Crack resistance, permeability, and microstructural analysis of recycled aggregate concrete with SAP

Abstract

High shrinkage and cracking performance are among the main problems that hinder the wide application of recycled aggregate concrete (RAC). To analyze the influence trend and mechanism of superabsorbent polymers (SAP) on the crack resistance of RAC, the effects of the recycled coarse aggregate (RA) content and SAP content on the crack resistance of RAC were studied via a slab cracking test. Moreover, the compressive strength, chloride permeability, pore distribution, and microstructure of the concrete in each mixture were also tested. The results showed that the incorporation of RA will have a negative impact on the crack resistance of concrete. The greater the amount of RA, the shorter the initial cracking time of the concrete, and the larger the crack width and area of the concrete. The initial cracking time, maximum crack width, and cracking area of the RAC tended to decrease with increasing SAP content. However, the crack resistance of the RAC does not seem to change much when the SAP content exceeds 0.3%, and even the local cracks in the slab specimens of the RAC increase in size. NMR and SEM analyses revealed that the water absorption-release and microreservoir characteristics of SAP affect the degree of the hydration reaction in the early stage, alleviate the internal drying of concrete, reduce shrinkage and inhibit the development of inner cracks, and effectively reduce the cracking sensitivity of RAC. This process can continuously provide water for the hydration reaction of cement when the SAP releases water in the later stage of curing, optimize the pore structure of the recycled concrete system, enhance its compactness, and improve its compressive strength and impermeability.