Yunjian He

Traditional fiber-reinforced polymer (FRP) retrofit methods can restore the strength of reinforced concrete columns well, but stiffness is also partly restored. To increase the initial stiffness of retrofitted columns, this study investigated the seismic behavior of retrofitted resilient reinforced concrete (RRC) columns that were retrofitted by different methods, including high-strength mortar retrofit, carbon fiber-reinforced polymer (CFRP) retrofit, and CFRP and steel plate retrofit. In addition, the effect of the axial load was also considered. Quasi-static tests were conducted twice on five specimens, i.e., before and after repairing. The first test was used to create earthquake damage, and the second test was used to compare the seismic behavior of the retrofitted columns. The experimental results indicated that the CFRP retrofit method, whether with a steel plate or not, can restore the lateral resistance capacity well; furthermore, the drift-hardening behavior and self-centering performance were well maintained. The residual drift ratio of the CFRP-retrofitted column was less than 0.5%, even at a drift ratio of 3.5%, and less than 1% at the 6% drift ratio. However, the initial stiffness was only partly restored using the CFRP sheet. The introduction of steel plates was beneficial in restoring the initial stiffness, and the stiffness recovery rate remained above 90% when CFRP sheets and steel plates were used simultaneously. The strain distribution of the CFRP sheet showed that the steel plate did work at the initial loading stage, but the effect was limited. By using the steel plate, the CFRP hoop strain on the south side was reduced by 68% at the 6% drift ratio in the push direction and 38% in the pull direction. The axial strain of CFRP cannot be ignored due to the larger value than the hoop strain, which means that the biaxial stress condition should be considered when using an FRP sheet to retrofit RC columns.

In the precast building structure, the joint is the key part that affects the mechanical performance of the assembly structure. By selecting coarse and fine fraction aggregate; adding high quality mineral admixture, expansive agent, and water-reducing agent; and optimizing the mixture ratio design, precast members joint concrete (PMJC) with self-compacting performance, micro-shrinkage, good volume stability, and strength matching with precast concrete is developed. In this study, the PMJC with a water-binder ratio, fly ash, ultra-fine silicon powder, sand rate, expansive agent, and water-reducing agent of 0.38, 30%, 3%, 47.7%, 8%, and 0.7%, respectively, showed a good workability which could meet the conditions of on-site construction. In addition, the strength matched the precast members, and the PMJC had certain ductility and stability, small early contraction, and met the requirements of the joint material.