Evaluating Large Diameter PVC Versus Fiberglass For Sewer Projects

1. Evaluating Large-Diameter PVC versus Fiberglass for Sewer Projects

2. Pipe Material Selection Selection of the most appropriate pipe material is critical for maximizing cost-effectiveness and ensuring long- term reliability of sewer systems. This document compares solid-wall polyvinyl chloride (PVC) and fiberglass (FRP) pipes, the two most commonly installed pipe materials for large- diameter gravity-sewer applications. Some engineers still consider 18-inch as large-diameter, but the 36-inch size was added to the ASTM F679 standard in 1986 (about 40 years ago). Comparisons in this paper are limited to 24- through 60-inch sizes which are commonly available for both products. The Primary Difference: Deflection As will be shown, the primary difference between PVC and FRP pipe is how each pipe is able to respond to ring deflection when buried. FRP can withstand only small deflections before the material fails while PVC pipe can endure significant deflections with no material failure. Thus, engineers and owners should consider controlling the deflection of FRP for their projects. Additionally, flexible pipe design demonstrates there is never a need to specify a higher stiffness for PVC than for FRP. In most cases, FRP should be specified at a higher stiffnesses than PVC. 48-inch PVC pipe 2

3. Pipe Stiffness: Importance of Direct Comparison EXTERNAL LOAD For flexible pipe, pipe stiffness is the ability to resist ring deflection from external load. A common misconception is that PVC pipes need higher pipe stiffness to match the performance of FRP. ASTM D2412 is the test method used to determine pipe stiffness for both materials, making a direct comparison appropriate. There can be confusion due to the different terminology used, but PVC’s pipe stiffness (PS) is the same as FRP’s stiffness number (SN). Both pipes are widely available in 46 psi stiffness, but both have higher-stiffness products if required by project parameters. For this document, 46 psi is the basis for comparison. This stiffness is suitable for most PVC installations, but not necessarily for FRP pipelines. For some projects with high loads or poor soils, FRP pipe may require a higher stiffness (i.e., SN 90 or SN 120) to keep deflections within allowable limits. Conversely, PVC PS 46 is acceptable for almost all projects, because anticipated deflection is less than its allowable deflection limit. Pipe stiffness and soil stiffness resist deflection for all flexible pipes. 3

4. Pipe Deflection: Not All Materials are Equal Both PVC and FRP are considered flexible pipes which are designed based on limiting pipe deflection. However, there is a noticeable difference in deflection limits for the two materials: • PVC pipe:7.5% (per ASTM D3034) • FRP pipe:5.0% (per ASTM D3262) This means that PVC pipe is better suited for a wide range of installation conditions. FRP pipe, with a lower deflection limit, is more prone to failure from deflection.Unlike FRP, PVC pipe does not experience material failure at any level of deflection. Another indicator of the difference in deflection capability is the quality-control flattening test required by both pipe materials: • ASTM D3034 PVC pipe requires flattening of 60% of the original diameter without damage. At 30% deflection, reverse curvature occurs (no material damage), which is considered the design limit for PVC. Allowable long-term PVC deflection is 7.5%, providing a safety factor of 4.0 against the design limit (no material failure). • ASTM D3262 FRP pipe also requires a flattening test (called ring-deflection testing) in which the pipe must reach a deflection limit before material damage (i.e., surface cracking). For a 46-psi stiffness fiberglass pipe, this limit is 10%. At a long-term FRP deflection of 5%, this equates to a safety factor of 2.0 for material failure. Above, a 48-inch PVC pipe is deflected at 40%, twice as much as the 36-inch FRP at 20%. The PVC pipe has suffered no damage, while the FRP pipe has failed. Site Conditions that Reduce Soil Stiffness Based on the previous sections, FRP pipe requires more stringent deflection control. There are many conditions where the soil embedment may not provide support for the pipe as designed: • Large-diameter sewer pipes are frequently installed in areas of low soil strength such as near rivers, lakes, or coastal areas, which may reduce trench wall support. • High water tables may significantly affect soil strength due to soil migration. • Inadequate backfill compaction can have a significant effect on soil support of the pipe. These conditions will likely reduce the soil stiffness assumed for the design. 4

5. Design Example: Pipe Deflection The following design example demonstrates the importance of the pipe-soil structure to control deflection and shows the long-term deflections that result from these conditions: • PS/SN 46 psi is used for both pipes. FRP SN 72 and SN 120 are included for comparison. • The Modified Iowa Equation is used to calculate long-term deflection for both materials under the same conditions. • Depth of bury = 22 ft • Composite modulus of soil reaction, E’ = 400 psi Calculated deflections are shown in the table below. PVC PS 46 5.9% FRP SN 46 5.9% FRP SN 72 5.2% FRP SN 120 4.3% Deflection Comparisons for PVC and FRP Pipes Calculated long-term deflection 7.5% YES 5.0%  NO 5.0% NO 5.0% YES Allowable long-term deflection per ASTM D3034 and D3262 Within allowable deflection In this example, PVC is below its allowable long-term deflection limit while FRP has exceeded its limit for both SN 46 and SN 72. For FRP not to exceed the deflection limit, SN 120 is required. If PVC pipe is required to have a higher stiffness than FRP in a project you are working on, please contact a PVCPA regional engineer for technical design support. FRP: More Stringent Embedment Requirements As shown in the design example, there are conditions that can lead to failure for FRP, whereas PVC PS 46 will not only be below its design limit, it will also not experience material failure at any deflection. Designers may need to specify ways to control FRP deflection by using more stringent embedment requirements (such as flowable fill with wider trench) and/or using higher pipe stiffness than what is required for PVC. Using these methods for decreasing FRP’s deflection allows the designs for both materials to be equivalent. PVC does not require either of these methods since it does not fail due to deflection and has a high safety factor for design. The adjacent figure shows how embedment can be specified for FRP and PVC to obtain similar overall performance. SPECIFIED BACKFILL OR FLOWABLE FILL TO ABOVE PIPE SPECIFIED BACKFILL TO SPRING LINE PVC FRP Example of trench detail that provides equivalency between PVC and FRP pipe. 5

6. Installation As explained, the main difference between FRP and PVC is that FRP is much more susceptible to failure when the pipe deflects more than designed. As a result, FRP requires much more stringent bedding and backfill specifications. To ensure that the embedment around the FRP pipe is properly placed and compacted, utilities should consider requiring enhanced inspection and testing during construction (such as laser-profiling and camera inspection). Utilities should also consider long-term monitoring programs for all FRP pipelines in service. Cost: The Importance of Long-Term Performance PVC and FRP often have differences in initial costs. However, during the evaluation process, it is important to consider not only bid costs but also factors that affect long-term performance, such as: • Trench design • Need for additional inspection • Failure mode • Likelihood of failure • Consequence of failure FRP Pipe-Failure Case Studies There are an estimated 3,500 miles of 24-inch and larger PVC solid-wall sewer pipe installed in North America, compared to approximately 1,200 miles of FRP. There is no history of failures of solid-wall PVC sewer pipe due to deflection. However, there are numerous well-documented instances of premature, catastrophic failures of FRP pipe that can be found from news articles or other publicly available information. Case #1: Bentonville, AR Over 47,000 feet of 30- and 36-inch FRP were installed in 2009 and 2010. Starting in 2016, the utility began experiencing failures. A comprehensive assessment of the line showed that only 1,700 feet of pipe were deflected below the 5% deflection limit for FRP. The remaining 45,000 feet were all over-deflected and required varying levels of repair or replacement. The estimate to replace the entire line was $26.7 million. The utility determined that this was the most cost-effective solution. In 2020, the utility replaced the entire pipeline with 30- and 36-inch PVC pipe. Click here for additional information. Case #2: Franklin, TN The utility installed approximately 25,000 feet of 18- to 54-inch FRP pipe in three phases. The project was completed in 2001. Only 13 years into the expected 50-year service life, the pipeline began failing. In the course of three years, three sections failed, costing the city more than $1.8 million in emergency repairs. Click here and here for additional information. 6

7. Case #3: Grand Prairie, TX A 104-inch diameter FRP interceptor was installed in 2008. The pipeline experienced catastrophic failure in 2020. Emergency repair of the pipeline, which carried up to 55 million gallons per day, was completed in two months and required installation of more than 4,000 feet of replacement pipe. Click here for additional information. Case #4: Murfreesboro, TN Beginning in February 2019, the utility had multiple failures of 36-inch FRP. These failures initially cost the utility over $350,000 in emergency repairs. Later, it was determined that a 1,400-foot section of the interceptor needed to be replaced, at an estimated cost of $3.0 million. Click here and here for additional information. Large-Diameter PVC Sewer Installations The map shows the locations of some large-diameter PVC sewer pipe installations across the U.S. and Canada. These utilities evaluated different sewer pipe materials and selected PVC pipe as the best option, demonstrating its ability to perform in all environmental conditions, including areas with poor soil stiffness or high water table. For information on any of these installations, contact a PVCPA regional engineer. Deflection: The Critical Consideration The primary difference between PVC and FRP pipe is how each pipe responds to deflection when buried. While both pipes have the same pipe stiffness and will deflect the same amount, FRP is more susceptible to failure. As a result, FRP is much more reliant on controlling deflection by conservative embedment design, strict installation specifications, and continuous installation inspection. Flexible pipe design demonstrates there is never a need to specify a higher stiffness for PVC than for FRP. In most cases, FRP may need to be specified at a higher stiffness than PVC. Owners, engineers, and contractors should consider these factors for large-diameter sewer projects. PVCPA regional engineers are available to address all aspects of this document and assist with project-specific questions. Please visit www.uni-bell.org/contact or call (972) 243-3902. UNI-PUB-21-25 7

8. UNI-BELL PVC PIPE ASSOCIATION 201 E. John Carpenter Freeway Suite 750 Irving, TX 75062 8