Why are Carbon Fiber Lining Systems Used Extensively on Pre-stressed Concrete Cylinder Pipe (PCCP)?
Carbon fiber systems can be utilized to provide structural repair and upgrade for nearly any type of type of pipe including Pre-Stressed Concrete Cylinder Pipe (PCCP), Concrete, Mortar Lined Steel, Fiberglass and all types of Metallic pipelines. Since the mid-1990’s, carbon fiber reinforced polymer (CFRP) composite systems have been increasingly used for internal rehabilitation and strengthening of large diameter pipelines. The primary type of pipeline rehabilitated using CFRP is PCCP because the segmental manner in which PCCP is constructed and the localized manner in which structural distress is observed allow for evaluation and targeted rehabilitation of individual pipe segments.
PCCP has had a long and diverse history since its earliest application in the United States in 1942, with many changes in standards and materials over the years1. There are two types of PCCP: lined-cylinder type (LCP), consisting of a steel cylinder with cast concrete core, wrapped with steel prestressing wire directly over the steel cylinder, and embedded-cylinder type (ECP) which has the prestressing wire embedded wrapped onto an outer concrete core, as shown in Figure 1. The prestressing wires are covered with an exterior mortar coating which protects the wires from the environment. ECP is made in larger diameters than LCP and has been constructed as large as 252 inches in diameter.
PCCP is designed for combined loads including internal working and transient pressure, pipe and water weight, soil load, and live load in accordance with the procedures outlined in ANSI/AWWA C304, Standard Design of Prestressed Concrete Cylinder Pipe2. The current analysis and design procedure is based on checking certain serviceability, damage, and strength limit states by calculating stresses and strains in the concrete core, mortar coating, steel cylinder, and prestressing wires.
Figure 1.1. Cross section of a prestressed concrete cylinder pipe, ECP-type
There are a variety of failure mechanisms for PCCP; however, a typical failure mechanism involves breakage of the prestressing wires on individual sections of pipe (Romer et al, 2008). When prestressed wires on an individual section of pipe break, the structural integrity of that pipe is compromised and the risk of a failure in the line may be significantly increased, particularly if a pressure surge occurs in the line. In the late 1990s, electromagnetics technologies were developed to structurally assess the integrity of the prestressed wires on PCCP3. These inspections are able to isolate the location of broken prestressed wires with an accuracy that allows pipeline owners to identify individual pieces of pipe which have been structurally compromised. Based on over 8,000 miles of PCCP electromagnetically inspected to date, the distress rate in PCCP is approximately 4%, with only a fraction of the distressed pipes having a significant number of broken wires. Failure risk of pipes with broken wires can be performed and repairs can be prioritized. Once distressed pipe sections have been identified for rehabilitation, either replacement or a structural repair is selected based on various constraints related to accessibility to excavate, downtime, and cost.
As mentioned above, Carbon fiber wrap lining systems have been used increasingly for internal rehabilitation of large diameter pressurized water and wastewater pipelines since the mid-1990s. The primary motivation for use of Carbon fiber linings as a rehabilitation technology is minimizing disruption to the surrounding environment, particularly for distressed pipelines which are difficult to excavate for conventional pipeline replacement. These fully structural lining systems are especially effective for segmental repairs of PCCP segments identified during the inspection and failure risk process because the CFRP materials are brought in through existing access points into the pipeline and can be transported to repair locations up to several thousand feet away from the access location. The CFRP lining system can be designed to meet a variety of loading conditions including as a stand-alone system resisting all internal pressure and external loads. This fully trenchless method of repair and upgrade is successfully utilized by municipal and power generation pipeline owners across the United States to extend the life cycle of critical pipeline assets.
1Romer, A.E., D. Ellison, G.E.C. Bell, and B. Clark (2008), Failure of Prestressed Concrete Cylinder Pipe, American Water Works Association Research Foundation, Report #91214, Denver, Colorado: AWWA 2ANSI/AWWA C304-07 (2007). Standard for Design of Prestressed Concrete Cylinder Pipe. American Water Works Association (AWWA) 3Zarghamee, M.S., R.P. Ojdrovic, and P.D. Nardini (2012) Best Practices Manual for Prestressed Concrete Pipe Condition Assessment: What Works? What Doesn’t? What’s Next? Water Research Foundation