Problem: Microbiologically induced corrosion severely damaged a critical concrete junction box.
Solution: Repairing and replacing the degraded concrete using specially formulated concrete and an antimicrobial solution to inhibit future microbial corrosion.
Repairing and relining large sewer lines is always a serious undertaking, especially when the lines in question run underneath a Texas bayou.
A national cured-in-place pipe (CIPP) contractor in Texas was working on such a project when the company ran into a serious snag: After designing and installing a temporary bypass system involving six high-volume diesel pumps and four temporary lines, system engineers were surprised to discover that a critical concrete junction box was badly decomposed by microbiologically induced corrosion (MIC) and in danger of failing structurally. To keep this project on track, the box needed to be cleaned and rehabilitated in a very short time frame, while CIPP work continued.
The contractor called on T.V. Diversified (Lake Worth, Fla.), a trenchless rehabilitation contractor, to complete the difficult project under extreme time pressure.
Working under pressure
MIC is a familiar evil to public works departments. Warm temperatures, turbulence, organic waste, and low oxygen levels commonly found in wastewater systems can make the system a near-perfect incubator for thiobacillus bacteria and the formation of hydrogen sulfide gas. Thiobacillus feeds on the sulfur in the gas, and converts it to sulfuric acid; some thiobacillus colonies can thrive in 7% acid solutions. The sulfuric acid attacks concrete and turns it into rotten, spongy calcium sulfate. Depending on the concrete’s condition, significant deterioration can take place in just a few months.
In Houston, the very large junction box — 6-m (20-ft) wide × 10-m (32-ft) long × 6-m (20-ft) deep — was surprisingly close to total destruction even though it had been protected by a polyvinyl chloride (PVC) liner.
“It was pretty bad,” said Tom Vitale Jr., owner of T.V. Diversified. “About 8 in. to a foot [200 to 300 mm] of the inner surface was completely corroded and eaten away … and the walls were only 18 in. [460 mm] to begin with, so structural integrity was compromised. And the PVC liner was falling off in sheets, because the corroded concrete just wouldn’t hold it up anymore.”
PVC lining is installed in many wastewater systems as an MIC-prevention measure — the lining is intended to keep sulfuric acids away from concrete. PVC lining can work well in some situations, but it depends on meticulous application and absolute structural integrity. Even a tiny hole can allow thiobacillus to set up colonies between the PVC and concrete walls. When that happens, the lining actually hides the corrosion, and operators may not notice MIC until serious damage has been done.
Vitale is an expert at rehabilitating MIC-damaged structures, but even he was surprised by the size, severity, and urgency of this project. “We got the call in Florida, and 4 days later we were onsite in Texas,” Vitale said.
Rebuilding and protecting in one pass
Given the size of the chamber, Vitale needed scaffolding. Rather than rent, he purchased scaffolding on eBay from a seller in Texas. With the scaffolding in place, he and his crew got to work with 34,500-kPa (5000-lb/in.2) pressure washers, thoroughly cleaning the concrete walls and removing all PVC lining and corroded concrete.
The cleaned walls were then rinsed with a 10% solution of ConmicShield®, an antimicrobial product made by ConShield Technologies (Atlanta). The antimicrobial product is not toxic to humans or animals, but creates an environment that permanently inhibits thiobacillus growth. It either can be used as a rinse, or mixed with concrete and applied structurally because the additive bonds with the concrete matrix and creates a permanent barrier.
After the antimicrobial rinse, Vitale used AP/M Permaform’s (Johnston, Iowa) Permacast® MS-10,000 to fill the voids caused by corrosion and to rebuild the junction box walls. Voids deeper than 130 mm (5 in.) first were filled with MS-10,000 UL, a fast setting underlayment. Both products are silica-modified Portland cement; the strong corrosion-resistant cement is designed to strengthen and repair concrete structures. Vitale applied the new concrete with low-pressure rotor-stator pumps to minimize backsplash and improve adhesion. The City of Houston performed visual inspections as work progressed.
For the top inch of concrete resurfacing, Vitale continued to use MS-10,000, but added ConmicShield® to the mix.
“You don’t need the antimicrobial protection in the deeper layers,” Vitale explained, “so to save money we just use it on the surface, which creates a barrier that protects the entire structure.”
Vitale used a thickness gauge to ensure even coverage. By using silica-modified cement and antimicrobial product together, Vitale was able to repair the junction box as well as permanently protect it without the expense or complication of additional PVC or epoxy linings.
T.V. Diversified spent 2 weeks onsite with an accelerated schedule. But the project would have gone even faster except for a massive rainstorm over the bayou.
“Some pump stations couldn’t handle the extra flow and backed up, so we were kept out of the box for a couple of days,” Vitale said. “It was just Mother Nature acting up — not much you can do about it, except clean up the mess and get started again.”