Carbon fiber reinforced polymer liners are becoming an increasingly popular option for municipalities that are looking to provide full structural repair to distressed sections of their underground pipes without having to address the entire pipeline.
The San Diego County Water Authority (SDCWA) has used carbon fiber wrap in recent years to address spot repairs to its large diameter pipes, including a recent urgent repair for two sections of a pre-stressed, concrete cylinder high-pressure water line in the City of Fallbrook, Calif.
Pre-stressed concrete cylinder pipe is constructed by welding steel joint rings to a steel cylinder; it is then coated internally with concrete. The outer surface is wrapped with high-strength steel wire directly over the steel cylinder and covered with a cement-based mortar coating.
The City of Fallbrook — with a population of approximately 30,000 — is located just inside the northern border of San Diego County and is considered an older, residential area with homes and properties spread out, dotting the landscape. The city installed the pre-stressed concrete cylinder pipe in the 1960s and today has many miles of it running underground, carrying treated and untreated water. However, in the years since their installation, SDCWA has determined that this type of pipe was not well suited for the corrosive soil conditions in San Diego County, resulting in replacement and repairs over the years to its pipelines that have become corroded and damaged.
Such was the case this spring for a 96-in. water line that required an urgent repair. Instead of relining or replacing the entire pipeline, SDCWA decided to use a carbon fiber wrap to rehab the sections in dire need of attention, negating the possibility of a catastrophic pipeline failure.
This particular pipe is one of five high-pressure water lines that run through San Diego County but the only one that carries untreated water. The troubled pipe carries millions of gallons of untreated water from Riverside County to San Diego County where it gets treated and then delivered to SDCWA’s member agencies. SDCWA, formed in June 1994, delivers water to 24 member agencies across 951,000 acres. SDCWA serves 3 million customers and maintains nearly 300 miles of pipeline.
SDCWA engineer Jack Neely said this 96-in. pipe was installed in 1982, making it a relatively young pipe to be facing so much damage. But as early as 2004, breaks in the high-strength wires were found during a routine inspection; however, nothing suggested that complete rehab of the pipe was warranted at the time. Neely said SDCWA continued to monitor the condition of the line until it reached a point that required that something be done.
Municipalities have several options when monitoring their pipes, including Eddy Current, in which an electronic receiver is inserted into the pipe to determine where and if wires are broken due to current jumps or it can find a discontinuous spot in the pipe where the wire is broken. SDCWA uses Eddy Current but mainly employs acoustic fiber-optic cables in the pipe for inspection and detection. The cable acts like sonar, allowing you to listen for when the wires break.
“Using acoustic fiber-optic cables allows us to monitor the pipe in real time and we can detect when the frequency of the breaks increases. Over the last five years, the frequency of the breaks [in that pipe] went up,” Neely said. “It’s because of that that we decided to fix it now before it unzips and goes into a catastrophic failure.”
SDCWA determined that two sections in the pipe approximately 25 ft each in length needed repaired. Neely said that there was no question that SDCWA would use a carbon fiber wrap system, as it had successfully used the rehab method in the past. “It’s very quick and has minimal impact, thereby reducing the impact to the community,” he said. “It doesn’t mean we couldn’t have dug the pipe up and replaced it, because we’ve done repairs like that. But [using carbon fiber wraps] is definitely great for doing repairs of 100 ft or less. In this case, it was approximately 56 ft [total] and it was pretty much a no-brainer to go with the carbon fiber wrap.”
SDCWA has worked on many projects with Fibrwrap Construction, which has performed pipeline renewal and rehabilitation projects since 1996 utilizing the Fyfe Company Tyfo Fibrwrap system. This project began with the pipe being shut down on May 3. After SDCWA assessed and determined which sections needed rehabbed, Fibrwrap Construction walked through the pipe and marked those areas and determined how many spools of fiber wrap was required.
With the Tyfo Fiber Wrap system, the pipe can be rehabilitated without it being dug up and is installed through existing manholes and/or other entryways. This cuts down on costs associated with an extensive excavation such as traffic control, extra heavy machinery, manpower and the amount of pipe downtime, said Fibrwrap Construction president Jason Alexander.
He also noted that some of the benefits of using a fiber wrap system include a high strength-to-weight ratio, corrosion resistance and relative ease of installation.
How It’s Done
Fibrwrap Construction prepared the jobsite by setting up the ventilation system and testing the pipe’s oxygen and CO2 levels, verifying that it was safe for crews to enter. Then, crews used a 40,000-psi Hydroblast system to clean the pipe and roughen its interior concrete by removing 1/16- to 1/8-in. of the top layer of the top layer of concrete. “We are preparing the surface, basically roughening its surface using high-pressure water, similar to sandblasting,” Alexander said. “We do this to prepare the surface for the epoxy so it can achieve a bond of the fiber wrap to the pipe.”
Then crews “dry out” the pipe using its climate control equipment so there is no free-standing water or moisture on the pipe’s roughened surface. Finally, crews are ready to apply the thickened epoxy and the fiber wrap liner. The process of applying the carbon fiber liner is done by hand rolling it manually by the crews, sometimes needing several layers to complete the job, Alexander said. These layers accumulate a wall thickness of less than 0.5 in., thereby not significantly reducing the pipe’s internal diameter and affecting flow.
The Fallbrook pipe was shut down on May 3 and work to repair it was done May 5-9. Neely said that Fibrwrap Construction applied four to five layers of the wrap. A post-inspection walkthrough was performed by SDCWA and the rehabbed pipe was given the thumbs up. The pipe was re-opened on May 12.
“It’s done very fast and this project was done in 96 hours over four days,” Neely said. “What it does to the pipe is that it reinforces it so it takes all the pressure off the existing pipe.”
Alexander agreed, saying “The fiber is designed to take 100 percent of the internal pressure of the pipe, as well as the external pressure. We are able to do a precision rehabilitation in a pipe, only addressing the areas that are in need of it as opposed to the whole pipeline.”
He said more municipalities are choosing a fiber wrap system as a method of rehabilitation, as the option of performing a precision repair only addressing the segment of pipe in need is very appealing. “The biggest reason though is that it gives them the ability to have just the pipe section in need strengthened using this method that will last a very long time. Its lifespan is about 50 to 75 years.”
This 96-in pipe was included in a much bigger relining and rehabilitation program being done through SDCWA. SDCWA has operated a Relining and Pipe Replacement Program since 1991, which targets the relining, replacement and urgent repairs of its pre-stressed concrete pipelines. Of the 82 miles of pre-stressed concrete pipelines in the care of SDCWA, approximately 25 percent of them have been through the program. The program was recently recognized by the American Public Works Association’s (APWA) San Diego Imperial Chapter for the work SDCWA has done between June 2008 and June 2009.
Sharon M. Bueno is managing editor of Trenchless Technology.