FRP for Structural Rehabilitation of Large Diameter Sewers

Carbon and glass fiber reinforced polymers (FRP) have been used for infrastructure for more than three decades, and applications in pipeline rehabilitation has gained momentum in recent years. The advantages these systems can provide include high strength, light weight, and ability to be installed on essentially any type of pipe material, size, and geometry. The common practice for an FRP system design entails accounting for carbon layers only. While carbon fiber is extremely strong in tension (200,000 psi ultimate tensile strength and above), it is not as strong in compression (typically about 50 percent of tensile strength or less). As such, if the external loads on a pipe is significant, such as deeper gravity sewers, then a stand-alone design CFRP system will result in too many layers of carbon with a price greater than other alternatives by multiple folds. This has been the primary reason for FRP systems not being able to find many applications in the wastewater and stormwater infrastructure rehabilitation. This is changing with latest advents in the FRP industry. One such example is sandwich structure system developed by QuakeWrap based on one of the co-authors' (Dr. Mo Ehsani) idea of utilizing the I-beam concept for the FRP liner profile to increase ring stiffness without using excessive layers of carbon fiber laminae. This concept was developed into a pipe (StifPipe®) and has been used in a number of applications over the years. The first tier StifPipe® applications were made using a 'mini I-beam' fabric sandwiched between glass and carbon fiber polymer layers. Then after much testing and evaluation, the manufacturer (QuakeWrap) moved onto a proprietary 3D fabric that has the capability to absorb more epoxy resin, thereby giving it more ring stiffness without a significant increase in weight. The first installation of StifPipe® with the sliplining method dates back to 2012 for a 48-inch wastewater pump station discharge line in Avalon, California. The outside diameter (OD) of the StifPipe® segments used was 47-inches and the one-inch annular space was filled with non-shrink grout. This first installation was followed by multiple other sliplining applications mostly in stormwater lines and culverts up to 70-inches of diameter. In 2018, QuakeWrap started entertaining the idea of building a StifPipe® inside a pipe as cured-in-place installation. In other words, this would be applying the conventional FRP installation technique (wet layup) with the resin rich 3D core fabric. Upon a couple of successful mock installations at QuakeWrap's R&D facility, the first project with the wet layup was carried out in New Jersey (for Middlesex County) to repair a 66-inch storm sewer pipe. The installation went smoothly and QuakeWrap has completed several projects ever since with this method including a 12-ft diameter, 100-120 ft. deep stormwater tunnel under I-35 in Minneapolis, Minnesota (see the image below). The latter case required a 1.47-inch thick StifPipe™, which is designed to withstand the entire soil and groundwater pressure at that depth. FRP systems' high stiffness, hence low thickness along with their smooth surface (Manning's n between 0.009 and 0.010), often results in improved hydraulic capacity in a sewer line after lining. A typical StifPipe® has a pipe stiffness (PS) value of 75 and weighs about a tenth and fifth of concrete and centrifugally cast glass fiber reinforced pipe (CCGFRP) of equivalent strength, respectively. This enables installation of StifPipe® without any jacking equipment for diameters up to 72-inch. Design considerations include factoring in any excessive axial forces, which could require additional layers to prevent axial buckling in a pipe that is otherwise has a small wall thickness to withstand external and internal pressure (for force mains and surcharge conditions). The main installation challenge for the wet layup installation of StifPipe® is making sure the surface prep is done properly. This applies to all types of FRP installation; nevertheless, a unique challenge for StifPipe® is making sure the thick and heavy 3D core layer is saturated with epoxy completely and applied properly on the preceding layer. There is no saturating machine produced for this type of thick fabric; and therefore, the current practice is infusing the resin with ribbed rollers. Quality assurance is performed by weighing the saturated fabric. FRP systems are continuing to provide a viable alternative for pipeline rehabilitation, and the use of FRP systems for particularly large diameter pipelines is growing. Nevertheless, better design practices and education of the end users are needed to get the best out of these high-tech materials in achieving cost competitiveness. With such design practices it is fair to assume that the use of these materials will also grow in the gravity flow pipe systems (mainly storm and sanitary sewers) given the advantages they provide such as high strength, minimal thickness, smooth surface, and ability to accommodate any geometry. Right design and installation practices will enable engineers and owners to enjoy the great benefits of these materials to maximum extent. This presentation will present design and installation practices for composite, as well as proprietary, FRP systems used for sewer rehabilitation. The presenter will also provide insight into NASSCO's Guideline Specification for sewer rehabilitation with FRP, which was published recently. (https://www.nassco.org/resources/guideline-specs).