Strain-rate sensitivity of cement paste by microindentation continuous stiffness measurement: Implication to isotache approach for creep modeling

Strain rate (ε̇) of concrete is crucial for structures due to its significant effect on the mechanical properties of concrete. Understanding strain rate effect can ensure safe design of concrete structures with enhanced performance. In this study, strain rate effect on time-dependent deformation and mechanical properties of hardened cement pastes is investigated by performing continuous stiffness measurement (CSM). The w/c ratios of hardened cement pastes are 0.3, 0.4, and 0.5, the applied strain rates are 0.01 s− 1, 0.05 s− 1, 0.1 s− 1, and 0.5 s− 1. Experimental results show that the contact hardness (H) increases with increasing strain rate, and their relationship can be well captured by an empirical power law equation. However, strain rate has a negligible effect on the elastic modulus (E). The viscoplastic depth (hvp, shown in Fig. 2d)-force (P) curve, which is controlled by the micro creep behavior of cement pastes, is also affected by the strain rate. Similar to the long-term creep behavior of soils, a unique isotache-type hvp-P-ε̇ relationship is found for hardened cement pastes at micro level. The results preliminarily confirm the applicability of isotache approach to characterize the time-dependent behavior of cement pastes at micro level.