Showing papers by "Radhakrishna G. Pillai published in 2009"

Parameters influencing corrosion and tension capacity of post-tensioning strands

Abstract: A 12-month long strand corrosion test program with 298 specimens was conducted to identify and quantify parameters influencing corrosion and tension capacity of strands in post-tensioned bridges. The parameters investigated were grout class, moisture content, chloride concentration, void type, and stress level. The test specimens were 41 in. (1041 mm) long, in unstressed or stressed conditions, partially or completely embedded in cementitious grout, and exposed to various environmental conditions representing possible field conditions. After the exposure period, the grout material was removed and the strand surfaces were cleaned and visually evaluated for corrosion damage. The tension capacities of the strands were then determined. Results indicate that the corrosion was most severe at or near the grout-air-strand (GAS) interface. Corrosion evaluation and statistical analysis of the strand tension capacity results show that orthogonal, inclined, and bleedwater void conditions caused more corrosion and tension capacity loss than parallel and no-void conditions. The change in grout class did not result in statistically significant changes in the tension capacity of the strand samples evaluated. Statistically significant changes in tension capacity were observed with changes in the GAS interface, stress level, moisture content, and chloride concentration.

Probabilistic Capacity Models for Corroding Posttensioning Strands Calibrated Using Laboratory Results

Abstract: The presence of air voids, moisture, and chlorides inside tendons or ducts was cited as a reason for the early age strand corrosion and failure in the Mid-bay, Sunshine Skyway, and Niles Channel posttensioned (PT) bridges in Florida, United States Although rare, these incidents call for frequent inspection and structural reliability assessment of PT bridges exposed to moisture and chlorides This paper develops and presents probabilistic strand capacity models that are needed to assess the structural reliability of such PT bridges and recommends a time frequency of inspection A total of 384 strand test specimens were exposed to various void, moisture, and chloride concentration conditions for 12 and 21 months; the remaining tension capacities were then determined Using this experimental data and a Bayesian approach, six probabilistic capacity models were developed based on the void type The mean absolute percentage errors of these models are less than 4%, indicating that reasonably accurate prediction of the strand capacity is possible, when void, aggressive moisture, and chloride conditions are present

Effect of Voids in Grouted, Post-Tensioned Concrete Bridge Construction: Volume 1 – Electrochemical Testing and Reliability Assessment

Abstract: Post-tensioned (PT) bridges are major structures that carry significant traffic. PT bridges are economical for spanning long distances. In Texas, there are several signature PT bridges. In the late 1990s and early 2000s, several state highway agencies identified challenges with the PT structures, mainly corrosion of the PT strands. The Texas Department of Transportation (TxDOT) performed some comprehensive inspections of its PT bridges. A consultant’s report recommended that all ducts be re-grouted. However, the environment in Texas is very different than the environments in which the corrosion of the PT strands were observed. The objective of this research was to evaluate the corrosion activity of strands for PT structures and to correlate this corrosion activity with general environmental and void conditions. To achieve this objective, time-variant probabilistic models were developed to predict the tension capacity of PT strands subjected to different environmental and void conditions. Using these probabilistic models, time-variant structural reliability models were developed. The probability of failure of a simplified PT structure subjected to HS20 and HL93 loading conditions was assessed. Both flexural failure and serviceability were assessed. Results indicate that the presence of water and chlorides can lead to significant corrosion rates and failure is dependent on this corrosion activity and the number of strands exposed to these conditions. Volume 1 of this report presents these results. To assist TxDOT with developing a plan to mitigate this corrosion, studies were performed to assess repair grout materials, inspection methods, and repair methods. In addition, a general methodology is presented on optimizing repairs. These topics are presented in Volume 2 of this report. An Inspection and Repair Manual was also developed from this research and is presented in a separate report. Results indicate that TxDOT should prevent water and chlorides from infiltrating the tendons; this can be achieved in part by repairing drain lines and ducts and protecting anchor heads, as these conditions can lead to early failure of PT bridges. Recommendations on inspections, repairs, and materials are provided; however, further research on the potential formation of galvanic coupling of strands embedded in both existing and new repair grouts needs to be assessed.

Effect of Voids in Grouted Post-Tensioned Concrete Bridge Construction: Inspection and Repair Manual for External Tendons in Segmental, Post-Tensioned Bridges

Abstract: Segmental, post-tensioned (PT) bridges are major structures that carry significant traffic. These bridges are designed and constructed because they are economical for spanning long distances. In Texas, there are several signature PT bridges. In the late 1990s and early 2000s, several state highway agencies identified challenges with the PT structures, mainly corrosion of the PT strands. The Texas Department of Transportation (TxDOT) performed some comprehensive inspections of their PT bridges. A consultant’s report recommended that all ducts be re-grouted. However, the environment in Texas is very different than the environments in which the corrosion of the PT strands was observed in the other bridges. Report 0-4588-1 summarized the research findings from a comprehensive study on the corrosion characteristics, reliability, materials, and repair for PT bridges. This document, an inspection and repair manual, was developed from information from this research program. This document provides an efficient approach to inspect and repair PT bridges. However, it should be noted that in this manual, repair does not include filling the voids in the tendons with grout. A recent failure of a tendon in a bridge in Virginia was suspected of being caused by repair grouting of the tendon, possibly due to the formation of a galvanic couple between the new repair grout and the original grout. Although a procedure for pressure-vacuum grouting of tendons is provided in Appendix A of this manual, this method should not be used until the potential issue associated with galvanic corrosion of the strands after repair is resolved.