Experimental investigation of the delamination behavior of curved post-tensioned concrete structures without through-thickness reinforcement

The recent delamination failure of the concrete containment structure wall at the Crystal River Unit 3 Nuclear Power Plant has led to increased interest in the mechanical behavior of curved post-tensioned structures. Extensive investigations have been performed to identify the causes of the delamination failure, but no experimental research has focused on the behavior of curved post-tensioned structures. Analytical studies focused on the through-thickness stress development stemming from prestressing forces have been performed, yet there is no experimental data to verify the results. This research examines the behavior of curved post-tensioned structures and their delamination failures through the structural testing of two curved post-tensioned wall specimens. Two 90° curved post-tensioned wall specimens were constructed and tested under monotonically increasing prestressing loads at the University of Texas at Austin. In an effort to gain insight into the size effect associated with delamination failures, the dimensions of the second specimen were doubled from the first. The specimens were well-instrumented to ensure the delamination behavior was adequately captured. Using the output from load cells positioned at the live-end and dead-end of each curved wall specimen, the friction losses, forces, and stresses developed at various locations along the curved wall sections were determined. In order to directly compare the behavior of both specimens, the applied stresses were normalized with respect to the measured concrete compressive strength. The test results were used to determine the underlying mechanisms of the curved post-tensioned concrete structures, such as the size effect of delamination failure and initiation of delamination cracking. The experimental results indicated an apparent size effect on the delamination resistances of the curved wall specimens, with a 32 % decrease in the normalized capacity from Specimen 1 to Specimen 2. In addition, the compressive stresses at the initiation of the delamination crack were 0.13f [subscript c]’ ~ 0.23f [subscript c]’, which are significantly less than the allowable stress limit of 0.35fc’ specified in ASME Boiler and Pressure Vessel Code (BPVC) Section III, Division 2 for the service load condition. Lastly, the measured friction losses were 38%~43% greater than the friction losses calculated based on ACI 343R-95.