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An overview of bonding agents
in decorative concrete
No matter what bonding agent is used, proper preparation of the substrate is essential for a successful, lasting bond.
by Jennifer G. Prokopy
Like acrylic resins, styrene butadiene (SBR) latex bonding agents are non-re-emulsifiable and are compatible with cementitious materials, and are sometimes used as an admixture when creating overlays. The materials are applied using a spray, brush, or roller. While they are used regularly for both indoor and outdoor applications, they are not recommended for areas that will receive significant water or chemical exposure.
Two kinds of polyvinyl acetate (PVA) bonding agents are used in decorative concrete applications: re-emulsifiable and non-re-emulsifiable.
 Re-emulsifiable PVAs produce a film over the substrate that can be allowed to dry, which later becomes soft again with the application of the overlay material, offering the benefit of significant advance application time. But the material is not recommended for outdoor use, where humidity or rain can trigger premature re-emulsification. Application can be done with a spray, brush or roller.
Tim Blankenship, division manager of Atlanta-based Precision Concrete Construction, stays away from re-emulsifiable PVAs. “There is an opportunity to contaminate the agent if you let it set before you apply (the overlay),” Blankenship says, noting that the bond may be compromised as a result. Others see the agent’s ability to re-emulsify as a benefit. “Its advantage is, you can let it totally dry,” says Kreielsheimer. On big jobs, this can be a boon to contractors, who “don’t have to work a few feet ahead of themselves.”
Non-re-emulsifiable PVAs offer good water resistance and wear well. They are applied with a spray, brush, broom or roller, and may be used in a wider variety of applications both indoors and outdoors.
Epoxy bonding agents contain no cement, and generally are more expensive than cementitious bonding agents, but offer excellent bonding strengths. “Government agencies specify them constantly,” says Kreielsheimer. With their incredibly strong bonds, epoxies generally are recommended for jobs that require significant repair work.
Epoxies must be applied to a dry substrate. Overlays should be applied while the epoxy bonding agent is still wet. Bob Pirro, branch manager of Structural Preservation Systems Inc.’s Hawthorne, N.J. office, says a common mistake contractors make when using epoxy bonding agents is allowing them to cure too long: “Then it acts as a de-bonding agent,” he says. “It becomes like glass and the fresh concrete doesn’t bond.”
There are even products that combine the best of both worlds. Pirro says his company regularly uses an epoxy-modified cementitious bonding agent, which also provides an anti-corrosion coating, for doing spall repair and other types of concrete repair work. The product allows his crew to coat rebar and concrete substrate at the same time.
Any discussion of bonding agents would be incomplete without addressing the importance of — and methods for — substrate preparation. According to Blankenship, the single biggest mistake a contractor can make is inadequate preparation: “There must be surface prep before the bonding agent. Not having it thoroughly cleaned, or if the preparation you do causes microfractures in the surface, can cause an overlayment to become delaminated.”
Surface preparation starts with cleaning the surface, clearing it of contaminants. A range of techniques is available for profiling the surface for bonding agent application, and Kreielsheimer says contractors should perform a needs analysis before making a decision on which material to use, asking questions like: “What is the substrate? What is the cementitious material you’re bonding to it? How much abuse will the finished surface receive?”
The surface can then be prepared using one of a variety of methods, including acid etching, sand blasting, shot blasting, scarifying, or bush hammering. In most cases, the rougher the prepared substrate, the better the bond. While contractors disagree on which method works best, all agree that preparation is critical to ensure adequate bond.
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Needs analysis and preparation for success
