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 such as 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 it 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 in-field 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 an 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 sends ConmicShield-infused mortar onto a structure in one direction, then the other, at 5000 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 1/2 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 3200, 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 3000 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 11/2 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, 1/2-inch of ConmicShield-infused PL-8000 cement was applied to half of the 7200-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.
Use in new construction
“When cities saw how the agent performed in sanitary sewers they asked if they can use it in new construction,” Cherry says.
After watching how a particularly problematic manhole replaced with additive-infused precast concrete performed over eight years, the Miami-Dade Water & Sewer Department added the product to both precast and poured-in-place mix specifications for 30,000 yards of pipe, manholes, clarifiers, and air entrainment tanks in a $600 million plant upgrade.
Because the liquid is delivered to the ready-mix producer, construction managers controlled quality by reviewing the mix tickets of each incoming concrete truck. To see how the mixes perform, stainless steel screws were inserted to measure the distance from screwhead to the concrete surface.
Associate director John Chorlog, PE, told Treatment Plant Operator magazine in January that after six months, “We found minimal concrete loss in some tanks below the liquid level, but we think that’s just surface latents, like form oil, that are being washed off. Overall, the treated concrete above the liquid waste level where corrosion occurs looks good.” The additive is designed to do just that: avert damage where the air-breathing bacteria produce sulfuric acid rather than submerged concrete.
Ever cautious, though, some specifiers require additional testing.
The Indianapolis Department of Public Works reviewed the product internally and asked ConShield Technologies to submit the additive to an independent technical analysis by Indiana University-Purdue University Indianapolis’ Construction Engineering Management Technology program (see Get a Second Opinion).
After six months, the program verified that ConmicShield additive-infused mixes meet the department’s ASTM-based sanitary sewer specifications for compressive strength, shrinkage, set time, hardened air properties, precast reinforced manhole sections, absorption, freeze/thaw, and scaling and protection against microbiologically induced corrosion. The research included the results of testing by Oldcastle Precast Inc. of Auburn, Wash., and Rinker Materials Corp. of Houston, which evaluated how the product affects curing and strength. Though the former reported longer set times, they still met department requirements.
In February 2009, the New Products Committee of the city’s public works department notified ConShield Technologies the product “may be specified as an alternate as DPW deems best as on a case-by-case basis.” The first project that allows for the product just received a notice to proceed.
About a decade ago, the Metropolitan St. Louis Sewer District used the product to repair a 50-foot drop shaft. Before repairs, one-third of the 18-inch-thick concrete walls had been eaten away. Ten years after lining with Permacast mortars with ConmicShield additive, the shaft remains in excellent condition. The utility has specified the additive repeatedly since then with similar results for new and rehabilitated manholes and new, precast pipe.
Some precast producers prefer dry-casting pipe because of its shorter mixing times. When a project requiring new pipe arose, materials engineer Ron Moore wanted to ensure the recommended formula of 1 gallon of additive per cubic yard of concrete performs equally well for both the dry- and wet-casting methods.
Two studies, one funded by the district and the other by ConShield Technologies, were conducted to ensure that the utility’s specifications for sampling dry-cast pipe mixes don’t affect the additive’s performance or pipe strength.
Situ Biosciences LLC, Skokie, Ill., tested three samples: one without the additive, one with the additive mixed for the standard dry-cast mixing time of 90 seconds, and one with the additive mixed for seven minutes. The latter killed the bacteria and thoroughly incorporated the additive into the mix.
The Hauser Laboratories Division of Microbac Laboratories Inc. in Boulder, Colo., confirmed the utility’s protocol for identifying the presence of bacteria is sound. ASTM D4783 modified for concrete uses Serrita Marcenes instead of Thiobacillus, which can take 28 days to cultivate. In addition to growing much more quickly, the presence of Serrita Marcenes is easily visible to the naked eye as a red stain.
The utility has specified the additive for new construction including projects with 42-inch through 96-inch pipe.
ConmicShield is a registered trademark of ConShield Technologies Inc., Atlanta.
This article originally appeared in the March 2011 issue of sister publication Public Works magazine.