Sanitary sewer rehabilitation, Charleston, S.C.
Charleston, S.C., is a charming city immersed in more than three centuries of history extending back to colonial America. Today, civil engineers serving the city help preserve the community’s past by turning to solutions that are designed for long-term durability. For an underground sewer system in need of repair, the forward-looking solution included cast polymer composite sewer interceptor structures. The structures combine the strength of steel reinforcing rod with a special polymer concrete formulated with corrosion-resistant resin. Considering Charleston is located in a seismic region, these high-strength characteristics are an additional asset.
The composite structures are 10-foot-diameter, vertical access shafts as deep as 110 feet. At the base of each shaft, the same resin-based composite was used for a foundation floor, as well as a vortex chamber with baffle wall to help control sewage flow. The composite installation was part of a major sanitary sewer rehabilitation project designed by the Charleston office of consulting engineers Black & Veatch. The interceptor structures, manufactured by U.S. Composite Pipe with Vipel resin from AOC, were contracted to Affholder, Inc., a subsidiary of Insituform Technologies, Inc., Chesterfield, Mo.
The deepest shaft on the Charleston, S.C., sewer interceptor project was 110 feet.
Dan Swidrak, on-site project engineer for Affholder, now with Independent Concrete Pipe, identified several advantages of the polymer concrete compared with traditional concrete. "Because the composite pipe was ready to install upon arrival, it took weeks off the time for installation," he said. "With conventional concrete, additional time would have been needed to pour the material in place, wait for it to cure, clean and prepare the surface, then apply a protective epoxy liner."
Swidrak also pointed out the durability and performance benefits of the composite pipes. "A protective liner requires regular inspection and maintenance," he said. "If the liner surface is abraded or nicked, the concrete is exposed to corrosive sewer gases. With composite, the protection against corrosion is inherent throughout the entire structure."
Each sewer interceptor shaft was installed by stacking cylindrical composite riser sections up from the composite floors. Sections were joined using steel end ring joints integrally manufactured into the structure. These special joint rings adhere to American Water Works Association (AWWA) C-302 standards for air and water tightness and were tested to 50 psi.
A worker sets formwork for casting a 12-foot-high vortex section from polymer concrete.
Casting procedures for both conventional and polymer concrete are similar. U.S. Composite Pipe components are manufactured by first placing a steel reinforcement cage into a formwork. Like conventional pipe, the steel reinforcing gives the finished product the ability to handle severe loading. The polymer concrete is then vertically cast into the formwork and vibrated for optimal compaction.
Instead of making its pipe with a traditional cementitous material, U.S. Composite Pipe uses a special resin, filler, aggregate, and additive mixture licensed from PPT, LLC, Des Moines, Iowa. The polymer formulation specifies a Vipel resin that, when compared with cementious binder, provides improved compressive, tensile, shear, bonding, and flexural properties.
Because of thinner walls, polymer concrete pipe sections weigh less than portland cement concrete pipe, allowing use of smaller cranes for installation.
Another advantage to using high-performance building materials is the ability to reduce the overall weight of the structure by using thinner wall sections. Since polymer concrete has nearly the same unit weight as portland cement concrete, reduced wall thickness can significantly lower the total weight of the structure. Lower weight can decrease the overall cost of the finished product, the cost of shipping it to the job site, and the cost of a larger crane for installation.
Eric H. Davidson, P.E., vice president of U.S. Composite Pipe, said, "We took the original design from Black &Veatch and were able to reduce the wall thickness by 40 percent or more through our own U.S. Composite Pipe in-house engineering".
According to Davidson, the Vipel resin technology comes with excellent technical support. "We are part of a company that has more than 20 years experience making pipe with conventional concrete," Davidson said. "When we started developing composite materials, AOC representatives were on hand to check our gel times and see how the resin was cooking off. Now we have a formulation that, with the help of AOC resin consistency, we just ’dial in’ to make polymer concrete."
This article was contributed by AOC, Collierville, Tenn., a supplier of resins, gel coats, colorants, additives, and synergistic systems for composites and cast polymers.