Study on the sulfate freeze thaw resistance of recycled coarse aggregate self-compacting concrete

Concrete buildings in outdoor environments in northern China will suffer from sulfate freeze-thaw effects year-round, which will seriously threaten the long-term safe use and service life of concrete. The construction industry is considered to be one of the key sectors for achieving sustainable development goals, and this view is widely recognized in industry and academia. In this paper, the sulfate freezethaw resistance values of recycled coarse aggregate self-compact concrete (RCASCC) under the influences of three recycling coarse aggregate (RCA) replacement rates, four freezethaw environments, and six freezethaw cycles were studied. It was concluded that the larger the RCA substitution rate was, the more severe the surface degradation and failure morphology of RCASCC. Among the four freeze-thaw environments, the surface deterioration of RCASCC was the most serious in the 5% MgSO4 solution freeze-thaw environment, and the surface deterioration and failure morphology of RCASCC were the slightest in the clear water freeze-thaw environment. In addition, the compressive and splitting tensile strength loss rate of RCASCC decreases the most in a 5% mass fraction MgSO4 solution freeze-thaw environment. Among the six groups of RCASCC, RCASCC with RCA substitution rate of 0 had the largest relative peak stress and the smallest relative peak strain in the clear water freeze-thaw environment. RCASCC with RCA substitution rate of 50% had the smallest relative peak stress and the largest relative peak strain in the 5% mass fraction MgSO4 solution freeze-thaw environment. The uniaxial compressive stressstrain constitutive model was used to fit the stressstrain curves of RCASCC before and after sulfate freezethaw cycling. The elastic modulus and Poisson's ratio values of RCASCC in the six groups decreased with increasing number of freezethaw cycles. The larger the substitution rate of RCA was, the smaller the elastic modulus and Poisson's ratio of RCASCC. The elastic modulus and Poisson's ratio of RCASCC in clear water freeze-thaw environment are the largest, and the elastic modulus and Poisson's ratio of RCASCC in 5% MgSO4 solution freeze-thaw environment are the smallest.