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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
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Miami-Dade Water & Sewer Department added ConmicShield to specifications for new assets such as four 200-foot-diameter clarifiers.

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 box on page 42).

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 Pre-cast 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 in 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.

WEB EXTRA

For more information, including an excellent explanation of what causes microbiologically induced corrosion, click here.

GET A SECOND OPINION

In addition to Indianapolis, Fort Wayne and Zionsville, Ind., have asked manufacturers to submit products for independent technical analysis by Indiana University-Purdue University Indianapolis (IUPUI).

Construction Engineering Management Technology Director Tom Iseley says the New Product Review program is slated to roll out nationally within several months. Manufacturers will be charged $7,000 to participate; public agencies can subscribe to results for an annual fee of $500.

For more information, contact New Product Review Manager Behnam Hashemi in Indianapolis at 317-274-7082 or shashemi@iupui.edu.