WSSC Working Experience with Structural Rehabilitation of PCCP Using Carbon Fiber
December 28, 2011Pre-stressed Concrete Cylinder Pipe (PCCP) was once the most common material used by many water authorities for large diameter water transmission mains nationwide. The Washington Suburban Sanitary Commission (WSSC), the eighth largest water and sewer utility in the United States, has 350 miles of PCCP water transmission mains ranging from 16- to 96-in. diameter. The WSSC owns more than 5,500 miles of water mains in the 1,000-sq mile service area, with approximately 1.8 million customers in Montgomery and Prince Georges Counties, Md.
As pipelines age, their risk of failure increases. PCCP mains fail catastrophically and often without warning if left unmonitored due primarily to long term corrosion and failure of the high tensile pre-stressed wire reinforcement. The consequences of failure for large diameter water mains can be devastating (Figure 1), depending on the pipeline location, and have a major impact on the water supply for a large number of customers. WSSC, as well as many other water authorities, have experienced such failures. For these reasons, in order to minimize the occurrence and risk of failures, an effective PCCP management program is necessary, which includes routine inspections, pipe section repairs or replacements and ideally, acoustic monitoring to manage the inevitable infrastructure deterioration.
The WSSC manages its inventory of large diameter PCCP pipelines with this asset management strategy designed to maintain safe and continuous operation of the mains and realize the practical useful life of the pipelines. By the end of 2013, WSSC plans to have 77 miles of mains 48 in. and larger equipped with continuous AFO monitoring, the largest PCCP monitoring program in North America.
It has been WSSC’s experience, that routine inspections identify an average of 2 percent of the total number of pipe sections in a pipeline as requiring repair. This low percentage of deteriorated pipe sections warrants the use of an asset management strategy versus total replacement. Deteriorated pipe typically are either replaced or repaired internally by reinforcing with Carbon Fiber Reinforced Polymer (CFRP) composites depending on their location. This later method is a widely accepted and proven means to repair PCCP pipelines and has the advantage of being a trenchless solution. The WSSC has rehabilitated many sections of PCCP using CFRP through its PCCP Management Program.
Large diameter water mains often are subject to limited available shutdown time due to the lack of redundancy. The success of a PCCP Management Program requires significant planning to coordinate the shutdowns at periods of lesser water demand, prepare the mains for inspection and an effective rehabilitation program to assure quality control and specification compliance during construction. Figure 2 shows a flow chart developed and implemented by WSSC for the Condition Assessment and PCCP Rehabilitation Process. This plan establishes the priority and sequence of inspections. The assessment begins with detailed planning by the review of pipeline as-built drawings, pipe lay schedules and pipe design specifications. The condition assessment includes initial leak testing, followed by shutdown and complete dewatering, then internal inspection. The WSSC internal inspection protocol consists of mapping and pipe numbering, visual inspection and sounding, electromagnetic testing, and sonic/ultrasonic testing. The results are analyzed and repair or replacement recommendations are developed. The process of prioritizing repairs correlates all inspection data and compares the number of estimated wire breaks found to the safe limit of wire breaks as determined by modeling each pipe class using Finite Element Analysis (FEA).
Once pipe sections requiring rehabilitation are selected, WSSC engineers prepare the scope of work for the prospective on-call repair contractors. Concurrently, WSSC engineers enter the dewatered pipe to conduct concrete testing of the inner core of pipe sections to be repaired with CFRP. This task is necessary for future design of the carbon fiber reinforcement. WSSC engineers use a digital Schmidt hammer to determine median compressive concrete strengths for design.
The WSSC PCCP rehabilitation Quality Assurance and Quality Control (QA/QC) process is managed using a web-based system by WSSC engineers, the construction contractor, the contractor’s engineer preparing the CFRP designs and the WSSC consultant inspector. This provides for continuous quality assurance and control, providing clear and transparent information for the entire team. It has proven to be the ideal tool for submitting and reviewing contractor designs and storing a large amount of project design information. It allows the contractor to send electronic submittals for the WSSC Engineers to easily retrieve and review CFRP designs, conduct quality assurance and confirm compliance of each procedure while being very responsive to the contractor and the expedited construction process.
To expedite the construction process, the WSSC uses experienced general contractors under an on-call unit price contract, which saves significant start up time. The notice to proceed initiates the design and submittal of the CFRP shop drawings and design details on the web-based system. The WSSC engineering office provides pipe design and record data and reviews design submittals from the structural engineer working for the general contractor. The WSSC consultant inspector participates in the quality control review of the submittals so they are familiar with the construction documents in advance of construction.
Using CFRP composites for PCCP repair requires a thorough construction specification, specially trained construction inspectors as well as a detailed inspection protocol for quality assurance.
Prior to the installation of the CFRP reinforcement, a pull-off test is performed (Figure 4) to confirm design assumptions. This procedure is necessary in order to confirm the existing condition of the internal concrete core and designed bond between concrete core and CFRP material. Results of the pull tests are reported to WSSC engineers and added to the web-based quality assurance and control records.
WSSC engineers initiate a record in the web-based system for each repaired pipe section, identifying pipe number, location and specifics of the rehabilitation. This record will include all design, inspection, repair and as-built information.
The major field preparation and installation procedures for CFRP are noted below. Each procedure is controlled and documented for quality assurance and compliance with specifications.
Preparation of the deteriorated pipe before CFRP reinforcement includes surface preparation concrete core by high pressure water blasting, drying, patching and repairing any damaged concrete core, and preparation of the pipe joints for termination of the CFRP. These procedures are controlled by the consultant inspector, who closely examines each of the prepared pipes for compliance to specifications and approved construction shop drawings. Approvals and inspection comments are documented.
The installation of the CFRP structural reinforcement (Figure 5) includes mixing of the epoxy material, saturation of the glass and carbon fiber mesh with epoxy mix, application of the saturated sheets on the internal surface of the pipe, detailing of the liner termination at the pipe joints.
Each of the CFRP installation procedures is subject to continuous quality assurance and control, which includes testing and production of test sample panels. The cured sample panels are produced by the carbon fiber contractor per standard WSSC specifications, collected by the inspector and sent to a certified testing laboratory. Following completion of the installation, the inspector performs a thorough examination of the install FRP system confirming compliance with design and checking for defects. Any deficiency is recorded specifying longitudinal and circumferential location, size and character of the defect. The contractor makes any repairs and resubmits the installation report, identifying date and method of the repair.
A final walk-through inspection confirms completion of any defects and results in the final approval of the work. Upon approval, a final one page QA/QC report is generated for each pipe section.
Based on WSSC’s working experience by inspecting and doing rehabilitation repairs on 12 to 18 miles of PCCP transmission mains annually, the use and development of an efficient web-based QA/QC system for the rehabilitation of PCCP with CFRP proved to be a vital tool to ensure consistency, specification compliance and to meet the demands of an expedited construction process.
Michael P. Gipsov, P.E., M.ASCE, Principal Structural Engineer, Washington Suburban Sanitary Commission and David M. Burke, P.E., Group Leader, Washington Suburban Sanitary Commission.