The first step in preparing new concrete for the aggressive exposure conditions found in water and wastewater construction is to “repair” it. That’s the advice of Vaughn O’Dea, director of Sales, Water & Wastewater Treatment for Tnemec, who has written extensively on the topic. “Cast-in-place and precast concrete contains an abundance of bugholes, pinholes, and other surface imperfections that form during concrete placement.” O’Dea explained. “Resurfacing new concrete establishes a level substrate for topcoating and improves the film quality of the subsequent protective lining system by filling voids, bugholes, and other surface defects and irregularities in new concrete.”

The failure to provide a contiguous surface for topcoating can result in entrapped air within the substrate escaping into the protective coating. “These small voids in the coating provide avenues for gasses or corrosives to penetrate the otherwise impervious coating material and attack the underlying concrete substrate,” O’Dea acknowledged. “In order for these protective coating systems to be effective, it is essential for contractors to properly repair the concrete surface prior to topcoating.”

Mechanical methods such as wet/dry abrasive blasting and waterjetting/high-pressure waterjetting are effective in removing laitance from a concrete or shotcrete surface prior to applying a cementitious resurfacer. Laitance is a thin, weak layer of aggregate fines and non-reactive cement residue that is loosely bonded to the substrate. O’Dea recommends abrading concrete to produce a minimum surface profile of CSP-5 as noted in ICRI Guideline 310.2 to create an anchor profile to enable topcoat adhesion.

Given the expanding market of hydraulic cementitious resurfacing and repair materials, O’Dea advises specifiers to be cautious when considering commercially available products. O’Dea cited several advantages offered by Series 218 MortarClad, an epoxy-modified cementitious repair material, which was used at the Central Water Reclamation Facility in Pensacola, Florida:

• The epoxy polymerization does not form a laitance layer on the surface of the mortar.

• The dual-curing mechanism of the hydrating cement particles and chemically-crosslinked epoxy resin allows the mortar to be applied as a thin parge coat down to 1/16-inch.

• The need for external curing in accordance with ACI 308R (American Concrete Institute Guide to Curing Concrete) is eliminated.

• The Coefficient of Thermal Linear Expansion (CTLE) properties are similar to concrete, preventing cracking or disbanding of the repair system when exposed to thermal cycling.

• The epoxy polymerization increases compressive and tensile strength and provides a chemical bond for topcoat adhesion.

“These properties are unique to epoxy cementitious overlays,” O’Dea added. “All other hydraulic repair materials require increased minimum thicknesses, external curing in accordance with the ACI 308R, and removal of the laitance layer on the surface of the mortar.”

For more information on the importance of resurfacing concrete, contact your local Tnemec coating consultant. 

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