An electrical generation station in western Colorado was experiencing corrosion in the prestressing wires through sections of their precast concrete cylinder pipes. The water transfer pipes supply water between the power station and the cooling towers. During a 2-week shutdown period, testing was conducted to determine the extent of the corrosion in the wires. Testing found that one spool, an 18 ft (5.5 m) section of pipe, was compromised to the point that external post-tensioning was required. The spool ran 18 ft (5.5 m) in length, with a 9 ft (2.7 m) diameter, and was located approximately 20 ft (6.1 m) below grade. A large excavation was required to gain access to the damaged spool. The excavation was made approximately 25 ft (7.6 m) deep with a 60 ft (18 m) diameter, with sloped sidewalls for safety.
The water transfer pipe containing the damaged spool ran parallel and next to another water transfer pipe. Once the damaged spool was exposed, a trench box was placed between the two water transfer pipes for safety. Prior to complete excavation, each end of the spool was excavated and tendons were installed to help support the dead load. The spool was then fully excavated and the outer concrete shell was sound tested with the engineer of record. The investigation found that the exterior concrete shell had debonded from the interior and a large crack spanned half the circumference of the spool. It was determined that the outer shell should be removed and further examination of the crack was necessary.
The exterior shell was excavated using pneumatic hammers. Upon inspection, it was found that a significant number of prestressing wires were corroded and broken. The large crack that ran around the exterior shell continued into the interior concrete shell. Core samples were taken to determine the depth of the crack within the internal concrete shell. If the crack extended the full depth to the steel fiber liner allowing corrosion, spool replacement would be required. The core sample showed that the crack did indeed extend full depth, however, the liner showed no signs of corrosion. With a corrosion-free liner and solid inner core, a post-tension repair was viable.