How Coatings Protect Concrete

An Introduction to How Coatings Protect Concrete
By: Charles H. Holl, Texas Industrial Floors Inc., (formerly with Phoenix Engineering Services Inc.) and Scott A. O'Connor, Phoenix Services, a division of MAC-PRODUCTS, Inc.
(Published in Journal of Protective Coatings & Linings, August 1997)

Concrete: What It Is and Why We Coat It

As we noted last month, concrete is a mixture of Portland cement, water, and fine and coarse mineral aggregates, and sometimes various admixtures. When all of these materials are mixed in the correct proportions, an complex chemical reaction takes place. This reaction is known as cement hydration, the process by which concrete hardens and cures.

Even though it can be a very strong, hard substrate, bare concrete is also subject to deterioration. Concrete can be very porous, so chemicals can penetrate the pores and attack the paste. The paste and aggregate can also be worn down by physical impact and abrasion. Water can penetrate concrete, freeze and expand inside it when the temperature drops, and ultimately weaken the concrete from within. In addition, if the concrete has reinforcing steel bar (rebar) to impart additional strength and other properties, the rebar can corrode if moisture, oxygen and chloride ions penetrate the concrete. Corrosion or rebar contributes tot he deterioration of concrete.

Thus, as noted last month, we coat concrete to protect it from chemical and physical attack. We also coat it to protect products stored or processed in direct contact with the concrete from contamination caused by dust from the substrate. And we coat concrete to improve its appearance, ease of maintenance, and light reflectance. As long as coatings are properly applied to an adequately prepared concrete surface, they can prolong the life of the structure.

What Makes Concrete Difficult To Protect?

As you might expect, it is difficult or impossible for any coating to protect concrete if the coating is incompatible with it, or if the concrete itself is in some way weakened, contaminated, or inherently defective. Here are some characteristics of concrete that can make it difficult to protect.

  • Alkalinity: The cement paste that holds concrete together is made up of materials called alkaline hydrates. These materials create a property called alkalinity. Alkaline materials are highly reactive to all acids and compatible with some coatings, like alkyds.
  • Porosity: Because it has man pores, concrete can be attacked by water, gas, and other contaminants. In addition, the porosity of concrete can contribute to the formation of coating defects called pinholes. Pinholes, which are cause by the movement of gas or moisture vapor through an uncured material, give contaminants in the environment an easy way to reach the concrete.
  • Moisture content: Water is a key ingredient in concrete. Water left from the reaction that forms concrete (called "waters of convenience") makes its way to the surface, leaving behind capillaries and void spaces. When a finished concrete structure contains large amounts of water in a liquid or vapor state and transmits water to the environment, the escaping water can prevent a coating from adhering to the surface, curing properly, or performing as needed.
  • Tensile strength: Tensile strength relates to the ability of the material to withstand tension without fracturing. If concrete is too weak in tensile strength, the top layer of it can delaminate, taking the coating off with it.
  • Laitance: Laitance is a weak, thin layer of the concrete's surface that forms when cement paste and fine aggregate are carried to the surface during the cure of the concrete. If the laitance is not removed before coating application, the coating will most likely disbond.
  • Surface defects: Concrete may contain many surface defects, including dusting, grooves, depressions, cracks, holes (called bugholes), and fins (sharp protrusions from the concrete).
Before the concrete is prepared and coated, its condition should be assessed, either by a representative of the facility owner who is paying for the work or the project manager from your company. For our purposes in this Bulletin, it is important for you to know that like steel, concrete requires careful surface preparation and strict adherence to the specification and the manufacturer's recommendations for coating application. Future Bulletins will describe procedures for surface preparation of concrete and application of coatings, linings, and floor toppings.

How Coatings Protect Concrete

In the May 1997 Bulletin, we described the 3 ways that coatings protect steel.

  • Barrier protection: The coating prevents moisture and oxygen from reaching the surface. Inhibition: The coating interferes with the electrochemical process of corrosion. Sacrificial action: The zinc pigments in a zinc-rich coating are attacked instead of the steel when corrosion occurs.
  • Unlike coatings for steel substrates, protective coatings for concrete do not in most cases require or include inhibitive or sacrificial pigments to provide protection. Coatings applied to concrete are typically barrier coatings. They provide protection by becoming a physical barrier, or shield, isolating the concrete from its immediate environment. A barrier coating must prevent aggressive liquids and gasses from passing through it and reaching the concrete.
Barrier Coatings

You probably have heard of many of the coating types that we classify as barrier coatings. Examples include epoxies, vinyls, polyesters, methyl methacrylates, and polyurethanes. These coatings are named from the resin type sued to make them. (remember, coatings are made of resins, pigments, and solvents). As we discussed in the May Bulletin, the resin provides many of the protective properties of a coating.

One important property of a barrier coating is called permeability. The permeability of a barrier coating's film depends on its moisture vapor transmission (MVT) rate. The MVT ate is determined by how fast water molecules pass through and move around the spaces between the resin molecules. The effectiveness of a coating in preventing permeation depends on how closely and tightly bound the molecules of the resin are to one another. The coating's effectiveness also depends on the type of resin molecule and the amount and type of pigment. Cross-linking is a measure of the degree of intense bonding of coating resins.

The lower the permeability of a barrier coating, the more protective the coating is. Basically, the higher the degree of the coating resin's cross-linkage, the lower the permeability, the better the adhesive bond of the coating to the surface, and the better the overall protective barrier.

By the way, these intermolecular spaces between the resin molecules are much larger than the water molecules and should not be confused with physical holes (pinholes) in the coating film. Pinholes in the coating film are generally considered defects and should be repaired. Spaces between resin molecules are not defects.

The barrier properties of coatings can be improved by adding reinforcement fillers to the resin. Fillers come in a variety of forms, such as silicate aggregates (sand), glass or mica flakes, fibers, ad woven fiberglass ( incorporated as a mat in the resin system as it cures). The addition of fillers physically increases the length of the path that the intruding liquid or gas molecules must take to penetrate the coating. Flake materials form layers of overlapping platelets, parallel to the concrete surface, somewhat like shingles on a roof. Fillers and fiberglass mat can also be added to improve the barrier coating's physical properties, such as impact and abrasion resistance.

Coatings that Breathe

As discussed earlier, the curing of new concrete often results in the release of substantial quantities of water. If this water is trapped between the coating and the concrete, it can cause the coating to loose adhesion or form blisters. It is sometimes necessary, therefore, to use coatings that "breathe." These coatings allow water vapor (the gas form of liquid water) to pass through them. However, care should be taken when selecting a more permeable coating to ensure that the service conditions are not beyond the range of the coating. The higher the permeability, the lower the resistance in preventing water or other chemicals from the outside environment from passing through the coating. It is the coating manufacturer's and the specifier's responsibility to select and furnish the coating with the right degree of " breathability" and "permeability rating" for the intended service (use) of the coated concrete.

What about Coating the Steel Rebar?

Coating the steel rebar in concrete is a special case. New rebar can be coated directly, before it is incorporated in a concrete structure being built. Conventional barrier and inhibitive coatings for steel, as well as special barrier coatings called powder coatings, can be used to protect new rebar.

Conclusion

Coatings can be used to add chemical resistance to concrete structures, such as the secondary containment you see surrounding fuel storage tanks. Coatings can also be used to make areas such as floors more resistant to abrasion and wear from foot and equipment traffic.

You don't have to know all of the details of how coatings protect concrete, but some knowledge will help you as you learn to prepare concrete surfaces and apply coatings to them.

For a complementary subscription of Journal of Protective Coatings & Linings courtesy of General Polymers, please email Ray Mutchler, JPCL publisher.