Concrete clarifiers and storage tanks may lose up to 1/8-inch of material each year when they're exposed to treatment chemicals, which cause biogenic sulfide corrosion. Many plant managers apply protective coatings to ward off degradation, not knowing that — when improperly applied — the coatings speed the destruction of those valuable assets.
Studies have shown that pinholes that create a discontinuous protective film are the major cause of concrete breakdown at water treatment plants. Cast-in-place and precast concrete sometimes contain imperfections that were formed during placement. Those small voids, caused when entrapped air within the substrate is released into the coating, allow gasses or corrosives to penetrate the otherwise impervious coating and attack the underlying concrete substrate.
Although resurfacing the substrate improves film quality by eliminating imperfections, applying a coating directly to an unprepared surface causes pinholing, outgassing, and bughole-induced outgassing of the barrier film.
Fortunately, resurfacing materials and repair methods have been developed exclusively for use under high-performance protective coatings. The best choices for repair materials are cementitious mortars using ingredients with properties that closely match those of the host concrete.
Four composite cementitious mortars are available for water and waste-water structures:
READ BEFORE ASSEMBLING
Regardless of repair material, though, the first step to a long-lasting, effective coating is to create a proper surface profile.
Manufacturer recommendations for surface preparation range from high-pressure power washing to very specific surface profile requirements. This inconsistency causes confusion — and often failures — for contractors who must sort through the manufacturer's instructions and compare them to the requirements of the protective coating system and specification documents.
Enlightened manufacturers recommend industry-accepted surface preparation standards, such as the ones created by the Society for Protective Coatings (SSPC), National Association of Corrosion Engineers (NACE), American Concrete Institute (ACI), and International Con crete Restoration Institute (ICRI), to create consistency and clarity.
The goal is to meet the prescribed adhesive specification. For example, most requirements in typical water or waste-water repair applications call for the adhesion of the protective coating system, including repair material, to exceed the concrete's tensile strength (generally around 350 to 450 psi). Using a portable adhesion tester according to ASTM D7234, for example, will confirm it.
MAKE SURE IT'S THICK ENOUGH
Cementitious resurfacing mortars applied at less than minimum recommended thickness may be too weak and friable to support high-build, chemical-resistant protective coating systems specified for water and wastewater environments. Therefore, double-check the “prepared” surface before installing the protective coating.