| The strain-rate sensitivity of the cement paste and mortar constituents of concrete is studied both experimentally and analytically. Cement paste and mortar specimens are loaded in compression using seven strain rates, ranging from 0.3 to 300,000 microstrain/sec. Water-cement ratios of 0.3, 0.4, and 0.5 are used. Specimens are loaded to 15,000 microstrain 27 to 29 days after casting. Strain-rate sensitivity is measured in terms of the initial modulus of elasticity, Poisson's ratio, and stress and strain at failure.; An analytical model of a porous solid is developed to study and simulate the rate sensitive behavior of the materials. The model consists of spherical grains and saturated spheroidal pores in communication with unsaturated regions. Movement of pore fluid results in a strain-rate sensitive response to load. The model is used to duplicate the strain-rate sensitive initial elastic moduli, and to simulate basic creep strains of cement paste.; The initial elastic moduli at low stresses and the strength of cement paste and mortar increase by 7 percent and 15 percent, respectively, with each order of magnitude increase in strain rate. Poisson's ratio is more strain-rate sensitive, as the strain increases. The strain at the maximum stress is the greatest for the lowest strain rate. With an increase in strain rate, the strain at the maximum stress first increases and then decreases.; To simulate the strain-rate sensitivity of cement paste, the analytical model requires the representative pore shape to be a flat oblate spheroid. The model duplicates the strain-rate sensitive initial elastic moduli of cement paste, and explains most of the nonlinearity of the stress-strain curve at low stresses. The model closely simulates the short-term basic creep strains under low sustained stresses. In the long-term, the analytical creep strains are expected to be lower than the experimental values due to continued hydration, and maturation creep in cement paste. |
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