Poor bonding of the topping to the base course can be a primary reason for spalling. Bond coats can prevent this type of damage.
Portland Cement Association Poor bonding of the topping to the base course can be a primary reason for spalling. Bond coats can prevent this type of damage.

Q: In the August issue, you stated that new concrete does not stick well to existing concrete (Spying Out Bond Coat Problems, Concrete Surfaces, August 2011). From my experience, most contractors do not use bond coats anymore, as concrete adheres just fine to existing concrete. Most engineers would not recommend them either. Can you clarify?

A: You are correct that some contractors are choosing not to use bond coats for repair work, resurfacing, or new construction jobs. The article in the August issue was responding to a question about the proper selection and use of polymer-based bond coats.

Generally, the use of bond coats is a matter of preference. However, industry organizations such as the American Concrete Institute (ACI) recommend using bond coats to avoid potential future aesthetic and structural damage and to achieve a strong bond. The previous article was based on the recommendations of key industry organizations, as well as the results of our own testing.

In most application scenarios (unless the fresh concrete is already modified with an appropriate polymer), fresh, wet concrete does not bond well to existing dry concrete. Extensive testing has demonstrated that a stronger bond between fresh concrete and existing concrete can be created by using bond coats that use acrylic resins, styrene butadiene (SBR) latex, polyvinyl acetate (PVA), or a wide range of copolymers and epoxies. These types of bond coats are preferred, especially in northern and temperate climates where they are proven to sustain a more durable bond during repeated freeze/thaw cycles.

In fact, ACI 302.1R-04, section 11.8, “Guide for Concrete Floor and Slab Construction,” indicates that one of the primary reasons for spalling in new construction of deferred, two-course concrete floors where the base dries before the application of the top layer is poor bonding of the topping to the base course.

Bond coats can help prevent this kind of damage by strengthening the bond between the layers to ensure strong adhesion. Aside from new projects, bond coats (polymer modified or not) are especially useful for repair work where the older concrete is still structurally sound and worth preserving.

An important thing to keep in mind with bond coats is timing. If a contractor adds a new layer of concrete over another layer on the same day the original concrete was placed, a bond coat may not be needed. But any time after that initial application, a bond coat is a good choice.

Another key point to consider is surface preparation. Good surface preparation and advance cleaning is a necessary step with any kind of bonding activity.

No matter how a contractor chooses to bond concrete, there is an easy way to test to see if the bond is strong. Simply gently tap on the concrete patch using a hammer after at least 24 hours. If the sound you hear is a hollow echo, you know that you have a strong bond between the layers.1

In summary, good adhesion between the top/repair concrete and the base/original concrete substrate is critical, especially where a monolithic character is required for the transfer of heavy loads. A strong bond can be achieved through a combination of good workmanship, surface preparation, consolidation and curing, and a bond coat.

For more information, visit www.concrete.org.

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

1 www.sakrete.com/media-center/blog-detail.cfm/bp_alias/Bonding-to-Existing-Concrete