State of Technology Report for Force Main Rehabilitation, Final ...

Most of the controversy over the application of the above formula stems from one’s interpretation of the
time corrected value for the modulus of elasticity, EL. The note to Appendix X1.1 in ASTM F1216 states
that the choice of value depends on the estimated duration of the application of the load (P) in relation to
the design life of the structure (and should be obtained from the manufacturer’s literature). If the total
duration of the load is estimated to be 50 years, either continuously or over the sum of intermittent
periods, then the appropriate choice for EL will be that given for 50 years of continuous loading.
However, no method of test for determining the long-term modulus, EL, is referenced in ASTM F1216 or
any of the product standards. Therefore, the method of determination could vary considerably depending
on the method employed by the manufacturer. A flexural creep test under constant load would be one
way of arriving at a value, but the value here will also be dependent on the load. The ISO standards for
GRP pipe do contain requirements for the manufacturers to carry out long-term flexural creep or
relaxation tests, and then to report the results for design purposes. The creep and relaxation tests
specified have been set up to yield similar results independent of the method chosen. It would seem
logical that the CIPP industry should consider adopting a similar methodology for their liner materials
given that they too are composites made from the same resins (polyesters, vinyl-esters and epoxies). As
this long-term property is key in the design of all gravity CIPP and deformed/reformed thermoplastic
pipes, a great deal of development effort is exerted by renewal liner manufacturers to enhance this value.
The addition of glass fiber is one example of such enhancement.
The other criteria that must be met in a liner for a partially deteriorated host pipe is to bridge over any
holes or gaps. Here, depending on the ratio of the hole diameter to pipe diameter, the liner is assumed to
behave like a circular flat plate with fixed edges covering an open hole and subjected to transverse
pressure only. This condition applies when the following equation is satisfied:
where:
d = diameter of the hole or opening, inch
D = mean inside diameter of the original pipe, inches
t = thickness of CIPP liner, inches
When this condition applies, the liner acts as a flat plate in bending to span the hole and the following
formula applies:
where:
σL = long-term (time corrected) flexural strength for CIPP, psi
A similar note is included for the selection of σL as EL, namely choosing a value that is consistent with the
load duration. Assuming that a 50-year design life is expected from the liner, then a value for the longterm
flexural strength of the liner material under constant load for 50 years is needed. As in the case of
EL, no test method is stipulated. This is another example where the CIPP liner industry could take a page
out of the GRP pipe handbook and adopt one of their test methods. The ISO standard for GRP pipe
includes a test method for determining the long-term ring bending strength of a composite ring subjected
to constant loading in an aqueous environment.
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