Linearity and assessment of micro-scale creep in C-S-H via nanoindentation

This paper focuses on quantifying the micro-scale creep compliance of the C-S-H phase in well-hydrated, 1.5-year-old cement paste, measured using grid nanoindentation with a Berkovich tip.Four different load levels were tested to induce varying levels of deformation and stress.The results showed that the measured creep compliance is independent of the maximum vertical force within the range of 1 mN to 3 mN, standardly used in nanoindentation of cement pastes.The same trends and conclusions were observed for a reference polymeric material, PMMA, chosen for its homogeneity.In both cases, the creep falls in the linear viscoelastic regime.The linearity was further checked by calculation of the stress developed beneath the indentation tip.The stress was estimated using a finite element numerical model simulating the contact problem.The calculated stress reached 61.4% of the compressive strength measured by micro-pillar testing, 17.1%-46.1% of the strength measured by micro-beam testing, and 29.0% of the strenght estimated by the multiscale model.