New insights into linear-nonlinear tensile creep mechanisms of UHPC: Stress level-dependent creep-active site transitions

This study elucidates the micromechanical origins of tensile creep in ultra-high performance concrete (UHPC) under various stress levels by systematically investigating the distribution and evolution of creep-active sites (high-strain micro-zones governing creep behavior) across multiple scales. Results reveal that, at stress levels below 30 %, creep-active sites are randomly distributed throughout the specimen. The slip of C-S-H gel induces synchronized deformation rates in both the matrix and weak zones (e.g., interfacial transition zone (ITZ) and macroscale defect). As stress levels increase (40 %–60 %), creep-active sites localize at 60–800 nm wide microcracks, with strain accumulation rates in weak zones progressively exceeding those in the matrix. At stress levels above 70 %, intensified strain concentrations in ITZ and macroscale defect trigger fracture-dominated nonlinear creep. This work unifies micro-crack theory and C-S-H gel slip mechanisms, demonstrating that the development from linear to nonlinear tensile creep behavior originates from stress level-dependent creep-active site transitions.