Effects of compaction method and rubber content on the properties of

Effects of compaction method and rubber content on the properties of concrete paving blocksTung-Chai Ling ⇑Faculty of Construction and Land Use,The Hong Kong Polytechnic University,Hung Hom,Kowloon,Hong Konga r t i c l e i n f o Article history:Received 28May 2011Received in revised form 19August 2011Accepted 21August 2011Available online 13October 2011Keywords:Compaction method Crumb rubberConcrete paving block Propertiesa b s t r a c tA wide variety of recycled waste has been successfully used in the production of concrete paving blocks.It is known that the mechanical properties of these concrete products tend to be inconsistent,which is understandable in view of the range of mix designs as well as the variety of materials and compaction methods that were adopted in the production.In this study,recycled waste tyre (crumb rubber)was used to replace sand by volume at the level of 0%,10%,20%and 30%in order to investigate how the soft rubber particles behave under plant-machine compaction method during the production of rubberized concrete paving blocks (RCPB).In the hardened stage,the physical properties as well as mechanical properties of RCPB including density,compressive strength,bending strength and skid resistance were studied.The results showed that as a small proportion ($10%)of soft rubber particles was included in the mixture,the particles easily distorted and filled the voids between the solid particles.This filling mechanism reduced the porosity of concrete mixtures and effectively developed an adequate adhesion between the particles,resulting in higher gain in strengths.On the contrary,as the rubber ratio increased more than 10%,which the deformability is more predominant than the filling mechanism,this results in higher total stress concentrations and rebound stress of rubber particles,thus,increasing the porosity and micro-cracks,resulting in loss in strengths.Nevertheless,the presence of rubber in concrete did not dem-onstrate brittle failure,but rather a ductile which had an ability to withstand post-failure loads.In com-parison,the mechanical properties of plant-made RCPB performed better than that of corresponding manually-made RCPB.Therefore,plant-compaction method is recommended for future RCPB production and crumb rubber content used to replace sand by volume should be kept at or less than 10%.Ó2011Elsevier Ltd.All rights reserved.1.IntroductionSemi-dry concrete block has become increasingly important material in the construction industry.The fabricated concrete blocks can vary in size,strength and durability,depending upon their usage and the need in construction.To achieve better durabil-ity,high utility,consistent quality,and good appearance of SCB,the design concept of the production is mainly based on the combina-tion of low water–cement (w/c)ratio and high compaction meth-od.Presently,there are three general methods for the production of semi-dry concrete blocks at high and uniform standards:(1)hand ramming compaction;(2)manual or machine tamping;(3)high frequency of vibration and compaction methods.Current practice at most of the commercial plants is the (3)method,high frequency of vibration and compaction method which is fully auto-mated,fast and efficient.The final product also has higher density,better strength as well as lower permeability and lesser pore structure.2.Research backgroundOne of the major consequences of the rapid growth of popula-tion,economic and industrialisation is the massive generation of solid wastes and by-product materials.Most of these waste mate-rials are currently landfilled worldwide.As a result,an innovative solution to meet these challenges is necessary.Owing to the advantages and the successful development of the production methods of semi-dry concrete block,it is expected that introduc-tion of solid waste and by-product materials in concrete blocks could considerably reduce the waste management problem.Previous studies have shown that it is possible to utilise most of the solid wastes or by-products in the production of concrete block products.Extensive works have been conducted by Poon et al.[1–6]on the use of construction and demolition wastes such as recycled concrete aggregates and contaminants (tiles,brick,glass and wood)for the production of concrete block products.In their works,a combination of compaction method (manual and high compression pressure)was used for the production of concrete paving blocks (CPB)at laboratory scale.The CPB were prepared with little amount of water in such a way the mix was cohesive0950-0618/$-see front matter Ó2011Elsevier Ltd.All rights reserved.doi:10.1016/j.conbuildmat.2011.08.069Tel.:+852********.E-mail addresses:tcling611@ ,cetcling@.hkbut with zero slump.The semi-dry cohesive concrete mix was poured into a200Â100Â60mm mould in three layers at equal depth.For thefirst two layers,manual compaction was applied using a hammer and a wooden stem.After the third layer was poured,a compression force at a rate of600kN/min was applied until the force reached500kN.The test results showed that it was feasible to produce CPB with50%recycled concrete aggregate and50%of crushed clay brick that satisfied the minimum require-ment of ETWB of Hong Kong.In order to test the feasibility of the production technique,the same mix proportion of CPB were pro-duced at a commercial plant setting.An automatic block making machine with a combined vibration and compaction force of 80psi for12s was used.They concluded that the properties of plant-produced CPB were comparative with those laboratory-pro-duced CPB except a slightly higher compressive strength was found in the plant-produced CPB.Several authors[7–11]have reported that incorporating recy-cled glass in CPB is one of the promising solutions for waste glass recycling.According to the reported data[7],one conventional block making manufacturer could use as much as1000tonnes recy-cled glass per day(as a replacement of natural aggregates)in the CPB production.Previous research studies data have shown consid-erable scatter on the maximum possible content of recycled glass that could be used as natural aggregates replacement in CPB.Meyer et al.[7]found that it was possible to produce plant-CPB containing up to100%glass as aggregate.However,studies conducted by Byars et al.[8]indicated that if the glass aggregate exceeded40%,the amount of moisture could not be absorbed by glass aggregate, therefore,that may result in slump and the produced plant-CPB may not be able to retain its original shape.Turgut[9]demon-strated that no damage was observed for all the concrete blocks prepared with recycled glass at laboratory setting under a con-trolled w/c and moderate compaction technique(160MPa).Other studies have been carried out[12–14]which aimed to create useful by-products such asfly ash and bottom ash in the production of CPB and pressed blocks.Naik et al.[12]investigated the effect of using15%and25%offly ash as cement replacement in the production of paving stone at a plant setting.Results showed that none of the produced paving stone satisfied the minimum strength requirement according to ASTM C936.They concluded that the presence of a frog(groove)on the bottom part of the brick mould had affected some of the physical properties of the blocks during the production.Holt and Raivio[13]conducted a prelimin-ary laboratory to look on the effect of different residues as cement replacement in CPB.In the laboratory,an Intensive Compaction Tester(ICT-1000R)with a pressure of100kPa,simulated to the plant manufacture setting was used.Based on the laboratory test results,one residue was selected to replace10%and15%of cement in CPB to assess its feasibility during the production at full-scale plant setting.In terms of fresh and hardened properties,the CBP containing residue at10%and15%showed comparable results as compared to reference CBP.Karasawa et al.[14]worked on the use of large amount offly ash asfine aggregate replacement in making compacted CPB.The results revealed that as the amount offly ash increased to25%in the mix,a large plastic deformation and deterioration of dimension quality was observed during demoulding.They further noted that it was possible to control the plastic deformation by increasing thefineness modulus in the fine aggregate compositions.Freidin[15]used manual block-mak-ing machine(Cinva–Ram)with compaction pressure of4MPa to prepare building blocks containingfly ash and bottom ash.Results showed that there was no complication for the produced blocks to meet the specified requirement for shape,surface appearance as well as strength.Among the waste,waste tyre is one of the major environmental problems faced by most municipalities in the world due to its not readily biodegradable nature.Majority of such tyres were disposed at landfill and eventually will become mosquito breeding places and the worst case when it is burnt.In recent years,concerning the possible use of recycled waste tyre(crumb rubber)in the pro-duction of CPB has been investigated[16–21].Preliminary results [16]demonstrated that the production process of making CPB mixed with crumb rubber by using manually-operated hand press machine(Cinva–Ram)was quick and simple.Test results have shown that partial replacement of natural aggregates by an equal volume of crumb rubber varying from10%to20%resulted in a bet-terflexibility and energy absorption.This improvement may be due to the ability of the crumb rubber to undergo large elastic deformation before the failure of the CPB took place.Moreover, some of the unique properties of CPB produced using rubber aggre-gate were light weight,higher impact resistance,higher toughness and plastic deformation,which might offer offering a great poten-tial for it to be used in sound barriers and pavement structures.An attempt was made by Ling et al.[17]to evaluate how the crumb rubber contents and w/c ratios affected the properties of manual-compacted CPB.The laboratory test results of CPB incorporating crumb rubber ranging from0%to50%showed that the higher the rubber content in the concrete mix,the lower was the com-pressive strength.Sukontasukkul and Chaikawe[16]also noticed that as the rubber content increased to20%,there was a great reduction in strength at approximately82.5%.Thus,the demand for more practical and effective use of crumb rubber to its maximum possibility in the production of rubberized concrete paving blocks(RCPB)to improve some of the engineering properties while maintaining strength has become intense.The influence of soft rubber particles in the production of RCPB under different compaction methods are investigated in this present work.The quality assurance from raw materials to manufacturing process andfinishing products is critically evaluated.The hardened properties of RCPB including density,compressive strength,bend-ing strength and skid resistance specimens made by plant-machine and hand-ramming method are studied and compared.3.Details of research studyFig.1shows theflowchart of the present study.This study was divided into three major parts.In thefirst part,the physical prop-erties of raw materials used for the production of rubberized con-crete paving blocks(RCPB)were assessed.In the second part,the influence of soft rubber particles and high compaction force on the fresh and physical properties of RCPB produced was monitored and assessed.Immediately,a same concrete mix was used to pro-duce manually-made RCPB using hand-ramming compaction method.In thefinal part,the density,skid resistance as well as compressive and bending strengths of both plant-made and man-ually-made RCPB were determined and compared.A total of approximately4300RCPB including control specimens were pro-duced in this study.3.1.Raw materialsThe RCPB comprised ordinary Portland cement(OPC),granite aggregate,coarse sand,fine sand and crumb rubber.OPC used throughout the study conformed to BS12[22].The superplasticizer (SP)used was Rheobuild1000(an aqueous solution of a Ca–Nap-thalene Sulphonated).Due to the high viscosity,the SP was dis-solved with mixing water before being added into concrete mix. Granite aggregate which had afineness modulus of5.29andflak-iness index of17.07%was used as coarse aggregate.Coarse andfine sand had a maximum particle size less than9.50mm and4.75mm andfineness modulus of3.02and1.78were used asfine aggregateT.-C.Ling/Construction and Building Materials28(2012)164–175165in base and face concrete mixes,respectively.Crumb rubber was a fine material produced by mechanical shredding with the grada-tion close to that of sand.Two particle sizes of crumb rubber were used:1–3mm and1–5mm as a partial substitute for sand in the production of face and base layers of RCPB,respectively.The den-sity of1–3mm and1–5mm dense crumb rubber were596kg/m3 and606kg/m3,whereas theirfineness moduli were4.52and4.74, respectively.3.2.Mix designA total of four mixes were prepared in a commercial plant set-ting.Each of the RCPB produced consists of two layers,base(bot-tom)and face(surface)layer.The raw materials used for base concrete mix were cement,granite aggregate,coarse sand and1–5mm crumb rubber.The mix proportion for control base concrete mix was1:1.9:3.8(cement:granite aggregate:coarse sand),with an SP/cement ratio of0.06.The weight ratio of granite aggregate to coarse sand of all the mixtures was kept at1:2.The w/c was in the range0.39–0.45for290–330kg/m3cement content,as appro-priate to the plant production setting.To produce a good surface appearance(approximately5mm thick),the raw materials used for face concrete mix consisted of high volume cement,fine sand and1–3mm crumb rubber.The mix proportion for control face mix was1:2.3(cement:fine sand). The w/c was in the range of0.23–0.29for570–620kg/m3cement content,as appropriate to the smooth facing layer production.The volume fraction of crumb rubber was varied at approxi-mately0%,10%,20%and30%for sand replacement in the mix for RCPB-0,RCPB-10,RCPB-20and RCPB-30,respectively.The details of all the RCPB mixes design are shown in Table1.3.3.Manufacturing process of plant-made RCPB3.3.1.Concrete mixingSemi-dry concrete mixing was used at plant production setting for the production of rubberized concrete paving blocks in this study.During the concrete mixing,two independent mixers wereTable1Mix composition and ratio of RCPB mixtures.Mix notation Mix ratio Cement content(kg/m3)w/c ratio Rubber content(%)Demoulded fresh density(kg/m3) Face(C:FS)Base(C:A:CS)Face Base Face Base Face BaseRCPB-01:2.31:1.8:3.86173280.230.45002170RCPB-101:2.11:1.8:3.45853170.230.438.89.72140RCPB-201:1.91:1.8:3.06042740.290.4821.619.42100RCPB-301:1.71:1.8:2.65742860.260.3930.429.02030C-cement,FS-fine sand,CS-coarse sand.166T.-C.Ling/Construction and Building Materials28(2012)164–175used with appropriate capacity and worked in face layer was added on the base layer for a better tially,granite aggregate,coarse sand,cement and rubber were mixed in a base concrete mixer for 1min.After mixing for 1min,the required thoroughly pre-mixed)was added to the dry for another 1min until the desired moisture Similar mixing process was also employed for face 3.3.2.Fabrication of plant-made RCPBAfter concrete mixing process was completed,face concrete mixes were transferred from pan hoppers,respectively.The amount in the feed trolled by an automatic weighting system.The a correct amount of concrete into steel mould internal dimensions of 210mm length,10560mm depth.When the mould was filled by the mix,preliminary vibrations together with high ing were applied (see Table 2).After that,the was poured into the mould again for the second intermediate compaction and vibration were ond.Finally,face concrete mix was filled up the (final)compaction of 1.2second was applied.released after the compaction and the head was early stripping of RCPB from the steel moulds.Previously,because the mechanics of RCPB particles had not been fully researched,placed on experience (trial and error)to select an ing time and w/c for the production of RCPB ting.The optimum time of vibration at 5.2second in the plant.But good compaction was more RCPB when the concrete mix contained 20%or 30%ber.Therefore,trail and error approach was taken ratio that can optimise the use of 20%and 30%of ticles in the production of RCPB.After various parameter’was adopted (see Tables 1and 2).3.3.3.Monitoring of fresh plant-made RCPBAfter demoulding,pallets were removed from the lowerator and all the RCPB were stood individually and were separated from one another by thickness of the mould walls.Fig.2shows 54fresh RCPB specimens on a pallet that was produced by one compactions pressing.All the RCPB produced were visually checked and their height was monitored by height control device.Any significant change in the physical appearance indicates something awry in the parameters setting during the production.Fig.3indicates some of the ‘‘expanded’’RCPB that were being rejected as the pallet passed through the height control device.Besides,the appearance and colour variations of RCPB were also checked by manual sampling of five samples from the pallets (see Fig.4).Weight and dimensions of the sampling samples were determined as shown in Fig.5.The thickness of the face layer was also measured in accordance to the requirement of MA 20[23].Table 3shows the results of physical properties of fresh plant-made RCPB.Because of low specific gravity of rubber particles,weight of fresh plant-made RCPB decreases with the increase in the percentage of rubber content.However,the decrease in weight of rubber was found to be less important when rubber content was at 10%of the total sand volume.Based on the visual observations of the fresh plant-made spec-imens,no honeycombs,cracks,and outstanding deformation were found on RCPB containing 0%and 10%of crumb rubber.For RCPB containing 20%or 30%of crumb rubber,some cracks and delamina-tion between the face and base layers were clearly observed.This may be attributed to the increase in degree of compressibility byTable 2Vibration time and speed was adopted during the production of RCPB.Sequence of vibration applied Time (s)Speed (Hz)Preliminary 1.860Intermediate 2.258Final (for face)1.260Total Vibration Time (s)5.2–Fig.2.Height checking.1 2345T.-C.Ling /increasing the rubber content (during compaction).Therefore,once the RCPB were released from the compaction force (after demoul-ding),which in turn changed the volume and dimensions (expan-sion takes place due to the stress released from rubber particles)of the RCPB specimens.The deformability mechanism of the pres-ence of high elastically deformable crumb rubber during the com-paction and moulding is illustrated by Fig.6.Fig.7shows a typical pattern of cracks of varying orientations and dimension change of fresh RCPB-30specimens.Another possi-ble reason for the cracks development may be due to the increase in shear plane between the mould wall and all sides of fresh RCPB specimens.This is because the sand–rubber mixture could created higher shear resistance compared to sand alone at the same com-paction [24].Aslantas [25]also reported similar problem for con-crete block produced by a combined vibration and pressure method.He stated that high water content in the mix had caused the surface problems and instability of concrete at the demoulding stage.Also,care should be taken with the use of high content of rubber in the concrete mix because it can gradually affected the concrete slump [26].3.3.4.CuringAll the plant-made RCPB samples were cured under an elevated curing temperature at temperature of 25±3°C with humidity of 95±3%for the first day.After 1day,the 1-day hardened plant-made RCPB samples on pallets were then removed from roller-con-veyors mounted on the outlet side of the press,and placed on to an elevator.The samples were then collected and further cured at room temperature of 32±3°C with 65±5%relative humidity until the testing on the 28th day.The effects of different curing condi-tions on the properties of plant-made RCPB can be found in [20].Fig.5.Dimensions measurement.Table 3Physical properties of fresh plant-made RCPB.Mixnotation Rubber content (%)Total depth (mm)Thickness of facing layer (mm)Weight (kg)Visualobservations RCPB-0059.6 5.5 2.82Very good,no cracking RCPB-101059.8 5.0 2.82Good RCPB-202059.2 5.0 2.74Some cracking RCPB-303059.83.32.68Some cracking,delaminationRCBPRCBPSteel plateMould wallH1 < H2When compaction force is applied When compaction force is releasedB1 < B2Materials 28(2012)164–1753.3.5.Investigation of 1-day hardened plant-made RCPB3.3.5.1.Physical appearance.The visual properties and physical attribute of 1-day plant-made RCPB samples were checked in accordance to the requirement of the quality specified by MA 20[23].Figs.8and 9show some of the finished products that rejected due to some faults present.For each mix design,more than 1000RCPB specimens were pro-duced.The total number of samples produced and rejected is given in Table 4.The rejection rate increased with an increase in the rub-ber content which mainly due to the decrease of its feasibility dur-ing the production.The rejection samples of RCPB-20and RCPB-30shown in Table 4were included in the redundant samples sourced from those failed trial and error mixes.3.3.5.2.Surface colour.Fig.10shows surface colours of plant-made RCPB containing different content of crumb rubber.The RCPB mixed with 20%and 30%of crumb rubber were slightly darker than those controlled RCPB (0%)and RCPB mixed with 10%of crumb rubber.This slight colouration would not cause significant problem when it was applied in pedestrian area or low traffic volume pavement.3.4.Manufacturing process of manually-made RCPBThe production of manually-made RCPB has been studied in a side experiment.Only base concrete mix collected from wood pal-let at the end of each batch plant production was used to prepare manually-made RCPB.The compaction method used to produce manually-made RCPB was a modified method from ASTM D 698[27](see Fig.11).This manually-made method was proven to achieve minimum target strength of 30MPa [17],which stratified for the application in trafficked area less than 3tonnes gross weight.The procedure of the modified manual-compaction method is described below.Manual compactions were applied by using a hammer at each layer of concrete mixture after the steel moulds were filled in two layers of about equal depth with internal dimen-sions of 200Â100Â60mm.To ensure the mixture was well com-pacted at each layer;50drops (five drops in horizontal direction and ten drops in vertical direction)of 1.86kg square hammer (25mm Â25mm)were uniformly applied from a height of up to 15cm onto the mixture directly.After the second layer was com-pacted,a final mix will fill up the empty space within the mould.Excessive materials were removed from the blocks surface and the surface was then flattened with a trowel.The manually-made RCPB were removed from the steel moulds 1day after castingFig.9.Rejected RCPB.Table 4Number of total samples produced and rejected for each mix design RCPB.Mixnotation Total production sample Rejected sample %of rejection rate RCPB-0111660 5.4RCPB-1010121009.9RCPB-20112644639.6RCPB-30104664862.0RCPB-0 RCPB-10RCPB-20 RCPB-30Materials 28(2012)164–175169and cured in air at an average room temperature of 30±3°C with65±5%relative humidity until the 28days of testing.3.5.Mechanical properties test methodsThe compressive strength was determined in accordance with MA 20[23].Prior to the testing,RCPB were soft capped with two pieces of 4mm thick plywood to ensure a flat surface during load-ing.A modified British Standard (BS 6073-1)[28]method was used for the three-point bending strength test.Load was applied to a central line of RCPB while being simply supported over a span of 150mm until rupture occurred.Deflection and energy absorption were automatically recorded in the data acquisition system and the modulus of rupture (MOR)was then calculated.The MOR is gi-ven according to equation:r ¼32LFBD 2and expressed in MPa.Where,L is the span length (mm),F the maximum applied load (N),B the average width of the sample (mm),and D its average thickness (mm).The dry density of RCPB was determined according to BS 6073-2[29].The density of the RCPB is simply the mass of air dried sample divided by its volume,expressed in kg/m 3.The skid resistance was determined in accordance with ASTM E 303[30].Four swings were made and an average result was calculated for each specimen.All the testing at hardened stage was conducted at 28days after cast-ing.An average result of three samples for manually-made RCPB and five samples for plant-made RCPB were reported.4.Results and discussion on the 28-day hardened properties pressive strengthFig.12shows the effects of percentage of crumb rubber replac-ing fine aggregate on the compressive strength of RCPB.In general,the compressive strength results obtained in this study for both plant-made and manually-made methods are in agreement with previous studies,in which the compressive strength of RCPB de-creased as the percentage of crumb rubber increased [16–19].A study by Sukonrasukkul and Chaikaew [16]reported that the inclu-sion of 20%of crumb rubber with fineness modulus of 4.98,3.77and 2.62into concrete paving blocks decreased the compressive strength as much as 84.4%,78.1%,85.0%,respectively.In this study,when crumb rubber with a fineness modulus of 4.74was used as 20%of sand replacement decreased the compressive strength of 50.0%and 52.6%for plant-made and manually-made RCPB,respec-tively.As for a study by Ling et al.[17],20%of crumb rubber incor-porated in concrete paving blocks with water-to-cement ratio of 0.45,0.50and 0.55caused a decrease of 33.6%,18.3%and 38.0%in compressive strength,respectively.It is worth to note that the compressive strength of plant-made RCPB increased,as the proportion of crumb rubber in concrete was 10%.This point of inflexion can be explained by the mechanism behaviour shown in Fig.13.As a small proportion ($10%)of crumb rubber was included in the mixture,the soft rubber particles easily distorted and filled the voids between the solid particles (natural aggregates)under a compression force of plant-made machine.This filling mechanism was found to reduce the porosity by filling up the free pore volume in the concrete mixtures (see Fig.13b).Furthermore,under this circumstance,the rubber particles had bonded well with cement matrix (see Fig.14a)which in turn re-sulted in a better compressive strength of RCPB.A perfect adher-ence between rubber and cement matrix has also been observed in other work [31].However,it is impossible to achieve ‘‘zero’’porosity by increas-ing the rubber particles in the mixtures.A larger porosity was ob-served with an increasing rubber ratio higher than 10%,because at this point the deformability is more predominant than the filling mechanism.This indicates that the deformability of RCPB in-creased significantly after an optimum close packing is reached.Also,as the crumb rubber content increases beyond this limit,it in-creased the total stress concentrations in the concrete mixture and therefore rebound stress of rubber particles occurred.This,in turn,results in micro-cracks between the interfaces in concrete matrix and considered to loss the strength significantly.Fig.14b shows the loss of adhesion between the rubber particles and the sur-rounding cement paste.As can be seen in Fig.12,the compressive strength of RCPB greatly depends on the compaction methods used.Regardless of crumb rubber content,the compressive strength of plant-made RCPB was higher than the corresponding manually-made RCPB.This finding can be explicated through the packing behaviour of the particles in the RCPB whereby the higher level of compactionRammer pattern for compaction in 200×100×60 mm steel mouldTest method ASTM D 698 [19] Modifiedmethod Hammer weight (N) 24 18 Hammer size 50 mm in diameter25×25 mm inrectangular Drop height (cm) 30 15Total number of drops75 100 Number of drop per layer25 50 Compaction rate (drops/min)25 50H1 H2 H3H1 - H525mmV 1–V 1025m mV 7 V 6 V 5 Fig.11.Summary of modified hand-ramming compaction method for manually-made RCPB.170T.-C.Ling /Construction and Building Materials 28(2012)164–175。