Steel reinforced polyethylene (SRPE) pipe is the latest product that has entered the already overcrowded flexible plastic drainage and sewer pipe market with manufacturers claiming to address several concerns that have plagued traditional profile wall high density polyethylene (HDPE) pipe. The marketers of SRPE pipe claim that this new product combines the high strength of helically wound steel ribs surrounded by HDPE, however after a thorough evaluation, design engineers may discover that SRPE will not provide a significant improvement over conventional HDPE pipe and may actually be cause for new concerns.
Properties of HDPE and SRPE Pipe
The design of SRPE pipe should include an extensive review of the material properties of its two main components, HDPE and steel, and their compatibility with each other. The long-term behaviour of viscoelastic materials like HDPE is very different from elastic materials like steel. Due to stress relaxation, the tensile strength and modulus of elasticity of HDPE decrease significantly as shown in Table 1, therefore a design engineer must understand how and when to use short term and long term HDPE material properties. Conversely for steel, which is typically needed for its tensile strength, exhibits a linear stress-strain relationship up to a yield point when it becomes permanently deformed and elongates under tension until it reaches failure. The amount of elongation and how suddenly the steel fractures depends on its chemical composition and ductility, therefore test data and Mill Certificates should be reviewed by those with a competency in steel material properties.
TABLE 1 – HDPE Short – and Long – Term Modulus and Tensile Strength
|Initial||75 – Years|
|Product||Tensile Strength, Fu (ksi)||Modulus of Elasticity, E (ksi)||Tensile Strength, Fu (ksi)||Modulus of Elasticity, E (ksi)|
|Profile HDPE Pipe||3.0||110.0||0.90||21.0|
The flammability of HDPE may be an important consideration in areas where drought and wildfires are prevalent, especially under critical roadways with high traffic counts. Any plastic with a Limiting Oxygen Index (LOI) less than the 21% atmospheric oxygen concentration is considered to be flammable. Plastics such as polyethylene and polypropylene both have a LOI of 17% which means it will easily support combustion in the open air.
Storm and sanitary sewers are exposed to a range of temperatures such as the low temperature runoff from snow melt to the heated water discharged by household appliances such as laundry machines and dishwashers. For this reason, a critical aspect of a non-homogeneous pipe material is the thermal compatibility of the individual material components.
HDPE is significantly more susceptible than steel to expansion and shrinkage from temperature changes so this may be cause for concern for the long-term service life of SRPE pipe due to a loss of bond between the HDPE and steel ribs, material fatigue, and environmental stress cracking after many cycles of temperature fluctuations.
Structural Design Method
A fundamental responsibility for engineers specifying a pipe product is to have an understanding of the applicable structural design method in order to determine the appropriate trench width, backfill material type, and compaction efforts required for specific site conditions. While design methods for reinforced concrete pipe, corrugated steel pipe, and even traditional plastic pipe products have been established through research and development, the structural design method for SRPE is not as well documented. Engineers would expect the SRPE pipe manufacturers to provide clarity on a design method, however various SRPE pipe products are significantly different from each other and manufacturers only publish height of cover tables that mention an ambiguous design method based on both Section 12.7 Metal Pipe, Pipe Arch, Arch Structures and Steel-Reinforced Thermoplastic Culverts and Section 12.12 Thermoplastic Pipes in the AASHTO LRFD Bridge Design Specifications. This raises a fundamental question about SRPE pipe – is it simply a profile wall HDPE pipe or is it a steel pipe with a plastic coating?
The performance, and the potential for structural failure, of a flexible pipe can be predicted from pipe deflections. If a thermoplastic pipe is designed and installed properly, deflections should be less than 5% of the pipe’s base inside diameter. If excessive deflections are permitted, joint leakage might occur which can lead to the weakening of the backfill material and eventual collapse of the plastic pipe. One manufacturer of SRPE pipe indicates that a minimum pipe stiffness (PS) of 320 kPa is provided for all diameters, which traditional HDPE pipe and another SRPE pipe manufactured to CSA standards were not able to achieve for diameters greater than 900mm. Engineers may be tempted to conclude that in terms of overall pipe deflections, the SRPE pipe with a higher PS will perform better than traditional HDPE pipe. Equations, like the Modified Iowa Formula, used to estimate deflection for a flexible pipe can be expressed in general terms as:
Where the “pipe stiffness” term in the denominator is extremely small compared to the “soil stiffness” term. A proper installation of a plastic pipe in accordance to CSA B182.11 and ASTM D2321 with a well compacted granular material can easily provide a soil stiffness greater than 6,900 kPa while the pipe stiffness of the SRPE pipe is only 320 kPa. This demonstrates that the soil stiffness has the biggest effect on a plastic pipe’s structural capacity, while the pipe stiffness contributes very little.
|Units for Pipe Stiffness
A fact that may not be obvious to many engineers is that while the units of measure for PS are the same as those for pressure and stress, they are very different quantities. According to ASTM D2412 Standard Test Method for Determination of External Loading Characteristics of Plastic Pipe by Parallel-Plate Loading, the PS value is actually pounds-force per inch of length per inch of deflection (lbf/in.-in. or pii) which can also be expressed as psi, but is not the same as poundsforce applied to an area of one square inch. Similarly in SI units, PS is expressed as newtons of force per metre of length per millimetres of deflection, which is dimensionally the same as kilopascals (kPa) but is not equivalent.
Steel reinforced polyethylene (SRPE) pipe is shipped to jobsites across Canada from the state of Utah and from southern Ontario. Inevitably the pipe gets damaged when shipping long distances. Damage includes perforations to the pipe, scratching and damage to the polyethylene coating and damage to the reinforcing ribs. Damage to the polyethylene coating compromises the service life of the pipe and negates all the longevity claims of the manufacturers. Damage to the reinforcing ribs compromises the structural integrity of the pipe. In both cases the owner of the new infrastructure is not getting the product that was paid for and rejection of the damaged pipe is the only option.
In addition to adhering to CSA B182.11 and ASTM D2321 for the proper installation of plastic pipe, designers and field inspectors should demand that the suppliers of large diameter plastic pipe provide proper instructions to installation contractors on how to properly use movable trench boxes or shields. Both installation standards specify provisions to avoid trench boxes from disturbing the installed pipe and its soil embedment, which becomes far more difficult with plastic pipe diameters greater than 600mm due to most local health and safety regulations for excavations.
The proper method for field cuts to adjust the SRPE pipe length and field installations of watertight fittings for service connections must also be clearly detailed for the installation contractor.
Post Installation Inspection
As with all buried flexible pipe, both CCTV inspection and ring deflection testing must be mandatory in order to verify that the soil embedment was properly constructed. As discussed, the SRPE pipe design must specify the appropriate trench width, backfill material type, and compaction efforts required to minimize the pipe deflection to less than 5%. The post installation inspection is the only means of verifying that the contractor achieved these design specifications.
Deflection measurements must also become a part of regular routine inspections throughout the service life of the SRPE pipe. The suppliers of SRPE pipe should also provide the materials and training of maintenance personnel required to repair damage to the plastic liner, corrosion of any steel ribs, or any damage to the spiral weld locations.
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