Review Utilization of oil palm kernel shell as lightweight aggregate in concrete – A review U. Johnson Alengaram ⇑ , Baig Abdullah Al Muhit, Mohd Zamin bin Jumaat Department of Civil Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia highlights " Seventy-four recent and past papers have been reviewed on oil palm kernel shell concrete (OPKSC). " Physical, mechanical, durability, functional and structural behaviors of OPKSC reviewed. " Data of past 28 years on OPKSC are tabulated for reference. " Properties of lightweight concrete (LWC) compared with OPKSC. " Discussion on very recent paper on foam concrete with OPKS included. article info Article history: Received 16 October 2011 Received in revised form 25 July 2012 Accepted 11 August 2012 Keywords: Oil palm kernel shell Lightweight aggregate Mechanical properties Structural behavior Functional properties abstract This paper reviews previous research carried out on the use of oil palm kernel shell (OPKS) as lightweight aggregate (LWA). OPKS is a waste material obtained during the extraction of palm oil by crushing of the palm nut in the palm oil mills. It is one of the most abundantly produced waste materials in South East Asia and Africa; OPKS has been experimented in research as lightweight aggregates (LWAs) to produce lightweight concrete (LWC) since 1984 and today there are many researchers working in this area. In this paper the physical and mechanical properties of OPKS are summarized along with mechanical, durability and functional properties and structural behavior of OPKS concrete (OPKSC). Recent papers on foamed and fiber reinforced OPKSC are also included. It is seen from the results that OPKSC has comparable mechanical properties and structural behavior to normal weight concrete (NWC). Recent investigation on the use of crushed OPKS shows that OPKSC can be produced to medium and high strength concrete. Sustainability issues combined with higher ductility and aggregate interlock characteristics of OPKSC compared to NCW has resulted in many researchers conducting further investigation on the use of OPKS as LWA. Ó 2012 Elsevier Ltd. All rights reserved. Contents 1. Introduction ......................................................................................................... 162 2. Physical properties of OPKS as aggregate .................................................................................. 163 2.1. Specific gravity ................................................................................................. 163 2.2. Shape, thickness and texture ...................................................................................... 164 2.3. Bulk density .................................................................................................... 164 2.4. Water absorption and moisture content ............................................................................. 164 3. Mechanical properties of OPKS and comparison with NWA ................................................................... 164 4. Fresh concrete properties of concrete with OPKS as coarse aggregate ........................................................... 164 4.1. Materials used by researchers ..................................................................................... 164 4.2. Slump ......................................................................................................... 164 4.3. Flow table ..................................................................................................... 164 4.4. Air content ..................................................................................................... 164 5. Physical properties of OPKSC............................................................................................ 165 5.1. Plastic density .................................................................................................. 165 0950-0618/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.conbuildmat.2012.08.026 ⇑ Corresponding author. Tel.: +60 379677632. E-mail address: [email protected](U.J. Alengaram). Construction and Building Materials 38 (2013) 161–172 Contents lists available at SciVerse ScienceDirect Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
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Construction and Building Materials 38 (2013) 161–172
Contents lists available at SciVerse ScienceDirect
Utilization of oil palm kernel shell as lightweight aggregate in concrete – A review
U. Johnson Alengaram ⇑, Baig Abdullah Al Muhit, Mohd Zamin bin JumaatDepartment of Civil Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia
h i g h l i g h t s
" Seventy-four recent and past papers have been reviewed on oil palm kernel shell concrete (OPKSC)." Physical, mechanical, durability, functional and structural behaviors of OPKSC reviewed." Data of past 28 years on OPKSC are tabulated for reference." Properties of lightweight concrete (LWC) compared with OPKSC." Discussion on very recent paper on foam concrete with OPKS included.
a r t i c l e i n f o
Article history:Received 16 October 2011Received in revised form 25 July 2012Accepted 11 August 2012
This paper reviews previous research carried out on the use of oil palm kernel shell (OPKS) as lightweightaggregate (LWA). OPKS is a waste material obtained during the extraction of palm oil by crushing of thepalm nut in the palm oil mills. It is one of the most abundantly produced waste materials in South EastAsia and Africa; OPKS has been experimented in research as lightweight aggregates (LWAs) to producelightweight concrete (LWC) since 1984 and today there are many researchers working in this area. In thispaper the physical and mechanical properties of OPKS are summarized along with mechanical, durabilityand functional properties and structural behavior of OPKS concrete (OPKSC). Recent papers on foamedand fiber reinforced OPKSC are also included. It is seen from the results that OPKSC has comparablemechanical properties and structural behavior to normal weight concrete (NWC). Recent investigationon the use of crushed OPKS shows that OPKSC can be produced to medium and high strength concrete.Sustainability issues combined with higher ductility and aggregate interlock characteristics of OPKSCcompared to NCW has resulted in many researchers conducting further investigation on the use of OPKSas LWA.
The high demand for concrete in construction using normalweight aggregates (NWAs), such as gravel and granite, has drasti-cally reduced natural stone deposits and this has caused irrepara-ble damage to our environment. As a result, the emphasis onsustainable materials has intensified recently. The growing needfor sustainable development has motivated researchers to focustheir investigation on the use of waste or recycled materials intopotential construction material. Lightweight aggregates (LWAs)from industrial waste materials such as fly ash, expanded slag cin-der, and bed ash has led for sustainable materials. However, thelack of production techniques in developing and underdevelopedcountries has not brought much advantage to them. A substantialamount of cost can be reduced if the weight of the structure is de-creased. LWA had been in use for a long period of time in devel-oped countries and it proved cost effective. It served the purposeof both the structural stability and economic viability. The lowerthe weight, the more versatile are the structures. Since 2nd A.D.different types of LWA such as clinker, foamed slag, and expandedclay has been used as construction material [1]. Recently, becauseof growing environmental concerns, waste materials are beingused as aggregates for construction [2]. During the last 27 years,oil palm shell (OPS) or palm kernel shell (PKS), has been used byresearchers as LWA to replace conventional NWA in structural ele-ments and road construction [3–6] in Africa and Southeast Asia. Forsimplicity, oil palm kernel shell, abbreviated as OPKS is used torepresent OPS or PKS in this paper. OPKS is one kind of organicaggregate with better impact resistance compared to NWA.Numerous articles on the physical, mechanical, structural andfunctional properties using OPKS as LWA have been published.
OPKS is a waste product at the time of extracting oil from oilpalm tree [3,7]. Oil palm tree, being as in the same genera as Coco-nut palm tree, shares many features with it. Its scientific name isElaeis guineensis and is found mainly in East Africa [8]. Previously,cultivation of oil palm tree was remained secluded in the East
Africa because trace of oil palm tree have been found in the eraof Pharaohs some 5000 years ago but now-a-days, its cultivationis focused in South East Asia, in countries such as Malaysia andIndonesia. Olanipekun et al. [9] reported that oil palm trees canbe found in large quantities in America, Asia and Africa, especiallyin Nigeria. Malaysia alone produces 52.8% of the total production ofpalm oil and Malaysia and Indonesia produce about 80% of the to-tal palm oil of the world. Furthermore, these two countries exportabout 90% of the total palm oil produced altogether. There are twokinds of oil in palm nut; one is palm oil, which remain in outer coreof the nut and the other is palm kernel oil which is extracted fromthe inner core, known as palm kernel. Palm kernel is covered by ahard endocarp which is called palm kernel shell and is alternativelyknown as oil palm shell [8,9]. However, the term oil palm kernelshell is also adopted by the researchers to avoid confusion andunnecessary debate.
Malaysia produces 4 million tons of OPKS annually [3,4,10–14]and according to Ramli [11] nearly 5 million hectare (ha) of palmoil trees are expected by the year 2020. Being the second largestpalm oil producing country in the world, Malaysia is also respon-sible for producing a large amount of palm oil wastes. To preservethe environment, researchers have taken initiative to utilize OPKSas LWA [2,13,15]. Proposals were made to substitute OPKS asroad based materials instead of asphalt on various occasions[6,7,9,15,16]. Teo et al. [4,15] used OPKS as LWA to build a one-storey building and a foot bridge which are being monitored fortheir structural behavior. OPKS is also used as granular filtermaterial for water treatment [9,17], floor roofing and road basedmaterial [15]. Okpala [3] reported the thermal conductivity of0.19 W m�1 K�1 for OPKS which is much lower than the valueof 1.4 W m�1 K�1 for conventional stone aggregate. Thus thelightweight concrete (LWC) made with OPKS having low thermalconductivity and high insulation capacity may result in low en-ergy consumption and greener environment. Recently, attemptshave been made to incorporate OPKS as a substitute for poor lat-eritic soil. But the result shows that the composite mix of OPKS
U.J. Alengaram et al. / Construction and Building Materials 38 (2013) 161–172 163
with asphalt is inadequate for sub-grade, sub-base and basecourse in road construction [18].
The mechanical and structural properties of OPKS concrete(OPKSC) have been compared with normal weight concrete(NWC) by many researchers to show the effectiveness of OPKSC[2,4,10,19–22]. Physical and mechanical properties, and structuralbehavior with respect to bond, flexure and shear, have been inves-tigated and reported [10,23]. Time-dependent behaviors of OPKSCsuch as creep [23] and shrinkage [13,23] were also compared withNWC. These properties are discussed in this paper in concise but ininformative form.
Fig. 2. Oil palm kernel shell dumped at the factory yards [4].
2. Physical properties of OPKS as aggregate
The mechanical properties of OPKSC change depending on thephysical properties of OPKS. The established physical propertiescompared with NWA are specific gravity, thickness and shape, sur-face texture, loose and compacted bulk densities, air and moisturecontent, water absorption and porosity.
2.1. Specific gravity
Specific gravity of a material expressed as the ratio of the den-sity of that particular material and that of water [24,25]. Specificgravity of OPKS varies from but has never crossed the value of2.0 as reported by various researchers (Table 1). The range of spe-cific gravity for OPKS is around 1.17–1.62. The highest value of spe-cific gravity of OPKS from Table 1 is reported to be 1.62 by Ndoke[6] who tried to use the OPKS for soil stabilization. Okpala [3] re-
0
20
40
60
80
100
0.1 1 10 100Sieve Size (mm)
Per
cent
age
fine
r
Mining sand Palm kernel shell
Fig. 1. Particle size distribution of sand and OPKS [27].
Fig. 3. Oil palm kernel shell of different sizes [22].
ported the lowest value of specific gravity of 1.14, while Teoet al. [4], Mannan and Ganapathy [14], and Basri et al. [15] reportedthe same value of 1.17. This can be compared to the specific gravityNWA of 2.6 [13]. Specific gravity of other artificial LWA such asLECA and Lytag and natural LWA such as pumice and expandedshale is found in the range of 0.8–0.9 and 1.30–1.7, respectively[26]. Particle size distribution of typical OPKS is shown in Fig. 1[27] and it is seen from the figure that OPKS has wide range of par-ticles from 3 to 14 mm.
Table 2Mechanical properties of oil palm kernel shell.
Name ofauthor (year)
Abrasion value(Los Angeles)(%)
Aggregateimpact value(AIV) (%)
Aggregatecrushing value(ACV) (%)
Okafor (1988) – 6.00 10.00Okpala (1990) 3.05 – 4.67Basri et al. (1999) 4.80 – –Mannan and Ganapathy
(2001, 2002)4.80 7.86 –
Olanipekun (2005) 3.60 – –Mannan et al. (2006) – 1.04–7.86 –Ndoke (2006) – 4.50 –Teo et al. (2006 and 2007) 4.90 7.51 8.00Jumaat et al. (2008) 8.02 3.91 –Mahmud et al. (2009) – 3.91 –
Table 3Typical physical properties of oil palm kernel shell and crushed granite normal weightaggregate (NWA) [13].
Physical properties OPKS NWA
Specific gravity 1.17 2.61Water absorption for 24 h (%) 23.30 0.76Bulk density (kg/m3) 590 1470Fineness modulus (FM) 6.24 6.33Flakiness index (%) 65.17 24.94Elongation index (%) 12.36 33.38
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2.2. Shape, thickness and texture
Fig. 2 shows the dumped OPKS that is left at the factory yard.The shape of OPKS aggregate varies irregular flaky shaped, angular,circular or polygonal as shown in Fig. 3; it depends on the extrac-tion method or breaking of the nut [3,7,15]. The thickness of OPKSvaries between 0.15 and 8 mm depending on the species [3,15].Generally, the surface texture remains fairly smooth in both theconcave and convex part of the shell. The broken edges show roughand spiky attire [7,15].
2.3. Bulk density
Loose and compacted bulk densities of OPKS aggregate varies inthe range of 500–600 kg/m3 and 600–740 kg/m3, respectively[3,4,6,7,13,15,20–23,27,28]. Generally bulk densities are also af-fected by sizes of OPKS [22]. Due to lower density of OPKS, the den-sity of concrete made of OPKS usually falls in the range of 1600–1900 kg/m3 [29].
2.4. Water absorption and moisture content
As OPKS is an organic aggregate, it contains many pores andhence the water absorption is high. Depending on the species ofthe trees and matured age of OPKS, the 24-h water absorption var-ies in the range of 14–33%, as seen from Table 1. Although OPKShas high water absorption, even higher water absorption of 37%was recorded for pumice aggregate [30]. Alengaram et al. [22]showed that with varying OPKS sizes, water absorption also variesin the range of 8–15% and 21–25% for 1 h and 24 h, respectively.
Table 1 shows that free moisture content of OPKS varies be-tween 8% and 15%. Water absorption of NWA is typically foundto be in the range of 0.5–1% [31]. Due to higher water absorptionof OPKS compared to NWA, the mix design does not follow the con-ventional mix design of NWC or LWC [31,32]. The water absorptionof OPKS aggregate is also similar to coconut shell aggregate whichis 24% as found by Gunasekaran et al. [28].
3. Mechanical properties of OPKS and comparison with NWA
Researchers have reported the mechanical properties of OPKSsuch as Los Angeles Abrasion, aggregate impact and aggregatecrushing values. Tables 2 and 3 show the results obtained by var-ious researchers on the above said mechanical properties. Gener-ally, the abrasion resistance of LWA is inferior to that of NWAdue to lower stiffness of LWA. The range of abrasion values forOPKS aggregate is 3–8% whereas that of crushed stone is about20–25%.
4. Fresh concrete properties of concrete with OPKS as coarseaggregate
4.1. Materials used by researchers
The fresh and hardened concrete properties of OPKSC are shownin the Table 4. Okafor [7] used superplasticizer in OPKSC and re-ported improvement in the workability of the concrete. As OPKShave different shapes-from angular to spike with rough edges, itadheres with the bulk cement paste more firmly along the edges.Okpala [3], Teo et al. [4], Mannan et al. [5]. and Okafor [7] used riv-er sand as fine aggregate. For comparison with NWC, crushed gran-ite stone is the most popular choice for the researchers. Class F flyash has been used by Basri et al. [15] and Alengaram et al. [22,33]for its pozzolanic reaction. As silica fume is 100 times smaller thanthe particles of cement, Alengaram et al. [33] used it as cementi-
tious material and found its influence effect on the mechanicalproperties of OPKSC. Moreover, it has pozzolanic property pro-duces more C–S–H which adds to the strength.
4.2. Slump
Slump test is the standard test for the workability of concrete. Itmeasures the consistency according to ACI 116R [34]. It is very use-ful in calculating the variations in the uniformity of mix of givenproportions [31]. It is seen from Table 4 that the slump value is in-creased when the water cement ratio is increased as with the nor-mal concrete. Mannan and Ganapathy [14], Okpala [3] and Okafor[7] found the slump to be very low (0–4 mm) indicating S1 work-ability (ENV 206: 1992) or a very low workability [31]. Abdullah[23] achieved slump in the range of 0–260 mm with a compressivestrength of 15 MPa. However, it does not necessarily mean that lowslump ensures high compressive strength [13,14,35]; Alengaramet al. [20,36] showed that by incorporating a small percentage ofsuperplasticizer a slump value of 105 mm (high workability) couldbe achieved [31]. High range water reducing admixtures (Superp-lasticizer or SP) are capable of dispersing cement grains whichare directed towards high slump value resulting in highworkability.
4.3. Flow table
Flow table test on OPKSC samples showed a flow value of400 mm [20]. Generally, flow is measured as the average diameterin both directions of the concrete and the mean is taken as the flowvalue. The addition of silica fume enhanced cohesiveness of theOPKSC. However, with the addition of 1% SP the flow of OPKSCtends to increase [20].
4.4. Air content
In fresh concrete, air content is relatively higher in OPKSC thanNWC. The irregular shapes of OPKS hinder the full compaction of
Table 4Fresh and mechanical properties of concrete (All the mechanical properties reported at the age of 28-day.).
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concrete, thus contribute to the higher air content. OPKS is alsoporous material which can be cause of higher air entrapment in-side concrete. Mannan and Ganapathy [12] showed that the addi-tion of FA in OPKSC can result in the lower air content than OPKSCwithout FA. The spherical particles of FA fill up the pores insidefresh concrete and lower the air content. They reported the air con-tent to be 4.8–5.1% depending on the mix proportions and FAcontent.
5. Physical properties of OPKSC
5.1. Plastic density
Fresh concrete densities of OPKSC are found to be in the range of1753–1763 kg/m3 as shown by Okafor [7] depending on the mixproportions and w/c ratio. Usually the fresh concrete density isabout 100–120 kg/m3 lower than the saturated density of LWC;
this might be attributed to water absorption of OPKS. Mannanand Ganapathy [12] also reported the fresh concrete density ofOPKSC in the range of 1910–1958 kg/m3 depending on the mixproportions.
5.2. Density of hardened concrete
Table 5 shows the mechanical properties of OPKSC. The com-pressive strength of concrete depends on density and it is one ofthe most important variables to consider in the design of concretestructures. ACI and ASTM specify a density less than 2000 kg/m3
for structural lightweight concrete (SLWC). According to Clarke[29], the density of SLWC is between 1200 and 2000 kg/m3
whereas Neville [31] observed the density of LWC to be between350 and 1850 kg/m3. Attempts have been made by variousresearchers to decrease the density of OPSKC without affectingthe strength. The density of OPKSC depends on various factors such
Table 5Densities of oil palm kernel shell concrete reported by researchers.
Abdullah (1988) – – 1725–2050Okafor (1988) 1753–1763a – –Okpala (1990) 1630–1780 (1:1:2)a – –Okpala (1990) 1600–1700 (1:2:4)a – –Okafor (1990) 1863–1910a – –Basri et al. (1999) – – 1801–1856Mannan and Ganapathy (2001) 1655–1910 – –Olanipekun et al. (2005) 1700–1850 – –Mannan et al. (2006) 1865–1930 – –Teo and Liew (2006) 1792 – –Teo et al. (2007) – – 1960Alengaram et al. (2008) 1850–1960 1639–1715 –Alengaram et al. (2008) 1888 – –Jumaat et al. (2009) – 1670 (foamed concrete) (varies) –Alengaram et al. (2010) 1880 – –Shafigh et al. (2011) – 1868–1937 –
a Note: Values vary for different w/c ratios and superplasticizer or mineral admixture dosages.
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as, the specific gravity of OPKS, water cement (w/c) ratio, sand andOPKS contents and water absorption of OPKS. Oven-dry densitiesare 200–250 kg/m3 lower than the saturated surface dry (SSD) den-sities [29]. Air-dry densities of OPKSC are in the range of 1725–2050 kg/m3 as shown in the Table 5.
6. Mechanical properties of OPKSC
6.1. Compressive strength
Compressive strength of concrete is the most desirable propertyfor any new material used in concrete technology. All othermechanical parameters such as flexural strength, splitting tensilestrength and modulus of elasticity directly depend on the compres-sive strength of the concrete. Attempts have been made to enhancethe compressive strength of concrete using OPKS as coarseaggregate.
Depending on the mix design and curing conditions differentresearchers have found different grades of strength. Abdullah[16,23] was the pioneer in using OPKS as LWA and a achieved acompressive strength of up to 20 MPa with w/c ratio of 0.4 asshown in Table 4. This value is almost equal to the specified cylin-drical compressive (fc) strength of 17 MPa by ACI. Teo et al. [4]achieved a compressive strength of 22 MPa. Mannan and Ganapt-hay [14,15] used the ACI method of mixed design for NWC and re-ported the compressive strength of 13.65 MPa which greatlydiffered from the target strength of 28 MPa. Though Mannan andGanapathy [14,15] used the method suggested by Short and Kinni-burgh [32] they found the compressive strength to be around14.40 MPa for a target strength of 25 MPa. So, it is evident fromthe results that neither of the previous methods is suitable forOPKSC.
It can be seen from Table 4, that most of the researchersachieved the cylindrical strength of 17 MPa (equivalent of20 MPa cube strength) benchmark for structural concrete set byACI [37] and ASTM C330 [38]. Recent researches show that up to30 MPa is achievable with [12,20,36]. Alengaram et al. [39] re-ported a strength of 37 MPa which is 85% higher than the mini-mum strength of 20 MPa. They used silica fume and class F flyash to enhance the early and later day strength and sulfonatednaphthalene formaldehyde condensate as superplasticizer to dis-perse the cement grains effectively [40]. It can be seen from otherresearches on LWC using sewage sludge, pumice, palm fiber, ex-panded polystyrene, ignimbrite, bottom ash, Lytag, Japanese, Chi-nese and Pinatubo, cenospheres, etc., as LWA, the compressivestrength of 10–30 MPa was reported with a density range of
1500–1900 kg/m3 [30,41–50]. Generally, it is seen from the exper-imental results that the mechanical properties of OPKSC increasedwith decreasing w/c ratio. Shafigh et al. [51] investigated the prob-ability of making high strength lightweight concrete (HSLWC) withcrushed OPKS and steel fiber. They achieved 28-day compressivestrength in the range of 41–45 MPa with steel fibers; however,they [52] achieved 28-day compressive strength of up to 48 MPawith crushed OPKS and lime stone powder as a filler.
Okpala [3] reported that the compressive strength of OPKSC de-pends on the bond between paste aggregate interface. Okafor [7]concluded that the compressive strength depended on the strengthof OPKS itself. Olanipekun et al. [9] showed that the compressivestrength of the concrete decreased as the percentage of OPKS sub-stitution increased. Mannan and Ganapathy [12] reported that thestrength, thickness and density of OPKS aggregate are lower thanthose of crushed stone aggregate which are the governing factorsfor the compressive strength in concrete. On their account, theirregular shape of OPKS is one of the factors for strength. They alsoreported that compressive strength is controlled by both thestrength of the aggregate and the strength of paste, and dependson one of these two that fails first. Mannan and Ganapathy [13]showed that at the earlier ages, the failure of OPKSC was governedby the failure of OPKS, but in the later ages the failure of OPKSCwas governed by the strength of OPKS-paste bond.
Alengaram et al. [39] reported that the suction of silica fumeinto the pores of OPKS enhanced the bond between OPKS and ce-ment matrix. They reported a compressive strength of about37 MPa. OPKS can also contribute to the mechanical propertiesby its aggregate interlock characteristic [21].
6.2. Modulus of rupture
Table 4 reports the modulus of rupture (MOR) by variousresearchers. Okafor [7] found that with different mix designs, theMOR varied in the range of 4.3–6.2 MPa and it was about 27% ofthe compressive strength. Okpala [3] also reported the MOR inthe range of 2.13–2.8 MPa which is about 14% of the compressivestrength. MOR reported by Teo et al. [4] also shows that it variedin the range of about 8–13% of the compressive strength. It canbe observed from the Table 4 that the MOR is affected by the mix-ture proportion. Okafor [7] and Okpala [3] used almost similar w/cratio, but with the use of higher fine aggregate content, Okafor [7]reported an increment of 1.20 times higher MOR than the latter.Similar trend was reported by Mahmud et al. [27], albeit withthe use of cementitious materials. Mannan and Ganapathy [13]concluded that as with compressive strength, MOR is also
Es = 0.30Ed1.25
R2 = 0.88
5
7
9
11
13
11 13 15 17 19 21
Dynamic modulus of elasticity, Ed (kN/mm2)
Stat
ic m
odul
us o
f el
asti
city
,E
s (k
N/m
m2 )
Fig. 4. Relationship between static and dynamic moduli of elasticity [39].
Fig. 5. Water permeability of OPKSC [13].
U.J. Alengaram et al. / Construction and Building Materials 38 (2013) 161–172 167
depended on physical strength of OPKS itself. Alengaram et al. [22]reasoned that the failure of the flexural specimen is governed byboth the strength of PKS and bond between OPKS and cementmatrix.
The relationship between the MOR and the compressivestrength as reported by Mahmud et al. [27] is given below:
fcfc ¼ 0:3ðf 2=3cuc Þ ðR
2 ¼ 0:92Þ; ð1Þ
Here, fcfc and fcuc are the 28-day flexural and compressive strengthsin MPa, respectively.
6.3. Splitting tensile strength
Okafor [7] showed that the splitting tensile strength varied inthe range of 2.0–2.4 with varying w/c of 0.65–0.48. Teo et al. [4]reported the values of splitting tensile strength of 2.24 MPa.Mahmud et al. [27] also reported similar values for the splittingtensile strengths. These values are about 6–10% of their respectivecompressive strengths. Mannan and Ganapathy [13] reported thatthe splitting tensile strength also depends on the curing conditionand physical strength of OPKS. Alengaram et al. [20,22] reportedthat the bond failure along the convex surface of OPKS of particlesize more than 10 mm in splitting failure specimens.
6.4. Static and dynamic moduli of elasticity
Young’s modulus or E-value of concrete is it is one of the mostimportant parameters in the design of structural members. How-ever, this is one of the least researched areas in OPSKC. There areonly three references available on this important property[4,27,39]. Teo et al. [4] showed the E-value of 5.31 GPa; However,Alengaram et al. [39] used cementitious materials such as 5% flyash and 10% silica fume and showed that E-value of OPKSC canbe enhanced. Furthermore, Table 4 shows that the OPKS contentof Alengaram et al. [39] was higher than Teo et al. [4]. Mahmudet al. showed E-values of up to 10.90 GPa [19] and this is abouttwice the value reported by Teo et al. [4]. They attributed the in-crease in E-value to the increase in sand content and subsequentreduction in OPKS content. They also concluded that the additionof silica fume in concrete mix enhanced the bond and betweenthe OPKS and the cement matrix, which enhanced the overallmechanical properties. Silica fume is almost 100 times finer thanthe cement particles [40]. Silicon di-oxide (SiO2) of silica fume re-act with the liberated calcium hydroxide (Ca(OH)2) of hydrated ce-ment, they produce more calcium silicate hydrate (C–S–H) gel thanusual. Microscopic analysis of OPKS surface confirmed the suctionof silica fume into the micro-pores of OPKS improved the bond[36]. Thus, Mahmud et al. [27] and Alengaram et al. [39] concludedthat the addition of silica fume enhances the mechanical propertiesof OPKSC. The static modulus of elasticity of other LWAC usingpumice, ignimbrite, or expanded polystyrene [30,43,44] as LWAvaries in the range of 7.69–11.4 GPa depending on mix designand age of curing; these results show the E-values of are quite sim-ilar to other LWAC. Shafigh et al. [51] showed that the use of steelfibers in OPKSC enhances the elastic modulus up to 16.1 GPa.
Dynamic modulus test is a simple non-destructive test andcould be performed to establish the relationship between staticand dynamic moduli of elasticity. The relationship between staticand dynamic moduli of elasticity is shown in Fig. 4. Alengaramet al. [39] suggested an equation to predict the static modulus ofelasticity from the dynamic modulus of elasticity,
Es ¼ 0:3ðEdÞ1:25 ðR2 ¼ 0:88Þ; ð2Þ
where Es (GPa) and Ed (GPa) are the static and dynamic moduli ofelasticity, respectively.
According to Mannan and Ganapathy [13] the E-value is influ-enced by the type of coarse aggregate (OPKS or granite aggregate),w/c ratio of the mix and curing age. The E-value depends on thestiffness of OPKS, the interfacial transition zone between the pasteand aggregate and the elastic properties of the constituent materi-als [13]. Alengaram et al. [39] reasoned that both fine aggregateand OPKS contents play role in the E-value; an increase in fineaggregate content with subsequent reduction in OPKS resulted inhigher E-values compared to concrete with high OPKS content.
7. Durability of OPKSC
7.1. Water absorption and permeability
Water absorption is defined as the transport of liquids in poroussolids caused by surface tension acting in the capillaries [53]. It isshown by Teo et al. [2] that the water absorption by OPKSC is11.23% and 10.64%, respectively for air-dry curing (CL) and fullwater curing (CC). The water absorption of LWC such as expandedpolystyrene concrete and pumice aggregate concrete is in therange of 3–6% [50] and 14–22% [54], respectively. And both showhigher water absorption than that of NWC [40].
Water permeability can be used as an indicator of durability ofconcrete. From Fig. 5, it is evident that OPKSC is less permeableover the time. At the age of 28 days, OPKSC under CC curing condi-tion has one-sixth of the permeability value compared to that ofconcrete cured under CL condition [13].
High water absorption of OPKSC can be explained throughmicroscopic analysis. Alengaram et al. [39] examined OPKSthrough microscopic analysis using scanning electron microscope
(a) Outer surface of palm kernel shell
b) Pores on
(b) Micro-pores on outer surface
Outer convex surface
Pore distribution
Fig. 6. Micro-pores on outer surface [39].
168 U.J. Alengaram et al. / Construction and Building Materials 38 (2013) 161–172
and the result is shown in Fig. 6. It shows tiny pores of size in therange of 16–24 lm on the convex surface of OPKS surface that areresponsible for high water absorption [36].
It is observed by Chia and Zhang [55] that high quality anddense interfacial transition zone in cement paste generally leadsto less permeable concrete. They also reported that 10% additionof silica fume generally decrease the water permeability of con-crete, supposedly because of the aforementioned reason. Teoet al. [2] reported that the permeability of OPKSC is due to crackingof the paste aggregate interface which in turn is attributed to theinternal stress.
7.2. Initial surface absorption (ISA)
Mannan and Ganapathy [13] compared the initial surfaceabsorption of NWC as control concrete with OPKSC cured undertwo different curing conditions, 6 days and 89 days in water. Inboth cases, the samples of OPKSC had shown greater absorptivecapacity than that of control concrete. The primary reason attrib-uted to higher surface absorption of OPKSC is high porosity ofOPKS. But interestingly, for OPKS and control concrete, the ISA val-ues remained the same at the age of 90-day for both curing condi-tions; however no apparent explanation was attributed to thisbehavior. On the other hand, early air drying of OPKSC also contrib-utes to higher ISA, which in turn is attributed to micro-cracksdeveloped in the concrete.
8. Functional properties of OPKSC
8.1. Thermal conductivity
Thermal conductivity indicates the conduction of heat throughmaterials. Thermal conductivity depends on the density of con-
crete specimen. Thus, it does not depend on the condition of thecuring process such as whether it is moist cured or autoclaved.The thermal conductivity also depends on the pores inside the con-crete [72]. Okpala [3] reported the thermal conductivity of OPKSand OPKSC of 0.19 W m�1 �C�1 and 0.45 W m�1 �C�1, respectively.Thus the thermal conductivity of OPKSC lies between 0.05 and0.69 W m�1 �C�1 of other LWA as reported by Neville [31], Demirb-oga and Gul [56] and Zach et al. [57]. Thermal conductivity ofOPKSC as reported by Okpala [3] is similar to that of other artificialinorganic LWA such as pumice, versalite or bottom ash(�0.43 W m�1 �C�1) [26].
8.2. Sound absorption
Okpala [3] showed that sound absorption coefficient increaseswith increasing w/c ratio for the same mix design of OPKSC. Thenoise reduction coefficients for OPKSC were found as 0.34 and0.35 for w/c ratio of 0.5 and 0.6, respectively; however, the noisereduction coefficient for NWC was only 0.02 which shows thatOPKSC has better sound insulation. He concluded that the higherreduction coefficient for OPKSC was mainly due to more pores in-side the concrete and also the OPKS in which the entrapped air actsas an insulator.
9. Time-dependent properties of OPKSC
9.1. Creep
Abdullah [16] investigated creep tests on three 150 mm ucylindrical specimens based on ASTM standards [16] and reportedthat OPKSC showed inconsistent constant creep values even after3 months. OPKSC showed a larger creep compared to that ofNWC and kept on increasing after 3 months; in contrast NWCshowed almost a constant value after 1 month. With 1:1:2 mixproportion and 0.55 w/c ratio, OPKSC showed a creep of about350 � 10�5 mm/mm after 1 month and 400 � 10�5 mm/mm after3 months. Conversely, NWC indicated a creep of about45 � 10�5 mm/mm with 1:2:4 mix proportion and the same w/cratio which is almost one-eighth creep value of OPKSC.
9.2. Shrinkage
Abdullah [16] used prisms of 50 � 50 � 250 mm3 as concretespecimens for the shrinkage test. The initial length of the specimenwas measured and after drying for at least 44 h in a relative humid-ity of 17%, the final length of the specimen was measured. Dryingshrinkage was reported as the ratio of the difference between thewet initial length and dry final length of the specimen to the dryfinal length of the specimen. It was shown by Abdullah that upto 1 month, the shrinkage of both OPKSC and NWC increased butafter that the shrinkage was found constant. Nonetheless OPKSCshowed about five times higher shrinkage strain than that ofNWC. Mannan and Ganapathy [13] carried out drying shrinkagetest on OPKSC and compared it with NWC on 7, 28, 56 and 90 days.They reported that the drying shrinkage of both the OPKSC andNWC increased with age but OPKSC showed higher increment. Atthe age of 28 and 90 days OPKSC showed 64 and 182 microstrainwhich was 6% and 14% higher increment of drying shrinkage com-pared to NWC, respectively. Drying shrinkage occurred due to lossof free water from concrete. It depends on variables such as w/c ra-tio, cement composition, type of aggregate, degree of hydration,curing condition, temperature of curing, relative humidity, mois-ture content and duration of drying [58,59]. Usually, the highershrinkage showed by OPKSC was attributed to the loss of water
U.J. Alengaram et al. / Construction and Building Materials 38 (2013) 161–172 169
in the early age of plastic concrete and the irregular surface ofOPKS [13].
Fig. 7. Crack paths (a) at earlier ages (b) at later ages [4].
Table 6Strength characteristics of the asphalt oil palm shell concrete [6].
Flexural behavior of reinforced beams was reported by Teo et al.[10] and Alengaram et al. [19]. Six beams, three each of singly anddoubly reinforced were tested with different reinforcement ratios[10]. Vertical flexural cracks were observed in the constant-mo-ment region and final failure occurred due to crushing of the com-pression concrete with significant amount of ultimate deflection.Since all beams were under-reinforced, yielding of the tensile rein-forcement occurred before crushing of the concrete cover in thepure bending zone. Eventually, crushing of the concrete cover oc-curred during failure, with a significant amount of deflection. Acomparison between the experimental ultimate moments (Mult)and the theoretical design moments show a closer relationshipfor doubly reinforced beams than singly reinforced ones [10]. Thetheoretical design moment (Mdes) of the beams was predictedusing the rectangular stress block analysis as recommended byBS 8110 [60]. For beams with reinforcement ratios of 3.14% or less,the ultimate moment obtained from the experiment was approxi-mately 4–35% higher compared to the predicted values. They con-cluded that for OPKSC beams, BS 8110 [60] can be used to obtainboth a conservative estimate of the ultimate moment capacityand adequate load factor against failure for reinforcement ratiosup to 3.14%.
The beam with the highest reinforcement ratio of 3.14% showedslightly higher mid-span deflection than the other two beamswhich indicates more ductile behavior. They also concluded thatthe ductility and moment curvature for OPKSC beams follows thesame trend as those of the NWC beams [10].
10.2. Shear behavior
Shear strength of concrete is one of the most controversial top-ics in structural behavior [61]. Alengaram et al. [62] comparedgrade 30 OPKSC beams with NWC beams. They concluded thatreinforced OPKSC beams showed higher ultimate shear strengthto density ratio. They found that both OPKSC and NWC beamsshowed independent shear cracks and concluded that ACI, BS andEurocode 2 (EC2) underestimate the shear capacity of OPKSC. Theshorter, narrower cracks in their investigation, proved higheraggregate interlock capacity and thus OPKSC beams showed 24%higher shear capacity and that of NWC beams [62]. The OPKS varyin shapes from angular to flaky. It is seen by visual observation thatmortar generally adheres to the concave portion of the aggregatewhich may contribute to the aggregate interlock and thus highershear strength was reported [62].
10.3. Bond strength
The bond between the reinforcement and OPKSC was investi-gated by pull out tests using deformed bars of various diameters[2,10,20]. The 28-day bond strength for plain and deformed barswas found in the range of 3–5.59 MPa and 6.32–9.36 MPa, respec-tively depending on the curing condition and size of the bar used.The experimental bond strength was always much higher than thetheoretical bond strength [2,10,20].
The bond strength of OPKS was 26–33% of the compressivestrength and comparable to the bond strength of other LWA suchas sintered pulverized fuel ash [63] and aerocrete [64]. It was alsoobserved that the compression failure at the earlier ages of samples
was caused by the failure of the bond between aggregate and thecement matrix, where the crack paths goes around the aggregatesas shown in Fig. 7a. But at later ages (56–180 days), the mortar-aggregate bond becomes stronger and as such, crack paths travelthrough the aggregate as illustrated in Fig. 7b.
11. Comparison of OPKSC with other agricultural wastes
Comparison of density and strength of OPKSC and concrete withother agricultural wastes such as oil palm clinkers, rice husk andcoconut shells as coarse aggregates was done by Abdullah [16].From the experimental test results, he concluded that concretemade with rice husk had the lowest bulk density with 136 kg/m3
and the lowest compressive and tensile strengths. Concrete madewith oil palm clinker showed the highest bulk density and 28-day compressive and tensile strengths. The 28-day compressivestrength of OPKSC with a density of 620 kg/m3 of 17.4 MPa wasfound lower than the concrete made with oil palm clinker that pro-duced 29.8 MPa. Gunasekaran et al. [28] showed that waterabsorption, specific gravity, impact value and bulk density of coco-nut shell aggregate were comparable to those of OPKSC. The 28-day compressive strength of coconut shell concrete was found tobe in the range of 5–27 MPa; however with a slump of only5 mm, the coconut shell concrete exhibited a very poor workabil-ity. Moreover, the compressive strength, modulus of rupture, split-ting tensile strength, theoretical and experimental bond strengthperformed on coconut shell concrete were comparable to OPKSC[28].
12. OPKS as partial replacement
Olanipekun et al. [9] studied the mechanical properties of OPKSusing 1:1:2 and 1:2:4 mix ratios with 0%, 25%, 50%, 75% and 100%replacement with OPKS. He showed that with the increasing ratioof OPKS, the compressive strength of concrete decreased. He alsoreported a cost reduction of 42% with OPKSC.
Fig. 8. Crack patterns and failure modes of OPKSFC beams without shear reinforcement [21].
170 U.J. Alengaram et al. / Construction and Building Materials 38 (2013) 161–172
OPKS was also used as partial replacement for coarse aggregateof up to 10% for heavily trafficked roads and up to 50% for lightlytrafficked roads. But for rural areas, replacement of up to 100%was used [6]. It can be seen from the Table 6 that the marshal sta-bility and flow values for 10% replacements of OPKS were found13.46 kN and 3.71 mm, respectively.
For 100% replacement i.e. with only OPKS, the marshal sta-bility value was 7.78 kN which reinforces the reason for its usein rural areas. The flow of asphalt concrete increases from3.42 mm to 4.32 mm for 0–100% OPKS replacement. Waterabsorption increases from 0.20% to 14% due to increase inOPKS ratio.
13. Oil palm kernel shell aggregate in foam concrete
13.1. Foam concrete
Foam concrete as defined by Ramamurthy et al. [65] is eithera cement paste or mortar, classified as LWC, in which air-voidsare entrapped by suitable foaming or aerating agent. Aeratedconcrete falls under the category of broader cellular concretewhose other member is microporite [66]. It possesses high flow-ability, self-compacting ability, controlled low strength, excellentthermal insulation capacity and moderate sound absorptioncapacity. Ever since foam concrete was patented in 1923, ithad been in use in the concrete industry. First comprehensive re-view on the composition, properties, and used of cellular con-crete worldwide was done by Valor [67] in 1954. This wasfollowed by Rudnai [68], Short and Kinniburgh in 1978 [32]and recently by Jones and McCarthy [69]. The density rangefor aerated concrete is 300–1800 kg/m3 depending on the pro-duction method which indicated greater flexibility for use.Though foam concrete has been traditionally used as insulatingconcrete without any coarse aggregates, it was Weigler and Karl[70] who combined both pre-formed foam and leca as LWA toproduce structural grade lightweight aggregate foamed concrete(LWAFC).
13.2. Mechanical properties OPKS foamed concrete
The use of OPKS for making foam concrete is relatively newand first attempted by Jumaat et al. [21]. They reported thatthe density of OPKS foam concrete (OPKSFC) remained in therange 1600–1700 kg/m3 which falls under the category of foamconcrete of density range of 300–1800 kg/m3 [66,71]. The cubecompressive strength of OPKSFC found to be in 16–24 MPa rangewhich satisfies the requirement for structural lightweight con-crete set by ASTM and ACI. But usually, the compressive strengthof foam concrete falls in the range of 1–43 MPa depending on thedensity and use of fly ash and silica fume as cement replacement[65]. Actually, air-void characteristics primarily determine the
strength of foam concrete. The filler materials like fly ash and sil-ica fume helps to make uniform distribution of air-voids by mak-ing uniform coating on each bubble, therefore, preventing themerging of the bubbles. For higher density of foam concrete,the compressive strength decreases with an increase in voiddiameter. If the pore diameter increases the air bubbles mergewith each other resulting in lesser paste volume and as a resultlower compressive strength [72,73]. Higher compressive strengthcan be obtained using fly ash up to 67% [74]. Small changes in w/c ratio has not affected the strength of foam concrete [69]. Themodulus of rupture and splitting tensile strength for OPKSFCwere found 9–11% and 7%, respectively of the compressivestrength as opposed to 12–16% for NWC. Modulus of elasticity(E-value) for OPKSFC was found one-fourth as that of NWC andgenerally the E-values of foam concrete falls in the range of 1–8GPa [65]. Jones and McCarthy [57] reported that the use of poly-propylene fiber in the foam concrete increased the E-value by twoto four times [69].
13.3. Shear behavior of OPKS foamed concrete
Jumaat et al. [21] investigated the shear behavior of reinforcedOPKSFC beams (see Fig. 8). They reported that OPKSFC beamshowed a greater number of flexural and shear cracks than the cor-responding NWC beams. Subsequently, the crack widths of OPKSFCwere found lower. The shear strength predicted using ACI code’sequation was close to that of experimental values; however, theBS code underestimated the shear strength. They also concludedthat the 10% increase of shear strength by OPKSFC beams com-pared to the NWC beams due to aggregate interlock property ofOPKS.
14. Conclusions
The utilization of OPKS as LWA to produce OPKSC was reviewedthrough 74 recent and past literatures. The physical, mechanical,durability, functional and structural behaviors of OPKSC were dis-cussed. The behavior of OPKSC was compared with NWC and con-ventional LWC. Based on the review, several conclusions can bedrawn and these are listed below:
1. Oil palm kernel shell (OPKS) is irregular shaped i.e. oval, cir-cular, polygonal or flaky shaped with 0.15–8 mm thickness.The surfaces of both convex and concave portions of OPKSare quite smooth with rough surfaces along the crackededges. OPKS can be termed as LWA as it has low specificgravity in the range of 1.17–1.6.
2. Loose and compacted bulk densities of OPKS falls in therange of 500–600 kg/m3 and 600–620 kg/m3, respectively.Thus, the saturated surface density (SSD) of OPKSC falls inthe range of 1600–1960 kg/m3.
U.J. Alengaram et al. / Construction and Building Materials 38 (2013) 161–172 171
3. The existence of numerous pores in the OPKS is responsiblefor high water absorption in the range of 14–33%. The freesurface moisture content is reported to be in the range of8–15%.
4. OPKS has very low abrasion of about 3–8% compared to 20–25% of natural crushed granite aggregate and thus showshigher resistance to abrasion.
5. OPKSC is reported to have very low workability as seen fromlow slump values due to angular and rough edges of OPKS.However, the problem associated with low workability canbe solved using superplasticizer to disperse the cementgrains.
6. The hardened density of OPKSC was found in the range of1650–1950 kg/m3 and various factors such as w/c ratio,inclusion of fine aggregate, water absorption and grain sizeof OPKS etc. affect the density. The oven-dry density wasfound 200–250 kg/m3 lower than that of SSD density.
7. Generally, the compressive strength of 13–22 MPa wasreported for OPKSC by many researchers. But with the inclu-sion of fly ash, silica fume and superplasticizer, compressivestrength of 37 MPa has been achieved. Using crushed OPKSand lime stone powder, a compressive strength up to48 MPa has been reported. MOR and splitting tensile strengthis reported to be 8–14% and 6–10%, respectively of the com-pressive strength. However, use of steel fibers in the fiberreinforced OPKSC enhanced the elastic modulus up to 16 GPa.
8. Water absorption of OPKSC was reported to be more than10% due to the micropores on the surface of OPKS. OPKSCshows higher initial surface absorption than that of NWCirrespective of curing conditions.
9. The reinforced concrete beams made of OPKSC showedhigher ductile behavior compared to the beams made ofNWC. The moment curvatures of the beams of OPKSC alsofollowed the same trend as that of NWC.
10. The aggregate interlock property of OPKS contributed tohigher shear strength in OPKSC compared to NWC.
11. The bond strength of OPKSC was reported to be 26–33% of itscompressive strength.
12. The creep value of OPKSC was found to increase even after3 months compared to NWC. Moreover, OPKSC showed eighttimes the creep value of NWC. The shrinkage value forOPKSC kept on increasing for 1 month and after 1 monthbecame constant, nonetheless, showed five times the valueobtained by NWC.
13. It was reported that OPKS can also be used in asphalt con-crete in pavement construction. It was shown that, in urbanareas where traffic load is heavy, a replacement up to 10%can be allowed whereas the replacement can be up to100% in rural areas.
14. Thermal conductivities of OPKS and OPKSC were 0.19 W/m Kand 0.43 W/m K, respectively and it was lower than 0.76–3.68 W/m K for NWC. The higher noise reduction coefficientof OPKSC proved its superior sound insulating property com-pared to NWC.
15. OPKS foamed concrete with a saturated density and com-pressive strength in the range of 1600–1700 kg/m3, and16–24 MPa, respectively can be considered as structuralgrade concrete.
Acknowledgement
This research work is funded by University of Malaya underMinistry of Higher Education (MOHE) research fund: High ImpactResearch Grant (HIRG) No. UM.C/HIR/MOHE/ENG/02 (D000002-16001) (Synthesis of Blast Resistant Structures).
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