Miami-Dade engineers tested the anti-microbial additive on the first manhole out of a force main that was replaced two or three times a decade despite treatment with epoxy coatings and plastic liners. Satisfied with the precast concrete's performance after eight years, they specified the additive for clarifiers and other upgrades. Photos: Miami-Dade Water & Sewer Department
Miami-Dade Water & Sewer Department added ConmicShield to specifications for new assets such as four 200-foot-diameter clarifiers.
By Stephanie Johnston
In 1996 Atlanta-based ConShield Technologies Inc. introduced a germicidal liquid that bonds molecularly with concrete, shotcrete, and repair mortars to keep single-celled organisms like Thiobacillus from growing and producing sulfuric acid. Nontoxic to humans and animals and EPA-registered as biocide #75174-2-47000, ConmicShield additive kills the bacteria by rupturing its membrane.
Unlike a coating or liner, the liquid is evenly dispersed throughout the concrete mixture of a pipe or other asset. As a result, maintenance-related wear and tear doesn't compromise a structure by scraping off and giving bacteria a foothold. According to Sales and Marketing Manager Joe Cherry, on average ConmicShield adds $3 to $4/square foot for a new 48-inch manhole compared to $12 to $14/square foot for a coating and $15 to $20/square foot for plastic liners.
Intrigued by a low-cost rehabilitation option that doesn't require infield welding and spark or adhesion testing, managers in Arizona, California, Florida, Michigan, Missouri, New York, South Carolina, Virginia, Washington, and Wisconsin use the product for new precast concrete pipe and structures and for manhole repair. If the structure's basically sound, the interior can be coated with special cementitious liners that includes ConmicShield additive. If beyond repair, a precast producer can be directed to include the product — typically at a rate of 1 gallon/cubic yard — in the design mix for a new manhole.
An early adopter, the Hampton Wastewater Division in Virginia opted to repair assets in-house through the “City Self-Install Program” offered by AP/M Permaform of Johnston, Iowa. A $100,000 investment (which the manufacturer can finance) buys a bidirectional device called the SpinCaster that flings ConmicShield-infused mortar onto a structure in one direction, then the other, at 5,000 rpm. The operator uses a winch to repeatedly raise and lower the device through the manhole's center to apply an even layer of mixture ranging from ½ to 2 inches thick. ASTM International developed standard F-2551 for lining manholes and is developing a similar standard for lining pipe — WK 26451 — using this technique, called centrifugally cast concrete pipe (CCCP).
According to the company, a three-person crew can do four to eight sanitary or storm manholes and catch basins a day. Since 2000, Hampton's crews have done 3,200, which is almost 30% of the city's 11,000 manholes; and Collection System Manager Barry Dobbins says the operation has “nothing but positive things to say about the product.”
This is how Chicago's Department of Streets & Sanitation is fixing 20,000 manholes, the world's largest such rehabilitation, though the $60 million project is being performed by two licensed applicators: Smith Maintenance Co. and Benchmark Construction Co. Inc., both of Chicago. Having specified an impermeable material that would provide a minimum of 3,000 psi compressive strength and 150,000 psi modulus of elasticity within 24 hours, engineers chose a 1- to 2-inch layer of AP/M Permaform's MS-10,000 Permacast cement with ConmicShield sealed and then coated with a layer of epoxy to protect against road salts in the storm sewer.
The City of Westlake, Ohio, will receive an achievement award from the North American Society for Trenchless Technology this month for one of the first horizontal rehabilitations.
Initially, managers considered cured-in-place pipe for a 40-year-old interceptor that had lost up to 1½ inches from 6-inch-thick walls. But most of the damage was within 25 feet of trunk lines. It wasn't cost-effective to tear apart extremely deep manholes to insert the lining when a SpinCaster could be inserted anywhere along the interceptor and adjusted to inside diameters ranging from 36 to 60 inches. In the end, ½-inch of ConmicShield-infused PL-8000 cement was applied to half of the 7,200-foot asset.
With ConShield Technologies right in its backyard, Atlanta's Department of Watershed Management used the additive in a major shotcrete pipe repair project in 1997. The city uses the same self-install program that Hampton, Va., does using Permacast with ConmicShield additive.