.01.071

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Fig. 1. Oil palm shell (left) and oil-palm-boiler clinker (right).

M. Aslam et al. / Journal of Cleaner Production xxx (2016) 1e102

method for treatment of OPS coarse aggregate. They reported thatthe advance in heat treatment methods has no significant effect onthe compressive strength. However, it enhances workability byabout 20% and slightly reduces the density of the OPS concrete.They achieved a 28-day compressive strength of 49 MPa for OPSconcrete using treated OPS. Alengaram et al. (2011) achieved thehighest compressive strength, reaching about 37 MPa by usingsilica fume and class F fly ash. Recently, high strength OPS light-weight concrete was successfully produced by (Shafigh et al.,2011a,b,c) with a compressive strength in the range of42e53 MPa. The key characteristic of mix proportions of a highstrength OPS concrete is the use of crushed OPS, smaller sizes ofcoarse OPS, low water to cement ratio and the use of limestonepowder in the concrete mixture as filler (Shafigh et al., 2011a).Recently, Mo et al. (2016) investigated the durability properties of asustainable concrete by using OPS as coarse and manufacturedsand as fine aggregates. They reported that the use of GGBFS aspartial cement replacement in the OPS concrete increased thecompressive strength gain compared to OPS concrete without anyGGBFS over the curing period of 1 year.

A report by Teo et al. (2009) revealed that when normal coarseaggregate is substituted with OPS, mechanical properties such ascompressive, splitting tensile and flexural strengths are reduced byabout 48%, 62% and 42%, respectively. However, a maximumreduction of up to 73% was observed for the modulus of elasticity.The modulus of elasticity of OPS concrete is in the normal range of5.5e11 GPa, which is considered a low value in this property(Alengaram et al., 2011; Mannan and Ganapathy, 2002). However,Shafigh et al. (2012c) reported that for high strength OPS concrete,it could increase up to 18 GPa. It should be noted that, generally, themodulus of elasticity of structural lightweight concrete ranges be-tween 10 and 24 GPa (CEB/FIP, 1977). A low value causes excessivedeformation in structural elements such as slabs and beams. Thedurability properties of different grades of OPS concrete have alsobeen studied by several researchers (Haque et al., 2004; Mannanet al., 2006; Teo et al., 2007), and their reports have shown thatthis concrete can be considered a durable.

Although previous studies have shown that OPS concrete hassatisfactorymechanical and durability properties, it does have somedrawbacks that need to be addressed before it can be used in realstructures. One of the main drawbacks is sensitivity of compressivestrength in poor curing conditions. Compared to normal aggregateconcrete, OPS shows a significant reduction in strength when notproperly cured (Mannan and Ganapathy, 2002; Shafigh et al.,2011a). Mannan and Ganapathy (2002) reported that OPS con-crete subjected to 7-day moist curing showed 17% lower compres-sive strength than OPS concrete under full water curing. Highstrength OPS concrete is also sensitive to poor curing. A minimumperiod of 7 days of moist curing is recommended for this type ofconcrete (Shafigh et al., 2012b). The sensitivity of compressivestrength of OPS concrete increases when the mixture contains ahigh cement dosage, lowwater to cement ratio or high OPS content(Shafigh et al., 2011a). Islam et al. (2016) investigated the fresh andmechanical properties of sustainable OPS lightweight by usingagro-solid waste materials. The OPS was used as coarse aggregatewhile ground POFA was used at partial cement replacement levelsof up to 25%. They reported that OPS concrete specimens subjectedto continuous moist curing showed higher compressive strengthcompared to partially cured and air dried specimens. Therefore,they recommended that the incorporation of POFA could reduce thesensitivity of OPS concrete towards poor curing. High cement con-tent and low water to cement ratio are needed to produce a struc-tural grade of LWAC with satisfactory durability properties,particularlywhen high strength is required. Therefore, changing thevolume of OPS content may reduce sensitivity.

Please cite this article in press as: Aslam, M., et al., Benefits of using bleaggregate concrete, Journal of Cleaner Production (2016), http://dx.doi.o

The aim of this study was to investigate the possibility ofreducing compressive strength sensitivity of OPS concrete byreducing OPS aggregate volume. For this purpose, OPS was partiallysubstituted with oil-palm-boiler clinker (OPBC). OPBC is a by-product of the burning of solid waste in the boiler combustionprocess in palm oil mills. It is like a porous stone, grey in colour,flaky and irregular in shape (Ahmad et al., 2007). Previous studies(Ahmad et al., 2008; Chan and Robani, 2005; Zakaria, 1986) haveshown that OPBC can be used as lightweight aggregate in concrete.The density and the 28-day compressive strength of OPBC concretefulfil the requirements of structural LWAC (Mohammed et al.,2014). OPBC was chosen as a partial replacement as it is a wastewithout any current application, as well as being a lightweightaggregate. Because there are no reports pertaining to thecompressive strength sensitivity of OPBC concrete in poor curingconditions. This study used two types of waste coarse lightweightaggregate to identify the optimum substitution level of OPBC in OPSconcrete.

2. Experimental programme

2.1. Materials

2.1.1. CementOrdinary Portland cement (OPC) with a 7- and 28-day

compressive strength of 36 and 48 MPa, respectively, was used.The specific gravity and Blaine specific surface area of the cementwere 3.14 and 3510 cm2/g, respectively.

2.1.2. AggregateThe OPS and OPBC (Fig. 1) used as coarse aggregate were

collected from a local palm oil mill, then washed and dried in thelaboratory. After drying, they were crushed using a crushing ma-chine and then sieved. Table 1 shows that these two types of coarseaggregate have almost the same grading. For each mix proportion,the OPS and OPBC aggregates were weighed in dry conditions,immersed in water for 24 h, then air dried in the lab environmentfor 2e3 h to obtain an aggregate with an almost saturated drysurface condition. The physical properties of OPS and OPBC areshown in Table 2; it shows that OPBC is heavier than OPS but hassignificantly lower water absorption. The crushing, impact andabrasion values of OPBC are significantly greater than OPS, whichshows that OPBC is weaker. In general, an OPBC grain is round inshape and has porosity on the surface, while, OPS is flaky withoutvisible surface porosity (Fig. 1).

nded waste coarse lightweight aggregates in structural lightweightrg/10.1016/j.jclepro.2016.01.071

Table 1Grading of OPS and OPBC aggregates.

Sieve size [mm] 19 12.5 9.5 8 4.75Cumulative % by weight passing OPS 100 96.80 84.24 61.20 2.98

OPBC 100 98.35 90.32 70.75 3.27

Table 2Physical and mechanical properties of aggregates.

Physical and mechanical properties Coarse aggregate Fine aggregate

OPS OPBC Normal sand

Specific gravity (saturated surface dry) 1.19 1.69 2.68Bulk density (compacted) [kg/m3] 610 860 165724 h water absorption (%) 20.5 7.0 1.2Crushing value (%) 0.2 21.2 e

Impact value (%) 5.5 36.3 e

Abrasion value (%) 5.7 23.9 e

M. Aslam et al. / Journal of Cleaner Production xxx (2016) 1e10 3

Local mining sand was used as fine aggregate. It had a finenessmodulus of 2.89, specific gravity of 2.68 and maximum grain size of4.75 mm.

2.1.3. Super-plasticizer (SP)Sika ViscoCrete was used as the SP in this study. This admixture

is chloride free according to BS 5075 and is compatible with alltypes of Portland cement including Sulphate Resistant Cement(SRC). It can be used at a rate of 500e2000ml per 100 kg of cement,depending on workability and strength requirements. Themaximum SP used in this study was 1% of total cement mass.

2.1.4. WaterThe water used was normal tap water. A fixed water to cement

ratio of 0.36 was used in all mixes.

2.2. Mix proportions

Six lightweight concrete mixes were prepared using OPS andOPBC as coarse aggregates; all mix proportions are shown inTable 3. The OPS concrete was considered the control concrete, andin all other mixtures, OPS was partially replaced with OPBC at 0, 10,20, 30, 40 and 50% by volume. The cement content for all mixes was480 kg/m3, which is the same content used in most OPS concretemixtures investigated in previous studies (Alengaram et al., 2008a;Mannan and Ganapathy, 2001a,b; Shafigh et al., 2011a).

2.3. Curing conditions

To determine the effect of the curing conditions on the 28-daycompressive strength of the concrete mixes, specimens werecured under eight types of curing conditions, shown in Table 4. Thecuring symbols and their descriptions are listed below:-

Table 3Mix proportions for concretes.

Mix code Content [kg/m3] SP (% cement)

Cement Water Sand Coarse aggregate

OPS OPBC

C-0 480 173 890 360 0 1C-10 480 173 890 324 51 1C-20 480 173 890 288 102 1C-30 480 173 890 252 153 1C-40 480 173 890 216 205 0.90C-50 480 173 890 180 256 0.85

Please cite this article in press as: Aslam, M., et al., Benefits of using bleaggregate concrete, Journal of Cleaner Production (2016), http://dx.doi.or

� FW: Specimens were immersed in water after demoulding untilthe age of testing.

� 3W: Specimens were cured inwater for 2 days after demouldingand then air cured in the laboratory environment.



� 2T2D: Specimens were watered twice a day (morning andevening) for 2 days after demoulding and then air cured in thelaboratory environment.

� 2T6D: Specimens were watered twice a day (morning andevening) for 6 days after demoulding and then air cured in thelaboratory environment.

� PS: Specimens were wrapped in 4 layers of a 1 mm thick plasticsheet after demoulding and then kept in the laboratoryenvironment.

� AC: Specimens were kept in the laboratory environment afterdemoulding.

Curing water was kept at a temperature of 23 ± 3 �C, and thetemperature and relative humidity of the lab environment were31 ± 3 �C and 84 ± 3%, respectively.

2.4. Test methods

To create each mix, the cement and aggregates were placedmixed for 2 min in a mixer. A mixture of 70% mixing water with SPwas added, andmixing continued for another 3min. The remainingwater was added and mixing continued for another 5 min. Afterthat, the slump test was performed. The concrete specimens werecast in 100 mm cube steel moulds to determine compressivestrength, cylinders of 100 mm diameter and 200 mm height forsplitting tensile strength, cylinders of 150 mm diameter and300 mm height for modulus of elasticity, and prisms of100 � 100 � 500 mm3 for flexural strength. Specimens werecompacted using a vibrating table.

After casting, specimens were covered with plastic sheets andstored in the laboratory environment, then demoulded one dayafter casting. Three test specimens were prepared to obtain averagevalues of mechanical properties at any age. For curing condition,four specimens were used to obtain an average value. The mainreason to prepare three or four specimens was to achieve a properresults of the property. The average values were only selected fromthose specimens which were giving 95e100% similar results.

To determine the water absorption of all mixes, specimens weredried in the oven at 105 ± 5 �C to reach a constant mass, then fullyimmersed in water kept at 23 ± 3 �C. Water absorption wasmeasured after 30 min, then after 24 and 72 h.

3. Results and discussion

3.1. Slump

The slump values of all the mixes are shown in Table 5. SinceOPBC is round in shape and has a lower water absorption rate thanOPS (about 66%), partial substitution of OPS by OPBC offers betterworkability. For example, mix C-30 showed a 39% higher slumpvalue than OPS concrete for the same amount of SP. A structuralLWAC with slump value in the range of 50e75 mm is considered tobe a lightweight concrete with good workability (Mehta andMonteiro, 2006). Excessive slump value causes segregation of theLWA from the cement matrix. Therefore, to avoid segregation, SPcontent in the C-40 and C-50 mixes was reduced. However, thesemixes had a much better slump value than the control mix, even


Table 4Curing conditions.

Curingcondition

Day in mould Day in water Watering (time/day) Day in labenvironment

FW 1 27 0 273W 1 2 0 255W 1 4 0 237W 1 6 0 212T2D 1 0 2 272T6D 1 0 2 27PS 1 0 0 27AC 1 0 0 27

y = 1.8218x + 1916R² = 0.9811

y = 1.9945x + 1790R² = 0.8737

1750

1800

1850

1900

1950

2000

2050

0 10 20 30 40 50 60

Den

sity

(kg/

m3 )

OPBC content (%)

Demoulded density

Oven dry density

Fig. 2. Relationship between density and OPBC content.


though they had lower SP content of 10% and 15%, respectively.Increasing workability and using less SP is a significant advantageof OPBC in OPS concrete.

15

20

25

30

35

40

45

50

0 10 20 30 40 50 60

Com

pres

sive

stre

ngth

(MPa

)

Age (days)

C-0 C-10

C-20 C-30

C-40 C-50

Fig. 3. Compressive strength development of concrete.

3.2. Density

The unit weight of all the concrete mixes was measured 24 hafter casting (immediately after demoulding), which is referred toas the demoulded density. The oven dry density was measured at28 days. Table 2 shows that OPBC density was about 40e45% lessthan that of conventional coarse aggregate. However, compared toother types of lightweight aggregate such as OPS and coconut shell(both from agricultural waste), the density of OPBC is about 23%and 30% higher, respectively. The density of coarse OPBC is 6e46%higher than that of artificial LWAs such as LECA and Lytag, and6e66% higher than that of natural LWAs like pumice, diatomite andvolcanic cinders. Therefore, as expected, by substituting OPS withOPBC, concrete density increased. The relationship between den-sity and OPBC content is shown in Fig. 2.When OPSwas replaced byOPBC, the demoulded density increased by around 2e4% comparedto the control concrete.

Although the use of OPBC in OPS concrete increases its density,even at a 50% replacement level, density is still in the acceptablerange for structural lightweight concrete. The C-0 and C-50 mixeswere at about 20% and 24% replacement levels, respectively, whichis lighter than conventional concrete. If oven dry density isconsidered, all concrete mixtures were lighter than the demouldeddensity by approximately 100e140 kg/m3. This density for OPScrushed, OPS uncrushed and scoria lightweight aggregate concretewas 70e120 kg/m3, 85e126 kg/m3 and 82e124 kg/m3, respectively(Kilic et al., 2003; Shafigh et al., 2011a,b).

3.3. Compressive strength

3.3.1. Under continuous moist curingThe 28-day compressive strength of OPS concrete without

OPBC aggregate (C-0) was about 36 MPa, which shows that thecontrol mix is a normal strength lightweight concrete. The effectsof OPBC aggregate addition on the compressive strength devel-opment of all mixes up to 56 days is shown in Fig. 3. As can be

Table 5The slump value of all the mix proportions.

Mix code Slump (mm)

C-0 55C-10 58C-20 65C-30 90C-40 100C-50 90


seen in this figure, using 10% OPBC did not affect the compressivestrength of OPS concrete. However, compared to the control mix,the compressive strength of the OPS specimens containing morethan 10% OPBC improved significantly, particularly at later ages.Compressive strength of specimens containing 20e50% OPBCincreased by 13e18%. The highest 28-day compressive strength,which was about 44 MPa was achieved for OPS concrete con-taining 30% OPBC.

The compressive strength of most OPS concrete types waswithin the normal range of structural LWC. The strength of struc-tural LWAC depends on the strength of the LWA used and on thehardened cement paste, as well as the bonding of the aggregate andcement paste in the interfacial zone (Lo et al., 2007). Mannan et al.(2002) stated that the failure of OPS concrete in compression oc-curs due to failed adhesion between the OPS and the cement paste.They studied various types of pre-treatment approaches toimprove the quality of OPS aggregate and the highest 28-daycompressive strength they achieved was about 33 MPa. Okpala(1990) also studied the failure mechanism of OPS concrete andreported that failure was dependent on the breakdown of the bondbetween the aggregate and the cement paste. He observed thatfailure was conditional upon the combination of the failure of theshell and aggregate paste interface. Due to the smooth nature of theOPS aggregate on both the convex and concave faces, the bondbetween the shells and cement paste was not strong enough tosustain high loads (Mannan et al., 2002; Okpala, 1990; Shafighet al., 2011b; Aslam et al., 2015). Hence, the concrete failure wasmainly due to debonding between the shells and cement paste, andthe aggregates did not fully utilize their potential strength. On the



other hand, OPBC is a porous aggregate with a rough surfacetexture (Fig. 1). It was therefore expected that it would have abetter interlock with the cement matrix, and, consequently, highercompressive strength. Fig. 4 shows a section of concrete containingOPS and OPBC aggregate, and their connection with the cementmatrix.

The 28-day compressive strength test results show that incor-porating 20e50% OPBC in OPS concrete enhances compressivestrength from grade 35 to grade 40, which can be considered highstrength lightweight concrete. As reported earlier, the C-0 and C-10mixes had almost the same compressive strength at all ages;90e95% of the 28-day compressive strength was achieved at 7 days,while the same ratio for mixes containing 20e50% OPBC was81e90%. For an artificial lightweight aggregate, this ratio was re-ported to be in the range of 76e87% (Wilson and Malhotra, 1988).The ratio for the 1-day and 3-day to 28-day compressive strength ofOPS-OPBC concrete was in the range of 49e57% and 71e83%,respectively.

3.3.2. Under air-dryingAll the concrete mixes showed a reduction on the compressive

strength at 28 days under air-drying conditions (AC). Compared tothe full water curing condition (FW), the reductions in compressivestrength of the OPS concrete containing 0, 10%, 20%, 30%, 40% and50% OPBCwere about 25%,10%,17%, 9%, 3% and 3%, respectively. Thereduction for the OPS-OPBC concrete was 3e17%, with an averagevalue of about 8.5%. The contribution of OPBC significantly reducedstrength loss in poor curing conditions, particularly when thecontributionwasmore than 30%. In the AC condition, the OPS-OPBCconcretes had 24e54% higher compressive strength than the con-trol mix (C-0), while this range was 3e21% under FW curing.

One of the reasons for these results is the better performance ofOPBC in terms of internal curing. During the process of internalcuring, saturated lightweight aggregates develop suction pressurein the hydrated cement paste due to self-desiccation and chemicalshrinkage. Henkensiefken et al. (2009) reported that in the processof internal curing, the biggest pores will lose water first as thedeveloped capillary stress decreases when the pores are emptied inthis order. The LWA pores are generally larger than those of thesurrounding cement paste, which increases cement hydration andstrength development, and restrains shrinkage cracking behaviour.The pore sizes of OPBC aggregate are larger than those of OPS, and,hence, the OPBC aggregate quickly loses water and minimizes

Fig. 4. Surface texture of OPS and OPB


capillary stress when pores are emptied consequently, its internalcuring performance is superior to that of OPS.

Weber and Reinhardt (1997a,b) studied the replacement ofnormal coarse aggregates by LWA of similar size in high perfor-mance concrete as a means of providing internal curing forimproved strength; they named this method autogenous curing. Intheir study, months of continuous hydration after casting wereobserved with X-ray diffraction; the prepared concrete improvedmechanical properties regardless of curing condition. They pro-posed a mechanism of water transport from the LWA to the hy-drating cement paste based on capillary suction. In addition to thebenefit of using LWA for increased compressive strength due tointernal curing, it also enhanced durability performance due to thehigher degree of hydration, improved density of the hydratedcement paste and reduced drying shrinkage, which minimized theeffect of self-desiccation and elimination of plastic and dryingshrinkage cracking (Shafigh et al., 2014; Weber and Reinhardt,1997a).

Fig. 5 shows the relationship between the 28-day compressivestrength of all different types of OPS-OPBC concretes in air-dryingand water curing conditions. As a comparison, this relationshipwas plotted for different types of concrete reported in previousresearch (Atis et al., 2005; Shafigh et al., 2012a, 2013a,b). As seen inthis figure, OPS concrete containing OPBC has a better compressivestrength than both OP and normal weight concrete containing silicafume as well as OPS concrete with fly ash or GGBFS.

3.3.3. Under partial early curingCuring is the practice of maintaining suitable moisture content

and temperature in concrete during its early stages to ensure itdevelops its desired properties (ACI-308, 1980). A minimum periodof 7 days of moist curing is generally recommended with concretecontaining normal Portland cement. However, for concrete mix-tures containing a mineral admixture, a longer curing period isdesirable to ensure the strength contribution from the pozzolanicreaction (Mehta and Monteiro, 2006). LWAs in concrete mixturescontain internal water, which helps increase the hydration ofcement; due to the advantages of internal curing, recommendedearly age curing may be reduced in the case of lightweight aggre-gate concrete, which reduces the curing cost.

Table 6 shows the effect of different curing conditions on the 28-day compressive strength of all mixes. The data shows that partialearly curing can improve 28-day compressive strength more thanair-drying. Compared to the AC condition, the effectiveness of the

C aggregates with mortar matrix.


y = 0.8957xR² = 0.6319

25

27

29

31

33

35

37

39

41

43

30 32 34 36 38 40 42 44 46

Com

pres

sive

stre

ngth

(MPa

) with

out c

urin

g

Compressive strength (MPa) with curing

Linear (OPS-OPBC)

Linear (OPS-SF)

Linear (NC-SF)

Linear (OPS-FA)

Linear (OPS-GGBFS)

Fig. 5. The relationship between compressive strength of OPS-OPBC concrete with andwithout curing and comparison with OPS concrete containing silica fume (OPS-SF)(Shafigh et al., 2012a); normal concrete containing silica fume (NC-SF) (Atis et al.,2005); OPS concrete containing fly ash (OPS-FA) (Shafigh et al., 2013a) and OPS con-crete containing GGBFS (OPS-GGBFS) (Shafigh et al., 2013b).


2T2D and 2T6D conditions on the improvement of compressivestrength of OPS containing 0e20% OPBC was more significantcompared to other mixes. Under these two curing conditions, thecompressive strength of OPS concrete (mix C-0) was still signifi-cantly lower than for the FW curing condition, while thecompressive strength of OPS concretes containing OPBC was veryclose to the FW curing condition. This is additional evidence thatthe compressive strength of concrete containing OPS as coarseaggregate is very sensitive to poor curing conditions. However,even a small amount of OPBC could significantly reduce thissensitivity. In general, partial early curing of 2T2D was better than2T6D; if the curing method is a type of watering that occurs a fewtimes a days, it should not continue over several days due to thenegative effect of the wettingedrying condition on concrete. Thetest results of these two curing conditions reveal that 2T2D wasbetter than 2T6D, with a savings in curing costs.

Compared to the AC condition, partial early curing of 2D, 4D and6D improved compressive strength. A longer curing time resultedin better compressive strength for all concrete types. However, therate of improvement for the OPS control mix was higher than forthe other types, which may be due to the C-0 mix experiencing asignificant reduction in compressive strength under the AC condi-tion compared to other mixes. Early water curing therefore causeda greater improvement in compressive strength. However, thecompressive strength of mix C-0 under 2D, 4D and 6D curingconditions was about 82e89% of compressive strength under FWcuring, while this range for all types of OPS-OPBC concretes was88e109%. This is additional evidence that concrete containing OPSas coarse aggregate needs special attention in terms of curing.

As can be seen in Table 6, the 28-day compressive strength for allconcrete types under 2D, 4D and 6D conditions is almost the same.However, among the OPS-OPBC mixes, the closer values are for theC-40 and C-50 mixes which means that the 2D initial water curingmay be sufficient for curing these two types of concrete.

Test results show that curing with a plastic sheet (PS) is a moreeffective method of improving compressive strength than air

Table 6Effect of different curing conditions on 28-day compressive strength.

Mix codes AC 2D 4D 6D 2T2D 2T6D Plastic (PS) FW

C-0 27.0 29.6 31.0 32.1 30.8 27.4 32.7 36.0C-10 33.5 36.9 36.7 33.9 37.3 38.3 37.6 37.1C-20 35.6 37.7 39.3 43.6 41.3 37.0 37.2 42.7C-30 39.8 41.3 38.2 42.7 41.2 41.3 41.9 43.5C-40 41.5 41.9 43.5 43.8 41.0 40.4 42.7 42.6C-50 40.0 45.2 42.2 44.8 42.5 38.9 43.9 41.3


drying. This method led to an improvement of about 21% in thecontrol mix (C-0), which was significantly higher than its effect onthe OPS concrete containing OPBC, which was 3e12%. The higherimprovement in the compressive strength of the control mix wasdue to a significant reduction (about 25%) in the compressivestrength of this concrete under dry conditions. For OPS-OPBCconcrete this drop was about 8%, on average. Any partial earlycuring therefore had better performance on the control mixcompared to the OPS-OPBC concretes. If all types of partial curingconditions are considered, the control OPS concrete had about 13%more compressive strength than the AC condition and 6.5% for theOPS-OPBC concrete specimens.

Most codes of practice recommend 7-day moist curing (Haque,1990). However for some structural elements such as columns, itcan be difficult to achieve. Nevertheless, as can be seen in Table 6,compressive strength is equivalent for 6D and PS, which means aplastic sheet can be used instead of 7-daymoist curing; thismethodof curing may be more practical.

3.4. Splitting tensile strength

Splitting tensile strength for all the mixes is shown in Table 7.The minimum 28-day splitting tensile strength required for struc-tural lightweight concrete to be used in structural elements is2.0 MPa (Kockal and Ozturan, 2011). Table 7 shows that all theconcrete mixes have more than 2.0 MPa splitting tensile strengthfrom a 3-day age and that splitting tensile strength increased withcompressive strength. The substitution of OPS with OPBC up to 30%did not affect splitting tensile strength at all ages (slightly reduced).The reductionwas significant in the 40% and 50% substitution levelsat early ages. However, it diminished with the increasing age of theconcrete; the 28-day splitting tensile strength of the concretescontaining 40% and 50% OPBC was similar to the control mix. It isinteresting to note that a significant improvement was observedfrom 7 to 28 days for the splitting tensile strength of the C-40 andC-50 mixes, while improvement for the other mixes was minor. Onthe other hand, compressive strength improvement with watercuring from 7 to 28 days was also significant, particularly for mix C-50.

The 28-day splitting tensile strength of concretes containingOPBC ranged from 3.05 to 3.31 MPa. Generally, the ratio of splittingtensile/compressive strength of normal weight concrete falls in therange of 8e14% (Kosmatka and Wilson, 2011; Shafigh et al., 2014).However, the ratio of splitting tensile to compressive strengthachieved for OPS concrete was 8e10% and 7e10.5% for OPS-OPBCmixes.

Fig. 6 shows a comparison of splitting tensile strength resultswith those predicted by the equations proposed by various stan-dards and researchers. ACI 318-05 (2005) proposed Eq. (1) fornormal weight concrete with a cylinder compressive strength inthe range of 21e83 MPa. Gesoglu et al. (2004) proposed Eq. (2) forcold-bonded fly ash LWAC with a cube compressive strengthranging from 20 to 47 MPa and Eq. (3) is the proposed equationfrom CEBeFIP (1990). Shafigh et al. (2014) proposed Eq. (4) for LWCusing two types of waste material from the palm oil industry.Neville (2008) reported Eq. (5) for pelletized blast furnace slagLWAC, with cube compressive strength ranging from 10 to 65 MPa.

ft ¼ 0:59�fcy

�0:5(1)

ft ¼ 0:27ffiffiffiffiffiffif 2cu

3q

(2)


Table 7Splitting tensile strength and flexural strength.

Mix no. Splitting tensile strength (MPa) Flexural strength (MPa)

1 day 3 days 7 days 28 days 7 days 28 days

C-0 2.20 2.89 3.03 3.29 4.43 5.38C-10 2.20 2.80 3.25 3.30 4.50 4.48C-20 2.00 2.81 2.92 3.05 4.17 4.98C-30 2.03 3.10 3.12 3.25 4.80 5.37C-40 1.80 2.48 2.64 3.31 4.54 5.30C-50 1.70 2.33 2.66 3.10 4.89 5.24


ft ¼ 0:301�fcy

�0:67(3)

ft ¼ 0:27ðfcuÞ0:63 (4)

ft ¼ 0:23ffiffiffiffiffiffif 2cu

3q

(5)

where, ft is the splitting tensile strength, fcu and fcy are the cube andcylindrical compressive strengths, respectively. As can be seen inFig. 6, Equations (2) and (3) showed values closer to the experi-mental results, with a reliability of about 90%.

3.5. Flexural strength

Flexural strength of all mixes at 7 and 28 days is shown inTable 7. The 28-day flexural strength of all OPS-OPBC mixes rangedfrom 4.48 to 5.52 MPa, an average of 12% higher than the 7-dayflexural strength. The flexural strength of normal weight concretewith a compressive strength of 34e55 MPa is in the range of5e6 MPa with a flexural/compressive strength ratio of 11.6e13.5%(Mehta and Monteiro, 2006). The 28-day flexural to compressivestrength ratios of the OPS-OPBC mixes were 12e12.7%. It cantherefore be concluded that all the OPS-OPBC concrete mixes had asimilar flexural strength, and flexural to compressive strength ratiowas similar to the NWC of the same grade. Holm and Bremner(2000) reported that the flexural strength of high strength light-weight aggregate concrete (HSLWAC) was generally 9e11% ofcompressive strength. The results of the study being described inthis paper showed that OPS-OPBC concretes had a higher flexural tocompressive strength ratio than HSLWAC. The ratio of splittingtensile to flexural strength for C-10, C-20, C-30, C-40 and C-50 was67%, 61%, 57%, 62% and 59%, respectively. However, for OPS concretewith compressive strength from 34 to 53 MPa, this ratio variedbetween 51% and 72% (Alengaram et al., 2008b; Shafigh et al.,

1.5

2

2.5

3

3.5

20 25 30 35 40 45

Split

ting

tens

ile st

reng

th (M

Pa)

Compressive strength (MPa)

ft (Exp) Eq: 1

Eq: 2 Eq: 3

Eq: 4 Eq: 5

Fig. 6. Experimental and theoretical splitting tensile strength of all concrete mixes.


2012b) depending mostly on the amount of OPS in the mixture(Shafigh et al., 2012b).

Fig. 7 shows the relationship between flexural strength andcorresponding compressive strength. The experimental resultswere compared with the predicted results using equations pro-posed by applicable standards and researchers. CEBeFIP (1977)proposed Eq. (6) for LWC made with expanded shale and clay ag-gregates, with cubical compressive strength ranging varying be-tween 20 and 60 MPa. Shafigh et al. (2012b) proposed Eq. (7) forcrushed OPS concretes, with cubical compressive strength between35 and 53MPa. Zhang and Gjvorv (1991) proposed Eq. (8) to predictthe flexural strength of high strength lightweight concrete. Theequation prediction of Lo et al. (2004) is for expanded clay LWACwith a cubical compressive strength of 29e43 MPa Eq. (9).

fr ¼ 0:46ffiffiffiffiffiffif 2cu

3q

(6)

fr ¼ 0:12f 1:03cu (7)

fr ¼ 0:73ffiffiffiffiffiffifcu

p(8)

fr ¼ 0:69ffiffiffiffiffiffifcu

p(9)

where, fr is the flexural strength, and fcu is the cube compressivestrength of the concrete in MPa. Equations (6) and (8) showed thatpredictions of flexural strength from cubical compressive strengthwere very close to the experimental results, with a reliability ofabout 99%. Equation (9) gave a more conservative estimate.

3.6. Modulus of elasticity

The modulus of elasticity of the C-0, C-10, C-20, C-30, C-40 andC-50mixtures was 7.9, 9.6, 10.2, 11.7, 13.0 and 15.0, respectively. Themodulus of elasticity value of OPS concrete was considered to below, while the contribution of OPBC aggregate into OPS concrete (at10%, 20%, 30%, 40% and 50%) significantly increased the modulus ofelasticity by about 18%, 23%, 32%, 39% and 47%, respectively. Themodulus of elasticity of concrete depends on the moduli of elas-ticity of its components and their proportions by volume (Neville,1971). The main difference between all the mixes was the typeand volume of coarse aggregates; it can therefore be concludedfrom themodulus of elasticity test results that themodulus of OPBCgrain is more than themodulus of OPS grain. On the other hand, thecrushing, impact and abrasion values of OPBC are significantly lessthan the OPS as shown in Table 1.

3.5

4

4.5

5

5.5

6

32 34 36 38 40 42 44

Flex

ural

stre

ngth

(MPa

)

Compressive strength (MPa)

fr (Exp) Eq: 6 Eq: 7

Eq: 8 Eq: 9

Fig. 7. Experimental and theoretical flexural strength results of all concrete mixes.


1

2

3

4

5

6

7

8

0% 10% 20% 30% 40% 50%

Wat

er a

bsor

ptio

n (%

)

OPBC content (%)

30 mints

24 hr

72 hr

Fig. 8. Relationship between OPBC content and water absorption.


Generally, the modulus of elasticity of structural LWC rangesbetween 10 and 24 GPa, and from 14 to 41 GPa in NWC (Shafighet al., 2014). The test results of this study showed that themodulus of elasticity of OPS concrete containing OPBC aggregateswas in the normal range for structural lightweight aggregateconcretes.

Tasnimi (2004) presented Eq. (10) for artificial LWA concreteswith a cylindrical compressive strength of about 15e55 MPa.Hossain et al. (2011) proposed Eq. (11), reporting data for LWC byincorporating pumice with a 28-day cylinder compressive strengthof 16e35 MPa and density of about 1460e2185 kg/m3 Alengaramet al. (2011) reported Eq. (12) for OPS concrete with 28-daycubical compressive strength of 25e39 MPa and air-dry densityof 1640e1890 kg/m3. The CEB/FIP model code (Short, 1978) pro-posed Eq. (13) to predict the modulus of elasticity of LWACs.

E ¼ 2:1684f 0:535y (10)

E ¼ 0:03w1:5f 0:5y (11)

E ¼ 5f 0:33cu ðw=2400Þ (12)

E ¼ 9:1ðw=2400Þ2f 2cu (13)

where, E is the modulus of elasticity (GPa),w is the dry density (kg/m3), fcy is the cylinder compressive strength (MPa) and fcu is thecubical compressive strength (MPa).

When compared to test results, all the equations provided veryconservative estimations with Eq. (12) being the closest to theexperimental results. Changing the constant value in equation (Eq.(14)) from 5 to 6 provided a good estimation with a reliability ofabout 90%.

E ¼ 6f 0:33cu ðw=2400Þ (14)

3.7. Water absorption

Water absorption of all concrete mixes was measured at the ageof 56 days for 30 min, 24 h and 72 h, as shown in Fig. 8. The waterabsorption of the concretes containing OPBC aggregates was lowerthan that of the control OPS concrete. Water absorption wasreduced when the amount of OPBC aggregate in the mixture wasincreased. Due to the high water absorption of OPS, substitution ofnormal coarse aggregate with this lightweight aggregate increasedits water absorption capacity. Therefore, reducing OPS volume inconcrete was expected to reduce initial and final water absorption.Table 1 shows that thewater absorption rate of OPBC aggregatewassignificantly lower than for OPS. Therefore, as shown in Fig. 8, OPS-OPBC concretes had lower water absorption rates than OPS con-crete. For OPS concretes with normal compressive strength, waterabsorptionwas higher than 10% (Teo et al., 2007). Water absorptionfor other types of structural lightweight concretes such asexpanded polystyrene aggregate concrete and pumice aggregateconcrete ranged from 3 to 6% and 14 to 22%, respectively (Babu andBabu, 2003; Gunduz and Ugur, 2005).

Ranjbar et al. (2013) categorized the quality of concrete as good,average and poor based on initial water absorption (absorption in30 min) values of 0e3%, 3e5%, and above 5%, respectively. Allconcretemixes showed initial absorption of less than 3%, which canbe categorized as “good”. Moreover, the moderate (24 h) waterabsorption was in the range of 5.3e6.3% and the final (72 h) ab-sorption was 6.3e7.1%. Neville (2008) reported that although


concrete quality cannot be predicted by water absorption, goodconcretes generally have a rate of less than 10% by mass.

4. Conclusion

In this study, oil-palm-boiler clinker (OPBC) was partially usedinstead of oil palm shell (OPS) as coarse lightweight aggregate in anOPS lightweight concrete and the effect of this substitution on themechanical properties of concrete was investigated. Based on thetest results, the following conclusions can be drawn:

1. Increasing OPBC coarse aggregates in OPS concrete increasedslump value.

2. Substituting OPS with OPBC aggregates increased the density ofOPS concrete by about 2e4%. However, even at the 50% substi-tution level, density was still in the acceptable range for struc-tural lightweight aggregate concrete.

3. The contribution of 20e50% levels of OPBC in OPS concretesignificantly improved compressive strength under standardcuring. At these substitution levels, grade 35 OPS concrete wastransferred to grade 40, which can be considered high strengthlightweight aggregate concrete. The optimum substitution levelwas determined as 30%.

4. Incorporating OPBC aggregate into OPS concrete reduces thesensitivity of compressive strength to lack of curing. OPS con-crete (without OPBC aggregate) under air-drying showed areduction of about 25% compared to continuous water curingwhile OPS concrete containing 30e50% OPBC only showed areduction of about 5%. This may be due to the better perfor-mance of OPBC lightweight aggregates for internal curing.

5. Partial early curing improves compressive strength more thanair-drying because it is more effective when OPBC content ishigher. However, if partial early curing occurs thoughwatering afew times a day (like 2T2D and 2T6D), it is recommended thatthis method is not used continuously over several days due tothe negative effect of thewettingedrying condition on concrete.

6. The 28-day splitting tensile strength of concretes containingOPBC was 3.05e3.31 MPa, which is in the usual range forstructural lightweight aggregate concrete.

7. The 28-day flexural strength ranged from 4.48 to 5.38 MPa,which was 12e15% of the 28-day compressive strength. Theseratios are equivalent to the normal weight concrete ratio.

8. The modulus of elasticity of OPS concretes increased about18e24% with the incorporation of OPBC aggregates. OPS-OPBCconcretes containing more than 20% OPBC aggregate had amodulus of elasticity in the normal range for structural light-weight aggregate concretes.



9. Thewater absorption of OPS concretewas reduced by increasingthe percentage of OPBC aggregates. All OPS-OPBCmixes showedinitial and final water absorption of less than 3% and 10%,respectively, and can be considered good concretes.

Acknowledgements

The authors gratefully acknowledge the financial support ofUniversity of Malaya, High Impact Research Grant (HIRG) No. UM.C/625/1/HIR/MOHE/ENG/36 (16001-00-D000036), “StrengtheningStructural Elements for Load and Fatigue”.

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