Q: I just applied a new layer of concrete over an existing concrete surface using a bond coat in between. Two weeks later, the new concrete started to crack and separate. What could have caused this and how do we prevent it in the future?

A: There are several possible reasons for failure when joining two concrete surfaces using a bond coat. The most common are inadequate surface preparation and using the wrong type of bond coat. Bond coats can be very versatile, but no single product is right for every job. So let’s start with a quick review of bond coats.

As many contractors know, new unmodified concrete generally does not stick well to existing cured concrete, whether it has just cured or has been in service for many years. If the new cementitious layer will be less than 1 inch thick, contractors often will use polymer-modified overlays to provide the needed adhesion (see “Polymer-Modified Mortars”).

If the new concrete layer will be thicker than 1 or 2 inches, and both the overlay and the existing concrete surfaces are unmodified, a bond coat typically ensures adhesion between them. A bond coat is essentially a layer of “glue” for joining unmodified concrete to unmodified concrete. This can be new to old concrete, or new to new concrete.

Vertical and horizontal applications for bond coats in residential construction range from self-leveling floors for patios and basements to walkways, stairways, and a variety of decorative concrete surfaces. Commercial applications include cementitious mortar applied to block walls, bridge deck overlays, and repair patches on bridge pilings.

Two types of bond coats

Although bond coats can be based on a number of different polymer chemistries—acrylic, polyvinyl acetate (PVA), vinyl acetate/ethylene (VAE), styrene butadiene (SBR), or various copolymers—they are all defined by ASTM C1059 as either Type I or Type II. These are the most important differences for contractors to know. All bond coat products should be labeled clearly as ASTM C1059 Type I or Type II.

ASTM C1059 Type I bond coats are redispersible, or sometimes called re-emulsifiable. Redispersible bond coats usually are based on PVA homopolymers, and can be applied by spray, brush, or roller. They create a film over the concrete surface that contractors can allow to dry. Whenever the new overlay concrete is placed over the bond coat, the moisture in the concrete causes the bond coat to redisperse, or become active, and create a strong bond between the two concrete surfaces.

Some contractors on large jobs prefer redispersible bond coats because they allow them the flexibility of applying the new unmodified concrete at any time. But if too much time elapses between applying the redispersible bond coat and placing the overlay, loose dirt or other contaminants may accumulate on the bond coat surface and compromise the adhesion. In this case, a second cleaning may be necessary.

ASTM C1059 defines Type I bond coats as “restricted for use in interior work not subject to water immersion or high humidity.” If a Type I bond coat is inadvertently used for an outdoor application and it rains the next week, delamination may begin, causing visible cracks in the overlay. Walking on the surface of a delaminated overlay may feel hollow and cause cracks. Eventually, small pieces of overlay will break off.

An ASTM C1059 Type II bond coat “may be used for areas subject to high humidity or immersion in water and is suitable for use in other areas,” ACI states. Usually based on acrylic, EVA, or SBR or copolymers, Type II bond coats are not redispersible. They can be used in most applications and be applied by spray, broom, brush, or roller.

Although Type II bond coats are far more versatile, overlays should be poured shortly after the bond coat is applied, while they are still tacky.

Sometimes, Type II bond coats can be designed as an admixture with portland cement, water and sand to create a bonding slurry. In effect, these slurries are a form of modified mortar because the Type II bond coat adds polymer content to the mortar, giving it strong adhesion properties.

Proper preparation

But even with the proper bond coat, never overlook surface preparation. This begins with a thorough cleaning, typically a power wash, to remove loose dirt and other contaminants. It is especially important to remove release agents that may have been used to form the underlying concrete because they are designed to prevent adhesion.

Then follow the manufacturer’s guidelines. This could include acid etching, sandblasting, shotblasting, scarifying, or brush hammering. Never take a short cut or try a method the bond coat manufacturer does not recommend.

When bond coats are properly selected and applied, and surfaces are properly cleaned and prepared, there is no reason they should not provide decades of reliable service. Acrylic- and other polymer-based bond coats make concrete to concrete adhesion much stronger and more durable when done correctly.

Contributed by Dow Construction Chemicals. Visit www.dowconstructionchemicals.com for more information.