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EFFECT OF AGGREGATE QUALITY ON THEPROPERTIES OF CONCRETE

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M. Maslehuddin

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King Fahd University of Petroleum and Min

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Construction and Building Materials 17 (2003) 97103

0950-0618/03/$ - see front matter 2002 Elsevier Science Ltd. All rights reserved.PII: S 0950- 0618 0 2 .0 0 0 9 7 - 1

Effect of coarse aggregate quality on the mechanical properties of highstrength concrete

H. Beshr , A.A. Almusallam , M. Maslehuddin *b,1 a b,

Department of Civil Engineering, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabiaa

Research Institute, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabiab

Received 25 February 2001; received in revised form 15 June 2002; accepted 28 August 2002

Abstract

This paper reports results of a study conducted to evaluate the effect of four types of coarse aggregates, namely calcareous,dolomitic, quartzitic limestone, and steel slag, on the compressive and tensile strength, and elastic modulus of high strengthconcrete. The highest and lowest compressive strength was obtained in the concrete specimens prepared with steel slag andcalcareous limestone aggregates, respectively. Similarly, the split tensile strength of steel slag aggregate concrete was the highest,followed by that of dolomitic and quartzitic limestone aggregate concretes. The lowest split tensile strength was noted in thecalcareous limestone aggregate concrete. The type of coarse aggregate also influences the modulus of elasticity of concrete.Weaker aggregates tend to produce a more ductile concrete than stronger aggregates do. 2002 Elsevier Science Ltd. All rights reserved.

Keywords: Coarse aggregate; Compressive and tensile strengths; High strength concrete; Modulus of elasticity

1. Introduction

The term high strength concrete is used for concretewith a compressive strength in excess of 41 MPa, asdefined by the ACI Committee 363 w1x. Others definehigh strength concrete as that possessing a uniaxialcompressive strength greater than that which is ordinar-ily obtained in a region, because the maximum strengthof concrete which is currently being produced variesconsiderably from region to region w2x. Use of highstrength concrete leads to smaller cross-sections and

hence, the reduced dead load of a structure. This helpsengineers to build taller buildings and bridges withlonger spans.

In conventional concrete (compressive strength -41MPa), the properties of coarse aggregates seldombecome strength-limiting, because this type of concretemixtures typically correspond to a watercement ratio

*Corresponding author. Tel.: q966-3860-2853; fax: q966-3860-3996.

E-mail address:[email protected] (M. Maslehuddin).Former Graduate Student, King Fahd University of Petroleum &1

Minerals, Dhahran 31261, Saudi Arabia.

(wyc) in the range of 0.50.7. Within this range, theweakest components in concrete are the hardened cementpaste and the transition zone between the cement pasteand the coarse aggregate, rather than the coarse aggre-gate itselfw37x.Similarly, when designing conventionalconcrete mixtures, the mineralogy of coarse aggregateis rarely of concern unless the aggregate contains someconstituents, such as opal which is a reactive silicamineral that could have a deteriorating effect on concretedurability w8x, while for a high strength (compressivestrength )41 MPa) concrete, researchers have noted

that the hardened cement paste and the transition zoneare no longer strength-limiting, but it is the mineralogyand the strength of the coarse aggregate that control theultimate strength of concrete.

The importance of the mineralogical characteristicsof coarse aggregates on the quality of concrete has beenpointed out by Farran w9x, Maso w10x, Alexander andDavis w11x, Baalbaki et al. w12x, and Giaccio et al. w13x.The effect of the coarse aggregate on elastic propertiesof high performance concrete (HPC) was also pointedout by Aitcin and Mehta w8x, Baalbaki et al. w12x, andAitcin w14x. These authors observed some significant
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98 H. Beshr et al. / Construction and Building Materials 17 (2003) 97103

Table 1Chemical composition of Portland cement

Constituent Wt.%

SiO2 19.92Al O2 3 6.54Fe O2 3 2.09

CaO 64.70MgO 1.84SO3 2.61K O2 0.56Na O2 0.28L.O.I. 0.73C S3 55.90C S2 19.00C A3 7.50C AF4 9.80

Table 3Grading of coarse aggregate

Sieve Percentageopening passing(mm)

19.00 100

12.50 909.50 304.75 102.36 0

Table 2Absorption and specific gravity of coarse aggregates

Type of aggregate Absorption Bulk (%) specific

gravity

Calcareous limestone 4.95 2.39Dolomitic limestone 2.20 2.54Quartzatic limestone 1.60 2.70Steel slag 0.85 3.51

Table 4Clay lumps in coarse aggregates

Source of aggregates Clay ASTM Clumps 142 limit(%) (%)

Calcareous limestone 0.78 1Dolomitic limestone 0.65Quartzitic limestone 0.41Steel slag 0.12

differences in the elastic modulus and hysteresis loop inthe case of HPCs prepared with different coarse aggre-

gates, but with the same water-to-cement ratio w8,14x.Aitcin et al.w15xinvestigated the effect of three differentcoarse aggregates in superplasticized concrete mixtureswith identical materials and properties (wycs0.24).They found that for a calcareous-limestone aggregate(85% calcite), a dolomitic-limestone aggregate (80%dolomite), and a quartzitic-gravel aggregate containingschist, the 91 day compressive strengths were 93, 103and 83 MPa, respectively. Moreover, they concludedthat the aggregatecement paste bond was stronger inthe limestone aggregate concrete than in the gravelconcrete due to the interfacial reaction effect w15x.Zhang

and Gjorv w16x investigated the effect of four coarseaggregate types, available in north California, on thecompressive strength and elastic behavior of a very highstrength concrete mixture. Based on their study, somesignificant differences in the elastic moduli and hyster-esis loop were observed. A formula was derived byChang and Su w17xfrom the theory of granular mechan-ics to estimate the compressive strength of coarseaggregate to be used in high strength concrete. Thisstudy w17x showed that there is a good correlationbetween the compressive strength of aggregates andsome of the engineering properties of concrete.

With increasing use of high strength concrete (HSC)

as a structural material, further information on itsmechanical properties is required, especially its stress

strain characteristics. Some studies w1,18,19x conductedto date have generally indicated that the empiricalrelationships between compressive strength and otherproperties, such as tensile strength, flexural strength,and modulus of elasticity, established for normal con-cretes cannot always be utilized for high strength con-cretes. Generally, it is noted that cracking of HSC is

more localized and approaches a homogeneous materialbehavior as compared to normal strength concrete. Also,HSC exhibits more linear elastic behavior and is morebrittle than normal strength concrete. Therefore, it isessential to develop data on the mechanical propertiesof hardened concrete, particularly its strength, in relationto the properties of the aggregates.

This paper reports the results of a study conducted toinvestigate the influence of coarse aggregate quality onthe compressive and split tensile strength and elasticproperties of high strength concrete.

2. Experimental program

2.1. Materials and test specimens

ASTM C150 Type I Portland cement, with the chem-ical composition shown in Table 1, was utilized inpreparing the concrete specimens. Four types of coarseaggregates, namely calcareous limestone (CC), dolomit-ic limestone (DL), quartzitic limestone (QZ), and steelslag(SS), were utilized to prepare the concrete mixtures.The physical properties of the coarse aggregates selectedfor this study and their grading are shown in Tables 2

and 3, respectively, while the quantity of clay lumps inthe selected coarse aggregates are shown in Table 4.
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99H. Beshr et al. / Construction and Building Materials 17 (2003) 97103

Fig. 1. Experimental setup utilized to evaluate the stress-strain characteristics of concrete.

Same aggregate grading was used in all the concretemixtures. Dune sand with a bulk specific gravity of 2.54and water absorption of 0.65% was used as fine aggre-gate. All the concrete mixtures were prepared with awater-to-cement ratio of 0.35 and a cement content of450 kgym , and a coarse aggregate to fine aggregate3

ratio of 1.63. A naphthalene-based superplasticizer wasused to improve the workability of the concrete mixtures.

Cylindrical concrete specimens, 75 mm in diameterand 150 mm high, were prepared for determining the

compressive strength, split tensile strength, and modulusof elasticity. Mixing was done in a revolving drum typemixer for approximately 3 5 min to obtain uniformconsistency. After mixing, the concrete was filled in thecylindrical moulds in two layers and consolidated on avibrating table to remove entrapped air. After casting,the specimens were covered with a wet burlap and curedin the laboratory at a temperature of 20"2 8C for 24 hprior to demolding and then cured under calciumhydroxide solution till the time of test.

2.2. Test techniques

2.2.1. Compressive strength

The compressive strength of concrete prepared usingthe selected coarse aggregates was determined by load-ing the specimens in uniaxial compression according toASTM C 39 at a constant loading rate of 3.3 kNysusing a servo-controlled hydraulic testing machine of3000 kN capacity. The compressive strength was deter-mined after 3, 7, 14, 28 and 180 days of curing.

2.2.2. Split tensile strength

The tensile strength of concrete can be experimentallydetermined by w20x: (1) uniaxial tensile test; (2) split

cylinder test; and (3) beam test in flexure. The firstmethod of obtaining the tensile strength is referred to asa direct test for determining the tensile strength, whilethe second and third methods are indirect tests. Theindirect method of applying tension in the form ofsplitting was suggested by Fernando Cerneiro, a Brazil-ian engineer w20x, and the test is often referred to as theBrazilian test, although it was also developed indepen-dently in Japan.

In this study, the split tensile strength of concrete

specimens prepared with the selected coarse aggregateswas determined according to ASTM C 496.

2.2.3. Modulus of elasticity

The static modulus of elasticity was determinedaccording to ASTM C 469. The specimens were testedin uniaxial compression at a constant rate of loading ofapproximately 3.3 kNys using a servo-controlled hydrau-lic testing machine. A portable data logger was used torecord the load and strain readings. Longitudinal strainswere measured using a compressometer that was fixedparallel to the direction of the applied load. The com-pressometer was centered at mid-height of the specimen

and the compressive deformation was measured usingtwo LVDTs located at diametrically opposite locationson the surface of the specimen. Fig. 1 shows theexperimental set up utilized to evaluate the stressstraincharacteristics of the concrete specimens prepared usingthe selected aggregates.

3. Results and discussion

3.1. Effect of aggregate quality on compressive strength

of concrete

The effect of aggregate quality on the compressivestrength of concrete was evaluated by testing concrete
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Table 5Loss on abrasion in the coarse aggregates

Type of aggregate Loss onabrasion(%)

Calcareous limestone 34.4

Dolomitic limestone 24.2Quartzitic limestone 19.2Steel slag 11.6

Fig. 2. Compressive strength development of concrete specimens prepared with the selected aggregates.

specimens prepared with CC, DL, QZ and SS aggre-gates.Fig. 2shows the variation of compressive strengthwith age for the concrete specimens prepared with thefour types of aggregates selected for this study. Asexpected, the compressive strength increased with agein all the concrete specimens. Further, the data in Fig.

2 indicate that the type of coarse aggregate has asignificant effect on the compressive strength of con-crete. The highest compressive strength was measuredin the concrete specimens prepared with the steel slagaggregates while the lowest compressive strength wasnoted in the concrete specimens prepared with calcare-ous limestone aggregates. After 28 days of curing, thecompressive strength of concrete specimens preparedwith calcareous, dolomitic, and quartzitic limestone andsteel slag aggregates was 43, 45, 47 and 54 MPa,respectively.

The data developed in this study indicate that in a

high strength concrete, i.e. concrete prepared with a lowwater-to-cement ratio and a high cement content, thecompressive strength is dependent on the quality ofcoarse aggregate. In such concrete, the bulk of thecompressive load is borne by the aggregates rather thanthe cement paste alone. The failure in such concretes isoften through the aggregates. Since the calcareous lime-stone aggregates are known to be weaker than the

dolomitic and quartzitic limestone aggregates the lowload carrying capacity of concrete prepared with calcar-eous limestone aggregates is understandable. Similarly,the dolomitic limestone aggregates are weaker than thequartzitic limestone aggregates, as such marginally lowercompressive strength of concrete prepared with formercoarse aggregates compared to that prepared with thelatter coarse aggregates is comprehensible.

The data on loss on abrasion summarized in Table 5of these aggregates provide ample evidence of the weaknature of the calcareous and dolomitic limestone aggre-gates compared to the quartzitic limestone and steel slagaggregates. The compressive strength of steel slag aggre-gate concrete was more than that of the quartziticlimestone aggregate concrete. This indicates that this
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Fig. 3. Split tensile strength of concrete specimens prepared with theselected aggregates.

aggregate is very suitable for producing high strengthconcrete.

When concrete is subjected to compressive loads,failure takes place at one or more of the followinglocations: (i) within the paste matrix; (ii) at the pasteaggregate interface; or (iii) within the aggregate. In a

rich concrete mix, as the one utilized in this study, thepossibility of failure within the paste matrix, alone, isvery rare, since this phase is very strong. Therefore, thefailure plain has to pass through the paste aggregateinterface or through the aggregate. In both modes offailure, the quality of aggregate significantly influencesthe mode of failure of concrete under compression.

Another point to be noted is that the porous aggre-gates, such as calcareous and dolomitic limestone aggre-gates, are capable of absorbing a significant amount ofwater. Hence, the cement aggregate bond in such aggre-gates is better than that of quartzitic limestone and steel

slag aggregates. Therefore, the failure of concretes pre-pared with the calcareous and dolomitic limestone aggre-gates is presumed to be within the aggregates since theinterface in these concrete specimens is strong due to agood bond between the aggregate and the cement paste.Such a failure was noted in the concrete specimens onvisual inspection after testing in compression. Highercompressive strength of steel slag and quartzitic lime-stone aggregate concretes compared with calcareous anddolomitic limestone aggregate concretes may thus beattributed to the quality of the aggregates. As shown inTable 5,the loss on abrasion of calcareous and dolomiticlimestone aggregates was more than that in the quartzitic

limestone and steel slag aggregates. The higher strengthof quartzitic limestone and steel slag aggregates, con-tributes to increased compressive strength of concreteprepared with these aggregates.

According to Wasserman and Bentur w21x, the physi-cal process that occurs at an early age is governed bythe absorption of water into the aggregate. Higherabsorption eliminates the accumulation of water in thefresh matrix in the vicinity of the aggregate; as a result,the interfacial transition zone in the aggregates withhigher absorption is denser. Aitcin and Mehta w8x notedthat in a high-strength concrete the hardened cement

paste and the transition zone are no longer strengthlimiting. On the other hand, the mineralogy and thestrength of coarse aggregates may control the ultimatestrength of concrete, particularly in a high strengthconcrete.

Baalbaki et al.w12x investigated the influence of threedifferent types of crushed coarse aggregates (dolomiticlimestone, quartzite, and sandstone) on the elastic prop-erties of high strength concrete. They found that thehighest and lowest compressive strength was of thesandstone and quartz aggregate concretes, respectivelyw12x. The low compressive strength of quartz concretemay be explained by the relative incompressibility of

the aggregates that improves the rigidity of concrete anddecreases the strength. On the other hand, the capacityof the sandstone aggregate to deform in one directionallows the stress to be more uniformly distributed;thereby, the whole section contributes to strength, asopposed to the rigid aggregates that tend to concentratestresses on certain regions of the section, therefore

creating premature ruptures w12x.The average rate of strength development, as shown

in Fig. 2, was higher at early ages, where the ratio of728 days strengths, ranged from 0.74 to 0.80, com-pared with 0.73 to 0.82 reported by Berke et al. w22x.Also, the ratio of 28180 days strength was in the range0.880.93 compared with 0.850.95 reported by Berkeet al. w22x. This observation is in good agreement withthat reported by Carette and Malhotra w23x and also bySlate et al. w18x. According to Carrasquillo et al. w24x,the ratio of 728 days strengths is between 0.60 and0.65 in the case of normal strength concrete. The higherrate of strength gain of HSC compared to normalstrength concrete is attributed to the high internal curingtemperature developed during hydration w18x. The highheat of hydration is associated with the high cementcontent in HSC mixes. Differences in the rate of strengthgain, however, becomes negligible at later ages w18x.

3.2. Effect of aggregate quality on split tensile strength

of concrete

The split tensile strength was determined after 14, 28and 90 days of curing. Fig. 3 shows the split tensilestrength of concrete specimens prepared with the four
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Fig. 4. Modulus of elasticity of the concrete specimens prepared withthe selected aggregates.

types of aggregates investigated in this study. The splittensile strength increased with age in all the concretespecimens. As can be seen in Fig. 3 the split tensilestrength of steel slag aggregate concrete was the highest,followed by that of quartzitic and dolomitic limestoneaggregate concretes. The least split tensile strength wasnoted in the concrete specimens prepared with calcare-ous limestone aggregates. However, the split tensilestrength of concrete specimens prepared with quartzitic

and dolomitic limestone aggregates was almost similarafter 28 days of curing. After this age, the split tensilestrength of steel slag aggregate concrete was 164% ofthat of calcareous limestone aggregate concrete. Simi-larly, the split tensile strength of concrete prepared withthe dolomitic and quartzitic limestone aggregates was129 and 130%, respectively, of that prepared with thecalcareous limestone aggregates.

3.3. Effect of aggregate quality on the static modulus of

elasticity of concrete

Fig. 4 shows the measured values of the modulus ofelasticity of concrete specimens prepared with the select-ed coarse aggregates. These data indicate that the typeof coarse aggregate has a significant effect on themodulus of elasticity of concrete. After 28 days ofcuring, the modulus of elasticity of calcareous, dolomiticand quartzitic limestone and steel slag aggregate con-cretes was 21.6, 24.5, 28.8 and 29.6 GPa, respectively.The modulus of elasticity of steel slag aggregate con-cretes was the highest, while that of calcareous limestoneaggregate concrete was the lowest. The lower values ofmodulus of elasticity of calcareous limestone aggregateconcrete may be attributed to the soft nature of these

aggregates. A more ductile failure results in the concretespecimens prepared with these aggregates.

The importance of coarse aggregate quality on elasticproperties of high strength concrete was also pointed byAitcin w14x, and Aitcin and Mehta w8x. Some significantdifferences in the elastic modulus and hysteresis loop

were noted in high strength concrete prepared withdifferent coarse aggregates. Baalbaki et al. w12x alsoevaluated the effect of coarse aggregates on the elasticproperties of high strength concrete. The elastic modulusof high strength concrete was noted to be stronglyinfluenced by the elastic properties of coarse aggregatesw12x. Similarly, Giaccio et al. w13xnoted that the highestmodulus of elasticity of concrete was achieved in basal-tic-HSC, followed by limestone-HSC and granitic-HSC.They w13x stated that this could be attributed to a higherpercentage of microcracking in limestone-HSC duringthe first loading applied to measure the modulus of

elasticity.As is apparent from the data in Fig. 4, the staticmodulus of elasticity was higher for the concretescontaining steel slag aggregates and quartzatic limestoneaggregate concretes, than for concrete prepared withcalcareous and dolomitic limestone aggregates. For high-er compressive strength concretes, the modulus of elas-ticity was higher because of the mortar stiffness andimproved mortaraggregate bond. Hooton w25x, Lutherand Hanse w26x,and Khatri and Sirivivatnanon w27xalsofound that elastic modulus is primarily a function ofcompressive strength. Based on the previous discussion,it can be concluded that the effect of the type of coarse

aggregate is more significant on the modulus of elasticityas compared to the compressive strength. According toAitcin and Mehta w8x, and Baalbaki et al. w12x, thenature of coarse aggregate significantly affects the mod-ulus of elasticity of high strength concrete. This influ-ence was attributed to the highly dense paste structureand pasteaggregate bond, which causes the concrete tobehave like a composite material. Therefore, aggregatecharacteristics could be important in determining theelastic properties of high strength concrete.

4. Conclusions

The quality of coarse aggregate has a significanteffect on the compressive strength of high strengthconcrete. The compressive strength of steel slag aggre-gate concrete was more than that of crushed limestoneaggregate concrete. The compressive strength of con-crete prepared with calcareous limestone aggregate wasthe least. These data indicate that in a high strengthconcrete, i.e. concrete prepared using a low watercement ratio and high cement content, the compressivestrength is dependent on the quality of aggregate. Insuch concrete, the bulk of the compressive load is borneby the aggregate rather than the cement paste alone.
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The failure in such concretes is often through theaggregates. Since the calcareous limestone is known tobe weaker than the dolomitic and quartzitic limestoneaggregates, its low load carrying capacity isunderstandable.

Split tensile strength increased with age in all the

concrete specimens. The type of aggregate also influ-ences the split tensile strength of concrete. The splittensile strength of steel slag aggregate concrete wasmore than that of limestone aggregate concrete. Theleast split tensile strength was noted in the calcareouslimestone aggregate concretes.

The quality of coarse aggregate also influences themodulus of elasticity of concrete. Weaker aggregatestend to produce a more ductile concrete than strongaggregates do.

Acknowledgments

The authors acknowledge the support provided by theDepartment of Civil Engineering and the ResearchInstitute at King Fahd University of Petroleum andMinerals, Dhahran, Saudi Arabia.

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