Study On Capillary Water Absorption Of Recycled Concrete Under Freeze-Thaw Cycle

As China’s old cities are being renovated and new cities are being renewed,a large amount of construction waste has emerged.Practising the concept of sustainable development and recycling construction waste into new green concrete has important economic,social and environmental benefits.Due to the unstable quality of recycled aggregates(RCA)and their complex internal structure,the recycled concrete(RAC)prepared from them has a very different media transport performance from that of ordinary concrete.The rate of moisture transport within the material matrix directly affects the rate of damage to RAC under freeze-thaw action.Capillary water absorption ’is the main mode of moisture transport within the matrix of most in-service concrete structures in engineering practice,therefore,capillary water absorption performance is of great significance to the study of the frost resistance of RAC.In this paper,the effects of RCA(0,20%,40%and 60%)substitution rates and the number of freeze-thaw cycles on the performance of RAC are investigated.The macroscopic capillary water absorption energy and microscopic internal structural characteristics are used to evaluate the frost resistance of RAC,and the factors affecting the capillary water absorption energy of RAC are analysed using grey correlation analysis,and the optimal mix ratio is determined.The main research elements are:(1)The frost durability of RAC was analysed by the changes in the indices of apparent morphology,mass,relative dynamic elastic modulus and compressive strength of the test blocks.As the number of freeze-thawing increases,the apparent morphology can be divided into four stages:intact,rough,pitted and peeled;its mass loss is the first to decrease and then gradually increases;its relative dynamic elastic modulus and compressive strength gradually decrease.(2)The capillary water absorption test was carried out by taking the RAC of freeze-thaw 0 times,25 times,50 times and 75 times,and by testing the mass of each group of specimens before and after water absorption,the cumulative water absorption curve of each group of specimens was obtained,and the initial water absorption rate,secondary water absorption rate and late water absorption rate could be obtained by linear fitting of this curve in stages.Based on the initial water absorption rate,a prediction model for the initial water absorption rate of RAC under freeze-thaw conditions was established to characterise the capillary water absorption performance of RAC and provide a theoretical reference for the freezing resistance of RAC in cold regions.(3)Electron microscopic scanning tests were carried out on RAC with 0 and 50 freeze-thaw cycles to investigate the internal structural changes and to analyse the effect of capillary water absorption on the freezing resistance from a microscopic perspective.The microscopic results coincided with the trend of the water absorption curve obtained from the water absorption test.Under the same freeze-thaw environment,there was a relationship between the degree of denseness of the internal structure of RAC and the RCA substitution rate,and the degree of denseness was related to the water absorption energy of RAC.(4)The grey correlation analysis was used to analyse the main factors affecting the frost resistance of RAC and to select the optimum mix ratio for this test.The most important factors affecting the frost resistance of RAC are the initial and cumulative water absorption rates,and the initial water absorption rate can reflect the overall water absorption performance of RAC.Therefore,a prediction model for the initial water absorption rate of RAC under freeze-thaw conditions was established to provide a theoretical basis for the frost resistance of RAC in cold regions.