Corrosion induced loss of capacity of post-tensioned seven wire strand cable used in multistrand anchor systems installed at Corps projects

Abstract

Over the past five decades, the U.S. Army Corps of Engineers has been upgrading its projects by installing high-capacity, post-tensioned foundation anchors, typically with seven-wire strand cables. The purpose of these anchors has been to achieve structural stability for Corps hydraulic concrete structures (e.g., locks, dams, approach walls) and/or to remediate cracked concrete monoliths. Substantial improvements to protect multistrand anchor systems from corrosion have been made in the past five decades, but the corrosion of older multistrand anchorage systems is still a major concern.This report discusses a laboratory-testing program for the estimation of post-tensioning (PT), seven-wire strand cable strength as a function of corroded cross-sectional material loss. Pull tests were performed to gather reduced cable strength measurements. An innovative morphological procedure using digital photography was developed by U.S. Army Engineer Research and Development Center (ERDC) researchers for quantifying the cross-section geometrical properties of cables near their failure locations. The laboratory-testing program also included a successful series pull test to failure on pristine specimens for a control set of data, and the issues encountered are detailed. A statistical assessment of pull-test data to failure of pristine and corroded cables is used to establish a correlation between cross-section properties, corroded and pristine, and the cable strength.An overview of the corrosion process and the variables, ranked by contribution in Corps structures, which determine corrosion rate at each of the multistrand cables, is provided. Further, methods for estimating cable capacity under load were developed using the provided best-fit curves from the laboratory pull tests.

Corroded Anchor Structure Stability/Reliability (CAS_Stab-R) software for hydraulic structures

“…Figure 4-32. Anchorage tab-Corrosion rate -Oxygenation level curve from Ebeling et al (2016). ........................................................................................................................................................ Tables Table 4- Over the last 40 years, a number of existing Corps hydraulic structures have been retrofitted with post-tensioned, multi-strand cable anchorage systems to prevent movement in the form of sliding or overturning, thereby enhancing the stability of these structures.…”

“…When the multi-strand anchor cables lose enough material to reduce the capacity of the cable below the tension force that was applied at installation, the anchor will fail, thereby reducing the stability of the structure. A series of unprecedented pull-test experiments conducted at the Engineer Research and Development Center (ERDC) with results published by Ebeling et al (2016) and Haskins et al (2016b) provided statistical relationships of reduced cable capacity to the reduced cross-section area and to the cross-section minimum short axis diameter of failed cable strands with varying levels of corrosion.…”

“…The CAS_Stab-R software incorporates the corroded cable reduced capacity information presented in Ebeling et al (2016) to compute probabilistic assessments of the expected remaining lifetime of structural anchor cables and the PUP of a navigation structure that employs posttensioned stranded wire cable anchorages. The wedge solution methodology of Engineering Technical Letter (ETL) 1110-2-256 (HQUSACE 1981) is utilized to compute the FOS results, which provide the PUP solution.…”

“…A series of experiments were conducted at the ERDC in 2013 to measure the loading capacity of seven-strand wire cables. The laboratory experiments and statistical processing of the resulting (reduced) strength data were discussed in Ebeling et al (2016) and summarized in Haskins et al (2016b). The seven strands were arranged with a center "king" wire surrounded by six "outer" wires that spiraled along the length of the king wire.…”

“…A scatter plot of the sampled cable capacities as a function of the minimum cross-sectional area is shown in Figure 2-1. As discussed in Ebeling et al (2016), the minimum area of the smallest wire in the seven-strand assembly was used to correlate to the reduced strength of the corroded seven-strand wire assemblage. Statistical analysis of the cable capacity as a function of the corroded wire area (of the smallest wire) yielded the second order equation for the mean trend line given in Equation 2.1. where: C = the mean cable capacity, in pound force A = the area of the most corroded wire, in square inches.…”