Fixing Cracks Before Overlay Application

An epoxy crack filler, feathered 2 inches on either side
of a crack.

Question:
What is the best method for fixing cracks in concrete prior to applying a decorative overlay?

Answer:
The term “fixing cracks” is a bit misleading since there is no way to “fix” cracks in concrete, only fill them. The proper term to use when treating cracks prior to placement of a decorative overlay is “bridging.” This is accomplished by filling or covering the crack with a rigid material so it becomes flush or slightly higher in elevation than the surrounding surface. (Avoid soft, flexible crack fillers.)

To prepare the crack for repair, use a crack-chaser diamond blade to remove any loose concrete and to straighten out the crack as much as possible. The repair material filling or covering the crack needs to bond tenaciously to the concrete surface and be applied so that it gradually feathers down to surface level. The crack filler also needs to bond well with the overlay, so it needs to have a rough profile. The three most common methods for bridging cracks prior to applying a decorative overlay are:

• -Rigid Epoxy Systems – These are usually two-part epoxies available in self-mixing cartridges or for mixing onsite. You apply them using an injection gun or putty knife to fill the crack to overflowing, and then you feather out the epoxy on either side of the crack anywhere from 2 to 5 inches (see photo). Placing mesh tape into the wet epoxy provides additional reinforcement. Once the epoxy begins to set, course sand is cast onto it to provide a rough bonding surface for the overlay.

• -Cement-Based Systems – Polymer-modified cement-based repair mortars are mixed onsite and applied with a trowel or putty knife (again, using the feathering technique). Multiple thin applications are usually required to avoid shrinkage cracking. To provide a good bonding surface for the overlay, roughen the surface by scratching it with a broom or trowel as it begins to set.

• -Crack Suppression Mats – These have been used successfully for years in the tile industry. Roll out the mat to cover the crack by 5 inches on either side (or cover the entire surface if cracking is extensive), and adhere the mat to the surface using a bonding primer or glue. This type of crack suppression method is also a great way to cover seams when applying decorative overlays over wood and other non-concrete surfaces.

No matter the type of crack treatment used, assuring that the crack is properly bridged is critical in eliminating shadowing and show-through of the original crack in your newly applied overlay.