volume 34, no. 11 november 2008

practical ideas for water operators

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PIPE REPaIR:CFRP to thE REsCuE

Treatment opTimize spenT-filTer

backwash waTer

online ToolsDaTabase Delivers

conTaminanT resources

Taste and odor controlreservoir circulaTion

reDuces geosmin

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DON’T STRESS OVER PRESTRESSED CONCRETE CYLINDER

PIPE FAILURES

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Technology is helping utilities locate water main breaks, estimate damage, and

repair specific sections quickly and cost-effectively. BY SHAH RAHMAN

Shah Rahman is vice president, Pipelines Division, Fibrwrap Construction

(www.fibrwrapconstruction.com), Ontario, Calif.

ARBON FIBER-REINFORCED poly-

mers (CFRPs) have been used to

structurally rehabilitate steel and

concrete structures in North America

for more than two decades. Using these advanced

composite systems, engineers routinely strengthen

bridges, buildings, waterfront piers, industrial

facilities, and many other civil works structures

that have been in service for many years. Struc-

tural renewal of large-diameter pressure pipe-

lines also has been performed with CFRPs in

municipal and industrial facilities for more than

a decade. The method is typically trenchless and

performed completely inside pipes with diame-

ters of 30-in. and larger—even pipes as large as

201 in. (about 17 ft). Pipe retrofits can also be

performed externally.

CFRP composites used in strengthening applica-

tions are additional tension members that bond to

and supplement the strength of an existing rein-

forced concrete structure. CFRPs consist of fibers

of high-tensile strength combined with a resin

matrix. Several thousand carbon fiber filaments

are twisted together to form a yarn that can be

woven or stitched into a fabric (Figure 1). CFRPs

are held together by a resin matrix—the “poly-

mer” component of the CFRP (Figure 2). CFRPs

are high-strength-to-weight ratio materials that are

ideal for high-tensile strength, low-weight, and

low-thermal-expansion applications.

C

CFRP repair requires the host pipe to be placed out of service and dewatered for entry by repair personnel and manual application of the composite layers.

The resin matrix serves as a load path between each fiber filament, mak-ing the composite pliable and protecting the fiber. Epoxy is the most commonly used resin in strengthening applica-tions. Each epoxy has unique properties and contributes to a composite’s unique strength, stiffness, and other physical/mechanical properties for engineered applications.

AWWARF RESEARCH ON PCCPMost water transmission mains in North America are large-diameter prestressed concrete cylinder pipe (PCCP) and steel pipe. Because these pipes are so large, their failures can be catastrophic, usually

affecting large populations. According to a recent Awwa Research Foundation study on PCCP, Failure of Prestressed Concrete Cylinder Pipe:

Almost 100 million ft (about 19,000 mi) of PCCP was manufactured between 1940 and 2006.A database of PCCP failures contains 592 independent entries, representing three categories of failure in 35 states and the District of Columbia.Within 50 yr of installation, one rup-ture and 66 other failures occurred for every 264,000 ft (50 mi) of pipe.A significant increased failure rate occurred for pipe installed between 1971 and 1979 because of the type of prestressing wire used during this period.Half of the recorded catastrophic leaks and breaks involved pipe manufac-tured or installed between 1971 and 1979.In the last decade, electromagnetic

and acoustic technologies have been developed that can help municipalities locate and accurately estimate the num-ber of wire breaks in pipe sections. Fur-ther analysis of the electromagnetic and acoustic data helps pipeline owners pri-oritize rehabilitation initiatives. The abil-ity to precisely isolate and repair specific deficient pipe sections results in tremen-dous cost and time savings.

ADVANTAGES AND DISADVANTAGESWhen a structurally deficient pipe section has been identified, it can be repaired with CFRP. Most pipes rehabilitated with CFRP have been PCCP, but large-diameter steel pipes also have been repaired using this technology.

CFRP repair provides several bene-fits. The CFRP repair method is trench-less, eliminating the need to dig around or through surface structures. All components of the system—fiber, epoxies, tack coats, primers, and top coats—must be NSF-61 certified and ideal for potable water appli-cations. The volatile organic compounds in all CFRP epoxies should be well below US Environmental Protection Agency limits and environmentally friendly.

The segmental repair system allows personnel to repair only a specific pipe section. Significant time savings result. For example, 20-ft pipe sections with diame-ters larger than 66-in. can be rehabilitated in as little as 72 hr, which is often less time than is required to remove and replace a pipe section or for other repair options.

Because one layer of CFRP is only 0.04-in. thick, pipeline flow losses are negligible. A typical high-pressure application usually renders only a !- to "-in.-thick compos-ite liner. In addition, the CFRP system pro-vides a significantly smoother flow surface than the host pipe. However, CFRP repair requires the host pipe to be placed out of service and dewatered for entry by repair personnel and manual application of the composite layers.

SYSTEM DESIGNCFRP composite systems are linearly elastic to failure, whereas steel yields and plastically deforms to failure. When designing a structural strengthening sys-tem, it’s important to consider stiffness, strength, and strain. In CFRP liners, elas-tic modulus and strain limitations control the system’s performance. Inside a PCCP line, the CFRP liner acts as a barrier that controls the level of strain experienced by the steel cylinder in the host pipe. The

Figure 1. Dry Carbon Fiber Fabric Carbon fiber filaments are twisted together to form a yarn that can be woven or stitched into a fabric.

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Figure 2. Cross-Sectional View of Carbon Fiber-Reinforced Polymer CompositeCFRPs are held together by a resin matrix—the “polymer” component of the CFRP.

Carbon Fiber

Single CarbonFiber FilamentResin Matrix

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In CFRP liners, elastic modulus and strain limitations control

the system’s performance.

composite liner enables the steel cylin-der to perform within its elastic range—at or below industry-recommended levels—to ensure the pipeline’s long-term performance is maintained.

AWWA-recommended strain limita-tions are 50 percent and 75 percent of the steel yield point for operating and surge conditions, respectively. Conse-quently, these limits result in low CFRP

stresses, providing a factor of safety greater than 5 for the CFRP liner. So when a composite liner is used, the retrofitted pipeline will have the ability to address standard operating and surge pressures

The wet lay-up carbon !ber-reinforced poly-mer (CFRP) process is a method by which the dry CFRP fabric is saturated in the !eld with epoxy resin, then applied internally to full sec-tions/pieces of structurally de!cient pipe.

1. Before commencing repairs, assess the host pipe’s condition, set up emergency plans, check for hazardous gases, and set up ventilation and dehumidi!cation systems.

2. In PCCP lines (2a), use a high-pressure hydroblasting unit to remove a minimum

1 / 16-in. of concrete from the inner core sur-face. In steel lines (2b), the cement–mortar lining or any other type of internal lining should be removed !rst, followed by a further abrasive treatment of the pipe to white steel.

3. In preparation for the !ber saturation process, epoxy resins are mixed on-site (3a). A specially designed saturator is used to sat-urate measured lengths of 2-ft-wide dry !ber fabric (3b). When saturation is complete, the composites must be applied within three hr.

4. After the surface is properly prepared and the walls have dried, additional repairs to the concrete surface can be conducted if necessary. Next, the surface is primed for application of composite layers.

5. The saturated rolls of composite are applied to the pipe wall. When the composite is applied in the circumferential direction, the pipe is strengthened for hoop stress (5a). When the composite is applied in the longi-tudinal direction, some level of resistance to bending moments is accomplished (5b).

6. A !nal topcoat further protects the composite layers.

1

3a

5a

2a

3b

5b

2b

4

6

STEP BY STEP

INSIDE A CFRP REHABILITATION PROJECT

Case studies demonstrate that carbon !ber-reinforced polymer (CFRP) repair saves time and money, eliminating the need for large open-trench repair projects.

Southwest Transmission Main, Mary-land. Following a 2006 condition assess-ment of Maryland’s Southwest Transmission Main—a PCCP line that serves the city of Baltimore and Baltimore, Howard, and Anne Arundel counties—it was determined that repairs were necessary to avoid future cata-strophic failures. De!ciencies identi!ed pre-stressing wire breaks, longitudinal cracking, and delamination of the concrete core.

Fully structural CFRP renewal was carried out on 68 sections of 54-in. and 36-in. pipe, with internal pressures ranging from 75 psi to 150 psi. Another 19 sections of pipe in Anne Arundel County with minor circumferential cracking were “stitched” with CFRP to prevent groundwater in!ltration through the cracks. Howard County’s 33 sections of 36-in. pipe were completed in 23 days. Feasibility stud-ies had shown that traditional repair methods would have taken four to six mo.

Conduit 94, Denver. In February 2008, Denver Water experienced a catastrophic failure in a 66-in.-diameter section of steel pipe in Conduit 94 because of a pump fail-ure. The failed section, located near an elbow where the steel pipe transitioned to PCCP, was replaced on an emergency basis with 40 ft of new cement–mortar-lined steel pipe that was procured from the inventory of a local steel pipe manufacturer. The two new sections were 0.307-in. and 0.375-in. thick, which didn’t meet the Denver Water Board’s speci!ed factor of safety of 2 for the design working pressure of 225 psi and surge pressure of 90 psi. Due to the location of the line 15 ft directly under-neath the highway, CFRP repair was the only available option to upgrade the pipe. CFRP design considered safety factors for long-term load, short-term load, core crushing, and radial tensile strength. The parameters were met by using eight layers of carbon-!ber composites. The cement–mortar lining was removed, and one lon-gitudinal layer of glass !ber served as a dielectric barrier. The project was com-pleted in a week during a scheduled main-tenance shutdown.

Superior Pipeline, Phoenix. The 60-in. PCCP Superior Pipeline conveys surface water from the Val Vista Water Treatment Plant to customers in southern Phoenix. It was designed for 175 psi, a working pres-sure of 125 psi and 50 psi surge pressure. In October 2006, a 24-ft section catastroph-ically failed. The failure was later attributed to corrosion of prestressing wire due to hydrogen embrittlement and possible pipe damage during installation.

Before placing the pipeline back into ser-vice, inspection revealed that 22 sections of the 60-in. line needed immediate structural repair, which was performed using CFRP. Due to high demand on the water main and the short time allowed for completing the work, repair schedules were tight. On all occa-sions, the projects were completed on time or ahead of schedule.

Author’s Note: The case studies on the Southwest Transmission Main and the Supe-rior Pipeline were adapted from technical papers presented at the American Society of Civil Engineers Pipelines International Con-ference 2008, Atlanta, authored by Frank Donaldson et al. and Aimee Conroy et al., respectively.

CASE STUDIES

CFRP TO THE RESCUE

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per industry recommendations, but will also have the ability to control short-term pressures common to unforeseen rapid valve closures that result in transient pressures up to five times operating con-ditions. Therefore, CFRP-liner designs are

based on strain compatibility between the liner and host pipeline. Controlling strain significantly increases long-term durability by ensuring the CFRP and host materi-als don’t exceed limits for stress, crack-ing, and creep.

ONGOING AWWARF RESEARCHAWWARF and USEPA are funding research being performed by the US Bureau of Reclamation on the use of CFRP for the structural renewal of PCCP. AWWARF Project No. 4114, Fiber Rehabilitation of

The Southwest Transmission Main called for internal visual and sounding inspections before surface preparation could begin.

Denver Water’s John Bambei (left) investigated the failed pipe section with Dr. Mehdi Zarghamee (right).

On the Superior Pipeline project, crews performed repairs farther from the access point than they had ever attempted.

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New CFRP research and development will continue to yield more ef!cient systems and economical solutions.

RESOURCES

AWWA Manual of Water Supply Practices M9: Concrete Pressure Pipe, 1995.ICC AC 125: Acceptance Criteria for Con-crete and Reinforced and Unreinforced Masonry Using Externally Bonded Fiber-Reinforced Polymer.ICC ESR 2103: Concrete and Masonry Strengthening Using the Tyfo Fibrwrap Fiber-Reinforced Composite Systems.Loera, R. 2006. PCCP Carbon-Fiber-Liner Repair Standard Specifications. In: Atalah, A., Tremblay, A., editors. Service to the Owner. Proceedings of the American Soci-ety of Civil Engineers Pipelines Interna-tional Conference, Chicago. Reston, Va.Romer, A.E., Bell, G.E.C. 2008. Failure of Prestressed Concrete Cylinder Pipe. AWWA Research Foundation. Report No. 91214, Denver.

Prestressed Concrete Pipe, is addressing CFRP material properties and installation issues, as well as taking an in-depth look at design issues and long-term material dura-bility. At the conclusion of the three-year study, an AWWARF report will be produced, along with a USBR specification on CFRP rehabilitation that can be used as a basis for designing and installing CFRP systems for PCCP repair and renewal. These efforts will supplement current design methods and may also serve to initiate what will eventually result in the publication of an AWWA Standard. In recent AWWA Con-crete Pipe Committee discussions, it has been suggested that the report may be a good basis for the inclusion of a chapter on the rehabilitation of PCCP in the AWWA Manual of Water Supply Practice M9, Con-crete Pressure Pipe.

ON THE HORIZONFaced with the challenges of structur-ally deficient large-diameter PCCPs, some of which fail catastrophically, there is a conscious effort today on the part of many large infrastructure owners to be proactive rather than reactive. Using

various electromagnetic and acoustic technologies, they are making worthwhile investments in investigating and analyz-ing their large-diameter buried transmis-sion lines and taking immediate action

to structurally rehabilitate the worst pipe sections with CFRP technology. New CFRP research and development will continue to yield more efficient systems and eco-nomical solutions.

CFRP INSTALLATION

QUALITY CONTROL IS KEY TO EFFECTIVE USEThe performance and long-term durability of carbon !ber-reinforced polymer (CFRP) sys-tems inside pipelines can be ensured by

using experienced installers who have more than !ve years of underground infrastruc-ture work experience and an equal amount of experience with CFRP applications.submitting structural and durability testing results by the materials manufacturer and the applicator (examples include full-scale external load testing and internal pressure testing of pipes, environmental durability testing, biological growth-support-potential testing, anticavitation testing, checking for volatile organic compounds in resins, and ultraviolet testing for external applications).material approvals from independent agencies such as International Code Council.ensuring that all components of the CFRP system, including the !ber, epoxies, tack coats, primers, and top coats are all certi!ed to NSF 61 requirements for potable water applications.con!rming in-!eld sample tensile strength for all applications per ASTM D3039, Stan-dard Test Method for Tensile Properties of Polymer Matrix Composite Materials and adhesion testing on bond-critical projects per ASTM D4541, Standard Test Method for Pull-off Strength of Coatings Using Portable Adhesion Testers.

CFRP repair can also be performed externally when appropriate.