Defining Concrete Compressive Strain in Unbonded Post-Tensioned Walls

The nominal flexural strength of a concrete member is determined when the strain at the extreme compression fiber reaches 0.003, which implies an accurate measurement or calculation of the concrete strains in the member's critical region. The challenges associated with defining this strain using experimental and analytical means were demonstrated for precast concrete walls with unbonded post-tensioning. A series of laboratory tests was conducted to verify the critical compressive strains in post-tensioned (PT) precast concrete walls, which had no special confinement reinforcement in the wall toe. Test results confirmed the expected behavior of the wall panels and indicated that the onset of visible crushing in the wall toe may not occur until compressive strains develop that are significantly greater than 0.003. Despite employing several techniques, however, including displacement gauges, strain gauges, and photogrammetry, obtaining reliable and accurate experimental concrete strain measurements in the toe region of the wall proved challenging. To improve the understanding of the strain quantification at the nominal limit state, finite element analyses were conducted and the analysis results were compared with the experimental data. While some of the measured and analytically estimated strains provided comparable results in the critical wall toe region, a large spread in measured strains was observed and the average from several measurement devices was required for an accurate estimation of the nominal limit state. Overall, the three-dimensional (3-D) finite element model enabled an accurate and reliable estimation of the average concrete compressive strain. Based on this research, a strain of 0.005 is recommended for defining the nominal flexural strength of PT concrete walls. [PUBLICATION ABSTRACT]