Impact of Waste Rubber Powder Size and Replacement Ratio on the Compressive Strength Anisotropy and Pore Structure of 3D Printed Concrete (2026-02)¶

Waste rubber powder, when incorporated into 3D-printed concrete, effectively improves its brittleness but simultaneously alters its compressive strength and pore structure resulting from the low elastic modulus and hydrophobic surface of rubber powder. In particular, the relationship between strength anisotropy induced by the layer-by-layer printing process and compressive strength remains underexplored. Present study explores the impact of rubber powder size and replacement ratio on the buildability and compressive strength anisotropy of 3D-printed concrete. Pore morphology and internal features were examined using mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM), and their relationships with fractal dimensions, compressive strength, and the anisotropy index (IA) were analyzed. Results show that rubber powder significantly enhances flowability; however, excessive incorporation of finer particles may cause segregation, compromising buildability and potentially leading to printing failure. The buildability, characterized by tanθ, deteriorated most at a 10% rubber replacement ratio, suggesting this threshold should not be exceeded in practice; meanwhile, incorporating 5% of 20-mesh rubber powder effectively minimized compressive strength anisotropy, indicating its potential as an optimal balance between buildability and structural integrity. Furthermore, while the fractal dimension of capillary pores positively correlates with compressive strength in the loading direction (FZ direction), that of macropores shows a strong power-law relationship with IA, highlighting its dominant role in the development of strength anisotropy.