Study on the deterioration characteristics and mechanisms of recycled brick-concrete aggregate concrete under load-freeze-thaw coupling

Abstract

Using construction waste to prepare recycled concrete is an important way to realize the resource utilization of construction waste and solve the shortage of sand and gravel resources in China. However, the current separation process makes it difficult to completely separate recycled brick aggregate (RBA) and recycled concrete aggregate (RCA), and the cost of high-precision separation is high. Therefore, the mixed-use of RBA and RCA to prepare recycled brick-concrete aggregate concrete (RB-CAC) can effectively reduce the pre-separation cost and has good economic and engineering applicability.In northern China, concrete is often subjected to load-freeze-thaw coupling. However, there are few studies on RB-CAC, and there are fewer studies on its degradation characteristics and degradation mechanism under load-freeze-thaw coupling. Therefore, based on the previous research on the mechanical properties of RB-CAC, RB-CAC was prepared by mixing RBA and RCA in a ratio of 1:1 and replacing natural coarse aggregate (NA) with a replacement rate of 30%. The macroscopic properties and degradation mechanism of RB-CAC under load-freeze-thaw coupling were studied by compressive strength tests, SEM, microhardness tests, and NMR tests. The quantitative relationship between microstructure and compressive strength damage was established, and the deterioration characteristics and mechanism of RB-CAC were explored. The results show that: (1) With the increase in stress ratio, the frost resistance of RB-CAC decreases gradually. Under the stress ratios of 0, 0.2, 0.4, and 0.6, the maximum number of freeze-thaw cycles that can be tolerated is 170 times, 150 times, 130 times, and 60 times (brittle fracture occurs when the stress ratio is 0.6, and freeze-thaws 70 times). After 60 freeze-thaw cycles, compared with 0 stress ratio, the mass loss rate under 0.2, 0.4, and 0.6 stress ratio increased by 0.33%, 0.42%, 1.61%, and the relative dynamic elastic modulus and compressive strength decreased by 4.73%, 2.71%, 14.13%, and 0.25 MPa, 1.07 MPa, 3.58 MPa, respectively. (2) The performance of RCA and the old mortar interface area is better than that of RCA and the new mortar interface area, and the performance of RBA and the new mortar interface area is better than that of RBA and the old mortar interface area. With the increase in stress ratio and freeze-thaw times, the interface thickness of RCA-old mortar is lower than that of RCA-new mortar, the interface thickness of RBA-old mortar is higher than that of RBA-new mortar, the interface hardness of RCA-old mortar is higher than that of RCA-new mortar, and the interface hardness of RBA-old mortar is lower than that of RBA-new mortar. (3) With the increase of the applied stress and the number of freeze-thaw cycles, the effective pore of the specimen shows a gradually decreasing trend, and the invalid pore shows a gradually increasing trend. The total porosity increased by 0.78%, 2.13%, 7.39%, and 0.71%, 2.766%, 3.50%, 4.83%, respectively, indicating that the size of the stress ratio has a greater influence on the porosity than the number of freeze-thaw cycles. (4) The fractal dimension was introduced to characterize the deterioration characteristics of RB-CAC, and the macro-micro relationship was established. The regression coefficients were 0.8784 and 0.9938, respectively. (5) The correlation analysis between the parameters of the multiple interface transition zone and the compressive strength was carried out, and the regression coefficients were all greater than 0.9. The greater the thickness of the interface zone, the smaller the hardness and the lower the compressive strength of the concrete.The research results provide a reference for reducing the separation process requirements of RBA and RCA, reducing the preparation cost of recycled concrete, and further developing high-frost resistance RB-CAC suitable for engineering applications in cold regions.