Effect of X-ray CT characterized pore structure on the freeze-thaw resistance of 3D printed concrete with recycled coarse aggregate

The application of concrete 3D printing technology in cold regions still faces durability challenges. The incorporation of coarse aggregates can significantly enhance the durability of 3D printed concrete, making it a priority consideration for optimizing the performance of 3D printed mortar. This work focuses on pore structure characteristics and investigates the effects of varying recycled coarse aggregate (RCA) replacement ratios (0 %, 50 %, 100 %) on the apparent morphology, mass loss, and relative dynamic elastic modulus of 3D printed recycled aggregate concrete (3DPRAC) under freeze-thaw (F-T) cycling conditions. Comprehensive analyses of the internal pore structure of 3DPRAC were conducted via X-ray computed tomography and scanning electron microscopy. The results showed that the F-T resistance of 3DPRAC was inferior to that of cast concrete. Although the incorporation of RCA exacerbates F-T damage in 3D printed concrete, this damage did not necessarily worsen with increasing RCA replacement ratios. The unique ellipsoidal pores in 3DPRAC, along with the porous old mortar and its ITZ contained in the RCA, were identified as key factors in degrading the F-T resistance of 3DPRAC by promoting the development of crack networks. Finally, the degradation mechanisms of the F-T resistance of 3DPRAC based on pore structure characteristics were discussed. This research deepens the understanding of the durability performance of 3DPRAC and may facilitate the application of digital concrete manufacturing technology in cold regions.