Grantee Research Project Results

The United States has a vast, underground drinking water distribution infrastructure. For large-volume water distribution, a particularly important type of pipe is the steel reinforced concrete water conduit (also called Prestressed Concrete Cylinder Pipe, or PCCP). There are thousands of miles of PCCP installed in sections with lengths up to 20 feet and diameters ranging from approximately 2 to 14 feet. In PCCP, the pipe consists of a thin steel cylinder lined with concrete and helically wrapped on the outside with a high-strength steel wire under tension to prestress the pipe. The reason for the helical wire wrap is to ensure that the concrete never goes into tensile stress during use because concrete loses all strength when not under compressive stress. As the pipes age, the stress windings can become defective due to causes such as corrosion and stress cracking. Figure 1 shows (clockwise from upper left) installation of PCCP sections, damage to the wires, a schematic drawing of the structure of PCCP and the individual components that make up PCCP.

 

Figure 1. (clowise from upper left) PCCP sections being installed, wires damaged by corrosion,
a schematic sketch of structure, and conduit components. 

 

Typically, the mechanism for degradation of the pipes would be corrosion that breaks one or more prestressing wires. When several defects occur in close proximity, the concrete may be placed into tension, resulting in cracking and a catastrophic leak. Leaks in water distribution networks are among the largest sources of water loss in the system. Technology capable of detecting damage to the prestressing wires of a concrete reinforced water conduit would make repair or replacement possible before a major leak forms or a pipe ruptures, and thus would provide a major environmental benefit to the nation and its water resources. While PCCP is critical to the Nation's water delivery system, the large size of the pipes, the thickness of the concrete (up to 10 inches) and the composite construction (which involves concrete, high-strength prestressing steel wires and sheet steel cylinders) make inspection particularly challenging.

 

The currently available instruments for inspecting the wires in PCCP utilize a nondestructive test (NDT) that relies on large wire coils to induce and detect electrical currents in the wires and is an adaptation of an approach that is a widely practiced and critically important for pipe and tube inspection. The current instruments, however, have limited sensitivity near the joints between pipe segments and a tendency to generate both false positive and false negative findings. This in turn can cause unnecessary repair expenses or fail to avoid catastrophic failure, which limits the willingness of municipalities and water districts to invest scarce resources in inspection.