1、Design Service LifeOfCorrugated HDPE PipeTR-43/20031825 Connecticut Ave., NW Suite 680 Washington, DC 20009P: 202-462-9607F: 202-462-9779www.plasticpipe.orgForewordThis report was developed and published with the technical help and financialsupport of the members of the PPI (Plastics Pipe Institute)
2、. The members haveshown their interest in quality products by assisting independent standards-making and user organizations in the development of standards, and also bydeveloping reports on an industry-wide basis to help engineers, code officials,specifying groups, and users.The purpose of this tech
3、nical report is to provide important information availableto PPI on the design life of corrugated HDPE pipe.This report has been prepared by PPI as a service of the industry. Theinformation in this report is offered in good faith and believed to be accurate atthe time of its preparation, but is offe
4、red without any warranty, expressed orimplied, including WARRANTIES OF MERCHANTABILITY AND FITNESS FORA PARTICULAR PURPOSE. Any reference to or testing of a particularproprietary product should not be construed as an endorsement by PPI, whichdoes not endorse the proprietary products or processes of
5、any manufacturer.The information in this report is offered for consideration by industry members infulfilling their own compliance responsibilities. PPI assumes no responsibility forcompliance with applicable laws and regulations.PPI intends to revise this report from time to time, in response to co
6、mments andsuggestions from users of the report. Please send suggestions of improvementsto the address below. Information on other publications can be obtained bycontacting PPI directly or visiting the web site.The Plastics Pipe InstituteToll Free: (888) 314-6774http:/www.plasticpipe.orgJanuary 2003D
7、esign Service Life Of Corrugated HDPE PipeIntroduction:Thermoplastic pipes began to be used over 50 years ago in Europe and NorthAmerica for water and sewer applications. Polyethylene pipe has been used fordrainage applications in the United States for over 35 years and for highwaypavement underdrai
8、n applications for 30 years. These pipe were generally150mm (6 inch) in diameter and smaller. As use and applications for thesematerials expanded and as diameters increased questions concerning designservice life and materials properties became significant issues. Designing withthermoplastic materia
9、ls requires an understanding of viscoelastic or viscoplasticmaterials. The mechanical strength of these materials has to be put in relation tothe forces acting on it, the stress, the loading time, and the temperature. Theproperties of primary concern are tensile yield strength and flexural modulus(b
10、ecause those are used in the design code); which are directly related to andproportional to density and, to a lesser extent, molecular weight. The values thattruly govern how well, and for how long, the pipe performs are compressionstrength and flexural modulus; because the compression strength dete
11、rmines thepipe hoop compression stiffness and the flexural modulus determines the pipe orring stiffness.Any discussion of buried pipe service life can not ignore installation quality andpractice. All buried structures, regardless of material, are structures of the soiland the pipe or structure, inte
12、racting with each other to carry the required loads.Poor installation practice will lead to poor performance of the soil-structureinteraction system. AASHTO provides conservative installation guidance inSection 30 of the “AASHTO Standard Specifications for Highway Bridges.”Similar installation requi
13、rements are provided in ASTM D 2321, “StandardPractice for Underground Installation of Thermoplastic Pipe for Sewers and OtherGravity-Flow Applications.” Proper installation practice can contribute greatly tolong-term system performance.Current Specifications:The current AASHTO (American Association
14、 of State Highway andTransportation Officials) specifications covering corrugated polyethylene pipeinclude Standard Specification for Corrugated Polyethylene Pipe, 300- to 1200-mm Diameter, AASHTO Designation: M 294-02 and AASHTO LRFD BridgeDesign Specifications, Section 12 Buried Structures and Tun
15、nel Liners.AASHTO M 294 requires that the pipe be manufactured from materials (resins)as defined in paragraph 6.1.1:“Extruded Pipe and Blow Molded Fittings Pipe and fittings shall be made ofvirgin PE that conform with the requirements of cell class 335400C as definedand described in ASTM D 3350, exc
16、ept that the carbon black content shall notexceed five percent, and the density shall be not less than 0.945 gm/cc norgreater than 0.955 gm/cc. Resins that have higher cell classifications in one ormore properties, with the exception of density, are acceptable provided productrequirements are met. F
17、or slow crack growth resistance, acceptance of resinsshall be determined by using the single point notched constant tensile load (SP-NCTL) test according to the procedure described in Section 9.5. The averagefailure time of the five specimens must exceed 24 hours with no single testspecimens failure
18、 time less than 17 hours.” From this resin description, Section12 of the AASHTO LRFD Bridge Design Specifications sets material propertiesfor design in Table 12.12.3.3-1 as follows:Type of Pipe MinimumCellClassASTM D3350AllowableLong-TermStrain %InitialFumin(MPa)InitialE min(MPa)50-YearFumin(MPa)50-
19、YearE min(MPa)Corrugated PEPipe AASHTO M294335400C 5 20.7 758 6.21 152These values represent the minimums for the cell classification from ASTM D3350 and do not represent what the industry is actually using. They alsorepresent initial material properties values based on a specific loading regimethat
20、 may or may not be representative of actual loading conditions. The initialflexural modulus value is based on tests conducted in accordance with ASTM D790, Method 1, Procedure B, loading a 50mm specimen, 3.2mm thick and12.7mm wide at a crosshead speed of 12.7mm/minute with the average value ofthe se
21、cant modulus calculated at 2% strain in the outer fibers. The initial tensilestrength at yield is determined in accordance with ASTM D 638 using a“dogbone” specimen 1.9mm thick loaded at a rate of 50 mm/minute. The 50-yearvalues for tensile strength and flexural modulus were selected based on resins
22、used in the 1980s with some additional safety factors, and do not apply to resinscurrently required and used by the industry. The use of a “50-year” value is anarbitrary carry-over from the gas pressure pipe industry, where results from HDBtesting, conducted for 10,000 hours (1.14 years), are projec
23、ted forward to 50years, with a safety factor. If 10,000 hours is projected forward two orders ofmagnitude, to 1,000,000 hours, that would equate to 114 years.Current Materials (initial properties):Resins currently utilized by the corrugated PE pipe industry exceed a minimumdensity of 0.948 gm/cc and
24、 have a melt index of less than 0.4 gm/10 minutes.These resins have a minimum initial tensile yield strength of 22.75 MPa (3,300psi) and a minimum initial flexural modulus of 900 MPa (130,500 psi), as testedin accordance with ASTM D 3350. As density increases, these values bothincrease, linearly. (S
25、ee Figures 1 and 2) (1)200 0250 0300 0350 0400 0450 0500 00.9350.940 0.945 0. 950 0.955 0. 960 0.965Density (g/cc)Tensile Yield Stress (psi)Figure 1. Tensile vs. Density2% Secant Flexural Modulus vs Densityy = 4E+06x - 4E+06R2= 0.9379050,000100,000150,000200,000250,0000.9400 0.9450 0.9500 0.9550 0.9
26、600 0.9650Density (g/cc)Flexural Modulus (psi)Figure 2. Flexural Modulus vs. DensityLong-Term Properties:1. Tensile Strength:Tests (a stress regression test) of a specific resin used by the corrugated PEpipe industry in the early 1980s, a 0.952 gm/cc density material with a 0.4 gm/10min melt index,
27、demonstrated a 100,000 hour hoop tension strength of 9.24 MPa(1,340 psi) and a 50-year value of 8.5 MPa (1,233 psi). If the test curve isprojected to 1,000,000 hours, the design tensile strength value would be 7.5 MPa(1,080 psi). (2)2. Modulus of Elasticity:Dr. Lars-Eric Janson has published several
28、 articles plus a textbook in which hepresents his work in developing a long term modulus of elasticity, or a relaxationmodulus. In his tests, one pipe is deflected to 5% vertical deflection betweenparallel plates and held, in some cases for 9 years, and the stiffness regressionrecorded, which direct
29、ly relates to the relaxation of the modulus of elasticity. Inanother series of tests, pipe was deflected to 4.3% and 13.6% and held for 8years, with the same regression being recorded on a log scale. The regressioncurves can be project to 100 years, which is slightly over one order of magnitudefrom
30、the period tested. Further, Dr. Janson found that tests conducted for as littleas 100 hours were sufficient to make safe extrapolations up to 50 years or morefor PE pipe. (3, 4)Similar tests were conducted by Dr. Lester Gabriel at California State Universityon 24 pipe samples utilizing both the para
31、llel plate test with samples deflected5% and held, and the curved beam test, developed by Dr. Gabriel and JimGoddard, with pipe wall samples subjected to 5% chord shortening. Pipesamples included 7 different diameters and represented 4 differentmanufacturers. From those tests, the relaxation curves
32、were developed andextrapolated to 100 years. The curves were consistent in shape and slope, andthe reduction in modulus from 50 to 100 years was a consistent 3% for both testmethods. (5, 6)Based on this testing, it should be appropriate to change the properties table inAASHTO Section 12 as follows:T
33、ype ofPipeMinimumCellClassASTM D3350AllowableLong-TermStrain %InitialFumin(MPa)InitialE min(MPa)50-YearFumin(MPa)50-YearE min(MPa)100-YearFumin(MPa)100-YearE min(MPa)CorrugatedPE Pipe AASHTOM 294335400C 5 22.75 900 8.5 179 7.5 174A corresponding change should be made to the AASHTO M 294 specificatio
34、n,increasing the minimum resin density to 0.948 gms/cc from the current 0.945gms/cc.These “long-term” values, at 50 and 100 years, are meaningful in design only ifthe stress is kept constant and there is no restraint on deformation. As stated byDr. Janson in discussing the soil-structure interaction
35、, “during the first years, thesoil undergoes the reconditioning and settling that the backfilling technique hasnot succeeded in achieving, but which nature, with the help of traffic loading,groundwater movement, soil creep, soil frost action, etc. finally takes care of.”“During this time the pipe im
36、pulsively fights against additional deflection by virtueof its short-term ring stiffness.” “When the surrounding filling has found its shapeand this shape fits the pipes deflected shape, no further change in the pipeshape of any practical importance takes place.” Therefore, it is really the initialv
37、alues that should have more impact on design of gravity flow pipe. (3) Two otherpapers of interest in this area include another by Dr. Janson titled “Short-TermVersus Long-Term Pipe Ring Stiffness in the Design of Buried Plastic Pipes,” (7)and one by Elzink and Molin titled “The Actual Performance o
38、f Buried PlasticPipes in Europe Over 25 Years.” (8) In the paper by Elzink and Molin, they statethat deflection becomes stable in a relatively short period of time and thenremains constant. Janson states; “deflection will reach a constant value within aperiod of 2 3 years.” He also states that, “in
39、traffic areas this period is found tobe shorter. Thus, stresses beyond this point tend to regress, while soil remainsstable around the pipe. This would not apply to pipe installed in soils orconditions that do not consolidate or stabilize, but those soils should not beplaced around buried pipes of a
40、ny type and are not permitted in the AASHTOconstruction standards.Further, using the AASHTO design procedure, as contained in Section 12 of the“AASHTO LRFD Bridge Design Specification,” as well as reviewing the researchon buried thermoplastic pipe, the pipe is found to be under hoop compression,with
41、 tension being limited and at a fraction of allowable levels. Thus, the point intime at which any tensile failure might occur is very far distant. In compression,Slow Crack Growth does not occur.Even with those “initial” values, the “instantaneous” modulus and tensile yield areconsiderably under-est
42、imated. For live loads, such as a truck crossing over aburied pipe at 95 KPH (60 MPH), the effect on the buried pipe takes less than asecond. Another way to look at these time effects is to return to the relaxationmodulus curves developed by Dr. Gabriel and Jim Goddard. For a standard pipestiffness
43、test per ASTM D 2412, run at 12.7mm per minute (0.5 inches perminute), the test on 1500mm (60”) pipe takes 6 minutes. In relation to the“instantaneous” modulus from the tests and the 100-year value, in 6 minutesapproximately 40% of the relaxation has occurred. That is consistent for all of theHDPE c
44、urves. (5)Figure 3 Relaxation Modulus Curve from Dr. Lester GabrielAnother report that is helpful when considering the implications of a deeplyburied pipe over a long period of time is the “Pennsylvania Deep Burial Study 15Year Summary Report” prepared by James B. Goddard and submitted to thePennsyl
45、vania Department of Transportation in August of 2002. This report coversthe inspection of a 600mm (24 inch) diameter corrugated polyethylene pipeinstalled in 1987 under Interstate 279 north of Pittsburgh, Pennsylvania. Thispipe has 30.5 meters of fill (100 feet) over it. Based on inspections complet
46、ed inthe summer of 2002, there has been no significant change to this pipe in the last7 years. The only documented problem with this installation is some cracking inthe Type C pipe under the couplings under 21 meters (70 feet) or more of fill,which have not spread to or appeared in the pipe wall out
47、side the couplings. Ifthe relaxation curves developed from the work by Les Gabriel and Jim Goddardare used to predict a potential spread of that cracking to couplings under lesserfill heights, and 70 feet of soil pressure for 15 years is used as the base, thencracking should appear under couplings w
48、ith 64 feet of fill at 100 years. Thismakes some interesting and conservative assumptions, primarily that thepressure on the pipe remains constant for that period of time, where it is morelikely that the pressure will decrease substantially, and probably has alreadysince no change in the cracking co
49、uld be identified over the last 7 years. Pipeshape and/or deflection has not changed in the last 10 years. A full summaryreport of this work has been delivered to PennDOT for their review andcomments along with a complete set of referenced reports totally over 1,000pages of data. (9)Another report that adds support to the very long design service life of corrugatedpolyethylene pipe is “Clogging of Perforations in Plastic Drain Pipe” by by GlenSanders and John Ellis of the U. S. Department of the Interior, Bureau ofReclamation. As the title indicates, the principle point of t
