Elastic and creep properties of young cement paste, as determined from hourly repeated minute-long quasi-static tests

Cement pastes are highly creep active materials at early ages. We here characterize both the elastic stiffness and the creep properties of ordinary Portland cement pastes conditioned at 20 °C. Three different compositions are investigated, defined in terms of initial water-to-cement mass ratios amounting to 0.42, 0.45, and 0.50, respectively. Implementing a new early-age creep testing protocol, we perform a series of 168 three minute long uniaxial macroscopic creep tests on the aging materials, with one such test per hour and corresponding material ages spanning from 21 h to approximately eight days. In this way, it is guaranteed that the material microstructure remains virtually unaltered during each individual creep test, while subsequent creep tests refer to clearly different microstructures. In order to minimize material damage, the compressive loads are restricted to at most 15% of the uniaxial compressive strength reached at the time of testing. The loading protocol consists of quasi-instantaneous compressive loading and unloading steps as well as a three minute long holding period in between. Representing the measured compliances very precisely by means of a power-law expression including elastic and creep moduli, as well as a creep exponent, while requiring the elastic and creep strains to be compressive at all times, yields concavely increasing time evolutions of elastic and creep moduli, as well as slightly decreasing or quasi-constant evolutions of the creep exponent. Combination of these results with calorimetry-based evolutions of the degree of hydration yields linear elasticity-hydration degree and over-linear creep modulus-hydration degree relations, while the creep exponents (slightly) decrease with ongoing hydration. The herein quasi-statically determined elastic moduli agree very well with those determined ultrasonically on the same cement pastes. This impressively underlines the fundamental characteristics of elastic properties being related to an energy potential, independently of loading paths and corresponding strain rates.