During the testing phase of constructing circular grit chambers in a wastewater treatment plant in Ft. Wayne, Ind., crews discovered serious leakage problems before the project was completed.
After meetings with the owner, design and construction engineering firms, painting contractor, and coatings manufacturer, the decision was made to apply 80 mils of pure polyurea to various interior concrete surfaces.
The grit chambers in this project were constructed using Type I portland cement. Concrete is commonly used in the construction of wastewater treatment plants. Concrete surfaces porous and permeable and may crack and leak even when skillfully placed.
If not specified in the new construction phase, polyurea protective coatings may be used to rehabilitate cracked, leaking concrete surfaces, such as the grit chamber in this example.
The city's original wastewater treatment plant, like many others across the United States, was built as part of the federal Work Projects Administration program in the 1930s. Over the years, this facility had been updated with new pumps and screens to meet increased demands caused by a growing population.
Increasingly strict federal Clean Water Act standards required rigorous treatment to assure that water being directed into the nearby Maumee River by these operations contained minimal amounts of pollutants. But after 65 years of service, it was time to update the old facility.
The new construction upgrade allowed output to be increased to 60 million gallons and more per day. To save on construction costs and preserve the city's budget for other projects, the new structure was built next to the old one, allowing existing administrative offices and utility rooms to remain.
In fact, the new and old facilities share a concrete wall. Part of that common wall houses the control room that contains electrical panels and computers, as well as the plant's administrative offices.
Before start-up and while still under construction, city engineers conducted tests to make sure all operations were running smoothly. Leaks in the system were quickly noted on some walls of the facility. Managers feared that water could eventually leak into the administrative and control rooms.
While the leaking did not indicate a structural problem, it was a concern. Leakage into the control room's electrical panels and computers could cause damage to expensive equipment that was designed to last for many years. Leaks were also found in the vertical concrete grit chamber surfaces.
A sodium silicate slurry coat was applied to leaking surfaces, but it did not prevent water from escaping. Both epoxy and urethane compounds were injected into cracks, but they also did not halt the seepage. Although remediation and repair efforts using these methods are often successful in stopping leaks, they were not effective on this project.
After several conference calls, fact-finding, research, and face-to-face meetings among the parties, the owner accepted the recommendation of the design and construction engineers, the material supplier, and a certified polyurea applicator to apply a polyurea coating system for these undesirable areas. The work was to begin immediately and, since it was winter and the site was in the Midwest, weather was a concern.
Crews first removed the build-up of ice and water from the grit chambers. Next, they provided warm, circulating air. The environment was maintained at about 50° F to 65° F with relative humidity about 40%.
Crews conducted abrasive blasting in accordance with ASTM D 4259, to achieve an ICRI CSP 3 to CSP 5 to remove the sodium silicate slurry coating, open bug-holes, and roughen the existing surfaces before applying the filler, primer, sealant, and polyurea topcoat.
After surface preparation, crews applied TPM 723 Vertical Repair Mortar, a polymer-modified, cementitious patching material, to fill voids.
They then applied a clear, low temperature, 100% solids epoxy primer—Corobond LT—at about 6 to 8 mils dry film thickness. This was followed by a moisture-cured urethane sealant, Pro Select Stampede Polyurethane Sealant, to smooth edges where metals, such as the aluminum lift gates inside the grit chambers, met concrete surfaces.
To complete the system, 80 mils of EnviroLastic AR425, a 100% pure polyurea coating from Sherwin-Williams, was applied. One hundred percent, pure polyurea coatings such as this one have the ability to create a seamless, flexible waterproof barrier and to “bridge” moving cracks up to 1/8-inch.
A test area of polyurea was sprayed onto polyfilm to verify proper equipment ratio and to confirm the system would be pinhole-free.
In addition, these coatings may be used as linings in many other wastewater tanks, such as digesters. They withstand exposure to many of the chemicals and solutions used during the wastewater treatment process.
This wastewater treatment plant is now operating, and the grit chambers are not leaking. Crack-bridging, flexing, and leak-deterring properties are features of pure polyurea elastomers. This is one of the many successful and suitable uses for these types of coatings.