In this paper, compressive and splitting tensile strengths, modulus of elasticity and
steel rebar-concrete bond strength of lightweight aggregate concretes (LWAC) produced at
0.40 w/c with sand and mixture of crushed stone and lightweight fly ash coarse aggregates are
presented. Lightweight fly ash aggregates (LWA), plain (LC) and reinforced with 0.1% (L1P)
and 0.5% (L5P) crumb rubber and0.1% (L1F) polypropylene fiber are utilized. Test results
revealed that the compressive strength of all LWAC conformed to the limitation for structural
use whereas a reduction in density up to 20% was achieved. Whereas the mechanical
properties decreased with the use of lightweight aggregates, some increases were observed for
the steel-concrete bond strength

Yıldırım, Hasan

Özturan, Turan

Epoka University

2nd International Balkans Conference on Challenges of Civil Engineering, BCCCE, 23-25 May 2013, Epoka University, Tirana, Albania.612Mechanical properties of lightweight concrete made with cold bondedfly ash pelletsHasan Yıldırım1, Turan Özturan11Department of Civil Engineering, Bogazici University, TurkeyABSTRACTIn this paper, compressive and splitting tensile strengths, modulus of elasticity andsteel rebar-concrete bond strength of lightweight aggregate concretes (LWAC) produced at0.40 w/c with sand and mixture of crushed stone and lightweight fly ash coarse aggregates arepresented. Lightweight fly ash aggregates (LWA), plain (LC) and reinforced with 0.1% (L1P)and 0.5% (L5P) crumb rubber and0.1% (L1F) polypropylene fiber are utilized. Test resultsrevealed that the compressive strength of all LWAC conformed to the limitation for structuraluse whereas a reduction in density up to 20% was achieved. Whereas the mechanicalproperties decreased with the use of lightweight aggregates, some increases were observed forthe steel-concrete bond strength.INTRODUCTIONThe use of fly ash as cement replacement in concrete and the large scale utilization offly ash for producing artificial lightweight aggregates remarkably reduces the emission ofgreenhouse gasses and rapid depletion of the natural aggregate resources. Besides, LWACproduced with LWA provides several advantages. Structural lightweight concrete is usuallydefined to have oven-dry density of 1680-1920 kg/m3[1] and the type and properties of thelightweight aggregates determine the density, strength and durability characteristics ofLWAC. Two methods namely sintering and cold bonding have been used practically toproduce lightweight fly ash aggregate [2,3].Cold bonding as a method of lightweightaggregate production is considered to be more economical than sintering which is an energyintensive process. However, cold bonding depends on the pozzolanic reactivity of the fly ashand it usually results in lower strength aggregates. Gesoğlu [4] and Gesoğluet. al [5-7]produced cold-bonded lightweight fly ash aggregates with specific gravities in the range of1.72 and 1.80. Water absorption of LWA was 27% by weight which was decreased to 3.0 and18% by treating the surface of pellets with water glass and cement silica fume slurry,respectively. LWCs produced with water glass treated LWAs gave rise to higher compressiveand splitting tensile strengths and modulus of elasticity. LWCs with untreated LWA had thelowest mechanical properties. Kockal [8] and Kockal and Ozturan [9-12] investigated thephysical and mechanical properties of LWCs made with sintered lightweight fly ashaggregates containing glass powder(LWGC)and bentonite (LWBC) as well as cold-bondedlightweight fly ash aggregates (LWCC).Air content and density of fresh LWCC, LWBC,LWGC were 3.9, 4.3, 4.1% and 1991, 1960, 1975 kg/m3, respectively. Compressive strength,splitting tensile strength and modulus of elasticity of LWCC, LWBC, LWGC were 42.3, 53.5,2nd International Balkans Conference on Challenges of Civil Engineering, BCCCE, 23-25 May 2013, Epoka University, Tirana, Albania.61355.8 MPa and 3.7, 4.8, 4.9 MPa and 19.6, 26.0, 25.7 GPa, respectively. Mor [13] observedperfect bond-slip behavior in high strength lightweight concretes due to better coherencebetween LWA and the cement paste.EXPERIMENTAL STUDYMaterialsTwo types of CEM I 42.5 R portland cement with the same specific gravity of  3.14g/cm3 and specific surface of 3910 and 3770 cm2/g were used for the production oflightweight aggregates and concrete, respectively. An F type fly ash from Çatalağzı ThermalPower Plant, Zonguldak, Turkey with specific gravity of 2.06 g/cm3 andspecific surface of2880 cm2/g was usedin the production of lightweight aggregate.Polypropylene fiber (PP) of12 mm length and 32µ diameter having a tensile strength of 250 MPa and crumb rubberwith amaximum particle size of 4 mm were also used in the production of lightweightaggregates.Natural and crushed sand (0-4 mm)with specific gravities of 2.65 and 2.70 g/cm3,respectively, and crushed stone No-1 (4-8 mm) and No-2 (8-16 mm) withspecific gravityof2.70 g/cm3as well as cold bonded lightweight fly ash (4-16 mm) were used in concreteproduction. A naphthalin sulfonated formaldehyde superplasticizer (SP) was used to providethe required workability of fresh concrete.Deformed steel bars (S420) with a diameter of 12mm were used to determine the bond strength through pull-out test.Preparation of Lightweight Coarse AggregatesLightweight fly ash pellets were produced through the cold-bonding agglomerationprocess by a pelletization disc shown in Figure 1.The pelletizer disc has diameter of 40 cmand height of 15 cm. To produce plain and fiber reinforced lightweight pellets, dry fly ash-cement mixtures were fed into the disc with an inclination of 43˚ and rotated at 45 rpmaccording to the results of Baykal and Döven [2].In the next step, water was sprayed onto thepowder mixtures during the first 10 minutes of the agglomeration process at an amount of 23-27 % of the total weight of material to get the spherical pellets. Extra 10 minutes wasallocated to further compaction of the fresh pellets to increase their strengths. Lightweightaggregates were produced with a cement-to-fly ash ratio of 0.1 by weight. Polypropylenefibers (PP) and crumb rubber were added to the dry fly ash-cement mixture of plain/controllightweight aggregates (LC) at 0.1% (L1F) as well as 0.1% (L1P) and 0.5% (L5P),respectively, to produce fiber reinforced fly ash pellets. Fresh pellets were preserved in plasticbags and left for the final hardening inside a curing room at a temperature of 20 ˚C and 80 %RH for 28 days.Tests and Measurements on Lightweight Fly Ash Aggregate GrainsAfter the curing period the fly ash pellets were sieved into 4-8mm and 8-16mm sizefractionsto be used as replacement of coarse aggregates No-1 and No-2, respectively. Afterdetermining the specific gravity, unit weight and water absorption, the crushing strength ofthe lightweight aggregates were determined on individual pellets that were placed one by onebetween two parallel loading plates (Figure 2) by Eq.(1).2nd International Balkans Conference on Challenges of Civil Engineering, BCCCE, 23-25 May 2013, Epoka University, Tirana, Albania.614(1)where P is the failure load and X is the distance between loading points[9]. Fortyrandomlychosen pellets with an average diameter of 12 mm were tested in order to calculatethe average crushing strength for each type of lightweight fly ash aggregates.Figure 1. (a) Pelletizer and (b) A schematic view of the disc.Figure 2.Aggregate crushing strength configuration.Concrete Mix DesignControl concrete mixture was produced using 450 kg/m3 portland cement, 228 kg/m3and 570 kg/m3 natural and crushed sand, respectively, 490 kg/m3 No I and No II crushed stoneeach and 180 kg/m3 water as well as sufficient amount of superplasticizer. Lightweightconcretes were produced by replacing the No I and No II crushed stone coarse aggregates byLC, L1P, L5P, and L1F lightweight fly ash coarse aggregates on volume basis as given inTable 1.Casting and Curing of Concrete SpecimensA special procedure was applied for concrete casting to minimize the slump lossowing to the high water absorption of the lightweight aggregates. For this purpose,lightweight aggregates were first submerged in water for 24 hours to be saturated and thenkept on large scale sieves for 1 hour to be surface dried before mixing. Slump, unit weight andair content of fresh concrete were measured. Three cylinder moulds of 100*200mm size werecast for testing the mechanical properties. After casting, the specimens were left in laboratoryenvironment for 24 hours. After demouldingcylinder specimenswere stored in the curingroom at a temperature of 20 ˚C and 85 % relative humidity for 56 days before testing forcompressive, splitting tensile and steel-concrete bond strengths as well as modulus ofelasticity.2nd International Balkans Conference on Challenges of Civil Engineering, BCCCE, 23-25 May 2013, Epoka University, Tirana, Albania.615TEST RESULTS AND EVALUATIONProperties of Lightweight Fly Ash AggregatesUnit weight, specific gravity, water absorption and crushing strength of LWAs arepresented in Table 2. Water absorption of LWAs in the size range of 8-16 mm are lower thanthat of LWAs in 4-8 mm size. When the size of the grain grows, the pellets become denserwith less and mostly closed pores resulting in decreased absorption which was also observedby Koçkal and Özturan [10] for cold bonded fly ash aggregates with the same fly ash butdifferent portland cement.Crumb rubber addition increased crushing strength of pellets from3.78 MPa to 4.19 and 3.90 MPa, which decreased with increasing rubber content. Averagecrushing strength of L1F pellets, on the other hand, was slightly lower than that of LC.Fresh Concrete PropertiesConcrete mixes were cast by using a high range water reducing agent at requireddosage for achieving a slump of 14±2 cm. Lightweight aggregate concretes were workableand cohesive and segregation was not observed. Fresh concrete properties are presented inTable 3. When LWAs replaced the normal weight aggregate at 50% volume fraction ofcoarse aggregate, unit weight decreased from 2425 kg/m3 to 2170 kg/m3 and air contentincreased from 1.10% to 2.10%. However, when natural coarse aggregate was fullyreplaced with lightweight fly ash aggregates, there was a significant reduction in thedensity of fresh concretes till the value of 1975 kg/m3.2nd International Balkans Conference on Challenges of Civil Engineering, BCCCE, 23-25 May 2013, Epoka University, Tirana, Albania.616Compressive Strength and Modulus of ElasticityCompressive strength and modulus of elasticity of concretes presented in Table 4varied between 49.0 and 29.8 MPa and 25.7 and 19.7 GPa, respectively, when crushed stonewas replaced with LWA. Similar results were obtained by Gesoglu et al. [7] as compressivestrength and modulus of elasticity decreased from 40.1 to 29.1 MPa and 22.4 to 18.2 GPa,respectively when cold bonded lightweight fly ash aggregate content was increased inconcrete with w/c of 0.35.Variation of compressive strength and modulus of elasticity with the type oflightweight fly ash coarse aggregate are shown in Figures 3 and 4, respectively. Among thelightweight concretes, higher compressive strength was obtained with the pellets containing0.1% crumb rubber. Increase in crumb rubber content caused a decrease in strength. With theuse of polypropylene fibers in the fly ash pellets, higher strength was obtained with coarserpart of lightweight aggregates. When all coarse aggregate was replaced by all types oflightweight aggregate, the compressive strength and modulus of elasticity were the lowest.Modulus of elasticity was not affected much by the type of LWA where as the lowest valueswere obtained when all coarse aggregate was replaced by LWA.Splitting Tensile StrengthSplitting tensile strength of concretes given in Table 4 changed between 4.71 and 2.82MPa when crushed stone coarse aggregate was replaced with LWA. Figure 5 shows thevariation of the splitting tensile strength by the type of coarse aggregate. Concrete mixturesconsisted of both LWA and crushed stone coarse aggregate in equal volume fractions hadlower splitting tensile strength compared to the concretes produced with normal weight coarseaggregate. The use of LWAs as a substitute of all crushed stone coarse aggregate gave rise toa considerable decrease in splitting tensile strength. Lightweight concretes with 0.5% crumb2nd International Balkans Conference on Challenges of Civil Engineering, BCCCE, 23-25 May 2013, Epoka University, Tirana, Albania.617rubber and 0.1% polypropylene fibers showed higher tensile strength when LWA replaced 8-16mm fraction of the coarse aggregate.00.511.522.53 x 104CCSt(4-16)LC(4-8)CSt(8-16)L1P(4-8)CSt(8-16)L5P(4-8)CSt(8-16)L1F(4-8)CSt(8-16)LC(8-16)CSt(4-8)L1P(8-16)CSt(4-8)L5P(8-16)CSt(4-8)L1F(8-16)CSt(4-8)LC(4-16)L1P(4-16)L5P(4-16)L1F(4-16)E (MPa)Production PlanFigure 4.Variation of elastic modulus with the type of coarse aggregate.00.511.522.533.544.55CCSt(4-16)LC(4-8)CSt(8-16)L1P(4-8)CSt(8-16)L5P(4-8)CSt(8-16)L1F(4-8)CSt(8-16)LC(8-16)CSt(4-8)L1P(8-16)CSt(4-8)L5P(8-16)CSt(4-8)L1F(8-16)CSt(4-8)LC(4-16)L1P(4-16)L5P(4-16)L1F(4-16)Splitting Tensile Strength (MPa)Production PlanFigure 5.Variation of splitting tensile strength with the type of coarse aggregate.Steel-Concrete Bond StrengthPull-out tests were conducted at proper loading speed until the maximum load wasreached where the specimens of all concretes split without yielding of steel bars. Steel bar-concrete bond strength of concretes given in Table 4 varied from 11.44 MPa to 8.52 MPa withthe use of LWA as similarly reported by Gesoğlu [4] that a gradual decrease in ultimate bondstrength was observed with the increase in volume percent of cold bonded lightweight fly ashaggregate. Figure 6 shows the variation of bond strength with the type of lightweightaggregate. When LWA and crushed stone were used together in equal parts, ultimate bondstrength increased without any trend. Crumb rubber and polypropylene fiber containinglightweight aggregate replacing the 8-16mm fraction of coarse aggregate resulted in higherbond strength. The positive contribution of LWAs may be accounted for a better coherenceand interlocking of the LWA with the surrounding concrete that minimizes the cracking andthus utilizes fully the adhesion between the concrete and rebar [4,13]. L1F(4-8) CSt(8-16)concrete had the highest (16.11 MPa) bond strength which may be attributed to the extra2nd International Balkans Conference on Challenges of Civil Engineering, BCCCE, 23-25 May 2013, Epoka University, Tirana, Albania.618adherence between LWAs and matrix phase that prevents possible cracking by the help of thepolypropylene fibers protruding from the surface of pellets.024681012141618CCSt(4-16)LC(4-8)CSt(8-16)L1P(4-8)CSt(8-16)L5P(4-8)CSt(8-16)L1F(4-8)CSt(8-16)LC(8-16)CSt(4-8)L1P(8-16)CSt(4-8)L5P(8-16)CSt(4-8)L1F(8-16)CSt(4-8)LC(4-16)L1P(4-16)L5P(4-16)L1F(4-16)Ultimate Bond Strength (MPa) Production PlanFigure 6.Variation of bond strength with the type of coarse aggregate.CONCLUSIONLightweight concretes produced with lightweight fly ash aggregates were cohesiveand workable and didn’t show segregation. Replacing the crushed stone coarse aggregate fullywith lightweight fly ash aggregates significantly reduced the density of fresh concretes. Theuse of LWAs as partial or total substitute of crushed coarse aggregate resulted in slight toconsiderable decrease in compressive and splitting tensile strength and modulus of elasticityof concretes. The ranking of compressive strength of lightweight concretes from highest tolowest [ L1P(4-8)CSt(8-16) > L5P(4-8)CSt(8-16) > LC(4-8)CSt(8-16) > L1F(4-8)CSt(8-16)]comply with the decreasing order of crushing strength of lightweight aggregates ( L1P> L5P>LC> L1F ).When LWAs were used to totally replace the crushed stone coarse aggregate bondstrength slightly decreased. However, when LWA and crushed stone were used together inequal parts as coarse aggregate ultimate bond strength increased without any trend. Inclusionof crumb rubber and polypropylene fibers in the fly ash pellets replacing the coarse (8-16mm)part of crushed stone resulted in higher bond strength.REFERENCES[1] ACI Committee 213R-03, (2003)Guide for Structural Lightweight-AggregateConcrete, American Concrete Institute, Farmington Hills, MI, USA.[2] Baykal, G. and Doven, A.G. (2000) Utilization of Fly Ash by Pelletization Process;Theory, Application Areas and Research Results.Resources Conservation andRecycling, 30(1), 59-77.[3] Harikrishnan, K.I. and Ramamurthy, K. (2006) Influence of Pelletization Process onthe Properties of Fly Ash Aggregates.Waste Management, 26(8), 846-852.[4] Gesoglu, M. (2004)Effects of Lightweight Aggregate Properties on Mechanical,Fracture and   Physical Behaviour of Lightweight Concretes, Ph.D. Thesis, BoğaziçiUniversity, Istanbul, Turkey.2nd International Balkans Conference on Challenges of Civil Engineering, BCCCE, 23-25 May 2013, Epoka University, Tirana, Albania.619[5] Gesoglu, M., Ozturan, T. and Guneyisi, E. (2007) Effects of Fly Ash Properties onCharacteristics of Cold-Bonded Fly Ash Lightweight Aggregates.Construction andBuilding Materials, 21(9), 1869-1878.[6] Gesoglu, M., Ozturan, T. and Guneyisi, E. (2006) Effects of Cold-Bonded Fly AshAggregate Properties on the Shrinkage Cracking of Lightweight Concretes.Cement &Concrete Composites,28(7), 598-605.[7] Gesoglu, M., Ozturan, T. and Guneyisi, E. (2004) Shrinkage Cracking of LightweightConcrete Made with Cold-Bonded Fly Ash Aggregates.Cement and ConcreteResearch, 34(7), 1121-1130.[8] Kockal, N.U. (2008) Effects of Lightweight Fly Ash Aggregate Properties on thePerformance of Lightweight Concretes,Ph.D. Thesis, Boğaziçi University, Istanbul,Turkey.[9] Kockal, N.U. and Ozturan, T. (2011) Characteristics of Lightweight Fly AshAggregates Produced with Different Binders and Heat Treatments.Cement & ConcreteComposites, 33(1), 61-67.[10] Kockal, N.U. and Ozturan, T. (2011) Durability of Lightweight Concretes withLightweight Fly Ash Aggregates.Construction and Building Materials, 25(3), 1430-1438.[11] Kockal, N.U. and Özturan, T. (2010) Effects of Lightweight Fly Ash AggregateProperties on the Behavior of Lightweight Concretes.Journal of Hazardous Materials,179(1-3), 954-965.[12] Kockal, N.U. and Özturan, T. (2011) Strength and Elastic Properties of StructuralLightweight Concretes.Materials and Design, 32(4), 2396-2403.[13]Mor, A. (1992) Steel-Concrete Bond in High-Strength Lightweight Concrete.ACIMaterials Journal, 89(1), 76-82.

Mechanical properties of lightweight concrete made with cold bonded fly ash pellets

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Mechanical properties of lightweight concrete made with cold bonded fly ash pellets

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