Prediction model for the axial compressive stress-strain relationship for recycled aggregate concrete considering size effect

Over the past decades of extensive research, recycled aggregate concrete (RAC) has become a well-established material for structural engineering applications. However, the size effect (SE), a critical factor in concrete structural design, has not been sufficiently studied in RAC. The use of recycled aggregates (RAs) influences the SE not only on compressive strength but also on peak strain. To date, experimental studies on the SE of the axial compressive stress-strain relationship of RAC have been limited to 200 mm specimens, and none have successfully captured the descending branch or proposed a corresponding model. To address this gap, this study conducted axial compression tests on 36 cylindrical RAC specimens to investigate their stress-strain relationship with a descending branch. The test variables included the specimen size (varying between 150 mm and 400 mm) and the replacement ratio of coarse recycled aggregate (CRA) (0–100%). A specialized experimental setup was developed to successfully capture the stress-strain curves, including the descending branch, for large-size RAC specimens. Based on the test results, the impact of the SE on key parameters such as the peak strain and the shape parameter of the descending branch of RAC was analyzed, and the impact of RAs on the SE was quantified. Based on the test data, this study derived an SE prediction model for the axial compressive stress-strain relationship of RAC by integrating the crack tip opening displacement (CTOD) model with the crack band theory. The model was verified against the test data from this study, along with 32 additional datasets on the axial compressive stress-strain relationship of natural aggregate concrete (NAC) and RAC collected from the literature. The test results indicate that, within the tested size range, the SE had a considerable effect on the peak strain of concrete, whereas its effect on ductility and elastic modulus was relatively minor. Within the tested size range, the relative size-induced reduction in peak strain for specimens with 100% CRA replacement was 66.3% larger than that for NAC specimens. The predictions of the proposed model are in good agreement with the test results. The mean ratios of predicted-to-experimental values for peak strain, elastic modulus, and ductility index range from 0.985 to 1.025, with corresponding coefficients of variation ranging from 0.037 to 0.082.