By Tim Frazier
Concrete is an incredibly strong and durable building material, and the quality control measures involved with precast concrete only accentuate those characteristics. But, like every other building material, concrete is not invincible. A high quality precast product – with low permeability and a dense matrix – paired with a protective coating, however, will remain protected from detrimental elements and provide the long service life owners have come to love. Before applying a protective coating, though, it’s important to understand the significant areas where concrete degradation can be expected and how to best protect your product.
Reviewed here are four common mechanisms of degradation.
When concrete is first hydrated, its pH is 12.5 or higher. Exposed to an acid source, with a pH below 7.0, the cement components dissolve and form chloride, sulfate and/or nitrate salts. As the cement paste continues to dissolve, coarse aggregate is exposed which leads to reduced strength and greater porosity. This process continues as fresh concrete surfaces are exposed to more acid – leading to more concrete degradation. Acid resistant types of cement help, but no concrete can resist sustained exposure to flowing acidic conditions indefinitely. It should be noted that pH values do not always correlate to the aggressiveness of an acidic solution in regard to its ability to dissolve cement paste. The aggressiveness of the acid depends on the solubility of the calcium salts formed when the acid reacts with calcium hydroxide. The higher the solubility of the resulting calcium salt, the greater the acidic attack on cement.
Prolonged exposure to acid manifests itself as rust bleeding from corroding reinforcing steel, cracking, exposed coarse aggregate, spalling and a highly weakened concrete matrix.
Fresh concrete, with a pH between 12-14, is typically unaffected by alkaline solutions unless the solutions are hot. Under sustained exposure, a hot 25% solution of sodium hydroxide may slowly degrade concrete over time and the same is true for hot potassium hydroxide. Under such conditions, the concrete undergoes destructive alkali-silica reactions (ASR). To enhance resistance, use aggregates low in reactive forms of silica, testing them to ASTM C289.
Chloride induced deterioration
When concrete is placed around reinforcing steel bars, the steel surface initially corrodes, forming a tightly adherent oxide film over the surface to provide protection from further corrosion. The highly alkaline environment maintains this protective film provided it remains free from exposure to moisture, oxygen and chloride ions. If exposed to those elements, the result is the formation of large amounts of iron oxide (rust) being generated from the steel with concurrent expansion. When these expansive forces exceed the tensile strength of concrete, concrete cracks develop. Cracks further perpetuate the cycle and further degradation occurs. Rust bleeding, cracking and spalling are all signs of chloride-induced corrosion.
Both naturally occurring and manufactured sulfate products can attack concrete. Such an attack can occur when concrete is in contact with sulfate containing water (seawater, swamp water, groundwater or sewage water). Their exposure to concrete structures causes extremely destructive reactions with hydrated portland cement paste. Over time, if evaporation cycles occur, sulfate concentration increases and cause microcracking, then larger cracking and concrete matrix disintegration. Sulfate attack usually occurs in two stages:
1. Sulfate ions react with calcium ions in the cement paste forming calcium sulfate (gypsum).
2. Gypsum and calcium aluminates form calcium sulfoaluminates (ettringite).
Both reactions cause cracking and disintegration. When concrete is frequently exposed to sulfates in an environment of cyclic wetting and drying, the concentration of destructive sulfates increases sufficiently to cause rapid concrete deterioration.
A utility vault with a waterproof coating is lowered into place on Georgia Street in Downtown Indianapolis.
Coating considerations and types
Protective barrier coatings are primarily used to protect concrete surfaces from chemical attack, and the entrance of liquids and gases. The coating must be alkaline compatible, and must resist the chemical it will be exposed to. Alkali resistance in coatings has become increasingly important because fast track construction often will not permit a 28-day cure prior to coating.
Understanding and defining actual exposure conditions – chemical type and concentration, duration and type of exposure, wet/dry cycling, and temperature – is critical in selecting the proper coating system. Chemical resistance is a function of molecular weight or crosslink density. Chemical resistance is typically greater with increasing crosslink density. However, flexibility can be lost in the gain for increased chemical resistance.
The three most often used generic coating types include:
• Epoxies – Epoxies are two-component systems consisting of an epoxy resin and a curing agent and are known to have good resistance to alkalis and organic acid. The cure rate of epoxies is affected by temperature and should not be applied below 40 F. Many cured epoxies soften above 150 F and are not resistant to ultraviolet light (UV). Epoxy novolacs have a greater crosslink density and an excellent all around resistance to alkalis, acids and solvents. The downside to having a higher crosslink density is they are more viscous, making application more difficult.
• Urethanes – There are two general types of urethane coatings: two-component and moisture cure. Mixing a polyisocyanate and a polyol creates a two-component urethane. Moisture-cure polyurethanes are one-component systems that cure by reacting with moisture in the environment. Two-component systems have a greater crosslink density and barrier properties; however, moisture-cure polyurethanes are still widely used since they perform well and offer the convenience of a one-component system. Urethanes perform best when applied on dry concrete as water can interfere with cure and film formation.
• Acrylics – Acrylic coatings will not attain the crosslink density of epoxies or urethanes but do offer chemical resistance in many situations. They are UV resistant and are available in lower volatile organic compound content (VOC) than alternative systems. In recent years, self-crosslinking acrylic coatings have emerged within the acrylic family, which provide a higher degree of crosslinking and improved resistance properties.
Other protective coating options include:
- Polymer fortified concrete overlays
- Polyesters and vinyl esters
Coatings are not a guarantee of 100% success in protecting from dentrimental elements. However, coatings significantly reduce the contact between the concrete and potentially damaging substances. Coatings used to protect concrete
against water and aqueous chemical solutions should have a very low water transmission rate (MVT). This is particularly true for immersion grade coatings or secondary containment coatings. ASTM E96, Test methods for Water-Vapor Transmission of Materials, gives one such way of evaluating coating permeability.
Tim Frazier is technical director of Concrete Sealants Inc. He has been involved with coating-related products for 27 years and with concrete coatings for the past 20 years. Frazier holds a bachelor’s degree in chemistry from Wilmington College, and a master’s degree in chemistry from Wright State University.
ICRI Guideline No. 03732. Selecting and Specifying Concrete Surface Preparation for Sealers, Coatings, and Polymer Overlays; ICRI: Sterling, VA.
Hooker.KA., Epoxy and Urethane Coatings. Concrete Repair Digest, April/May 1992.
Randy Nixon, The Fundamentals of Cleaning and Coating Concrete, SSPC: The Society for Protective Coatings, Pittsburgh, PA.