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Characterization of the early age properties of concrete is an ongoing area of research particularly important in understanding longer-term behavior of pre-stressed and conventional concrete members subjected to varying load histories, including strain reversals, such as may occur during placement of a concrete deck atop pre-stressed girders. Models currently in widespread use specifically exclude cases where strain reversals are present. Research presented in this paper was undertaken to refine the understanding of the viscoelastic behavior of cement paste at an early age when subject to strain reversals. A series of small-scale beams were tested in 3-point loading with the center point displaced at irregular time intervals. At some time intervals, beams were displaced negatively, inducing a strain reversal. Results indicate significant load loss in the first few days followed by steady relaxation throughout the length of testing. Results further indicate that load relaxation after reversal is significantly slower than that with same-direction increases in load. Finally, measurements of Young’s modulus of elasticity throughout the testing program indicate that sustained loading leads to an apparent increase in beam stiffness compared to unloaded beams of the same age. The explanation and significance of these phenomena are explored.

Effects of strain reversal on elastic and viscoelastic properties of early age cement paste beams