Typically, epoxy-modified cementitious materials can be applied at a minimum thickness of 1/16-inch, compared to the ¼-inch to 1/8-inch minimum for acrylic-modified resurfacers, ¼-inch to 3/8-inch minimum for portland-based mortars, and ½-inch minimum thickness for calcium aluminate-based materials.

A 6-mgd wastewater treatment plant in Louisiana recently averted what would have been an expensive mistake. A contractor who was about to apply the protective coating doubted the quality of the surface repair, suspecting that the portland-based cementitious mortar used to fill voids and bugholes was inadequate to support the epoxy lining system specified for the plant's severe wastewater headspace environments.

A test was performed using a Type II fixed-alignment adhesion tester, or elcometer, on a 5x5-foot section of concrete wall coated with the portland-based cementitious mortar, a common testing protocol for field-applied liquid coatings.

Results revealed 100% cohesive failure (or splitting) of the mortar at less than 80 psi. Adhesion was well below the anticipated 350- to 450-psi minimum generally obtained when applied over a cementitious substrate. The portland-based resurfacer applied at 1/16-inch thickness to the new concrete did not develop the proper calcium silicate hydrate matrix, ultimately yielding poor cohesive properties.

Given those findings, the decision was made to remove and replace the portland-based mortar with an epoxy-modified cementitious resurfacer.

CURING IS CRITICAL

Research indicates that bond strength develops more slowly than compressive strength. Sometimes crews mistakenly abandon curing procedures prematurely when the traditional cementitious mortar seems strong. But once the mortar dries, strength development stops and the mortar patch may fail.

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This is less likely to happen at a treatment plant where concrete was properly resurfaced before a protective coating was applied. Photos: Vaughn O'Dea

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Cast-in-place concrete after SSPC-SP13/NACE 6 surface preparation to expose bugholes and create an anchor profile according to ICRI CSP 5 (International Concrete Repair Institute Concrete Surface Profile).