Creep Behavior of Limestone Calcined Clay Cement Concrete under Axial Compression

To advance the practical application of limestone calcined clay cement (LC3) concrete in engineering structures, this study examines its creep behavior under axial compression for 180 days. Nineteen LC3 concrete cylinders were tested, considering different strength grades (high-performance and normal-strength LC3 concrete), calcined clay contents (20% and 30%), and stress levels (0.3–0.5 stress-to-strength ratios). Results revealed accelerated creep development during initial loading, stabilizing after two months. FRP jacketing at a low stress-to-strength ratio may only provide a sealed condition for concrete core rather than confinements. The estimated drying creep strains approximately accounted for 20% of the total creep strains. Normal-strength LC3 specimens demonstrated 20–30% greater creep coefficients than high-performance LC3 concrete counterparts, attributable to its higher water-to-binder ratio, porous interfacial transition zones, and differences in hydration products. Microstructural analysis showed that 30% calcined clay content in high-performance LC3 concrete produced a less dense C-(A)-S-H matrix with floc-like morphology compared to the 20% mixture, explaining its 12% higher creep. The nonlinear creep threshold exceeded 0.5 for high-performance LC3 concrete with 30% calcined clay, though this limit may vary with different replacement ratios. Whereas existing ultra-high performance concrete models could predict creep in high-performance LC3 concrete with reasonable errors, applications of existing creep models to normal-strength LC3 concrete required substantial modifications. These findings provide critical insights for implementing LC3 concrete in structural applications while highlighting the importance of LC3 composition-dependent creep characteristics.