The Viability of Partially Post-Tensioned Concrete Members in an Aggressive Environment, including Cyclic Loading

Open Access
Author:
Volz, Jeffery
Graduate Program:
Civil Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
July 11, 2008
Committee Members:
  • Andrea J Schokker, Committee Chair
  • Jeffrey A Laman, Committee Member
  • Daniel G Linzell, Committee Member
  • Howard W Pickering, Committee Member
  • Peggy Ann Johnson, Committee Member
Keywords:
  • post-tensioned
  • prestressed
  • durability
  • corrosion
  • fatigue
  • concrete
  • chloride exposure
  • corrosion-fatigue
  • prestressing strand
Abstract:
Partially prestressed concrete offers several advantages over fully prestressed de-signs including in¬creased ductility, increased energy absorption, decreased cost, de-creased camber, and decreased anchor zone congestion. However, design codes have been slow to adopt pro¬visions 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. 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 ex-perimental program followed the specific sequence of exposure (corrosion) followed by fatigue to failure. With regard to the individual strand testing, there was a significant drop-off in fa-tigue capacity at relatively small pit depths. On average, the fatigue capacity was re-duced 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 me-thod 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. Fur-thermore, 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 perfor-mance 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 du-rability of partially post-tensioned concrete members in an aggressive environment, in-cluding 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 objec-tive, 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.