Nine employees provide water, wastewater, solid waste, and street-related services for the 2,400 people who live in St. Pauls, N.C., a town that sprouted around a Presbyterian church built in 1799. While not all assets are that old, the 500,000-gallons-per-day treatment plant was in danger of losing its ability to disinfect wastewater.
The plant performs primary anaerobic digestion of wastewater sludge to reduce the volume of waste for disposal. After more than 50 years, the concrete inside the plant’s double-sided chlorine contact chamber was thinning and risked crumbling. Water also was penetrating the chamber’s floor, indicating a failing structure. Faced with a lack of funds and declined for a state grant to repair the structure, public works applied to a charitable program launched by an infrastructure rehabilitation contractor in 2013.
Every year, Carolina Management Team (CMT) in Asheville, N.C., donates time, talent, and material to help one municipality complete a much-needed project. (Last year’s lucky winner was the Town of Marshall, N.C.) In 2016, judges for CMT Gives Back decided to rehabilitate the chamber at no cost: a project valued at $46,000.
CMT is a woman-owned company that specializes in corrosion control, surface preparation, coatings and linings, tank maintenance, and manhole rehabilitation. The challenge in St. Pauls was to restore severely deteriorated concrete without shutting down the 11,500-gallon contact chamber. The solution earned the town and the contractor the 2017 Sherwin-Williams Impact Award, which honors demanding water and wastewater projects and recognizes infrastructure professionals committed to enhancing public health and safety.
Making a ‘split decision’
“Restoring the chamber almost wasn’t an option because of the concrete’s state,” says CMT Co-owner David Van Zee. “But upon close inspection, we confirmed the structural integrity of the baffles and basin, and that the concrete could be restored without starting from scratch.”
Completely shutting down the chamber wasn’t possible because the plant has no other disinfection vessels onsite and sewage would continue to enter the plant for processing. An effluent trough common to both sides of the chamber added another complication. To keep water from infiltrating the side being restored while enabling the other half to continue operating, CMT devised a plan based on a temporary dam and drain mechanism.
“We had to treat the restoration as two separate projects to accommodate continued operation,” says Van Zee.
After draining one side of the chamber, crewmembers built the temporary dam and drain and refilled that side so the plant could continue disinfection. Then they drained and fully restored the other side. After returning that side to operation, they drained and rehabilitated the first side. Finally, they drained the originally restored side again to remove the temporary dam and patch and recoat the 18-inch-by-4-inch area where it had been located.
CMT also restored the steel effluent box, piping, and valve stands using Macropoxy 646 fast-cure epoxy and Acrolon 218 HS (high solids) acrylic polyurethane. (All products referenced in this article are made by The Sherwin-Williams Co.)
Restoring severely deteriorated concrete
The first step was to remove contaminants from the substrate and apply mortar to rebuild the surface profile to its original state.
Crewmembers prepared the surface per SSPC-SP 13 requirements to achieve a concrete surface profile (CSP), the average distance between surface peaks and valleys when looking at a cross section, that matched the International Concrete Repair Institute’s (ICRI) CSP 9 guideline. A high CSP was required to accommodate application of a thicker mortar system. CSP 9 provided more surface area for the mortar to mechanically bond to the substrate, which was important because crews had to apply an average of 1 inch throughout the chamber.
Exterior deterioration was limited to minor flaking, peeling, and discoloration of the existing coating with no need to repair the concrete. The crew therefore only needed to prepare the exterior surface to a CSP 2 or CSP 3 before applying protective coatings.
Stopping water infiltration – fast
After restoring the baffles and basin, CMT addressed the moisture penetrating the chamber’s floor. It was critical to completely stop this intrusion to keep the subsequent lining and coating systems from blistering after application.
Using trowels, crewmembers applied FasTop MVT (moisture vapor transmission) moisture control system, a self-leveling coating that doesn’t require post-application moisture testing. Around cold joints – areas where the chamber’s concrete baffles met its bottom concrete slab – the crew built a 1-inch-thick-radius cove using FasTop MVT. The 45-degree-angle cove allows for tensile movement of the cold joints to ensure the waterproof seal holds over time.
Locking in the repairs
With a waterproof floor in place, the crew then built a protective lining on the chamber’s entire interior to prevent water, waste, and chemicals from reaching the repaired concrete.
First, they sprayed a primer coat of Dura-Plate 235 multipurpose epoxy at 4 mils to 8 mils dry film thickness (DFT) over the entire first half of the chamber to create a monolithic film. The moisture-tolerant primer was used to promote adhesion of the topcoat and to prevent outgassing from the concrete substrate. The primer seals pores in the concrete so air bubbles can’t push through the finish coat.
For that topcoat, they sprayed one coat of SherFlex elastomeric polyurethane on the walls and floor at 100 mils to 125 mils DFT. Applying a single coat at this high-build thickness created a monolithic liner with no seams, voids, or pinholes that could allow corrosive chamber contents to reach the concrete substrate. In addition, the thick application helps to bridge any cracks that may occur in the future; should cracking occur, the flexible coating won’t let the crack telegraph all the way through.
Crewmembers repeated this process on the second half of the chamber and then again on the small area of concrete that was re-exposed when they removed the temporary dam.
“Ensuring as monolithic a film as possible over the entire structure was critical to achieving a long-term repair,” says Van Zee. “We had no choice but to sacrifice a bit of film continuity because of the temporary dam structure. However, by carefully overlapping coating applications in those small areas, we’re confident the repairs will hold for years to come.”
To protect the chamber’s exterior coating, CMT applied Sher-Crete waterproofer at 7 mils to 9 mils DFT. A combination of acrylic resin and mineral aggregate, the product allows any dampness in the concrete substrate to permeate through to the atmosphere to prevent blistering and keeps exterior moisture from penetrating through to the concrete and promoting corrosion.
Doubling the asset life
The project is expected to add 40 years to 50 years to the chamber’s service life. In addition to doubling asset life, CMT’s benevolence freed funds that public works spent on other pressing needs.
The repairs were put to an unplanned test when Hurricane Matthew dumped more than a foot of rain on the town three weeks after project completion. Flooding fully immersed the newly restored chamber, but the repairs held.
“With the CMT Gives Back program saving nearly a third of our $150,000 in available utility fund reserves, we were able to address other necessary repairs without having to make tough decisions on what to postpone,” says Wastewater Department Supervisor Tim Lyde. “We’re so thankful for the big heart of CMT to help protect our community.”
