High-build coatings are much more than paint over concrete block or other concrete surfaces. They are high-tech products characterized by properties such as resistance to wind-driven rain, chalking and checking after weathering, high ultimate (breaking) elongation, elongation recovery and crack bridging capability, and water vapor permeability.
This article focuses on the benefits in using high-build coatings as surface treatments over concrete, points out their unique performance properties, and explains applicable ASTM test methods.
Coatings vs. paints
At a glance, acrylic high-build coatings are often mistaken for ordinary paints. They come in a pail, are applied on the wall by roll or spray, and are available in a variety of colors, either directly from the factory or tinted at the local distributor. However, the similarities end here.
High-build coatings are generally applied in one or two coats to a total dry film of 10 mils to 15 mils, while ordinary paints have average thicknesses from 3 mils to 5 mils. The difference in thickness is directly related to the viscosity of the two materials and their percent of solids. High-build coatings contain about 50% solids by volume, versus 30% for ordinary paints.
Solids, which are a balanced combination of resins and fillers, determine most high-build coating properties. The high percent of solids allows the incorporation of sand aggregates to achieve a variety of textured finishes.
A high amount of resin creates a dense net that provides for waterproofing against rain, when film coalescence is complete. In the case of elastomeric coatings, it is a combination of thickness and type of resin that allows the product to stretch over moving cracks. Typically, high-build coatings use a blend of acrylic resins, such as methyl methacrylate, which provides top performance properties for exterior wall coating applications over concrete and masonry.
Performance properties —protection
The performance properties that set high-build coatings apart from ordinary paints include resistance to wind-driven rain and enhanced crack-bridging capability with elastomerics. Water vapor permeability also is important since only the best quality high-build coatings can match the permeability of ordinary paints.
Wind-driven rain damages concrete in several ways. Freeze-thaw cycles experienced by the water cause spalling and exposure of steel bars, which leads to corrosion. Water also can carry contaminants, such as chlorides, that accelerate and further induce serious damage in steel reinforced elements.
ASTM D6904 (formerly known as Federal Standard TT-C-555) measures the ability of high-build coatings to provide an impermeable barrier against wind-driven rain over time, due to the absence of voids such as pinholes, which are entry ways for water. The test utilizes porous patio blocks (8 in. x 16 in. x 2 in.) that are coated on one side and waterproofed on the perimeter. Once the coated side is cured, it is exposed to rain at 98 mph for 24 hrs.
The test evaluation uses a visual inspection of the rear face of the masonry left untreated. The material passes the test if no dampness is detectable. In-house test evaluation may also include visual inspection at the cross of the broken block and/ or evaluation of weight gain, which should less than 0.2 lb.