Protective Coatings for Precast Concrete Structures

By Angus W. Stocking

Understand the basics of surface preparation, environmental conditions and coating resources for protecting precast concrete.

Editor’s Note: This article is intended to be a reference guide for entry-level production employees.

(NPCA file photo)

Concrete is one of the most versatile materials used in construction today, but that versatility comes at a cost. Concrete’s various uses also mean it’s exposed to many different factors that could cause damage over time. As a precaster, you must be aware of these factors and resourceful to properly protect and maintain your product.

Making a selection

Selecting and applying the proper protective treatment for precast concrete structures can be daunting. There are numerous options and each coating has different strengths, weaknesses and appropriate uses. There are also times when a coating may not be needed at all. But how are you to know when or when not to use a coating?

The first step is to recognize that a coating’s success relies heavily on two factors: the environment in which the concrete will be used and the application procedure. You must understand both to ensure proper protection and to maximize the concrete’s service life.

Environment

Degradation of concrete includes the following common mechanisms: acidic reactions, alkaline reactions, chloride-induced deterioration and sulfate deterioration. Each is produced by different environmental conditions. Concrete is also highly alkaline and contains moisture, which can prevent a coating from adhering to the surface, curing properly or performing as needed. For these reasons, a precast manufacturer may choose a coating from dozens of coating classes, including acrylics, polyureas, epoxies and urethanes. Each of these has subsets and, except for polyureas, come in solvent and water-based formulas that are alkaline resistant.

Scott Morris, vice president of Coopers Creek Chemical Corp., cited two environments in which it can be difficult to choose coatings that must be chemical, physical and thermal resistant: manufacturing plants and agricultural facilities.

“Manufacturing plants of all kinds produce aggressive discharge streams,” Morris explained. “The streams are often highly variable – coated precast structures might be contending with different chemical attacks from day to day. The concentrations of chemicals can vary too, from 10% acid to 30%. It’s a hard problem.”

In agricultural uses, the problem is the effluent, not variability. Morris feels that the challenges here are sometimes underestimated.

“Agricultural environments can be surprisingly aggressive,” he said. “The runoff from corn fields, for example, is very hard on coatings. The biological activity and the fertilizers used can create environments that can defeat many commonly used coatings.”

So what’s a precast manufacturer to do when producing a new structure for an unfamiliar environment? The first step is to find out why the coating is needed by requesting details about the environment before making a selection. Next, precasters can take advantage of resources from the National Precast Concrete Association and coating providers for additional insight and assistance.

Proper application

Morris said another important factor is concrete needs to be properly prepared before applying a coating. More specifically, he sees laitance as a common surface condition affecting coating adhesion.

“What happens is, after forms are pulled away from newly poured structures, the structure continues to cure and laitance forms,” he said. “That’s basically powdery limestone on the surface of the structure.”

In addition to laitance, factors that affect coating adherence include:

• “Eggshell,” a concrete imperfection related to laitance, can cause coatings to eventually pull away.
• Rods or snap ties removed during stripping can cause holes or protruding wire. These must be cut back and/or filled prior to coating application.
• Release agents can affect bonding and must be removed prior to coating.
• Curing agents’ effects are variable; some are compatible and some are not. Check with manufacturers and other users for more information.
• Glossy, hardened concrete cannot be coated successfully unless special methods are used.

Several methods of surface preparation are commonly used. The most common is dry abrasive blasting. Wet blasting scarifying and high-pressure water jetting also work in most cases. But grease, oil and other penetrating materials can be complicated. They cannot be removed with blasting and must be stripped before other preparation takes place.

The plant’s interior environment and safety also need to be considered when applying coatings. Tim Frazier, technical director for Concrete Sealants, said low temperatures dramatically affect the performance of water-based coatings. And where temperatures are warm, very high humidity can affect bonding. Morris said regulations in some states can also affect a precaster’s coating selection decision.

“Regulations are relatively tight in some states, California for example,” he said. “In the last 5-to-10 years a lot of coatings have been reformulated to contain less solvent. That means they’re safer, but thicker, but that’s not really a problem for precast manufacturers.”

Still, even in the absence of regulation, precasters should be careful and know their materials well.

“Lower percentages of volatile organic compounds is a trend,” Frazier said. “You have to know what’s being applied and have good practices.”

Know your variables

A great variety of concrete coatings are available, but identifying the right coating for a situation requires detailed knowledge of the environmental conditions your structure will face and the chemical expertise that is available. Applying coatings so that they adhere well is better understood, but still requires careful and thorough surface preparation that may vary according to regional conditions. Knowledge, experience and help from coating experts, as well as employee training, all play a critical role in ensuring the success of protective coating for precast concrete structures.

Angus W. Stocking, is a licensed land surveyor who has been writing about infrastructure since 2002.