Retrospective Evaluation of Cured-in-Place Pipe - (NEPIS)(EPA ...

impregnated with resin and still handled easily prior to installation. When the liner is inverted, this
sealing layer becomes the inner surface of the CIPP liner. The uncured resin can then flow into cracks
and openings in the host pipe to lock the liner in place. For structural purposes, a small amount of excess
resin ensures that sufficient resin is available to give the required liner thickness. However, too much
resin can cause problems such as blocking sewer laterals. A second advantage of the inversion approach
is that the liner is not dragged, relative to the host pipe, as it is installed; rather, the liner unfurls itself
along the pipe, sealing off infiltration and displacing standing water in the pipe as it moves along the pipe
as well as reducing the potential for physical damage to the liner. In early CIPP installations, the coating
layer was a sacrificial polyurethane layer expected to hydrolyze over time. Today, more permanent
coating layers are used – either a different polyurethane (PU) layer or a polyethylene (PE) layer. A future
area of research could be the performance of the PU/PE layers during installation and over the long term.
Variations of each method (inversion or pull-in-and-inflate) are used, depending on the circumstances.
For example, a PE tube, or a separate layer of coated felt, can first be inverted into the host pipe as a
“preliner” and then the actual liner inverted inside the first tube. This will eliminate concern about resin
washout if high groundwater inflows are present.
2.2.4 Tube Construction. Initially the CIPP tube (also called “bag”) construction was made of a
needled polyester felt and served only as a carrier for the resin. In this construction, the resin is the
dominant contributor to the mechanical properties of the system. Other forms of tube construction
entered the marketplace in the U.S. during the 1990s. These may involve the inclusion of reinforcing
materials such as fiberglass, aramid fibers or carbon fibers in some configuration. The reinforcement may
be positioned at selected points within the thickness of the tube wall or the wall may consist primarily of
braided reinforcing layer(s) (Rahaim, 2009).
Reinforced tube construction has been in use in Europe for longer than in the U.S. and allows the
designer/contractor to design a thinner CIPP liner and one with a wider range of application. The
reinforcing layers within the resin become a significant contributor to the mechanical properties of the
finished liner. This leads to a more complex mechanical behavior of the liner and the reduced thicknesses
are more susceptible to the effects of host pipe and liner imperfections on the structural analysis. Studies
of new composite tube materials can be found (e.g., Akinci et al. [2010]).
Liner thicknesses may vary from around 0.12 in. (3 mm) in small-diameter shallow pipes to over 2 in. (50
mm) in large-diameter deep pipes. In the construction planning for the CIPP project, consideration needs
to be given to the forces that will be exerted on the tube during the installation process. For the inversion
process, sufficient pressures must be exerted to allow the liner to “invert”. For the pull-in process, the
liner tube construction must provide sufficient tensile strength for the pull-in and resistance to damage or
tearing during the insertion. Liners to be installed on steep-gradient pipes pose particular challenges for
water inversion and curing because sufficient pressure must be available at the upper end of the pipe to
allow the inversion to occur, but the pressure at the lower end of the pipe must not be so high as to cause
liner tearing or excessive thinning. In designing the tube thickness, consideration needs to be given to the
maximum pressure exerted on the liner as it cures so that the final thickness of the liner meets the project
specifications. The contractor or supplier calculates these parameters based on the site information and
planned installation procedures.
2.2.5 Choice of Resins for CIPP. The following discussion of resin chemistries is summarized
from a paper on “Resin Choices for Cured-in-Place Pipe Applications” by Rose and Jin (2006).
According to Rose and Jin (2006), there are three main chemistries of thermoset resins that are wellsuited
for use in CIPP applications. These are polyester, vinyl ester, and epoxy resins.
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