PVC VS. Fiberglass (FRP) Gravity Sewer

1. PVCVS.FIBERGLASS(FRP)GRAVITYSEWER: ADVANTAGEPVC • There are common misconceptions among design engineers regarding pipe stiffness,pipe • deflection, impact strength, abrasion, and design life when comparing PVC and fiberglass (FRP)sewerpipes.Whencorrectevaluationsaredone,PVCistheproductofchoice. • Standards • PVCsewerpipehasbeenusedformorethan60years,at first insmallerdiametersbutnowinsizesupto60-inch. FRPsewerpipehasbeenavailableforashortertimeperiod,primarilyindiametersof24inchesandlarger. • FRPandPVClarge-diametersewerpipesaremanufacturedpercurrentASTMstandardsasfollows: • FRP–ASTMD3262“StandardSpecificationforFiberglass(Glass-Fiber-ReinforcedThermosetting-Resin) • SewerPipe” • PVC–ASTMF679“StandardSpecificationforPolyvinylChloride(PVC)Large-DiameterPlasticGravity • Sewer Pipe andFittings” • PipeStiffness(PS):NoNeedtoSpecifyHigherPSforPVC • AcommonmisunderstandingisthatPVCpipemustbespecifiedwithahigherPStobeequaltoFRPpipe.BothPVC andFRPemploythesametesttodeterminePS:ASTMD2412“StandardTestMethodforDeterminationofExternal Loading Characteristics of Plastic Pipe by Parallel-Plate Loading.” Also, both standards include the most common PS of 46 psi and a less flexible product with PS of 72 to 75 psi. The F679 standard for PVC also provides an even stiffervaluewithPSof115psi.SincePSvaluesareequivalent,thedeflectioncalculationsshouldprovidethesame percentagedeflectionnomatterwhichpipeisused. • Inflexiblepipedesign,thepipeembedmentqualityusuallyhasagreaterinfluenceonlimitingpipedeflectionthan doesthepipestiffness.However,fiberglassismoresensitivetoembedmentqualitysinceithaslessflexibilityandit issubjecttostraincorrosion. • Insummary, bothstandards includestiffness-equivalentpipes at PS=46psiandPS=72to75psi;PVCincludesathird productlinethatismuchstifferatPS=115psi.FRPisabletoproduceproductswithhigherthan72psi stiffness, butthis requiresareductioninallowablepipedeflection. • PipeDeflection–FRPSubjecttoDamageatLowerDeflectionLevels • Both FRP and PVC pipes are considered flexible pipes and use the same analysis method – the Modified Iowa Equation.ThismeansthatthesamepipedeflectionoccursforbothproductsforthesameloadingandthesamePS. However,FRPhasmuchtighterlimitsonhowmuchdeflectionisallowed: • FRP: 4% (short-term);5%(long-term) • PVC: 7½% (short- andlong-term) • Becauseofitsgreaterallowabledeflection,PVCpipecanbeburiedtogreaterdepths.Expressedanotherway,the same deflection occurs for both pipes, but FRP is subject to damage at lower deflection levels. The FRP standard, ASTM D3262, requires that strain corrosion be considered, which can limit the maximum deflection of FRP pipe to4%whenthepipeisdesignedfora50-yearlife.Also,FRPpipes arefrequentlyspecifiedwithvinylesterinterior lininglimitingthemaximumlong-termdeflectionto4%andshort-termdeflectionto3%. • ImpactStrength–ConstructionImpactsCanLeadtoFailureofInstalledFRP • ASTM F679 requires that PVC gravity sewer pipe be tested for impact resistance in accordance with ASTM D2444 “StandardTestMethodforDeterminationoftheImpactResistanceofThermoplasticPipeandFittingsbyMeansof a Tup (Falling Weight).” FRP has no such test requirement, which is surprising since FRP’s inside-diameter liner can crackwhensubjectedtoimpactforces.CrackingoftheinteriorlinerofFRPpipecancauseprematurefailureofthe pipeduetostraincorrosion. • Uni-Bell PVC Pipe Association | 2711 LBJ Freeway, Suite 1000 | Dallas, TX 75234 Phone: (972) 243-3902 | Fax: (972) 243-3907| info@uni-bell.org

2. PVCVS.FIBERGLASS(FRP)GRAVITYSEWER: ADVANTAGEPVC AbrasionResistance–FRPAveragesGreaterAbrasionWear Based on abrasion testing of different pipe materials by the Institute for Hydromechanic and Hydraulic Structures of the Technical University of Darmstadt (TUD), Germany, PVC pipe had about three to four times the resistance to abrasionasFRPpipe.TUD’stesthasnowbecomeastandardtestmethodforabrasiontestingforpipes. DesignLife–FRPSubjecttoStrainCorrosion PVCsewerpipedoesnotdeteriorateinservice–itsdesignlifeis100+years.Infact,arecentAustralianstudyshowed theresidualexpectedlifeofinstalledPVCsewerpipetobeasmuchas288years.Toreadthepaper,clickhere. Incontrast,FRPissubjecttostraincorrosion.FRP’sinteriorlayersaresubjecttocorrosionifcracksoccurontheinside or outside layers. These cracks can occur if the pipe experiences too much deflection. According to their industry publicationsandASTMD3262,FRP’sdesignlifeisonly50yearsatitsmaximumdeflectionlevels. One reason for PVC pipe’s long life is that it is a homogeneous material, meaning that the same material is used throughout the pipe wall. In contrast, FRP is a composite material consisting of multiple layers of fillers, resin, and fibersthatvaryintheirabilitytoresistcorrosion. AdvantagePVCSewerPipe Asshownabove,whencomparingfiberglassandPVCsewerpipes,PVCistheclearwinnersinceit: • Provides more pipe-stiffnessoptions • Ismoreforgivingregardingpipedeflections • Hassignificantlygreaterimpactresistance • Hasaboutfourtimestheresistancetoabrasion • Hasalongerdesignlife ForLarge-DiameterSewerPipeApplications,PVCIstheProductofChoice Utilitieslookingforlarge-diameterpipingfortheirsewernetworkshaveprovenoptionsavailablefromthePVCpipe industry.Asmunicipalitiesandengineersattempttoreducecostsandimprovesystemperformance,consideration shouldbegiventosomeofthesignificantdifferencesbetweenPVCandFRPsewerpipematerials.Thetablebelow summarizessomeofthekeydifferencesbetweenthetwomaterials. References: ASTM D2412; ASTM D2444; ASTM D3262; ASTM F679; Handbook of PVC Pipe, Uni-Bell PVC Pipe Association; “Wear Data of Different Pipe MaterialsofSewerPipelines,”UniversityofDarmstadt;WhittleandTennakoon,“PredictingtheResidualLifeofPVCSewerPipes;“TheCompleteHobas Guide,” Hobas PipeUSA Uni-Bell PVC Pipe Association | 2711 LBJ Freeway, Suite 1000 | Dallas, TX 75234 Phone: (972) 243-3902 | Fax: (972) 243-3907| info@uni-bell.org 09.25.15