The Transport Mechanism Of Moisture And Chloride Ions In Recycled Concrete

There are many reasons why buildings fail to meet their service life,many of which are associated with durability failures,while most buildings in coastal areas are more susceptible to water and chloride.At the present stage,nonrenewable resources are gradually lacking.In response to the national call of environmental protection and energy conservation,the recycled aggregates,are widely applied in actual projects,i.e.cross-sea bridge,harbor and artificial reefs.Due to the unstable quality of recycled aggregates and complex internal structure,there are many differences of transport performance between recycled and ordinary concrete.At present,there is still a lack of research on the medium transport performance in recycled concrete at home and abroad.With help of the National Natural Science Foundation of China and the Natural Science Foundation of Shandong Province,based on the above considerations,in this paper the study on the transport of water and chloride ions in recycled concrete was carried out.The influence of recycled aggregate quality and substitution rate on the medium transmission process was analyzed.The specific contents of the research work and main conclusions are summarized as follows:(1)Preparation of class II?III recycled coarse aggregate in accordance with the test requirements.The capillary absorption test of different types of recycled concrete specimens was carried out.The influences of these factors,such as the quality and substitution rate of recycled coarse aggregate,water-to-binder ratio and chloride salt concentration,on the capillary absorption performance of recycled concrete were analyzed.As a result,it was found that the cumulated absorbed water volume and capillary absorption coefficient of the recycled concrete specimens with large replacement ratio(50% ? 100%)and poor quality were larger than those with small substitution rate(30%?0%)and good quality.(2)The water permeability coefficient is fitted with the square root of the water absorption time according to the hyperbolic decay model.The curve of the recycled concrete specimens is fitted.The results show that the specimen with large substitutionrate(50% ? 100%)and poor quality has a penetration depth greater than that of the specimen with small substitution rate and good quality.Under the same conditions,the transmission of chloride ions in the specimen lags behind the water transport,and the penetration depth of chloride ions and the depth of water intrusion exhibits the linear trend.The penetration depth of water in the specimen during capillary water absorption is approximately equal to that in the salt solution.(3)The intrusion depth and content of water and chloride ions under capillary absorption conditions were obtained by digital image processing,and the relationship of the quadratic roots with time was established.The experimental study on the rapid chloride migration(RCM)for different types of recycled concrete specimens was carried out.The resistance of chloride ion penetration in the different types of recycled concrete was discussed.The results show that the diffusion coefficient of chloride ion in recycled concrete increases with an increase of the replacement rate of recycled aggregate.For the first 12 hours,it exists the linear relationship between the transport rate of water and chloride ions.(4)The relationship between chloride ion diffusion coefficient,water intrusion rate and capillary water absorption coefficient was established,and the relationship between chloride ion diffusion coefficient,water intrusion rate and specimen strength was established.It is found that the chloride ion diffusion coefficient increases with the increase of water intrusion rate and capillary water absorption coefficient,and the chloride ion diffusion coefficient and capillary absorption coefficient are linearly related to the strength of the test piece.