Why polyurea is a good choice for coating older concrete wastewater infrastructure

At U.S. wastewater industrial plants, the Environmental Protection Agency (Washington, DC) enforces requirements to ensure that operators pretreat pollutants in their wastes to protect local sanitary sewers and wastewater treatment plants. Such extreme wastewater handling environments operate under continual chemical exposure and heavy abrasion that deteriorate concrete and corrode steel in clarifiers, containment pits, anaerobic digesters, manholes, tanks, and other infrastructure assets. Polyurea coatings are proving ideal for rehabilitation.

Concrete Cracking and Deterioration

A variety of factors can cause these structures to develop sizable cracks, through which wastewater can escape by exfiltration. Groundwater could also enter the wastewater system through infiltration. Cracks can develop in aging concrete wastewater infrastructure for many reasons, including earth movements or daily and seasonal temperature changes that cause expansion and contraction, such as freeze-thaw. Additionally, harsh chemicals used to treat wastewater can deteriorate the concrete, as can exposure to hydrogen sulfide (H2S) gas, a form of sulfuric acid (H2SO4) present in sewers created by anaerobic organisms. Leaks, cracks, and damage to existing coating systems not only top the list of challenges in facility maintenance and new construction but can also trigger severe financial penalties. Since the EPA regulates municipal wastewater and stormwater management, concrete cracks or leaks that lead to wastewater exfiltration or groundwater infiltration can put the municipality in violation of a judicial consent decree.

“If the EPA finds that a municipality has old leaking sewer systems, putting them on a consent decree can mean that they don’t receive federal tax money until the problem is fixed,” says Jim Osborn, president of Osborn Contract Services, Inc. (Greer, South Carolina), a certified applicator of spray applied coatings and rehabilitation products.

According to Osborn, a cement material is traditionally used to repair wastewater-related sewer leaks. But this process has disadvantages, as does the use of other coating materials. For concrete repairs in wastewater infrastructure from municipal manholes and lift stations to clarifiers, trenches, and sumps, Osborn has transitioned to an advanced polyurea system.

“Hydrogen sulfide [H2S] gas eats away at the cement in sewer concrete, so it may only have a lifespan of a few years,” says Osborn. “Because epoxies are limited by much lower elongation properties, they crack and don’t bridge cracks well. Polyurethanes have more elongation, but nothing like polyureas.”

Advanced polyurea coatings and liners are proving ideal for wastewater infrastructure rehabilitation by delivering strong, flexible, abrasion- and chemical-resistant waterproofing that not only bridges existing cracks but can elongate up to 400% without cracking. Because these systems set and cure rapidly and can be installed and used in many temperatures, they also minimize downtime. The spray-applied waterproof coating creates a seamless, waterproof, durable protective liner that stops leaks and strengthens the integrity of the entire structure. It exhibits superior physical properties such as elongation up to 400%, crack bridging, hardness, and tensile strength to create a robust industrial liner that protects, strengthens, and waterproofs the infrastructure.

Characteristics of Polyurea

According to Osborn, a number of the polyurea’s characteristics help to extend wastewater infrastructure longevity, as well as prevent wastewater exfiltration and groundwater infiltration.

“Since the polyurea system provides superior elongation, it bridges cracks up to 0.125 in [3.18 mm],” says Osborn. “With tensile strength higher than traditional materials, it has much lower permeability for better waterproofing. Its impact, abrasion, and chemical resistance are excellent, so it resists hydrogen sulfide. Instead of years between the replacement of cementitious coatings, the polyurea coating can cost-effectively provide decades of protection.”

With proper crack repair and surface preparation, the polyurea coating can be a thick film applied directly to the concrete or similar substrate. An alternate application method that can sometimes mitigate the need for surface or crack repairs is to prespray the polyurea to geotextile fabric panels placed on top of the substrate and fuse the panel edges together with more polyurea.

Because of the polyurea’s ability to set and cure quickly, it also minimizes wastewater treatment plant or infrastructure downtime. This can translate into thousands of dollars of savings per hour, all while avoiding days of service interruption.

In one recent case, when extensive cracks were found in a concrete clarifier shortly before a chemical plant’s ribbon-cutting ceremony, Osborn turned to polyurea for superior protection and fast installation.

After repairing all cracks over 0.125-in wide in the clarifier with a concrete repair grout, Osborn prepared and primed the floor and walls with the manufacturer’s primer. He then applied 80 to 100 mils (2,032 to 2,540 µm) of polyurea.

“The polyurea allowed us to provide a warranty against potential future cracks opening in the concrete because of its crack bridging ability,” says Osborn. “We were able to spray the polyurea as thick as needed in one application. Since it sets and cures rapidly, structures can put be back in service very soon after spraying.”

Wider Temperature Range

While traditional coatings such as cementitious, epoxies, and polyurethanes will prematurely fail if not installed under a relatively narrow range of temperatures, the polyurea used by Osborn is designed for installation and use from -40 to 350 °F (-40 to 176.7 °C). It will withstand decades of freeze-thaw cycling, and wide variations of temperature and humidity.

“When you’re aboveground in the Midwest, you have freeze and thaw with concrete expansion, contraction, and cracking,” says Jennifer Hoop, president of Conco Spray Solutions (Indianapolis, Indiana), a contractor specializing in the rehabilitation and protection of municipal infrastructure including potable, storm, and wastewater systems. “We needed a lining that would move with the structure through the different seasons and tank temperature differentials. For that, polyurea works very well.”

To withstand extreme weather conditions at a northern Indiana wastewater treatment plant, Hoop selected polyurea for a secondary containment area around ferric chloride tanks. “After filling in cracks with grout and restoring the concrete, we sprayed the polyurea on geotextile to provide a really nice containment area for the ferric chloride tanks,” she says.

Hoop, whose company does a large amount of wastewater-related work on containment, clarifiers, tanks, and wet wells, says that polyurea has an added benefit in reducing continual clarifier maintenance.

“Typically, clarifier maintenance crews need to use high-pressure power washers for hours to clean solid waste from concrete surfaces,” says Hoop. “Since the polyurea provides waterproofing and has a cleanable surface, crews can simply hose down the clarifier to clean it. This can cut required weekly clarifier cleaning by two-thirds.”

“For wastewater-related rehabilitation, polyurea is a superior coating for any application that requires crack bridging, longevity, chemical and temperature resistance, as well as fast turnaround,” Hoop concludes.

For more information on corrosion in the water and wastewater industry, sign up for a free subscription to NACE's WaterCorr, a triannual, digital publication covering this industry’s technologies and best practices. 

Source: Originally appeared on materialsperformance.com as “Polyurea Coatings Protect Aging Concrete Wastewater Infrastructure;” adapted from VersaFlex press release.

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