| Recycled aggregate has been used as a replacement for known natural aggregates for a number of years.The future benefits of using recycled aggregate cannot be overemphasized.To make such benefits feasible,the engineering characteristics of recycled aggregate concrete(RAC)must be investigated and assured. Thus,this study was an attempt to evaluate extensive engineering characteristics of RAC.In order to minimize the number of samples required per experiment,Taguchi orthogonal optimization technique was adopted for this investigation.In this technique,only a few combination values of control factors required were chosen. The theory behind this technique,which needs to consider both the process of designing the experiment and the way of statistically analyzing the experimental data of response,is based on the usage of the orthogonal arrays,the analysis of variance (ANOVA),and the significance test with F statistics.Also,signal(S)to noise(N) ratio(S/N)were also applied in the analysis.The objective of S/N analysis is to determine the most optimum set of the operating conditions,from variations of the influencing factors within the results.All the tests were performed in accordance with Chinese codes and American Society for Testing and Materials(ASTM)standards.The experiments were carried out in two parts;in part one,the influence of varying the different contents of the recycled aggregate and fly ash on the physical properties of RAC was investigated.The properties investigated were the compressive strength,modulus of rupture(flexural strength),splitting tensile strength and modulus of elasticity.Orthogonal table with four levels and 3 factors was used to prepare the mixing proportions.The main factors used were the changes in water-cement ratio, the percentage of recycled coarse aggregates replacement with natural aggregate and percentage replacement of cement by fly ash.The levels represent the replacement ratios,and the replacement ratios used were;(ⅰ)Water/Cement Ratio(WC)=0.45,0.50,0.55,and 0.58(ⅱ)Percentage of recycled aggregate replacement=0,30,60 and 100(ⅲ)Percentage of fly ash replacement=0,20,30 and 40(ⅳ)Addition of steel fiber at 1.5%per m~3 of concrete to part of the mixtures so as to know the effect on the strength at 28 days.The mix target design strength was 30 Mpa at 28 days and the four design strengths were tested at 7,28,and 90 days.Mixtures containing steel fibers were tested only at 28 days.Regression analyses were performed(based on the orthogonal array results)between various strength values to obtain mathematical models for each tested property and also to relate one property to another,in order that the best fit equation be obtained.Results indicated that the compressive strengths,flexural strengths,splitting tensile strengths,and modulus of elasticity values of the concrete at all ages decreased as the recycled aggregate and water contents increased,but the differences in the measured properties of mixtures containing natural aggregates and recycled aggregate at each water/cement ratio was not very high,ranging from 0.5-20%for concrete mixtures with not more that 60%replacement of recycled aggregates.The addition of fly ash had little impact on the measured properties of the concrete but it reduced the water demand and improved the workability.Also,the addition of steel fibers enhanced the 28-day compressive and splitting tensile strengths with increment in the ranges of 10-30%and 27-41%respectively. In part two of this research,the durability of concrete with recycled aggregates exposed to freezing and thawing cycles was examined.Orthogonal table with three levels and four factors was used to prepare the mixing proportions.The main factors used were the changes in water-cement ratio,the percentage of recycled coarse aggregates replacement with natural aggregate,percentage replacement of cement by fly ash and percentage of total fresh air.Fresh air percentage variation was achieved through the addition of air-entraining agent in order to stabilize the air entrapped during the mixing in the form of very small,discrete bubbles known as entrained air. The levels also represent the replacement ratios,and the replacement ratios used were;(ⅰ)Water/Cement Ratio(WC)=0.45,0.50,and 0.55.(ⅱ)Percentage of recycled aggregate replacement=0,50 and 100(ⅲ)Percentage of fly ash replacement=0,20,and 30(ⅳ)Percentage of entrained fresh air=2.5,3.5 and 4.5The mix target design strength was also 30 Mpa at 28 days.Parts of the samples(100mm cubes and 100×100×400 prisms)were tested for compressive and modulus of rupture(flexural strength)strengths at 28 days while the remaining samples(100×100×400 prisms)were placed in the freezing and thawing machine to determine their frost resistance and factors that could affect their durability.The results showed that in saturated RAC,the recycled aggregate has negative effects on its durability performance.It is not satisfying,and their use in structures exposed to severe climate is not strongly recommended.But with the use of mineral and chemical admixtures,particularly air-entraining admixtures and fly ash,the recycled aggregate concrete could exhibit some level of durabilityRegression analyses were performed(based on the orthogonal array results) on the results between various strength values to obtain mathematical models for each tested property and also to relate one property to another,the best fit equations were obtained.In General,from the strength point of view,it may be concluded that recycled aggregates compared well with natural aggregates up to about sixty percent replacement particularly at lower water-cement ratio and therefore could be considered for various potential applications.However,from the frost resistance point of view,there is need for more research into how to improve the durability of recycled aggregates exposed to freezing and thawing cycles.
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