Journal of Composite Materials Torsional and cracking ... · Torsional and cracking characteristics ... material used as aggregate in lightweight concrete are oil palm shell ... coir

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JOURNAL OFC O M P O S I T EM AT E R I A L SArticle

Torsional and cracking characteristicsof steel fiber-reinforced oil palm shelllightweight concrete

Soon Poh Yap, U Johnson Alengaram, Mohd Zamin Jumaat andKuan Ren Khaw

Abstract

The study presents the investigations of the torsional behavior and crack resistance of a sustainable lightweight oil palm

shell (OPS) concrete. In the comparison between the OPS concrete (OPSC) and normal weight concrete (NWC), the

torsional failure of the NWC was sudden and brittle, while the OPSC exhibited post-cracking behavior. The second part

of the study examines the effect of steel fibers of 0.25–1.00% in the OPSC to produce OPS fiber-reinforced concrete

(OPSFRC). Improved mechanical properties and reduced brittleness were reported in the OPSFRC mixes. The addition

of 1% steel fibers in the OPSC-100 mix produced the highest compressive and flexural strengths of 47 MPa and 8.2 MPa,

respectively. The steel fibers up to 1% enhanced the ultimate torque, torsional toughness, and twist at failure of the

OPSFRC specimens by 60%, 1000%, and 550%, respectively. The addition of steel fibers significantly improved the crack

resistance of the OPSFRC specimens.

Keywords

Fiber-reinforced concrete, lightweight concrete, oil palm shell, steel fiber, torsion, cracking resistance, mechanical

properties

Introduction

The vast and expanding construction industry indeveloping countries, including Malaysia, is facing therapid depletion of concrete materials, such as graniteaggregate and mining sand, together with the environ-mental problems associated with their extraction. Thisprompted the idea of producing sustainable concreteusing waste materials derived from agriculture andindustry as a substitute for conventional concretematerials. Among the popular categories of wastematerial used as aggregate in lightweight concrete areoil palm shell (OPS), coconut shell, palm oil clinker,pumice, expanded clay, and expanded slag.1,2 The useof such lightweight aggregate to replace conventionalgranite aggregate leads to the production of lightweightconcrete (LWC) with the benefit of a reduction indensity relative to the normal weight concrete(NWC). The decreased density of the concrete permitsdead load savings for the structural design and founda-tion, which, ultimately, reduces the constructioncosts.3–5

Research has proven that lightweight OPS could beused to produce OPS concrete (OPSC) with a densityreduction of up to 30% and compressive strength ofabout 25–40MPa.2,6,7 In addition, OPS has greatpotential as a replacement for the coarse aggregatedue to its high impact resistance and low aggregateimpact value.6,8 In terms of durability, study fromMannan & Ganapathy showed that there is no retro-gression in strength in OPSC over a period of 365days.9 Hence, over the past decade, different research-ers have been prompted to conduct research work ondiverse applications of OPSC using foamed concrete10

and geopolymer concrete.11 However, the focus of suchresearch was to reduce the density and carbon dioxideemissions. Similar to other lightweight aggregate

Department of Civil Engineering, Faculty of Engineering, University of

Malaya, Kuala Lumpur, Malaysia

Corresponding author:

U. Johnson Alengaram, Department of Civil Engineering, Faculty of

Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia.

Email: [email protected]

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concrete, the development pace of the OPSC is ham-pered due to its low mechanical properties.

The most effective method to improve the mechan-ical properties of LWC is by the addition of steel fibers.Fibers are generally added into a concrete mix toimprove the toughness, ductility, post cracking per-formance, shrinkage, crack resistance, impact strength,fatigue, and torsional strengths as the plain concrete isbrittle and less resistant to impact and tensile load-ing.12–18 The benefits of steel fibers have paved theway to an increasing number of studies on OPS fiber-reinforced concrete (OPSFRC).3,6–8 The improvedmechanical properties permit the recommendation ofOPSFRC for structural applications. Furthermore,the studies on use of natural fibers such as flax andcoir fibers in LWC reported positive impact on themechanical properties, toughness, ductility, crackresistance, shrinkage, and more, with the additionaladvantage of environmental friendliness. Hence, thestudy on the steel fibers in the OPSFRC could pavesway for the production of greener OPSFRC with thesenatural fibers.

The present study aims to study the application ofOPSC and OPSFRC as structural members subjectedto torsion, by comparing to NWC. Previous studieshave already proven that OPSC showed comparablestructural behavior under flexural and shear load-ing.19,20 It has been reported that the ductility ratio ofOPSC was twice that of NWC and that the flexural andshear cracks are shorter compared to the NWC.However, no study on the torsional behavior ofOPSC has been conducted. The structural elementsthat are subjected to torsional forces include utilitypoles, eccentrically loaded box bridge girders, spiralstaircases, spandrel beams in the frames of buildings,and beams that are curved in plan, as well as earth-quake-resistant structures.21–23 The recent trend instructural design has changed and the aesthetic designconcept with an increase in the number of curved mem-bers incorporated. In such a case, the torsion charac-teristics of the structural members have to beconsidered to prevent the occurrence of torsionalcracking.

The present study reports the effect of hooked-endsteel fibers up to 1% (by volume) on the torsional andcracking characteristics of OPSFRC prism specimenswithout longitudinal and transverse reinforcementunder pure torsion loading. It is shown that the unre-inforced concrete produced a similar torsional responseto the reinforced concrete beams without stirrups. Thisis due to the torsional behavior of concrete beingmainly linear until the first cracking torque and theinitial stiffness being independent of the presence andamount of reinforcement.24 However, the addition ofsteel fibers enhances the ultimate torque, torsional

ductility, and crack resistance of the concrete,22,24,25

Therefore, before the study on the effect of the longitu-dinal and transverse reinforcement on the torsionalbehavior of the OPSC and OPSFRC, it is importantto study the effect of the variations in the steel fibervolume on the torsional and cracking resistance of theOPSFRC. The production of OPSFRC could furtherexpand the applications of OPS-based concrete as tor-sion-resistant members.

Experimental program

Materials

Cement and supplementary cementitious materials. Type 1ordinary Portland cement was used in all the mixes.The Blaine specific surface area and specific gravity ofthe cement were 3450 cm2/g and 3.13, respectively.Silica fume (SF) of 10% cement weight was addedinto OPSC and OPSFRC mixes as additional cementi-tious material.

Coarse and fine aggregates. OPS and crushed graniteaggregate were used as coarse aggregate in this studyas seen in Figure 1. The OPS is smooth with concaveand convex surfaces, while the granite is angular. Thecomparison between the physical properties of bothtypes of aggregate is shown in Table 1. The low bulkdensity and aggregate impact value of OPS enable OPSto be potentially used to produce LWC with lowerdensity and higher energy absorption capacity. TheOPS and granite aggregate below 2.36mm were sievedand removed. Mining sand with a specific gravity andfineness modulus of 2.65 and 2.7, respectively, was usedas fine aggregate.

Water & superplasticiser. Potable water (pH¼ 6) wasused for both mixing and curing processes. A polycar-boxylate-based superplasticiser with a constant amountat 0.65% of cement weight was added in the OPSC andOPSFRC mixes to improve the workability.

Steel fiber. In this study, hooked-end steel fibers confirmto ASTM A820 and BS EN 14889-1 were added intoOPSC to produce OPSFRC mixes. The aspect ratio andlength of steel fibers were 65 and 35mm, respectively.The tensile strength and modulus of elasticity of thesteel fibers are 1100MPa, and 205GPa, respectively.

Mix proportions

The mix proportions of all the mixes are shown inTable 2. For the OPSFRC mixes, all the constituentmaterials were kept constant and the variation betweenthe mixes is the fiber content.

2 Journal of Composite Materials 0(0)




Specimen preparation and testing

For each mix proportion, 100mm cubes,100� 100� 500mm3 prisms and 150f� 300mm cylin-ders were prepared for compressive strength (BS EN12390:2009), flexural strength (ASTM C78-10)/torsiontesting, modulus of elasticity/Poisson’s ratio (ASTMC469-10), respectively. The specimens were removedfrom the moulds 24 hours after placement into themoulds. All the specimens were then cured in wateruntil the age of testing. The compressive strengthswere tested at the age of 1, 3, 7, and 28 days, whileother specimens were tested at the age of 28 days.For each testing, the mechanical properties weretaken as the average values of three specimens.

The torsion test was conducted on the100� 100� 500 mm3 prism specimens using an auto-mated torsion machine (Figure 2). The torque–twistcurves were collected directly from the software ana-lysis. The important parameters from the torque–twistcurves are cracking torque, Tcr/twist, Øcr, ultimatetorque Tult/twist, Øult, and failure torque Tf/twist, Øf,which could be directly obtained from the curves. Whilethe initial and cracked torsional stiffnesses were calcu-lated as the gradients before and after the first cracking

commenced, respectively. Torsional toughness, which isthe total energy absorbed by the specimens throughoutthe torsion testing, was calculated by the area under thetorque–twist curves. Finally, the crack resistance of theOPSFRC mixes was investigated by measuring thecrack width after the specimen failed using a digitalmicroscopic camera.

Results and discussion

Workability (slump)

The slump values of all the mixes are reported inTable 3. The comparison between the slump values ofboth control mixes, NWC-0 and OPSC-0, without anyfibers indicated that the NWC produced slightly higherworkability than the OPSC. However, Mehta &Monteiro stated that the slump values of LWC withinthe range of 50–75mm showed comparable flowabilityand compactability similar to NWC having a slumpvalue of 100–125mm.26 The slump values of bothNWC-0 and OPSC-0 were 100mm and 85mm, respect-ively. In addition, both mixes showed comparable andsatisfactory compaction and finishability.

It is a well-known fact that the addition of steelfibers into concrete significantly reduces the workabilityof the concrete. The addition of 0.25–1.00% steel fibersinto the OPSFRC mixes produced a remarkable reduc-tion of workability of about 30–70%, compared to thecontrol OPSC-0 mix (Table 3). The dispersion of thesteel fibers in the fresh concrete results in the formationof a cement matrix–fiber network structure, which leadsto an increase in concrete viscosity, and, eventually,reduces the workability of the concrete.3,27 Moreover,the increase in the amount of steel fibers in the freshOPSFRC mixes requires a higher amount of cementmortar to wrap around the fibers, which further reducesboth the concrete viscosity and workability. Therefore,this could explain that the increase of steel fibers from0.25% to 1.00% in the OPSCC-100 mix produced the

Figure 1. (a) Crushed granite aggregate and (b) OPS used as coarse aggregate.

Table 1. Comparison of the physical properties between OPS

and granite aggregate.

Physical property OPS

Granite

aggregate

Maximum size (mm) 15 19

Moisture content (%) 10.53 0.35

Fineness modulus 6.41 6.55

Compacted bulk density (kg/m3) 635 1545

Specific gravity 1.37 2.65

24-hour water absorption (%) 24.3 0.6

Aggregate impact value (%) 2.11 13.83

Yap et al. 3




lowest slump value. Nevertheless, all the OPSFRCmixes attained acceptable compaction with goodfinishing.

Density

The significant density reduction of LWC relative to theNWC is the key factor for the OPS to be selected as apotential substitution of conventional granite aggre-gate. Based on the densities shown in Figure 3, froman oven-dry density (ODD) of about 2310 kg/m3 foundin the NWC-0 mix, the OPSC-0 mix that contains100% replacement of coarse aggregate by OPS pro-duced a 20% ODD reduction to about 1830 kg/m3.

The reduced density reported in the OPSC could beadvantageous in reducing the dead load and cost inthe fabrication of the structural members.

However, it has been reported that the addition ofsteel fibers increases the density of LWC,3,6–8,28 andreduces the benefit of density reduction in the LWC.EN206-1 states that concrete having an oven-dry dens-ity (ODD) of between 800 and 2000 kg/m3 could bedefined as LWC. The ODD of the OPSFRC mixesreinforced with 0.25–1.00% steel fibers increased tothe range of 1890–2000 kg/m3 (Figure 3), which isattributed to the high specific gravity of the steelfibers (S. G.¼ 7.9). Although the densities of theOPSFRC mixes increased, the OPSFRC still produceda density reduction of about 14–18% corresponding tothe NWC mix and could be identified as LWC.

The ODD of the OPSC and OPSFRC showed astrong correlation between the volume fraction (Vf)and the ODD, as shown in Figure 3. Equation (3.1) isproposed to predict the ODD of OPSFRC with up to1% steel fibers, which could help in the selection of themaximum fiber content when a certain density isfavored.

ODD ¼ 1830e0:09ðVfÞ ð3:1Þ

where ODD and Vf are the oven-dry density (in kg/m3)and volume fraction of steel fibers (in %), respectively.

Mechanical properties

Compressive strength. The compressive strengths of allthe mixes are reported in Table 3. Both the controlmixes, NWC-0 and OPSC-0, were designed to yield asimilar compressive strength of about 35MPa for aclear comparison between the torsional strengths ofboth NWC and OPSC.

Previous studies on LWC have shown that the add-ition of steel fibers into LWC resulted in a significantenhancement in the compressive strength.4,7,29,30 The

Table 2. Mix proportions.

Mix

designations

Cement

(kg/m3)

Coarse aggregate

Mining sand

(kg/m3)

Water

(kg/m3)

Silica fume

(kg/m3)

Superplasticiser

(kg/m3)

Steel fiber

(%vol. concrete)

Granite

aggregate

(kg/m3)

OPS

(kg/m3)

NWC-0 320 810 0 1030 200 0 0 0

OPSC-0 530 0 320 970 170 53 0.35 0

OPSC-25 530 0 320 970 170 53 0.35 0.25

OPSC-50 530 0 320 970 170 53 0.35 0.50

OPSC-75 530 0 320 970 170 53 0.35 0.75

OPSC-100 530 0 320 970 170 53 0.35 1.00

Figure 2. (a) Torsion machine and (b) illustration for the tor-

sion test set-up.





improvement in the compressive strength of OPSFRCby the addition of steel fibers was evident as enhancedcompressive strengths were observed in the OPSFRCmixes, compared to the OPSC-0 mix. As the volume ofsteel fibers increases, the compressive strength of theOPSFRC mixes increased significantly. It could beseen that the addition of 0.25% and 1.00% of steelfibers produced an improvement in the compressivestrength of about 16% and 40% compared to the con-trol OPSC mix, respectively. The highest compressivestrength of about 47MPa was recorded in the OPSC-100 mix, which contained 1.00% steel fiber.

The enhancement mechanism of the steel fibers onthe improved compressive strength in the OPSFRCmixes could be explained by the crack bridgingeffect.4,28,31–34 Under an increasing compressive load-ing, the crack is initiated and will propagate along theweaker component of the concrete. The weak compo-nent in the OPSC is the poor aggregate–cement pastebond.35 When the crack is approaching the randomly

orientated steel fibers, the fiber bridging will result incrack closure and transfer the crack tip stress betweenthe fiber–matrix interfacial bonding, substantiallyimproving the strength of the concrete. Additionalforce is required to pull the fibers out from the frac-tured cement paste for the crack to continue to propa-gate and open up, which eventually enhances theultimate compressive capacity of the concrete.

Flexural strength. The low mechanical properties, espe-cially the tensile strength of LWC, have limited theapplication of LWC, including OPSC as a structuralmember. The low tensile strength of LWC would leadto significant tensile cracking occurring at a much lowerloading capacity compared to that of normal concrete.6

In this study, the flexural strength is reported as thetensile strength of both NWC and OPSC in Table 3.The comparison between the flexural strengths ofNWC-0 and OPSC-0 mixes showed that the OPSC pro-duced 27% lower flexural strength than the NWC, even

0

250

500

750

1,000

1,250

1,500

1,750

2,000

2,250

2,500

NWC-0 OPSC-0 OPSC-25 OPSC-50 OPSC-75 OPSC-100

Den

sity

(kg

/m3)

ODD = 1830e0.09(Vf)

R2

= 0.94

Figure 3. Densities of all the mixes.

Table 3. Mechanical properties of all the mixes.

Mix

Slump

(mm)

Compressive

strength (MPa)

Flexural

strength (MPa) BrittlenessaModulus of

elasticity (GPa)

Poisson’s

ratio

NWC-0 100 34.7 (0.67) 4.45 (0.08) 7.80 21.65 0.176

OPSC-0 80 33.9 (0.37) 3.26 (0.16) 10.40 13.87 0.244

OPSC-25 55 39.3 (0.39) 4.28 (0.10) 9.17 15.67 0.286

OPSC-50 50 41.2 (0.53) 5.38 (0.05) 7.66 16.11 0.294

OPSC-75 40 45.9 (0.80) 6.70 (0.14) 6.85 16.29 0.275

OPSC-100 25 47.3 (0.20) 8.16 (0.16) 5.80 15.50 0.292

Note: The standard deviations of the corresponding mechanical properties are shown in the brackets.aBrittleness¼ compressive to flexural strength ratio.

Yap et al. 5




though both mixes yielded similar compressivestrength. In OPSC, the weaker component is the weakadhesion between the OPS and cement paste9; thisstatement is further supplemented by Figure 4. Basedon Figure 4(a), under the flexural loading, the NWCspecimen failed under the bonding failure, while thefailure of the OPSC specimen is mainly due to thebreakdown of the aggregate-cement paste bond,which is attributed to the smooth surface of OPSalong with some crushing of OPS. Hence, the tensilecracking could occur at the aggregate–cement pasteinterfacial zone, which has a lower flexural capacity inthe OPSC mix, relative to the NWC mix.

The steel fibers were added to the OPSFRC mixes toimprove the tensile and torsional strengths of theOPSC. The contribution of the additional fiber–matrix interfacial bonding produced the increment inflexural strength of about 30–150% compared to thecontrol OPSC. The addition of 0.25% steel fibers inOPSC-25 mix was sufficient to produce competent ten-sile strength relative to that of NWC. A further incre-ment in the steel fiber volume up to 1% resulted in thehigher flexural strength of the OPSFRC mixes, whichfell within the range of 5.4–8.2MPa. The highest flex-ural strength of 8.2MPa was obtained for OPSC-100mix, which is 83% and 150% higher than the NWC-0and OPSC-0 mixes, respectively. Similar to the discus-sions in the section on Compressive strength, theenhancement of tensile strength in the OPSFRCmixes could be explained by the crack bridging effect.The crack bridging of steel fibers was evident inFigure 4(c).The presence of steel fibers bridged acrossthe cross section of flexural crack eventually allowedthe concrete to retain in one piece without significantspalling. However, the enhancement effect of steel fibersis more evident in the tensile strength than the compres-sive strength. This could be observed in the increasedflexural to compressive strength ratio when the volume

fraction of steel fibers increases. The flexural to com-pressive strength ratios of OPSC-0, OPSC-25, OPSC-50, OPSC-75, and OPSC-100 mixes are 10%, 11%,13%, 15%, and 17%, respectively. The increasingratios indicated that the improvement in tensilestrength is higher than the respective increase in com-pressive strength.

Brittleness. Another concern of LWC is its brittleness,which is attributed to the porous lightweight aggre-gate.36 One of the methods to measure the brittlenessis by the compressive to flexural strength ratio37;the brittleness ratios of all the mixes are shown inTable 3. A lower brittleness value indicates that theconcrete is less brittle. In comparison to the brittlenessof about 8 in the NWC-0 mix, the brittleness of OPSC-0is 10.4, which means that, relatively, it is 33% morebrittle than the NWC mix.

The published papers on OPSC focused on increas-ing the mechanical properties of OPSC by usingcrushed OPS5,35 and cementitious materials.38

However, although LWC becomes more brittle as thestrength increases,4 the addition of steel fibers intoLWC transforms the LWC from a brittle to a moreductile material.24,28 Hence, the addition of steel fibersserves as an innovative method to improve both themechanical properties and brittleness of OPSC. Thebrittleness ratios of the OPSFRC mixes reduced signifi-cantly as the fiber content increased from 0.25% to1.00%. For every 0.25% increment of steel fibervolume, the brittleness of the OPSFRC was reducedby 8–12%. The OPSC-100 mix reinforced with 1%steel fibers showed the lowest brittleness ratio amongall the mixes at 5.8 and the brittleness reductionwas 44% compared to the control OPSC. Concrete isgenerally brittle as the concrete will fracture andhas complete and immediate loss of load-carryingcapacity once the ultimate stress capacity is reached,

Figure 4. Cross-sections of failed specimens under flexural testing of (a) NWC-0, (b) OPSC-0, and (c) OPSC-100 mixes.





especially under tensile loading. The crack-bridgingeffect of steel fibers blunts the crack propagation andthe fibers serve as a ‘‘bridge’’ for the fiber-reinforcedconcrete to exhibit residual strength after the ultimatestrength is reached, eventually increasing the post-cracking behavior of concrete.31–33,39 Hence, the add-ition of steel fibers reduces the brittleness of OPSFRCsignificantly.

Modulus of elasticity and Poisson’s ratio. The modulus ofelasticity (MOE) and Poisson’s ratio of all the mixesare reported in Table 3. The MOE of OPSC-0 mixwas found to be only 13.87GPa, which was 64% ofthat of NWC-0. In addition, the OPSC produced aPoisson’s ratio of 0.244 while the Poisson’s ratio ofthe NWC was 0.176. The lower MOE and higher lateraldeformation observed in the OPSC could be attributedto the reduced stiffness in the OPSC, which produceshigher deflection compared to the NWC.19 In compari-son to the normal aggregate, the OPS has a lower stiff-ness and restraining effect. Hence, under compressiveloading, the OPS undergoes higher strain, which pro-duces a lower MOE than the NWC.40

As seen from Table 3, the addition of steel fibersenhanced both the MOE and Poisson’s ratio of theOPSC. The OPSFRC mixes showed an increase in theMOE and Poisson’s ratio within the ranges of 1.6–2.4GPa and 0.03–0.05, respectively. The crack-bridgingeffect of steel fibers induced higher stiffness, and, there-fore, the OPSFRC can undergo higher stress and strainunder loading.22 Figure 5 shows the stress–strain curvesof the OPSC and selected OPSFRC mixes up to theupper stress. The upper stress of concrete is taken asone-third of cylinder compressive strength as stated inASTM C469. From Figure 5, the addition of 0.25%steel fibers imparted 16% and 2% increments in theupper stress and strain, respectively. The improvementof the upper stress is higher than the upper strain andthis resulted in a higher modulus of elasticity. However,both the MOE and Poisson’s ratio of OPSFRC speci-mens were found to be independent of the volume frac-tions of the fiber. By referring to Figure 5, it is evidentthat the increase in the fiber content from 0.25% to1.00% produced 20% higher upper stress and straincompared to the OPSC-100 specimen. The equalincrease in both upper stress and strain resulted in theminiature change in the modulus of elasticity when fibervolume increases. Similar explanation could be appliedon the Poisson’s ratio, at which the increase in fibercontent improved almost equally on both lateral andlongitudinal strains.

LWC generally produces lower strength andmodulus of elasticity compared to the normal concrete.CEB/FIP provided equation (3.2) to correlate themodulus of elasticity of LWC made from natural and

manufactured lightweight aggregates as a function ofair-dry density and compressive strength.

MOE ¼�

2400

� �2�f1=3cu �A ð3:2Þ

where MOE¼modulus of elasticity, r¼ air-drydensity, fcu¼ compressive strength, and A is coefficientof 9.1. The study from Alengaram et al.40 showed thatequation (3.2) was applicable to predict the MOE ofOPSC with the coefficient A¼ 5. However, the additionof fibers of enhanced the MOE and hence equation(3.2) was modified to equation (3.3) for OPSFRC rein-forced with polypropylene and nylon fibers3

MOE ¼�ODD

2400

� �2�f1=3cu �A ð3:3Þ

where rODD¼oven-dry density. The coefficient A forpolypropylene and nylon fiber-reinforced OPSFRC was7.7 and 6.1, respectively. Based on the results fromTable 3, the MOE of OPSFRC showed strong correl-ation with equation (3.3) if A¼ 7 is used. Equation(3.3) can be used to predict the MOE of OPSFRCwith steel fibers up to 1% within the error of �10%.

Torsional strength

The torque–twist curves of all the mixes are shown inFigure 6, while Table 4 reports all the results derivedfrom Figure 6 including cracking torque, ultimatetorque, and failure torque with their correspondingtwist, initial and cracked torsional stiffness, and tor-sional toughness.

Comparison between NWC and OPSC. The comparisonbetween the torsional strengths of the NWC-0 andOPSC-0 mixes showed that while both control mixesproduced a similar torsional toughness of about 4Nm/m and achieved an ultimate torque at a twistvalue of about 0.025 (Table 4), the torque–twistcurves of both mixes showed diverse behavior(Figure 6). The specimen of the NWC-0 mix producedalmost twice the ultimate torsional strength of theOPSC-0 specimen, but it did not exhibit post crackingstrength, as the specimens failed once the ultimatetorque was achieved at a twist value of 0.025.Although the OPSC-0 only produced about 55% ofthe ultimate torque of the NWC, the OPSC was capableof sustaining torsion beyond its ultimate torsionalstrength by producing a twist at failure of 0.039. Inaddition, the torsional toughness of both the controlconcrete mixes was comparable to the published resultson the NWC of similar compressive strength.22

Yap et al. 7




In terms of stiffness, the NWC-0 and OPSC-0 mixesproduced an initial torsional stiffness of about 11 kNm2

and 10 kNm2, respectively. The torsional behavior ofconcrete is mainly linear until the first crackingtorque, while the initial stiffness is independent of thepresence and amount of reinforcement.24,33 In addition,the torsional failure of concrete members is initiatedby the tensile stress developed due to a state ofpure shear, which arises due to torsion.22 This indicatesthat the initial torsional stiffness is mainly contributedby the concrete’s tensile strength, and, therefore,the NWC-0 of slightly higher tensile strength producedan initial torsional stiffness of 10% higher than the

OPSC-0 mix. In addition, both mixes have nocrack stiffness, which indicated that both the controlspecimens achieved ultimate torque once the speci-mens cracked. This is attributed to the absence ofreinforcement within the control specimens. Once thespecimens cracked, the brittle concrete could not sus-tain higher torque, and, therefore, the control speci-mens cracked and reached ultimate torque at thesame time.

Effect of steel fibers on the torsional strength of

OPSFRC. Torsional strength and toughness: Theenhancement of steel fibers on the torsional strength

0

50

100

150

200

250

300

350

0.00 0.05 0.10 0.15 0.20 0.25 0.30

Tor

que

(Nm

)

Twist (rad/m)

NWC-0

OPSC-0

OPSC-25

OPSC-50

OPSC-75

OPSC-100

Figure 6. Torque–twist curves.

0

2

4

6

8

10

12

14

0.00 0.20 0.40 0.60 0.80 1.00

Stre

ss (

MPa

)

Strain (x 10-3)

OPSC-0

OPSC-25

OPSC-100

Figure 5. Stress versus strain curves for the calculation of modulus of elasticity.





of OPSFRC is evident from the increased torsionalstrength of the OPSFRC mixes compared to theOPSC mix (Figure 6). It was observed that the increasein the volume fraction of steel fibers resulted in highertorsional strength (ultimate torque) of OPSFRC. It hasbeen reported that the steel fibers significantlyimproved the flexural toughness of concrete.6,34,41

Moreover, the benefit of steel fibers is also obvious inthe torsional toughness of OPSFRC, as shown inFigure 7. The highest torsional toughness in theOPSC-100 mix reinforced with 1% steel fibers wasabout 47Nm/m, which was tenfold higher than the cor-responding control mix. In addition, for every incre-ment of 0.25% steel fiber volume in the OPSFRCmixes up to 0.75%, the torsional toughness improveddrastically by about 11–15Nm/m. The torsional tough-ness achieved in the OPSFRC was found to be higherthan the torsional toughness of grade-50 NWCreported by Rao and Seshu.22

Similar to the compressive and tensile strengths, theincreased torsional strength and toughness in theOPSFRC mixes is mainly attributed to the crack brid-ging effect at which the additional energy must be con-sumed at breaking and pulling out of the fibers from thecement paste, which leads to higher failure load andadds toughness to the concrete.6,33

Twist at failure: The high torsional toughnessobserved in the OPSFRC mixes is mainly attributedto the excellent post-cracking behaviors, as shown inFigure 6 and Table 4. The good post-cracking behaviorof the OPSFRC specimens is clear from the signifi-cantly improved twist at failure of up to 550% and900%, compared to the control OPSC-0 and NWC-0mixes, respectively. From 0.25% to 1.00% steel fibersvolume in the OPSFRC mixes, every increment in thesteel fiber content of 0.25% produced about 150%increase in the twist at failure. This explained that thesteel fibers effectively enhanced the ductility/twist at

R² = 0.98

0

10

20

30

40

50

60

0 0.25 0.5 0.75 1

Tor

sion

al to

ughn

ess

(Nm

/m)

Volume fraction of steel fiber, Vf (%)

Figure 7. Torsional toughness against volume fraction (Vf) of steel fibers in OPSFRC mixes.

Table 4. Torsional strength for all mixes.

Mix

Cracking torque Ultimate torque Failure torque Initial

torsional

stiffness

(kNm2)

Cracked

torsional

stiffness

(kNm2)

Torsional

toughness

(Nm/m)

Torque,

Tcr (Nm)

Twist, Øcr

(rad/m)

Torque,

Tult (Nm)

Twist, Øult

(rad/m)

Torque,

Tf (Nm)

Twist, Øf

(rad/m)

NWC-0 341.7 (1.6) 0.025 (0.004) 342.1 (2.3) 0.025 (0.002) 0 (0) 0.025 (0) 10.95 0 4.27

OPSC-0 190.8 (2.1) 0.024 (0.007) 191.2 (2.8) 0.024 (0.002) 93.4 (3.2) 0.039 (0.006) 10.04 0 4.42

OPSC-25 221.2 (2.4) 0.024 (0.002) 225.4 (1.6) 0.030 (0.006) 114.5 (2.6) 0.104 (0.011) 14.10 700 16.57

OPSC-50 248.6 (2.2) 0.016 (0.005) 254.8 (1.8) 0.020 (0.005) 60.9 (3.1) 0.177 (0.007) 14.98 1550 27.78

OPSC-75 293.0 (0.4) 0.026 (0.009) 298.1 (3.0) 0.029 (0.005) 70.3 (1.5) 0.236 (0.009) 15.51 1700 42.83

OPSC-100 294.1 (2.9) 0.025 (0.010) 304.4 (1.7) 0.029 (0.004) 67.7 (2.5) 0.253 (0.015) 14.26 2575 46.55

Note: The standard deviations are shown in brackets.

Yap et al. 9




failure of the OPSFRC mixes. The steel fibers not onlyenhanced the ductility of concrete under flex-ural6,33,34,42 and impact,8 but the present study alsoproved that the steel fibers improved the torsional duc-tility of the LWC. The role of steel fibers in enhancingthe ductility of OPSFRC is by the crack bridging effect.After the first crack commences, additional energy isrequired to pull the fibers out from the fracturecement matrix for the crack to open and to propagateuntil the fibers are pulled out or broken. This results inthe post-peak tensile stiffening behavior and eventuallyenables the concrete to exhibit residual strength andincreased ductility after the ultimate strength isachieved.33,43 Moreover, the advantage of steel fibersin producing the post-cracking characteristic in OPSCis more evident, as seen from the study of Khaloo andSharifian25 who found that the addition of steel fibers inNWC did not produce any post-cracking behavior forthe concrete. Therefore, we can conclude that the add-ition of steel fibers further enhanced the post-crackingductility of OPSC.

Torsional stiffness: The initial and cracked torsionalstiffness of all the mixes is reported in Table 4. It wasshown that the addition of steel fibers improved theinitial torsional stiffness of the OPSFRC specimens to14–15.5 kNm2, compared to the initial torsional stiff-ness of 10 kNm2 in the OPSC-0 mix. However, thefiber content had no significant effect on the initial tor-sional stiffness. Following the discussions on the modu-lus of elasticity of OPSFRC, the presence of steel fibersinduced higher stiffness in the concrete matrix, whichexplains the improvement of the initial torsional stiff-ness in the OPSFRC mixes.

However, the observation on the cracked torsionalstiffness of the OPSFRC mixes varied from the initialtorsional stiffness (Table 4). When the volume fractionof steel fibers increased from 0.25% to 1.00%, thecracked torsional stiffness increased considerably from700 kNm2 to 2575 kNm2. This indicated that the highersteel fiber content allows the OPSFRC to sustain highertorque and twist after the cracking torque before reach-ing the ultimate torsional capacity. After the crackingtorque, the reinforcing steel fibers take over and holdthe concrete together through the crack bridgingeffect.33 The bridging of the fibers over the crackenabled a higher stress to be taken by the concretebefore reaching the maximum torque.

Cracking resistance

Comparison between the NWC and OPSC. The failure pat-terns of all the specimens are shown in Figure 8. Thefailure patterns of all the specimens showed skew bend-ing failure. A primary torsion crack with an inclinationangle of about 43–48o from the edge initiated upon

reaching the cracking torque and propagated to allthe faces of the specimens. The failure mode wasfound to be comparable to the published literature.22,25

The comparison between both the control mixes showsthat the failure of the NWC-0 specimen was sudden andbrittle as the specimen broke into pieces once the ultim-ate torque was achieved, as seen from Figure 8(a). TheOPSC-0 specimen (Figure 8b) broke into two partsalong the primary torsion crack without any fragmentsunlike NWC-0. When the ultimate torque was reached,the crack width of the primary crack was found to beabout 5mm, and the post-cracking behaviors of OPSCenabled the concrete to sustain higher twist until itfailed at a twist at failure of 0.039. The observationfrom the failure plane of the OPSC showed that theOPSC failed due to aggregate–cement paste adhesionfailure and breakdown of the OPS. It has been reportedthat OPSC is weak in aggregate–paste bonding.35

Under torsion loading, the cracks commenced at theaggregate–cement paste interface and joined up toform a primary torsion cracking, and hence the OPSCspecimen failed at 45% lower ultimate torque relativeto the NWC specimen.

Effect of steel fibers on the crack resistance of OPSFRC. Thecracking resistance of the OPSFRC specimens signifi-cantly bettered NWC and OPSC, as shown in Figure 8.Similar to the OPSC, the failed OPSFRC specimenretained its original shape with no fragments produced(Figure 8c–f). The OPSFRC failed along a primary tor-sional crack with an inclination angle of about 43–48�

from the edge. The addition of steel fibers did not affectthe inclination angle. When a structural member failsunder extreme loading, the formation of fragments likethe NWC might cause a severe hazard to the buildingresidents. Hence, the OPSFRC mixes, which produceda smaller crack width, could provide a higher degree ofstructural safety, serviceability and durability, com-pared to the NWC.

The crack arresting ability of steel fibers in OPSFRCis also evident in Figure 9. In the comparison of theVf of the steel fibers in the OPSFRC mixes, OPSC-25failed by producing a larger primary torsion crack witha crack width of 7.121mm (Figure 9a), and the crackwidths grew narrower to 5.743mm, 4.625mm, and4.375mm in the OPSC-50, OPSC-75, and OPSC-100mixes, respectively (Figure 9b–d). The crack width ofOPSC-100 mix reduced considerably by about 40%relative to the OPSC-25 specimen, which showed theeffectiveness of the crack bridging effect of steel fibersin arresting the crack growth. In addition, the reducedcrack width of the primary torsion crack is coupledwith the formation of minor cracks (Figure 8f). Theformation of multiple fine cracks in fiber-reinforcedspecimens allows for a higher strain capacity33 and





Figure 9. Crack width at failure of (a) OPSC-25, (b) OPSC-50, (c) OPSC-75, and (d) OPSC-100 specimens.

Figure 8. Failed specimens of (a) NWC-0, (b) OPSC-0, (c) OPSC-25, (d) OPSC-50, (e) OPSC-75, and (f) OPSC-100 mixes.

Yap et al. 11




contributes to the highest twist at failure in theOPSC-100 specimen. Following the Figure 9, the fiberpull-out and fiber debonding were observed across theprimary torsion cracking. Both fiber pull-out andfiber debonding mechanisms contributed to the signifi-cant crack bridging toughening for the OPSFRC. Thebridging of the steel fibers along the crack increasedthe tensile stress that could be withstood, which isattributed to the increase in the fiber–matrix interfacialbond, and resulted in a substantially reducedcrack width and increase in the number of finecracks.13,33,39,41,43

Proposed torsional model for OPSC andOPSFRC

The discussion from the section on Torsional strengthshowed that the torsional strength of the OPSCwas lower than the NWC, but that the OPSC producedsimilar torsional toughness to the NWC, whichwas attributed to its post-cracking characteristic.The diversity between the torsional behavior of theNWC and OPSC prevents the proposed model forestimating the ultimate torsional strength of NWCand LWC22,24,25 being applied for the OPSC andOPSFRC. Therefore, based on the discussion from sec-tions Torsional strength and Cracking resistance, theimportant properties that distinguish OPSC andOPSFRC from NWC are the cracking and ultimatetorsional strengths and twist at failure. Equations(4.1) to (4.3) are proposed for the estimation of thetorsional behaviors of OPSFRC with steel fiber contentup to 1%.

Equations (4.1) and (4.2) are proposed to predict thecracking torque and ultimate torque based on the flex-ural strength of the OPSFRC, as the addition of steelfibers improved both torques. In addition, equation(4.3) predicts the twist at failure based on the high cor-relation between the twist at failure and Vf of steelfibers.

Tcr ¼ 106:2ffiffiffiffiffiffiffiffifflex

pðerror ¼ �6%Þ ð4:1Þ

Tult ¼ 110ffiffiffiffiffiffiffiffifflex

pðerror ¼ �8%Þ ð4:2Þ

where Tcr¼ cracking torque (in Nm); Tult¼ ultimatetorque (in Nm), and fflex¼ flexural strength (in MPa).

�f ¼ 0:224Vf þ 0:005 ðerror ¼ �8%Þ ð4:3Þ

Conclusion

This study investigated the torsional behaviors ofOPSC as a new LWC from locally available waste

material compared to the NWC, and examined theeffects of steel fibers in OPSFRC. The following con-clusions could be drawn:

(i) The addition of steel fibers in OPSFRC mixesreduced the slump values by 30–70% relative tothe control OPSC mix. Nevertheless, all the mixesattained good compaction and finishing.

(ii) Both the OPSC and OPSFRC produced ODDwithin the range of 1830–2000 kg/m3 and couldbe categorized as LWC.

(iii) The control OPSC showed comparable compres-sive and tensile strengths as that of the NWC-0mix. Both mixes produced close torsional tough-ness but showed different torsional characteristics:NWC showed brittle failure with no post-crackingbehaviors, while the OPSC produced post-crackingbehaviors with a twist at failure of 0.039 rad/m;however, its ultimate torque is only about 55%of that of NWC.

(iv) The addition of steel fibers up to 1% significantlyenhanced the mechanical properties of theOPSFRC mixes. The highest compressive and flex-ural strengths of 47MPa and 8.2MPa, respect-ively, were reported in the OPSC-100 mixreinforced with 1% steel fibers.

(v) The crack bridging effect of steel fibers improvedthe torsional strength, post-cracking twist, andcracking resistance of the OPSFRC mixes.Compared to the control OPSC mix, the highestincrements of about 1000% and 550% were pro-duced in the torsional toughness and twist at fail-ure of the OPSC-100 mix.

(vi) Compared to the crack width of 7.121 in OPSC-25mix, the crack width of OPSC-100 mix was reducedto 4.374mm along with the formation of multiplefine cracks. This explained that both the fiber pull-out and fiber debonding mechanisms contributedto the significant crack bridging toughening for theOPSFRC, and eventually improved the crackresistance of the OPSFRC specimens.

It can be concluded that the OPSFRC withimproved mechanical and torsional properties, relativeto the NWC, is highly recommended for torsion-resis-tant members.

Funding

The authors are highly grateful to the financial support under

(i) University of Malaya Research Grant (UMRG) RP018-2012B: Development of Geo-polymer Concrete for StructuralApplication.

Conflict of interest

None declared.





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