Influence factors and prediction methods for flexural strength of recycled aggregate concrete

To investigate the influence of recycled coarse aggregates (RCA) on the concrete flexural strength, systematic experimental studies were conducted and the test parameters included the replacement ratio of RCA, water-to-cement ratio of parent concrete, service age of parent concrete, size distribution of RCA particles, water-to-cement ratio of recycled concrete and mixing method. The key influence factors were determined based on the test results. Existing equations to predict the flexural strength of recycled aggregate concrete (RAC) were summarized and benchmarked by the test results collected from this study and literatures. An equation to predict the flexural strength of RAC was proposed by incorporating the key factors. The results indicate that the replacement ratio of RCA and the size distribution of RCA particles are the key influence factors on the flexural strength of RAC. A reduction of 3.9% to 26.8% in the flexural strength is obtained for RAC prepared with 100% RCA when compared to natural concrete, and a variation of 15.5% is obtained in the flexural strength of RAC prepared with different size distributions of RCA particles. Larger influence magnitudes are obtained for RAC with larger water-to-cement ratios, and the difference in the reduction magnitudes of the flexural strength of RAC with different water-to-cement ratios can reach 14.7%. It is found that the use of RCA has larger influence on the compressive strength than on the flexural strength of concrete. Therefore, existing equations are not applicable to predict the flexural strength of RAC. The results obtained by the proposed equation, following the form of natural concrete equations and considering combined effects of RAC water-to-cement ratios and RCA replacement ratios, are in good agreement with the test results. The average ratio of predicted to experimental results is 0.991 and the correlation coefficient is 0.772. © 2019, Editorial Office of Journal of Building Structures. All right reserved.