New technologies are tackling old problems in sewer repair; composite materials are being used more often today to deal with sewer repair and rehabilitation in major cities. These new materials offer interesting advantages over more traditional poured concrete structures or lined structures. Additionally, composite structures hold the promise of longer life, reduced installation costs as well as reduced inspection and maintenance costs.
Naturally-decomposing wastewater components will release hydrogen sulfide gas; if there exists space between the water level and the ceiling of the pipe or culvert, bacteria above the waterline will ingest the gas and secrete sulfuric acid. This highly corrosive acid will aggressively attack the non-submerged surfaces. If left undiscovered or unrepaired, this reaction will eventually decompose the concrete to the rebar. To prevent this situation, civil engineers have long relied on liners – some of which have even been composites. The downside of liners is that they are relatively thin and once they are damaged, they become ineffective. Composite pipe sections can address this issue by offering chemical resistance throughout the entire structure; in many cases, composite pipe sections can be used without a liner. By choosing resins that exhibit excellent chemical resistance, the pipe manufacturer can save a city or water district inspection and maintenance costs down the road.
Composite materials can also reduce repair costs. Cured-in-place-pipe (CIPP) solutions are emerging that use either fiberglass or stainless steel with cement grout. In many cases, where both ends of a pipe are accessible, CIPP requires no excavation – short links of sleeves are coupled together and pulled through the host pipe by a cable from one end to the other, creating a continuous stainless liner. Then a cement grout or polymer resin (depending on the technology) is pumped to fill the annular space between the host pipe and the liner. The grout (or resin) flows into all cracks and joints including the cracks of the host pipe, joining together the liner sections and all old pipe sections into a solid and unified structure.
Many firms that are promoting CIPP technology are quoting a 100 year life with specific resin or grout combinations; this has the potential to reduce maintenance costs as well as the hassles associated with repairs. Beyond the reduced long term costs, CIPP may also offer the benefits of reduced on-site crew size and the elimination of a digger or excavating crew, depending on the specifics of the situation.
A third technology that is being used is cast polymer concrete. As an almost direct replacement for traditional Portland cement, the polymer concrete is cast over steel reinforcing members and can achieve improved shear tensile, compressive and flexural properties compared to cementious materials. This technology was recent used in Charleston, South Carolina to create sewer interceptors that were critical in the city’s sewer rehabilitation plan.
The interceptor structures were approximately 10 feet in diameter and some were 100 feet deep. To build these, polymer concrete sections were produced off site that included integral steel connecting rings. These connecting rings secured each pipe section to the next; the base piece was cast with an integral composite floor. Because the materials exhibited through-thickness chemical resistance, there was no need to apply a protective liner; typically with pour-in place concrete, there is a cure period that must occur before any liner can be applied. Additionally, because the polymer concrete exhibited higher mechanical properties, the sections could be made with a reduced wall thickness compared to a design utilizing traditional materials; this allowed for a smaller crane to be used on site which was critical given the narrow streets of historic Charleston.
With an ever-increasing materials base from which today’s civil engineers and planners can draw, new opportunities for cost and maintenance savings are possible. By selecting the materials that make the most sense for each particular job these professionals are ensuring that our municipal and regional wastewater systems will stand the test of time.