Ameron, Inc. (a concrete pipe manufacturer), and Peter Kiewit (a leading US contractor) retained Exponent to determine the cause of failures to one of the world’s largest pre-stressed concrete pipe (PCP) installations. Ameron had manufactured the PCPs, and Kiewit constructed the six inverted siphons on the Central Arizona Project (CAP), which carries water from the Colorado River to central and southern parts of Arizona. The CAP project is a waterway that transports water from the Colorado River to Central and Southern Arizona. At the time of construction it was the longest single water transportation project ever authorized by the United States Congress and combines an intricate system of canals, tunnels, pipelines, and pumping stations. The design and specifications for the project were provided by the Bureau of Reclamation (Bureau). The 190-mile Granite Reef Aqueduct portion of the CAP project comprised of an 80-foot wide concrete-lined canal carrying the largest flow of water in the system and crosses seven major river beds and washes by means of a 21-foot diameter inverted siphons. Six of these seven siphons, ranging in length from one-quarter mile to almost two miles in length, were constructed from what was at the time the largest precast pre-stressed concrete pipe ever manufactured. The pipe has an internal diameter of 252-inches, a wall thickness of 21-inches, and each segment was 22.6-feet long and weighted as much as 225 tons. In 1990, corrosion surveys indicated that the pre-stressing was corroding and failing. Extensive wire corrosion and mortar damage was discovered in four out of six siphons within a few years of service, and the Bureau was concerned that a preexisting defect in the pre-stressing wires and/or poor mortar quality were contributing.
Exponent investigated the cause of several failures of the pre-stressed concrete pipe (PCP). Exponent evaluated various hypotheses and identified the most plausible failure mechanism. The first phase of the 4-year project consisted of evaluating the reports and test data presented by the Bureau. The second phase of the project consisted of extensive on-site inspection and testing of the PCP siphons, additional in-house laboratory testing, and analytical modeling of the PCPs in their ambient environment.
Repairs being made to the CAP pipe
Location of pre-stressing wire in pre-stressed concrete pipe
Coroded and fractured pre-stressing wire in CAP pipes
Broken wires from pipe show little sign of ductility and right from tensile tests
SEM and energy dispersive spectroscopy (EDS) indicated the presence of chloride within the pre-stressing wire stress corrosion cracks.
Exponent’s analysis revealed that the siphon distress was due to inadequate protective measures to combat the highly aggressive service environment. Exponent found that SCC initiated distress was a result of chloride transport and concentration. The chloride was provided from pipe leaks, saline soil, and groundwater fluctuations. The chloride was concentrated at certain locations on the pipe as a result of “wicking” and evaporative concentration of chlorides in the desert environment. The distress to the protective mortar coating was mainly due to corrosion of the pre-stressing wires. Forces generated by rust formation and wire breakage fractured the mortar ligaments between the wires. Neither the mortar nor the pre-stressing wire was defective, nor were any mortar or wire defects necessary to cause the observed wire failures in the high-chloride environment that developed over time.
The Bureau had initially sued Kiewit and Ameron based on early testing that failed to identify and account for the aggressive chloride-rich environment. Based on Exponent’s investigation, the Bureau eventually settled the case for a fraction of the initial claim.