56 Underground Construction October 2004 www.undergroundconstructiononline.com

Technological advances in the pipingindustry over the course of a centuryhave made wooden pipes and brick sew-ers a concept of the past, while openingthe door to better engineered pipingproducts which provide the user withmore cost-effective solutions. Traditionalpiping materials for water and sewer sys-tems are steadily being replaced by mod-ern materials which provide better per-formance, ease of installation, and over-all, a better “bang for the buck.”

Environmental effects such as corro-sion pose a dramatic threat to the buriedpipe infrastructure in North America. Ithas been estimated in a study performedby the Federal Highway Administration(FHWA) that corrosion accounts formore than $36 billion of loss annually inwater and sewer systems throughout the U.S.

Today, polyvinyl chloride (PVC), athermoplastic material, is the most wide-ly used product in water and sewer sys-tems in the U.S. and Canada. This articlewill provide design engineers and speci-fiers with an information base on variousPVC pressure and gravity piping stan-dards and products, for open-cut andtrenchless construction and rehabilita-tion. The contents should contribute tothe decision-making process for standardand product selection when PVC hasbeen specified or is under consideration.Other pertinent information such asavailable joint-types is also discussed.

Plastic pipes

The availability of plastic pipe in thelate 1950’s in North America was thebeginning of an industry that wouldeventually revolutionize the field ofmunicipal water and sewer piping.Plastics are formed by the polymeriza-tion of molecules containing hydrogenand carbon. The three main types ofplastic pipes widely in use in NorthAmerica include polyvinyl chloride(PVC), high density polyethylene(HDPE), and glass-reinforced pipe (GRP,also called fiberglass pipe). PVC andHDPE fall into the group of thermoplas-tics, while GRP is a thermoset pipe. The

unique quality of thermoplastics enablethem to be heated, processed, formedand reshaped many times, without anypermanent changes taking place in thematerial’s physical/mechanical proper-ties. Thermoset plastics on the otherhand, are processed by a combination ofchemicals and heat, and once formed,can not be reshaped.

Thermoplastics, such as PVC used inpipe manufacture, are referred to as rigidplastics. The term rigid indicates thatthese materials do not contain any plasti-cizers, which would make them moreductile, and hence unsuitable for buriedmunicipal applications. Both thermoplas-tic and thermoset pipes are flexible con-duits (the pipe/soil interaction is identi-cal) and are designed accordingly.

Properties of viscoelastic

materials

Thermoplastics such as PVC are vis-coelastic. Viscoelastic materials exhibitelastic as well as viscous-like characteris-tics. A material that deforms understress, but regains its original shape andsize when the load is removed is classi-fied as elastic. Viscous materials, on theother hand, after being subjected to adeforming load, do not recover their orig-inal shape and size once the load isremoved. In reality, all materials deviatefrom the linear relationship betweenstress and strain (Hooke’s Law) at somepoint in various ways.

Defining the direct relationshipbetween stress and strain when a load isapplied to a material is the most commonway to evaluate the strength of thatmaterial. Graph A in Figure 1 illustratesthe linear relationship between stress-strain in elastic materials. In an idealelastic material, strain returns to zero assoon as the material is unloaded, and thelinear relationship is not typically time-dependent. But it should be noted that inall materials, this behavior is valid onlyup to a certain stress point, called theyield point, after which the strain in thematerial will increase dramatically bycreep, before finally failing.

U-Tech U n d e r g r o u n d T e c h n o l o g yCutting Edge Technical Information For Utility Construction & Rehabilitation

A Comprehensive Review Of Municipal PVC Piping ProductsThermoplastics At WorkBy Shah Rahman, EIT, Regional Vice President, Underground Solutions, Inc.

In the set of curves, B, in Figure 1, it canbe seen that the stress-strain relationshipis somewhat different for viscoelasticmaterials than it is for elastic materials.Clearly, you can no longer see a directlylinear relationship between stress-strain,and the gradients of the curves dependon the loading time. In other words, for agiven stress level, the longer the loadingtime, the larger the strain reached. Creepis defined as continuing deformation(increasing strain) with time when thematerial is subjected to a constant stress.The consequence of creep is that failureof the material will occur after load isapplied for a certain amount of time. Sotime dependency is a major factor to con-sider in viscoelastic material behavior.

An important fact is that the time tofailure is inversely proportional to theapplied stress. In thermoplastic pressurepipe, it is therefore possible to find andapply a stress level that is low enough toensure that the theoretical time to failurewill surpass the design life of thepipeline.

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In thermoplastic pipe applications,creep is not free, but is preventedbecause the deflection of the pipe iskept constant, as is the case in buriedPVC gravity (or pressure) pipe.Consequently, it can be seen fromFigure 1 that the initial stress decreaseswith time, and is referred to as the relax-ation property of thermoplastic pipingmaterials. These basic properties of vis-coelastic materials such as PVC andHDPE enable engineers to designpipelines that ensure both structural

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www.undergroundconstructiononline.com October 2004 Underground Construction 57

integrity and the long-term design life oftheir municipal piping systems.

Manufacture

In 2000, more than five billion pounds ofPVC resin was used in the manufacture ofPVC pressure and gravity pipe. A vinylchloride molecule is comprised of carbon,hydrogen, and chlorine, configured asshown in Figure 2a. PVC is obtained bypolymerization of single units of the vinylchloride molecule, which join to createlong chains, Figure 2b, and ultimatelyform PVC resin, Figure 2c.

PVC pipe is manufactured by firstblending the resin with stabilizers, pig-ments, lubricants, processing aids andfunctional additives, and heating thismixture to a temperature in the 400degree F range. This causes the compo-nents to properly fuse and convert into amalleable state. In this molten form, thematerial is mechanically extruded intopipe, whereby the pipe meets the physi-cal/mechanical properties outlined in var-ious ASTM, AWWA and other standards.

www.undergroundconstructiononline.com October 2004 Underground Construction 57

Following the completion of the extru-sion process, the pipe is allowed to cool,after which QA/QC testing is performedper requirements of the standards towhich they are made, before final deliv-ery to the end user.

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The Plastic Pipe Institute’s documentTR-2/2004, PPI PVC Range CompositionListing of Qualified Ingredients, lists theHydrostatic Stress Board’s (HSB) allow-able content of various ingredients forPVC pipe manufacture as defined by aminimum/maximum range. These listsare compiled through results of extensiveindustry-wide testing. In the earliest daysof PVC pipe manufacture, each PVC com-position was a fixed and very specificcomposition, with the use level and iden-

tity of each ingredient clearly “spelledout.” This is no longer the case; the list-ings of PPI TR-2 now enable manufactur-ers to produce pipe formulated withinthe allowable minimum/maximum ingre-dient ranges; consequently, it is likelythat no two manufacturers have the iden-tical formulation. This is a result of thesearch for more effective and less costlysources of ingredients. The ultimate goalis to produce pipe that meets all therequirements of the standards to whichthey are made.

Basic PVC properties

The basic properties of PVC compoundsare outlined in ASTM D1784, StandardSpecification for Rigid Poly (VinylChloride) (PVC) and Chlorinated Poly(Vinyl Chloride) (CPVC) Compounds.The specification defines a five-digit cellclass designation system that describesminimum characteristics for a particularcompound. The five designated proper-ties include: base resin, Izod impactstrength, tensile strength, elastic modu-

58 Underground Construction October 2004 www.undergroundconstructiononline.com

lus in tension and deflection temperatureunder loading.

For example, in the cell classification12454, the third digit, 4, indicates thatthe pipe has a minimum tensile strengthof 7,000 psi. The fourth digit, 5, indicatesa minimum modulus of elasticity in ten-sion of 400,000 psi. Similarly, a pipe man-ufactured to cell classification 12364would have a minimum tensile strengthof 6,000 psi, and a minimum modulus ofelasticity of 440,000 psi.

All conventional PVC pressure pipestandards in the U.S. are manufacturedto a cell class of 12454 only, highlightingthe importance of a minimum tensilestrength of 7,000 psi; this is the optimumvalue which yields the maximum long-term strength when factors of safety areapplied to PVC pipes operating underpressure. Gravity pipe standards on theother hand are made to both cell class12454 and 12364. For all practical pur-poses, gravity pipe manufactured toeither formulation perform very wellunder buried conditions.

Types of PVC pipe

There are three distinct types of PVCpipes manufactured in the world, eachdifferentiated by either the way in whichit is manufactured (which dictates thedirectional orientation of the molecules),or by the content of modifiers in itschemical formulation (which affect theability of the pipe to withstand largeimpacts by absorption and dissipation ofthe energy). They include:�� PPVVCC--UU:: Unplasticized PVC is the mostcommon type of PVC used in the manu-facture of pipe. That which we refer tosimply as PVC in North America is in factunplasticized PVC. The manufactureprocess described earlier in this article isthat of PVC-U. The molecular structureof PVC pipe is a random arrangement oflong chain molecules, where molecularentanglement is prevalent throughoutthe length of the pipe. In general, thePVC molecules do not exhibit any defi-nite directional orientation, and there-fore, a generally uniform strength pre-vails in both the radial (circumferential)and longitudinal directions. Testing hasshown that the modulus of elasticity infifteen year-old PVC is only slightly high-er in the longitudinal direction than inthe radial directions.�� PPVVCC--OO:: Molecularly oriented PVC ismade in the U.S. by expansion of conven-tional PVC pipe; during the expansionprocess, the molecules become orientedin a generally radial or circumferentialdirection. This molecular re-orientationincreases the strength of the pipe in the

hoop direction. Also, the resulting HDB isincreased from 4,000 psi to 7,100 psi.Consequently, this stronger material canhave a thinner wall than a conventionalPVC pipe of the same pressure capacity.The manufacture process of PVC-O in theU.S. is called an Offline Process, while asecond method, the Online Process, ismore widely used in Europe. In the U.S.,PVC-O if considered a proprietary product since only one company makesit. PVC-O pipe is not used in gravityapplications.�� PPVVCC--MM:: Modified PVC is produced byincorporation of additives or “impactmodifiers” to enhance the toughness ofthe material. Resistance to fracture byabsorption and dissipation of energy isevidence of the toughness of the pipematerial. PVC-M is made and used mainly in Europe and Australia, whileonly one manufacturer in the U.S. pro-duces this type of pipe for non-buriedapplications.

PVC pipe joints

There are various types of joints availablefor PVC pressure and gravity pipe stan-dards, used according to the application�� BBeellll--aanndd--ssppiiggoott ggaasskkeett jjooiinnttss:: Tra-ditionally, this has been the joint typemost associated with PVC pressure andgravity pipe standards used for open-cutmunicipal applications. The beveled spig-ot end of the adjoining pipe is slippedinto the bell up to the guide mark indicat-ed on the spigot; the elastomeric gasketis usually prepositioned in the bell andprovides a completely water-tight com-pression seal, Figure 3.

FFiigguurree 33:: EEllaassttoommeerriicc GGaasskkeett--JJooiinntt

The elastomeric gaskets are made perASTM F477, Elastomeric Seals (Gasket)for Joining Plastic Pipe. With properassembly, there is usually enough of a gapbetween the spigot-end and the neck ofthe bell to allow for expansion as well asangular deflection. Gravity pipe and fit-tings are made and tested to the require-ments of ASTM D3212, Joints for Drainand Sewer Plastic Pipes Using FlexibleElastomeric Seals. Uni-Bell’s Handbookof PVC Pipe: Design and Construction,recommends an infiltration/exfiltration

limit of 25 gal/in-dia/mile/day within asewer system to account for losses atmanhole connections and through otherappurtenances, not through the pipejoint itself. Most other gravity pipe mate-rial industries encourage infiltration/exfiltration limits in the 200 gal/in-dia/mile/day range.

ASTM D3139, Joints for PlasticPressure Pipes Using FlexibleElastomeric Seals, is typically the stan-dard to which PVC pressure pipe jointsare made and tested. AWWA C605,Underground Installation of PolyvinylChloride (PVC) Pressure Pipe andFittings for Water, also provides guide-lines for post-installation testing of PVCpressure pipe systems for potable waterdistribution and transmission.�� FFuussiibbllee JJooiinnttss:: Until recently, HDPEwas the only available thermoplastic pipeoption which used butt-fused joints inNorth America. In late 2003, thewater/wastewater industry saw the intro-duction of Fusible PVC/C-900/C-905. Afirst of its kind, this product combines aproprietary formulation with a uniquefusion procedure that allows lengths ofPVC pipe to be joined together in a con-tinuous string for installation through avariety of trenchless methods, includingHDD, sliplining, pipebursting, guidedboring, and jack and bore, as well asdirect bury applications. While FusiblePVC is primarily for non-drinking waterapplications like recycled water, forcemains, gravity drains and sewer applica-tions, Fusible C-900/C-905 are specifical-ly for use in potable water systems. Thepipes are manufactured to all require-ments of AWWA C900 and C905, and areNSF certified. Figure 4 shows a butt-fused PVC joint.

FFiigguurree 44:: BBuutttt--FFuusseedd FFuussiibbllee PPVVCC JJooiinntt

Fusible PVC pipe manufacture is possi-ble because of two significant develop-ments. First, a unique patent pendingformulation which meets all guidelinesfor components as detailed in the PlasticsPipe Institute’s (PPI) Technical ReportNo. 2, PPI PVC Range Composition ofQualified Ingredients — all ingredients inthe formulation meet the qualificationstandard. This allows PVC pipe to beextruded in standard diameters and wall

Thermoplastics At Work

www.undergroundconstructiononline.com October 2004 Underground Construction 59

thicknesses. The pipe exhibits all basiccharacteristics of conventional PVC pipefor pressure capability, tensile strength,external load capability, etc. —- its cellclassification is 12454B, per require-ments of ASTM D1784.

The second development was thefusion procedure. The UGSI fusible lineutilizes standard butt fusion equipmentfor the joining of the pipe. A unique setof temperatures, pressures and durationof fusion steps allow for the pipe to fuseand create joints that are essentially asstrong as the original pipe material. Thefusion procedure takes about the samelength of time as other thermoplasticsfusion joints such as HDPE.�� OOtthheerr JJooiinntt TTyyppeess:: Solvent-weldedjoints are typically used for plumbingapplications. However, some older utilitiesin the U.S. still allow solvent-welded PVCpipe to be used in municipal gravity appli-cations. ASTM D2672, Joints for IPS PVCPipe Using Solvent Cement, and ASTM D2855, Practice for Making Solvent-Cemented Joints with Poly (VinylChloride) (PVC) Pipe and Fittings, arecommonly used for solvent-welded joints.

Another type of joint, used only for

PVC plumbing products, is the threaded,screw-on type. ASTM D2464, ThreadedPoly (Vinyl Chloride) (PVC) Plastic PipeFittings, Schedule 80, is an example.

There are two proprietary restrained-joint PVC pipes, TerraBrute and Certa-Lok, details of which are provided inTable 4. Both products are used in pull-in-place trenchless applications.

PVC pressure pipe standards

PVC pressure pipes find their use inpotable water distribution and transmis-sion, as well as in sanitary sewer forcemains. The hydrostatic design basis(HDB) of PVC pressure pipe is the hoopstress value from which the long-termpressure rating of the material is estab-lished. It is the starting point for deter-mining the pres-sure capacity ofa given wallt h i c k n e s s .AWWA andASTM standardsfor conventionalPVC pressurepipe have an

HDB of 4,000 psi. For PVCO (molecular-ly oriented PVC) standards, an HDB of7,100 psi range is used. Due to this differ-ence, PVCO has a thinner wall than PVCpipe of the same diameter.

There are currently five widely usedPVC pressure pipe standards: ASTMD2241, AWWA C900, AWWA C905,AWWA C909 and ASTM F1483.

The first three are conventional PVC,while the latter two are PVC-O. Table 1summarizes some of the properties of each of the previously mentioned standards:

While ASTM D2241 was, and still is,used primarily in the rural water marketafter its introduction in 1964, AWWAC900 was the first widely specified PVC

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Standard HDB (psi) Factor of Surge Available Safety Allowance Diameters (in)

ASTM D2241 4000 2.0 NO 4 – 36*AWWA C900 4000 2.5 YES 4 – 12AWWA C905 4000 2.0 NO 14 – 48AWWA C909 7100 2.5 YES 4 – 24ASTM F1483 6810 / 6040 2.0 NO 4 – 16

* Diameters of 1/8-inch through 3.5-inch available with solvent-welded joints

60 Underground Construction October 2004 www.undergroundconstructiononline.com

pressure piping standard for larger,urban utilities. Introduced in 1975,AWWA C900 incorporated a higher factorof safety than its predecessor, and alsoincluded a surge allowance. In the diame-ter range of 4-inch through 12-inch, thisstandard is most widely used in distribu-tion systems, which is characterized bylooped systems, with numerous and fre-quent lateral connections; hence thisstandard incorporated a surge allowanceand a higher factor of safety. C905 waspublished in 1988, and because of itsdiameter range of 14-inch through 48-inch, the standard was designed for usein transmission and force main lines.Transmission systems are characterizedby fewer service connections than foundin distribution networks, generally highervelocities, and fewer barrier effects. LikeASTM D2241, it has a factor of safety of2.0 and no surge allowance. The fewerbarrier effects in transmission linesenable easier accounting for surges,which is incorporated into the systemdesign.

PVC gravity pipe standards

Non-pressure PVC pipes have been inuse in the U.S. since the early 1960’s.Today, PVC gravity pipe is used in sani-tary sewer, storm sewer and highwaydrainage and culvert applications. Thereare two main groups of PVC gravity pipe—- solid wall and profile wall. There areseven widely used solid and profile wallpipe standards: ASTM D3034, ASTMF679, AASHTO M278, ASTM F794,ASTM F949, ASTM F1803 and AASHTOM304.

Solid wall pipe, as the name suggests, ismade of a continuous wall of PVC of uni-form thickness. Profile wall pipe, on theother hand, is braced spirally or circum-ferentially with structural shapes, butprovides a smooth-wall interior. Profilepipes economize on the amount of mate-rial needed for fabrication; by altering theshape of the wall, the same stiffness assolid-wall pipe is achieved, using lessmaterial. Profile wall pipe generally fallinto three categories —- open profile(OP), closed profile (CP), and dual wallcorrugated profile (DWCP). OP pipehave their rib-enforcements exposed onthe outside of the pipe. CP pipe make useof a closed profile that provides a contin-uous outer wall where the wall sectionsare hollow and are often described as anI-beam or honeycomb. DWCP pipe has asmooth-wall waterway, braced circumfer-entially with an external corrugated wall.

While all PVC pressure pipe standardsare manufactured only to cell classifica-tion 12454 (tensile strength of 7,000 psi,modulus of elasticity of 400,000), some

sewer pipe standards allow manufactureof both cell classifications 12454 and 12364 (minimum tensile strength of 6,000 psi, minimum modulus of elastic-ity of 440,000 psi). For all practical purposes, pipe manufactured to eitherformulation perform very well underburied conditions.

Table 2 summarizes various propertiesof the previously listed ASTM standards.AASHTO M278 is similar in scope toASTM D3034, with the exception that theformer also allows for perforated wall. Asan AASHTO standard, the application ofthis type of pipe is for subsurfacedrainage and surface drainage (culverts)of transportation facilities. AASHTOM304 ranges in diameters of 4-inchthrough 48-inch, but what sets it apartfrom ASTM standards is the wide varia-tion of allowable pipe stiffness. While allASTM solid and profile wall standardshave a minimum pipe stiffness of 46 psi,AASHTO M304 pipe stiffness can go

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down to as low as 12 psi for the largestdiameter of 48-inch. In addition to therequirements for watertight joints, thisstandard also calls for soil-tight joints,per AASHTO Specifications for HighwayBridges, Division II, Section 23. Profilescan be of all three described for theASTM profile pipe standards: OP, CP, andDWCP.

Trenchless PVC products

The reality of underground piping con-struction today has evolved into a combi-nation of both open-cut and trenchlessprocesses. While open-cut installationscontinue to be the standard method ofinstalling pipe particularly for new con-struction, advances in the field of trench-less technology have given engineers andcontractors numerous tools with which toalleviate the challenges of undergroundconstruction in highly developed urbansettings. To meet the demand for trench-less construction, several PVC piping

Thermoplastics

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pressure after placement into a deterio-rating host pipe. In gravity lines, this pro-cedure ultimately provides a close-fitliner with the host pipe that gives somestructural support as well as improvedflow characteristics. The only PVC liningsystem for water distribution and trans-mission, Duraliner, gives a stand-alonestructural system capable of withstand-ing internal pressure of up to 150 psi,Table 6.

Conclusion

This comprehensive state-of-the-artreview of PVC piping products attemptsto provide design engineers and speci-fiers with up-to-date information, whichwill be helpful when using, or consideringthe use of PVC pipe, for water and sewercapital improvement projects as well asrehabilitation of deteriorating pipelines.With the rapidly rising need for rehabili-tation of failing buried piping systems,innovations particularly in the field oftrenchless technology are taking place,and consequently, new types of pipingsolutions are becoming available in themarketplace.

AAbboouutt tthhiiss aarrttiiccllee::

This paper contains excerpts from two

previous publications by the author,

Shah Rahman:

State-of-the-Art Review of Municipal

PVC Piping Products — presented at

the ASCE Pipeline 2004 Conference in

San Diego, CA, summer 2004.

Chapter 6 - Pipe Materials, in

Trenchless Technology : Pipeline and

Utility Design, Construction, and

Renewal by Mohammed Najafi, The

book will be published by McGraw-Hill

in winter 2004. Rahman wrote the

chapter in its entirety, and the materi-

al was reviewed by Dr. Reynold

Watkins of Utah State University.

AAbboouutt tthhee aauutthhoorr::

Shah Rahman is regional vice-presi-

dent of technical marketing for

Underground Solutions. Rahman is a

civil engineering graduate of the

Virginia Military Institute, and also

holds a graduate degree in marketing

from Southern Methodist University.

Prior to joining Underground

Solutions, he spend four years as the

marketing director and a regional

engineer for the Uni-Bell PVC Pipe

Association. Rahman can be contacted

at (972) 243-1300.

FOR MORE INFORMATION:PVC pipe: Uni-Bell PVC Pipe Association, (972) 243-3902, or circle reader service number #159

products have been developed in recentyears. Examples are listed in Table 3.

The three main types of trenchlessprocesses for which the available PVCstandards and products are designedinclude sliplining (rehabilitation), hori-zontal directional drilling (new construc-tion) and close-fit pipe lining (rehabilita-tion).�� SSlliipplliinniinngg PPrroodduuccttss:: Sliplining can becategorized into four unique types —segmental, spirally wound, man-entryprofile strip and continuous. The processis used for rehabilitation of both gravityand pressure systems. Table 4 provides a

listing of three widely used proprietary-joint segmental slipliners.�� HHoorriizzoonnttaall DDiirreeccttiioonnaall DDrriilllliinnggPPrroodduuccttss:: For the purpose of pull-in-place applications, in addition to FusiblePVC discussed earlier, there are twoother proprietary restrained-joint prod-ucts, Table 5.�� CClloossee FFiitt PPiippee LLiinniinngg PPrroodduuccttss:: Close-fit pipe lining is the generic name of atrenchless pipeline rehabilitation processwhere new thermoplastic pipe that ismodified in cross section before installa-tion, is reformed to its original size andshape by the application of heat and

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TTaabbllee 55:: RReessttrraaiinneedd--JJooiinntt PPVVCC PPrroodduuccttss ffoorr HHDDDD

TTaabbllee 66:: CClloossee--FFiitt PPiippee LLiinniinngg PPVVCC OOppttiioonnss

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