| The waste concrete is processed into recycled coarse aggregate(RCA)by crushing and screening,partially or completely replacing natural coarse aggregate,and recycled concrete(RAC)for structure is prepared by multi-generation cycle,which is a major demand for sustainable development of concrete industry.At present,the research on multi-generation recycling of waste concrete mainly focuses on the general atmospheric environment,and rarely involves the complex environment.This paper studies the potential and sustainability of multi-generation recycling of waste concrete in freezing and thawing environment in severe cold regions.Based on the environmental similarity theory,the cycle mode of ’commercial RCA of waste concrete-i generation RAC-i+1 generation RCA(i = 1,2,3)’ was adopted to simulate the working state of each generation of RAC in freeze-thaw environment for 50 years by loading fast freeze-thaw method.The mechanical properties,frost durability and microstructure evolution of multi-generation RAC were investigated.By studying the changes of RCA mechanical properties and microstructure,the potential of multi-generation recycling of waste concrete was analyzed.Based on emergy analysis theory,the sustainability of multi-generation recycling system of waste concrete was evaluated.Specific research work is as follows:(1)First of all,commercial RCA was used to prepare RAC.With the design life of 50 years,the number of accelerated freeze-thaw cycles of400(taking Changchun as an example)and the stress level of 30%ultimate tensile stress as the condition,the simulated freeze-thaw test for50 years in cold regions was carried out.RAC was prepared after crushing and recycling.The mechanical and frost resistance evolution of the three generations of RAC were studied and the corresponding maximum and optimal replacement rates were defined.The evolution of mechanical properties of the three-generation RAC showed that the compressive and splitting tensile strength of the three-generation RAC increased first and then decreased with the replacement of RCA,but its effect on the splitting tensile strength was more obvious.Under the same substitution rate,the strength of the second-generation RAC is the highest and the lowest.The maximum replacement rate of the first,second and third generation RAC based on mechanical properties was 100%,and the optimal replacement rates were 42%,63% and 50%,respectively.(2)The evolution of frost resistance and durability of three generations of RAC shows that the change of mass loss of three generations of RAC can be divided into three stages: mass increase(0-100freeze-thaw cycles),gradual decrease(100-300 freeze-thaw cycles)and accelerated decrease(300-400 freeze-thaw cycles).Relative dynamic elastic modulus(RDME)can also be divided into three stages in the same range.The second-generation RAC had the best frost resistance,and the mass loss and RDME remained gradually decreasing in the third stage.The maximum substitution rates of the first,second and third generation RACs determined based on frost resistance are 100%,and the optimal replacement rates are 25%,75% and 50%,respectively.(3)The physical properties of RCA obtained by crushing RAC after freeze-thaw cycle were tested to determine the utilization potential of multi-generation RCA.The results show that the yield of RCA in freeze-thaw environment is lower than that in atmospheric environment.With the increase of RCA cycle numbers,the particle size becomes finer,the water absorption,crushing value and adhesive mortar content increase gradually,but the apparent density change is not obvious.The increase of RCA replacement rate in parent concrete has a negative impact on the physical properties of the next generation of RCA,but they all meet the requirements of class III quality in the specification,which can be used for the preparation of structural concrete.The recycling potential of waste concrete can be improved by reducing the RCA replacement rate in the parent concrete.(4)The emergy analysis theory is used to evaluate the sustainability of RAC subsystem(single-generation RAC)and three generation RAC circulatory system(first,second,third RAC composite).The results show that the emergy input and output of each generation of RAC subsystem increase with the increase of RCA replacement rate and cycle numbers,and the sustainable development index ESI is greater than 1.Under the replacement rates of 25%,50%,75% and 100%,the ESI of the three generation RAC circulation system is 1.44E+00,3.26E+00,5.57E+00,and8.58E+00,respectively,which is greater than that of the subsystem.The results of emergy-value coefficient show that the overall performance of RAC system is the best under the 50 % replacement rate cycle mode,and the emergy-value coefficient is the highest under the 100 % replacement rate cycle mode.The concrete multi-generation cycle with 100 %replacement rate has the highest market competition potential and sustainable development ability.This study is of great significance to improve the concept of concrete recycling,promote the application of RCA in structural concrete in northern China,and improve the utilization level of recycled aggregate.However,there are also shortcomings in the study.The aggregate should be recycled under the optimal replacement rate to determine the maximum number of cycles.In addition,the multi-generation recycling of waste concrete under other durability environments should be systematically studied,and the applicable engineering categories of RCA should be defined. |
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