Sorting Through the Various Types of Concrete Coatings
Part 2 of a 3 part article covering coatings for concrete surfaces
Concrete coatings basically fall into thin and thick film types. Thin-film — about 1 mil — sealers can be acrylic-type or thin-film-penetrating sealer type. The penetrating type can permeate to a depth of 4 inches and is best for shielding against moisture and de-icing chemicals. Thick-film — 2-3 mils per layer and higher — coatings contain two-part epoxy, polyurethane and polyaspartic applications. Two-part products are resins combined with a hardener at application time.
Water-based acrylic coatings dry fast and clear, but they have lower performance characteristics and shorter lives. Solvent-based acrylics also dry fast and are easy to apply, but they penetrate better, resist stains more effectively, and provide a deep transparent finish for enhanced, rich color. They can bubble or turn yellow in high temperatures and emit moderate to high levels of odor as they dry.
Coatings in the siloxane and silane groups repel water well and provide good salt protection due to their excellent penetration. They have low chemical resistance and some odors but no color enhancement.
As concrete hardens, water and cement rise to the surface. As this bleed water dries, the surface concrete remaining is porous and not as wear resistant as the concrete in the center of the slab. Coatings called densifiers are used to fill surface pores, where the objective is to harden and compact concrete and reduce porosity. They penetrate well, are excellent for moisture and salt resistance, and withstand heavy traffic. Densifiers produce some sheen but do not enhance the color of the substrate.
Epoxy resins are bisphenol A, bisphenol F or phenolic/novolac. Co-reactants and hardeners consist of amine, polyamide, amidoamine, phenolic/novolac, siloxane, coal tar, water-based, and epoxy esters. Epoxies bond very well to properly prepared concrete, are self-leveling, and fill dents and cracks. They are water- or solvent-based or 100 percent solids, with the latter yielding 3-10 mils per layer.
Both epoxy and polyurethane coatings are long lasting thermosetting polymers that resist impact, stains, chemicals, oils, and wear. Polyurethane and polyaspartic coatings offer very good film-build and high gloss, and they are non-whitening.
Aliphatic polyurethane, 60-70 percent solids, is a good choice for concrete-floor protection. One layer is 2-3 mils thick. Compared to epoxy, it is: more flexible; more resistant to impacts, chemicals, and abrasion; and more UV stable. While not as hard as epoxy, polyurethane is more scratch-resistant, is three times more wear-resistant, and has larger temperature, humidity, and finish ranges.
Polyurethane does not bond well to concrete, is not as self-leveling, does not fill cracks and dents, and is not as moisture-tolerant during application as epoxy. It is solvent-based, so its higher levels of volatile organic compounds (VOC) might require respirator protection and more ventilation.
One solution that optimizes the best of both coatings is to apply epoxy first for excellent build, bonding, leveling, and crack and dent filling and then to apply polyurethane for excellent surface appearance and resistance to wear and impact.
Polyaspartic coatings offer very good performance results in a range of conditions. For example:
- They cure in 5-120 minutes depending on mix.
- Workers can apply them at -30 degrees to 140 degrees.
- Their water-like viscosity provides excellent wetting.
- They offer film build-up to 18 mils.
- They do not bubble, even in high humidity.
- They are UV-resistant.
- They have levels of high solids, so they have few if any VOCs.
- They can withstand temperature to 350 degrees when cured.
- They are oil and fat stain resistant.
- They offer higher abrasion resistance than urethanes or epoxies.
- They are very clear, with no whitening from moisture.
But application requirements are exacting as to surface preparation, and a moisture sealant might be required first. A polyaspartic coating also has short pot life, so mixing with expensive, high-pressure spray equipment must be done at the tip of the spray nozzle during application, and spray guns require careful cleaning.