(asce)mt.1943-5533.0000044

Using Recycled Concrete Aggregates in New ZealandReady-Mix Concrete ProductionWentao Zhang 1and Jason M.Ingham,M.ASCE 2Abstract:Motivations for adopting recycled concrete as an aggregate source include the preservation of natural resources,effective utilization of a growing waste stream,and financial and energy savings.Although current New Zealand practices include some use of crushed concrete in road construction,use of recycled concrete aggregate ͑RCA ͒in low specification concrete is currently infrequent and the use of recycled concrete as an aggregate source in structural concrete applications is rare.To make such recycling feasible,the properties of RCA must be related to the properties of new concrete that utilizes the recycled aggregates.In response to this need,a study was undertaken to investigate the feasibility of using RCA as a viable alternative to natural aggregate ͑NA ͒in the production of concrete manufactured in a conventional New Zealand ready mix concrete plant.Aggregate properties and hardened and fresh concrete properties of RCA concrete were studied and compared with the associated properties derived from NA concrete.Results indicated that RCA is a viable alternative to NA in the production of concrete.Furthermore,it was confirmed that the properties of RCA dictate the hardened properties of the reconstituted concrete and that RCA from certain strength placements limited the resulting possible strengths of concrete produced from it.DOI:10.1061/͑ASCE ͒MT.1943-5533.0000044CE Database subject headings:Recycling;Concrete;Sustainable development;Aggregates;New Zealand .Author keywords:Recycled concrete;RCA;Sustainability;Recycled concrete aggregate .IntroductionCurrently there is a lack of research on the performance of New Zealand recycled concrete aggregates ͑RCA ͒when the character-istics of the original concrete used in its production are known,and the general consensus within the New Zealand concrete in-dustry is that recycled concrete can only be used in low-end,nonstructural applications ͑Park 2001͒.Transit New Zealand,the national transport agency in New Zealand,allows some use of RCA in pavement base courses but there are no standards for the prescription and widespread use of RCA in New Zealand ͑Transit New Zealand 2006͒.Consequently,even though current New Zealand practices include some use of crushed concrete in road construction,use of RCA in low specification concrete is infre-quent and the use of recycled concrete in structural applications is rare ͓Cement and Concrete Association of New Zealand ͑CCANZ ͒2007͔.Numerous European demonstration projects have shown that RCA can be used in high strength applications ͑Limbachiya et al.2000͒,and reuse of concrete debris to produce new concrete is now practiced worldwide,with particularly extensive and stan-dardized implementation in Japan and Europe ͑Hansen 1992͒.Typically,experimental investigations have preceded the develop-ment of industry standards,such as the RILEM committee that conducted research commencing in the 1960s and developed in-dustry recommendations in the 1970s ͑RILEM 1994͒.Conse-quently,the study reported here was undertaken to address practical issues associated with determining the properties and behavior of New Zealand RCA,specifically pertaining to the pro-duction of 20–60MPa concrete,with a view to facilitating the implementation of RCA in New Zealand ready mix concrete plant.Review of Past ResearchIn ordinary fresh concrete,due to the presence of a water film on the aggregates,an interfacial transition zone ͑ITZ ͒is created be-tween aggregate and cement paste where the w/c ratio may be significantly higher than that of the cement paste.The ITZ is widely regarded as the “weakest link”in concrete ͑Alexander 1996;Jia et al.1986;Keru and Jianhua 1988;Li et al.2001;Popovics 1987͒,inhibiting the achievement of composite action in natural aggregate ͑NA ͒concrete.Obviously,the structure of RCA concrete is more complicated than NA concrete ͑Mehta and Aitcin 1990͒,with NA concrete consisting of only one ITZ,being the interface between the NA and the surrounding cement paste,whereas RCA concrete has two ITZ,located between the aggre-gate and old adhered mortar,and between the old adhered mortar and the new cement paste ͑Ryu 2002͒.The strength of RCA con-crete is usually dictated by the weaker of these two ITZs.The presence of adhered mortar generally degrades the quality of RCA and negatively influences the fresh and hardened properties1Civil Engineer,Humes Pipeline Systems,Level 3,Wright Stephen-son House,585Great South Rd.,Penrose,Auckland,New Zealand ͑cor-responding author ͒.E-mail:wentao.zhang@ 2Associate Professor,Dept.of Civil and Environmental Engineering,Univ.of Auckland,Private Bag 92019,Auckland,New Zealand.E-mail:j.ingham@Note.This manuscript was submitted on May 20,2009;approved on August 28,2009;published online on April 15,2010.Discussion period open until October 1,2010;separate discussions must be submitted for individual papers.This paper is part of the Journal of Materials in Civil Engineering ,V ol.22,No.5,May 1,2010.©ASCE,ISSN 0899-1561/2010/5-443–450/$25.00.D o w n l o a d e d f r o m a s c e l i b r a r y .o r g b y N o r t h C h i n a I n s t i t u t e o f W a t e r R e s o u r c e s a n d H y d r o p o w e r o n 02/25/14. C o p y r i g h t A S CE .F o r p e r s o n a l u s e o n l y ; a l l r i g h t s r e s e r v e d .of concrete ͑Alexander 1996;Jia et al.1986;Keru and Jianhua 1988;Li et al.2001;Popovics 1987͒.In order to improve the properties of RCA concrete,the two-stage mixing approach ͑TSMA ͒was developed by Tam et al.͑2005͒,where RCA and half of the mix water is combined and mixed for 60s.Cement,the rest of the ingredients,and the rest of the mix water is then added and mixed for a further 120s.The first stage of the mixing approach leads to the formation of a thin slurry at the ITZ,which allows microcracks on the surface of the RCA to be filled ͑Tam et al.2005͒.Ryu ͑2002͒and Otsuki et al.͑2003͒suggested that for higher w/c ratios,a double mixing pro-cess improves the quality of the ITZ on RCA by reducing the w/c ratio at the aggregate edge,and they reported improved compres-sive and flexural strength results of 17and 26%,respectively.RCA has high absorption due to the presence of adhered mor-tar.Kashino and Takahashi ͑1988͒suggested presoaking of RCA and Dhir et al.͑1999͒also examined an RCA pretreatment pro-cedure and its effect on fresh concrete properties.Presoaking showed a slight improvement in the fresh properties of concrete containing 100%coarse and 50%fine RCA when compared to the corresponding mixes with dry RCA.In practical concrete produc-tion,it is necessary to presoak RCA in order to produce workable concrete,with Hansen ͑1992͒determining that soaking of aggre-gates for 1h before use proved to be effective.Recycled concrete produced with coarse recycled aggregates and natural sand was found by Mukai et al.͑1978͒to require approximately 10L /m 3or 5%more free water than the control mix to achieve the same slump.Approximately 25L /m 3or 15%more free water was required when both fine and coarse recycled aggregates were used ͑Frondistou-Yannas 1977͒.Air contents of fresh concrete using RCA were higher and more varied than the air contents of control concrete made with NA,and the wet den-sities of RCA concrete varied from 2,200to 2,250kg /m 3,which was a 5%decrease when compared to the wet densities of NA concrete.Most researchers agree that there is a gradual reduction in compressive strength as the RCA content is increased beyond 30%͑Dhir et al.1999;Gómez-Soberon 2002;Limbachiya et al.2000͒.The general consensus is that the substitution of NA with RCA produces concrete with 0–35%less strength ͑Frondistou-Yannas 1977;Hansen 1992;Sri Ravindrajah et al.1988͒and the use of different qualities of recycled aggregates has been shown to result in an increase in the compressive strength variation co-efficient ͑Hansen et al.1986͒.Soshiroda ͑1983͒showed that in-creasing the recycled fines content negatively impacted on the resulting concrete strength.However,Dhir et al.͑1999͒found that replacement of up to 20%RCA fines did not significantly influ-ence the compressive strength of RCA concrete.Furthermore,there are reports of RCA leading to comparable or even higher compressive strengths than the control concrete ͑Yoda et al.1988͒.When replacing 100%of NA with RCA,Rao et al.͑2007͒showed a reduction in flexural strength of 15–20%.Gómez-Soberon ͑2002͒and Katz ͑2000͒reported a 10and 12%reduction in flexural strength with 100%coarse RCA replacement when compared to NA concrete,and Katz ͑2000͒determined the ratio between flexural and compressive strengths of RCA concrete to be 20%.When 60%of coarse RCA was used,flexural perfor-mance of RCA concrete beams was similar to that of NA concrete beams ͑Waleed and Canisius 2007͒.Likewise,Limbachiya et al.͑2000͒found no difference between flexural strengths of concrete produced with 100%RCA and flexural strengths of NA concrete.The optimum ratio of fine to coarse aggregate in RCA concreteis the same as that for conventional concrete ͓Building Contrac-tors Society of Japan ͑BCSJ ͒1977͔and in principle,an RCA concrete mix design is not dissimilar to mix designs used for conventional NA concrete.However,modifications were found to be required in practice ͑Hansen 1992͒.These modifications were:1.A higher SD must be employed when determining the targetmean strengths of RCA concrete than when designing con-crete with NA,due to greater strength variability in RCA concrete;2.For concrete durability reasons,a maximum RCA size of16–20mm is recommended;and3.Due to the higher free water requirement of RCA mixes,calculated cement content will be higher although savings associated with reduced haulage and landfill charges are ex-pected to readily exceed added costs associated with higher cement usage.Bairagi et al.͑1990͒recommended that the aggregate cement ratio should be modified to find the optimum ratio,and deter-mined that RCA concrete required an additional 8–13%cement in order to achieve design target strength.Experimental ProgramThe experimental program consisted of four stages:•Stage I:casting of three 15m 3NA concrete slabs having low ͑20MPa ͒,medium ͑40MPa ͒,and high ͑60MPa ͒target com-pression strengths;•Stage II:demolition and crushing of Stage I NA concrete slabs to produce RCA;•Stage III:production of RCA concrete having target compres-sion strengths of 20,40,and 60MPa;and•Stage IV:commercial production of RCA concrete in a ready-mixed concrete plant.In all stages of the experimental program,general purpose cement compliant with NZS 3122:1995͓Standards New Zealand ͑SNZ ͒1995͔was used.Preparation of NA Concrete SlabsMix designs used to manufacture the low,medium,and high strength NA concrete slabs were the standard mixes used by a commercial ready-mixed concrete plant,having specified 28-day cylinder compressive strengths of 20,4,and 60MPa ͑see Table 1͒and having a maximum aggregate size of 19mm.The 19mm,13mm,and Pap 7aggregates were crushed basalt,and the sand was sourced by suction dredges from Auckland’s offshore coastal de-posits.Pap 7is a crushed basalt aggregate having a maximum size of 7mm.Properties of the NA used in the slabs were as-sessed,and are reported in Zhang ͑2009͒.Cylinder compression and flexural strength tests were performed for each strength grade in accordance with NZS 3112:Part 2:1986͑SNZ 1986a ͒,and two concrete cores samples were extracted from the corner of each slab on the 14th,28th,and 35th day to ascertain the actual field strength of each concrete slab ͑see Fig.1͒,with the 35th day core strengths measured to be 21,43,and 58MPa,respectively.The slabs were then crushed and demolished over a 3-day period com-mencing on the 35th day after casting.Flexural strength testingTable 1.MTMS at 28Days ͑in Megapascal ͒Concrete grade ͑mix strength ͒,MPa 204060MTMS 28,MPa24.547.569.0D o w n l o a d e d f r o m a s c e l i b r a r y .o r g b y N o r t h C h i n a I n s t i t u t e o f W a t e r R e s o u r c e s a n d H y d r o p o w e r o n 02/25/14. C o p y r i g h t A S CE .F o r p e r s o n a l u s e o n l y ; a l l r i g h t s r e s e r v e d .was conducted on 3flexural beams on the 14th and 28th day ͑see Table 2͒.Fresh concrete tests involved initial slump,bleeding,air content,and density.The slabs were cured by spraying an even coating of a synthetic hydrocarbon resin antievaporation com-pound onto the top surface of the slab.Demolition and Crushing of NA Concrete Slabs Demolition and crushing of the NA slabs determined the quality and gradation of the RCA.This process was achieved by using a Komatsu 20t digger with D2500hydraulic rock breaker to de-molish the slabs into concrete pieces that had a maximum dimen-sion of approximately 300mm.The crushing and sorting process occurred at a mining quarry and involved a Lokotrack LT 1213S Impactor Crusher that broke the concrete pieces into one product with a maximum size of 20mm.A Nordberg SW348mobile screen then sorted the product according to size,which was nec-essary in order to obtain the 19-and 13-mm graded aggregate product.The impact crusher’s sieve module was set at a maxi-mum size of 20mm and all oversized crushed material was re-turned to the front of the crusher for recrushing.In practice,debris from demolished concrete structures will usually contain steel reinforcement and other contaminants,and additional steps to remove these deleterious contaminants would be required.Fig.2͑a ͒shows the measured 19mm grading curves,with both the 40and 60MPa RCA located within the grading envelope specified in NZS 3121:1986͑SNZ 1986b ͒.The deviation ob-served for the 20MPa RCA was attributed to the weaker concrete strength being unable to withstand the crushing power of the im-pact crusher,thereby producing a coarse product after one pass.The grading curves for the 13mm aggregate ͓see Fig.2͑b ͔͒showed similar characteristics for all strengths of RCA,with all curves having a higher percentage passing than the 13mm NA and being outside the envelope of coarse grading requirement for 13.2–4.75mm nominal size aggregates.About 90%of the 13mm RCA aggregates passed sieve size 9.5mm,while only 40%of NApassed the 9.5mm sieve.The difference between NA and RCA grading required significant differences in subsequent mix propor-tioning.As no recycled fines were used in the study because of concerns regarding excessive moisture absorption,varying quan-tity of NA sands and Pap 7were introduced to attain the required grading.Table 3shows that the RCA had lower dry density and higher absorption values than the NA.Furthermore,the dry densities of 19mm RCA were markedly higher than the dry densities of 13mm RCA.The higher absorption and lower dry densities exhib-ited in RCA 13mm,when compared to 19mm RCA,was attrib-uted to the higher percentage of adhered mortar on the 13mm RCA.Because adhered mortar is more absorbent and less dense,it results in increased absorption and reduced density.For both aggregate sizes,the dry density of the RCA increased slightly with increased strength of the original concrete.ThemeasuredFig.1.Concrete core compression results from NA slabsTable 2.Twenty-Eight-Day Flexural Strength of NA Slab Mixes ͑in Megapascal ͒Mix strength 20MPa 40MPa 60MPa Test 1 5.0 6.4 5.7Test 2 5.0 5.87.0Test 3 4.37.48.1Average4.86.57.0Fig.2.RCA grading curvesTable 3.Dry Density and Absorption Values of RCA and NAAggregate products Dry density ͑kg /m 3͒Dry densityAbsorption͑%͒Absorption NA dry density NA absorption NA 13mm2,804 1.00 2.1 1.0020MPa RCA 13mm 2,2800.817.6 3.6240MPa RCA 13mm 2,3200.837.4 3.5260MPa RCA 13mm 2,3300.837.2 3.43NA 19mm2,838 1.00 1.8 1.0020MPa RCA 19mm 2,5100.88 4.6 2.5640MPa RCA 19mm 2,5200.89 4.6 2.5660MPa RCA 19mm2,5300.894.42.44D o w n l o a d e d f r o m a s c e l i b r a r y .o r g b y N o r t h C h i n a I n s t i t u t e o f W a t e r R e s o u r c e s a n d H y d r o p o w e r o n 02/25/14. C o p y r i g h t A S CE .F o r p e r s o n a l u s e o n l y ; a l l r i g h t s r e s e r v e d .properties of RCA were in agreement with findings from other researchers as reported in the review of past boratory Production of RCA ConcreteSix mix categories with 28-day target mix strengths of 20,40,and 60MPa were tested,as detailed below and reported in Table 4.In all cases where cement content was increased or decreased,the water content remained constant.The six mix categories were:1.NA mixes as control mixes;2.Twenty MPa RCA mixes contained approximately 5%morecement than the NA mixes,to account for loss of workability and strength as reported by previous researchers;3.Forty MPa RCA mixes contained approximately 5%morecement,as for Category 2above;4.Sixty MPa RCA mixes contained approximately 5%morecement,as for Category 2above;5.Forty MPa RCA mixes with fly ash ͑FA ͒,with the cementcontent matching the NA mixes and the FA being a substitute for the supplementary 5%cement;and6.Forty MPa RCA mixes having a cement content replicatingthe NA control mix ͓referred to as decreased cement ͑D.C.͔͒.Each laboratory concrete mix had a volume of 50L and at least three mixes were made for each mix type ͑see Table 4͒,resulting in a total of 54mixes.Slump values were measured every 15min for a total duration of 90min.For each concrete mix,11cylinders having dimensions of 100-mm diameter ϫ200-mm length and three flexural beams having dimensions of 150ϫ150ϫ500mm were made,with one cylinder crushed on Days 1,3,7,14,and 56,and six cylinders crushed on the 28th day.One flexural beam was tested on the 14th day and two flex-ural beams were tested on the 28th day.Presoaking of RCA and the TSMA were adopted in the production of all RCA mixes,and 100%coarse aggregate replacement occurred for all RCA mixes.As mix quantities used in this study were similar to those of Tam et al.͑2005͒,no adjustment to the recommended TSMA mix times was required.Slump ResultsTable 5shows that there was no distinct observable trend in the rate of slump loss between different sources of RCA but that on average,RCA mixes exhibited a greater rate of slump loss when compared to mixes with NA.In addition,all concrete mixes with RCA experienced slump loss at a similar rate,as shown in Fig.3.FA is a much finer material than cement,and the inclusion of FA is believed to have coated the RCA with a thin layer that provided a “ball bearing effect”to the RCA and improved the slump per-formance.Generally,there was no significant difference in work-ability for the different source strengths of RCA.Compression ResultsFor each strength grade of concrete NZS 3104:2003͑SNZ 2003͒specifies the required 28-day compressive strength of laboratory cured cylinders ͑see Table 1͒,referred to as the minimum target mean strength at 28days ͑MTMS 28͒.This information is used in the following assessment.The compression strength gain curve shown in Fig.4͑a ͒indi-cates that all RCA produced 20MPa concrete with equal or greater 28-day strengths than the 20MPa NA concrete and that the use of 40and 60MPa RCA to produce 20MPa concrete led to higher strengths than obtained for the 20MPa NA concrete.Fig.4͑b ͒shows that when producing 40MPa concrete,compres-sion strengths depended on the strength of the RCA source.In addition,Fig.4͑b ͒shows that 20,40,and 60MPa RCA lead to the production of RCA concrete having 28-day compression strengths of 40,48,and 52MPa,respectively,which should be compared with the MTMS of 47.5MPa that is shown in Table 1.Conse-quently it was concluded that both 40MPa RCA and 60MPaTable boratory Mix TrialsAggregates RCA mix strengths ͑MPa ͒204060NA Type 1Type 2Type 320MPa RCA Type 4Type 5Type 640MPa RCAType 7Type 8Type 940MPa RCA+FA Type 10Type 11Type 1240MPa RCA+DC Type 13Type 14Type 1560MPaRCAType 16Type 17Type 18Table 5.Slump Loss after 90min ͑mm/min ͒Aggregates Mix strengthAverage 20MPa 40MPa 60MPa NA0.830.400.670.6320MPa RCA 0.600.670.800.6940MPa RCA 0.560.770.670.6760MPa RCA 0.500.890.810.73Average of RCA 0.550.780.7840MPa RCA+FA 0.830.500.610.6440MPa RCA+DC0.770.890.880.85Fig.3.Slump life of concrete mixesD o w n l o a d e d f r o m a s c e l i b r a r y .o r g b y N o r t h C h i n a I n s t i t u t e o f W a t e r R e s o u r c e s a n d H y d r o p o w e r o n 02/25/14. C o p y r i g h t A S CE .F o r p e r s o n a l u s e o n l y ; a l l r i g h t s r e s e r v e d .RCA can produce concrete that has compressive strength equal to or greater than the target 28-day strength for a 40-MPa mix.60MPa concrete produced using 60MPa RCA was found to only reach 60MPa at 28days ͓see Fig.4͑c ͔͒,and failed to attain the target 28-day mean strength of 69MPa ͑see Table 1͒.This represented a 10-MPa strength reduction when compared to the corresponding concrete mix using NA.At 56days,the 60MPa RCA concrete compressive strength results were comparable to those from the 60MPa NA concrete,indicating that the concrete produced from 60MPa RCA exhibited a delayed strength gain when compared to the NA concrete.Results from compression testing confirmed the influence of the RCA source strength on the resultant concrete produced from the recycled aggregates.Fig.4͑c ͒shows that independent of ce-ment content,20MPa RCA reached a ceiling strength of about 52MPa at 56days,indicating that it is not feasible to use 20MPa RCA in the production of 60MPa concrete.The compressivestrength of RCA concrete mixes was found to be governed by the cement content ͑with water content constant ͒,and not the com-bined binder content,as mixes with FA achieved comparable compressive strengths as mixes with D.C.͑see Fig.5͒.Comparing mixes designated NA with mixes designated D.C.,which had the same cement content,mixes with D.C.had a reduction in 28-day compression strength of 20–25%.Flexural Testing ResultsFor each mix,one flexural beam was tested on the 14th day and two beams were tested on the 28th day.Fig.6͑a ͒shows that when manufacturing 20MPa concrete,20MPa RCA produced concrete with flexural strength characteristics closely matching those asso-ciated with the 20MPa NA mix,whereas when producing 40and 60MPa concrete,20MPa RCA failed to attain the flexural strength of the corresponding mix using NAs.Also,whenproduc-pression strength gain of RCAmixes pression strength gain of mixes with FA and D.C.D o w n l o a d e d f r o m a s c e l i b r a r y .o r g b y N o r t h C h i n a I n s t i t u t e o f W a t e r R e s o u r c e s a n d H y d r o p o w e r o n 02/25/14. C o p y r i g h t A S CE .F o r p e r s o n a l u s e o n l y ; a l l r i g h t s r e s e r v e d .ing 20MPa concrete,it was possible to achieve higher flexural strengths when using 40or 60MPa RCA,than when using NAs ͓see Figs.6͑b and c ͔͒.The 40MPa concrete that was produced using 40MPa RCA resulted in similar flexural strength character-istics to the 40MPa NA concrete ͓see Fig.6͑b ͔͒.The effect that different source strengths of RCA had on mea-sured flexural characteristics was found to be more pronounced than for the comparable effect on compressive strengths.A pos-sible explanation may be due to the failure mode of the flexural beams,as flexural failure “peels away”the concrete and therefore the measured flexural strength is more indicative of the interface strength between RCA and the surrounding concrete.Flexural strengths of all RCA mixes were similar and comparable to mixes with NA,and in particular,mixes with FA exhibited the highest flexural strengths of all the RCA mixes ͑see Fig.7͒.Compression strength and flexural strength results indicated that a higher source strength of RCA is required in order to pro-duce higher strength RCA concrete.This finding was in agree-ment with previously reported findings,which identified that the strength of RCA concrete is governed by the weaker of the inter-face between the stony particle and adhered mortar,and the inter-face between the adhered mortar and the new cement matrix.It was determined that all source strengths of RCA can produce 20MPa RCA concrete,but that only the 40MPa RCA and 60MPa RCA can produce 40MPa RCA concrete.It was found that the production of 60MPa RCA concrete required an extended curing period of 56days.Full-Scale TrialsThe remaining RCA stockpile from “Stage II:demolition and crushing”was used to produce RCA concrete for thefull-scaleFig.6.Flexural strengths of RCAmixesFig.7.Flexural strength of mixes with FA and D.C.D o w n l o a d e d f r o m a s c e l i b r a r y .o r g b y N o r t h C h i n a I n s t i t u t e o f W a t e r R e s o u r c e s a n d H y d r o p o w e r o n 02/25/14. C o p y r i g h t A S CE .F o r p e r s o n a l u s e o n l y ; a l l r i g h t s r e s e r v e d .trials.The purpose of this component of the study was to confirm that laboratory results could be replicated at commercial scale using truck batching.As there are no provisions in the relevant New Zealand concrete manufacturing standards pertaining to the use of RCA to manufacture concrete,it was determined that initial implementation was limited to driveway and sidewalk applica-tions.Recognizing that most concrete debris should have com-pression strengths ranging between 20and 40MPa,it was decided that the two mixes that had most practical application were the 40MPa mix with 40MPa RCA and the 20MPa mix with 20MPa RCA.A “specials bin”was used at the ready mix plant to contain the RCA,with quantities determined by the batcher who observed the weight of the specials bin from the computer read out.The TSMA was again used,and after the correct slump was attained by the slump technician,a wheelbar-row of concrete was collected from the back of the truck for concrete testing.When placed,concrete workers noted that the concrete finished well and was workable,and it was concluded that the 20MPa RCA concrete was indistinguishable from NA concrete.Fig.3͑a ͒shows the average slump of concrete mixes with mix strengths of 20MPa at 28days,with the data associated with “field trial mix with 20MPa RCA”being the average of two truck mixes.The figure shows that all mixes exhibited similar slump life,including those for NA concrete.However,for 40MPa con-crete there was greater variation in slump loss ͓see Fig.3͑b ͔͒and the rate of slump loss was higher than for comparable 20MPa concrete.Fig.8indicates that the field trial mixes produced com-parable concrete strengths to the laboratory mixes and to NA concrete.ConclusionsFollowing a review of previously published literature,it was de-termined that the use of RCA for concrete production in New Zealand ready-mixed concrete plant was feasible,and that a study was necessary to understand the resultant material properties when using RCAs having a variety of characteristics.Recycled aggregates were intentionally manufactured from concrete that had been specially manufactured and cured to ensure that all rel-evant characteristics of the original concrete were known.Demo-lition,crushing,and sorting were performed using conventional commercial equipment,and the properties of the resultant re-cycled aggregates were compared with those from NAs,with dif-ference primarily attributed to the effect of adhered mortar.Amatrix of mix types was developed to investigate the outcome when using a range of recycled aggregate strengths to produce low ͑20MPa ͒,medium ͑40MPa ͒,and high strength ͑60MPa ͒concrete.It was confirmed that all strength grades of RCAs were suitable to produce low strength concrete,but that the production of medium and high strength concrete required the RCA to have a source strength matching or exceeding the strength of the new concrete.Inclusion of FA reduced slump loss but did not signifi-cantly improve compressive strengths.The study concluded with field trials,where results and anecdotal evidence indicated that RCA concrete was indistinguishable from NA concrete.It is hoped that the results from this study will contribute to a positive cycle of increased confidence in the use of RCAs and eventually lead to the development of standard specifications enabling wide-spread use of RCA in New Zealand concrete production.AcknowledgmentsThe writers wish to thank Timothy Walker of Conslab Ltd.,Ken Duncalfe of Fulton Hogan Ltd.,Opinda Saggi,Chris Munn,and Jim Shaw of Allied Concrete Ltd.,and Dr.James Mackechnie from the University of Canterbury for their invaluable assistance with this project.ReferencesAlexander,M.G.͑1996͒.“The effects of ageing on the interfacial zone in concrete.”RILEM Report ,J.C.Maso,ed.,V ol.11,150–174.Bairagi,N.K.,Vidyadhara,H.S.,and Ravande,K.͑1990͒.“Mix design procedure 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