The viability of partially post-tensioned concrete members in an aggressive environment, including cyclic loading.

摘要

Partially prestressed concrete offers several advantages over fully prestressed designs including increased ductility, increased energy absorption, decreased cost, decreased camber, and decreased anchor zone congestion. However, design codes have been slow to adopt provisions for the design of partially prestressed concrete because of concerns over fatigue and corrosion. The overall objective of this study was to determine the viability of partially post-tensioned concrete members in an aggressive environment, including the impact of cyclic loading.With regard to the individual strand testing, there was a significant drop-off in fatigue capacity at relatively small pit depths. On average, the fatigue capacity was reduced 50 percent for pitting that measured 0.010 to 0.015 inches in depth. Furthermore, an empirical relationship based on an exponential decay function was found to be the most reliable method of predicting the response of corroded prestressing strand for the specific sequence of corrosion followed by fatigue to failure. In particular, average pit depth offered the best correlation between fatigue capacity and the amount of localized corrosion.With regard to the full-scale beam testing, there were several conclusions reached as to the behavior of the tendon---duct, grout, and strand---under exposure and fatigue testing. First, a robust plastic duct is required as it serves as the primary protection method for the tendon and also performs very well under cyclic loading. Furthermore, the plastic duct requires steel or plastic saddles at tendon deviation points to eliminate the potential for puncturing the duct during the strand stressing operation. Next, without full encapsulation of the strand by the grout, the tendon will act as a conduit for chloride transport, thus spreading the potential for corrosion from a single breach in the duct. Furthermore, even within a well grouted tendon, the grout will contain both longitudinal and circumferential cracking, reducing the ability of the grout to protect the strand. Finally, grouting defects, such as voids and fine cracks, do not adversely affect fatigue performance of the strand, but these defects do adversely affect corrosion protection of the strand, allowing chlorides access to the strand.A set of best practices are also included in the recommendation section for the durability of partially post-tensioned concrete members in an aggressive environment, including cyclic loading. These best practices focus on tension and compression stress limits for the concrete, detailing issues for the tendon, and areas of concern with regard to the anchorage zones and long-term durability. With regard to the study's overall objective, partially post-tensioned concrete is a viable construction method in an aggressive environment, even with cyclic loading, but it relies on a robust plastic duct system as the primary protection method for the tendon.This objective was investigated through an experimental program, which included both individual strand testing and full-scale, post-tensioned beam specimens. The strand testing phase consisted of corroding samples to different amounts of section loss, and then testing them under cyclic loading until failure. The beam testing phase included 12 full-scale, post-tensioned beam specimens combining long-term static exposure testing with fatigue testing. One of the most important limitations of the study was that the experimental program followed the specific sequence of exposure (corrosion) followed by fatigue to failure.