This paper describes the development, testing and application of a continuous acoustic monitoring system to detect and locate corrosion-induced failures of tensioned steel elements in buildings, bridges and pipelines. The monitoring system utilizes acoustic sensors distributed about a structure. Data is processed on-site and transmitted over the Internet to a central processing facility where proprietary processing software is used to generate reports summarizing the time, location and classification of recorded events. Case studies describing the various applications and testing of the system are presented and discussed.
The use of high-strength steel wire has contributed greatly to advances in design and structural performance over the last hundred years or so. Unfortunately, this increase in strength has not been accompanied by a corresponding increase in durability. In addition to conventional dissolution corrosion (or rusting), these high-strength steels are susceptible to failure through brittle fracture caused by stress corrosion, hydrogen embrittlement and fatigue.
These corrosion mechanisms cause a significant loss of ductility in the steel, and failure can occur without a gradual loss of cross-section in the wire (Figure 1). Loss of ductility is not always accompanied by a reduction in ultimate strength. Studies have shown that galvanized wire is more prone to embrittlement than non-galvanized wire when the galvanizing is damaged or locally depleted.
It is important to note that, although chlorides and other contaminants can accelerate embrittlement, water and oxygen are the only ingredients necessary to initiate the process. For this reason, embrittlement has been found in post-tensioning strands in completely enclosed, climate controlled high-rise buildings where rainwater has entered the tendon system during construction.
The presence of corrosion in high-strength steel wire in bridges, prestressed pipelines and other structures can have serious consequences. The Ynys-y-Gwas bridge in the United Kingdom (U.K.) collapsed in 1985 as a result of corrosion of the post-tensioning system. A major program of inspection of the grouted post-tensioned bridge stock in the U.K. was then initiated. At least four other bridges were decommissioned and replaced because of serious corrosion. In 1992, the Highways Agency in that country banned the use of grouted post-tensioning in their bridges pending the development of improved detailing, grouting procedures and quality control measures l. Corrosion has also been found in post-tensioned bridges in France, Germany, Italy, Denmark and Japan. Severe corrosion and extensive wire failure has been discovered in the main cables and in the hanger systems of a number of suspension bridges in North America and Europe (Figure 2). This has resulted in expensive rehabilitation projects in many instances and replacement of the main cables. Corrosion-induced failures of stay cable wires have been documented in China, Argentina, Venezuela and Germany 2.
Prestressed Concrete Cylinder Pipe (PCCP) has been produced and installed throughout North America since 1942. Over 45,000 kilometers (28,000 miles) of PCCP is currently in service. While the percentage of PCCP failures has been low, the results can be catastrophic. Service interruptions for large pipe bursts are problematic, as often there is no system redundancy for big mains.
Beginning about 1970, a disturbing number of failures began to occur on PCCP, some in large diameter pipes with destructive results. During the fall of 1990, a meeting of concerned prestressed concrete pipe users was hosted in Denver, Colorado. Forty-six representatives fro
