Concrete is one of the most difficult surfaces to coat, due mainly to its wide range of properties. Steel is fairly uniform in texture, and workers are familiar with the proper ways to prepare and coat it. In contrast, concrete is quite porous and uneven in texture, with differences in density from one square inch to the next.
Why Coat Concrete?
Aesthetics
Concrete is one of the most commonly used structural materials for private and public buildings. Because its natural color of dull, light to medium gray is often considered unattractive or, at best, uninspiring, more colorful coatings are often an architectural preference.
Porosity
The porosity of concrete allows moisture to penetrate it quite readily. Besides the problem of moisture intrusion, chloride penetration can attack the steel bars and mesh that are typically used for structural reinforcement. Since the corrosion products tend to occupy greater volume than the original steel, expansive stresses can crack or spall concrete, which has poor cohesive strength compared to its compressive strength. In northern climates, this porosity often leads to destruction of the concrete structure during freeze-thaw cycles.
Decontamination
The normal porosity of concrete allows it to absorb almost any contaminant that comes into contact with it. Within nuclear facilities, coatings for concrete must be easily decontaminated in case of radiation spills. Within schools and hospitals, coatings must render the concrete surface resistant to bacteria. Within food establishments, coatings must resist growth of algae or other slime-type growths that could contaminate the food being processed.
Conductivity
Hospital operating rooms, solvent storage facilities, and microchip manufacturing facilities are among the many areas where conductive fillers have to be used in conjunction with organic polymers to dissipate static electricity.
Chemical Resistance
Petrochemical plants contain millions of square feet of concrete surfaces. In areas where aggressive acids are being processed, the alkalinity of the concrete must be preserved by isolation from these acids with chemically resistant mortars, bricks, tiles, or sheet membranes.
Properties of Concrete and Coatings
Before one can successfully coat a concrete structure, it is necessary to understand and overcome the natural properties of concrete. Concrete can be a simple mixture of Portland cement, lime, aggregate, and water, but it can also contain fillers selected for specific structural properties and resins selected for specific chemical-resistant properties, along with various additives selected to add specific strengthening properties to the otherwise simple mixture. Each variation creates a different surface with properties that can add or detract from a coating system’s ability to bond with the concrete.
Alkalinity
First and foremost, concrete is naturally alkaline, both in its curing stages and after its final cure. Therefore, any coating chosen must not only be able to withstand the environment that comes into contact with the concrete, but it must also be able to penetrate and bond with the alkaline surface of the concrete. Oil-based or alkyd coatings are unsuitable because the alkali in the concrete will react with the natural or synthetic oils in the coating, causing the creation of a soap-like film (saponification) that leads to delamination of the coating from the concrete.
Curing Mechanism
Regardless of the mix involved, or the additives in the mix, concrete cures by a process of hydrolyzing water with the ingredients in the mix. The commonly used rule of thumb is that concrete needs 28 days to cure at an average temperature of 20 °C (75 °F). It should be recognized that this traditional 28-day cure cycle allows the concrete to achieve the compressive strength designed into the mix ratio that was selected for the specific service conditions. Many coatings can be applied satisfactorily before the cure cycle is completed; however, this must be done in accordance with the manufacturer’s recommendations. ACI 308, “Guide to Curing Concrete,” provides recommendations on temperature, moisture conditions, and time of cure for various concrete structures.
This article is adapted from The Protective Coating User’s Handbook, Second Edition, Louis D. Vincent (Houston, TX: NACE International, 2010), pp. 163-165.