Optimization Of Recycled Concrete And Study On Flexural Behavior Of Geopolymers Recycled Concrete Beams

With the rapid development of the urbanization,the shortage of natural sand & gravel and the environmental problems brought by the production of cement have become bottlenecks in the development of urban construction.In this paper,a kind of energy-saving and environment-friendly geopolymer recycled concrete,in which cement is replaced with geopolymer and natural aggregate is replaced with recycled aggregate,is introduced.This new kind of concrete could ease the current situation of the shortage of sand & gravel and the environmental problems caused by the cement production.Experimental research is conducted from the point of material level and component level;the recycled concrete is optimized and the flexural behavior of geopolymer recycled concrete beams is studied.The main contents and conclusions of this paper are as follows:(1)Through comparing the current classification standards for natural coarse aggregates(NCA)and recycled coarse aggregates(RCA),and analyzing a great amount of experimental data from previous literatures,based on the experimental results from the validation test in this study,a uniform classification method was proposed for NCA,RCA and MCA(mixed coarse aggregates,namely the mixture of NCA and RCA with different proportions).Water absorption and crushing index of aggregates were used as the main classification indexes to investigate the effect of RCA and MCA on compressive strength of recycled aggregate concrete when applying this method.The results from the study showed that when RCA and MCA of classⅠ,class Ⅱ and class Ⅲ were used to produce C35 concrete or below,the compressive strength of the recycled concrete would reach not less than 100%,90% and 80% of the ordinary concrete with NCA,respectively.However,when RCA and MCA were used for the preparation of C40-C60 concrete,the compressive strength of the recycled concrete may decrease by 10% further.(2)The effects of different replacements of recycled aggregate on the compressive strength of recycled concrete were studied.When the replacement ratio of recycled aggregate was relatively small,the performance of recycled concrete made by recycled aggregate with small particle size(large coarse aggregate remains the NCA)was found better.While if the replacement ratio was relatively large,the performance of recycled concrete made by recycled aggregate with large particle size(small coarse aggregate remains the NCA)was found satisfatory.(3)The effects of water-binder ratio,concentration of sodium silicate solution,proportion of metakaolin and fly ash on the workability and compressive strength of geopolymer concrete and geopolymer recycled concrete were studied.The results showed that when the concentration of sodium silicate solution was 49% and the ratio of MK to FA is 1:1,the workability and compressive strength of geopolymer concrete and geopolymer recycled concrete is favorable,and the compressive strength of geopolymer recycled concrete is closer to that of geopolymer concrete.(4)Three ordinary concrete beams(including new and recycled ones)and seven geopolymer concrete beams(also including new and recycled ones)were tested under static load,to study the effects of cementitious materials,the replacement ratio of recycled aggregates,the replacement way and reinforcement ratio of steel bar on the the crack load,the mid-span deflection and the ultimate bearing capacity of the beams.The results showed that when calculating the flexural capacity of concrete beams using the formula of flexural capacity of ordinary concrete beams,it is necessary to multiply the a reduction factor of 0.9.Since the elastic modulus of geopolymer concrete is lower than that of ordinary concrete,the deflections of geopolymer concrete beams(including new and recycled)are greater than that of ordinary concrete beams(including new and recycled)under the same experimental conditions.The flexural capacity of recycled aggregate concrete beams is not affected by the replacement ratio of recycled aggregates.Under the same replacement ratio,the cracking moment of ordinary recycled concrete beams and geopolymer recycled concrete beams made by recycled aggregates with large particle size is higher than that of which made by recycled aggregates with equivalent particle size.