Using Epoxy to Fill Seams

Question:
Can I use epoxy to fill the seams where two precast countertop slabs abut?

Answer:
Seams are often necessary in larger countertop slabs to make it possible to manufacture, transport and install them. Seams serve another important function as isolation joints, allowing movement at key areas where cracking would otherwise occur.

Some of your clients may object to the visual presence of a seam, even if it’s only 1/16 inch wide or less. An advantage of an epoxy filler is that it can be color matched to the concrete and finished flush with the surrounding surface. Epoxies can be tinted with finely powdered stone, mineral admixtures and dry concrete pigments, as well as pigments specifically for use in epoxy. While not completely invisible, epoxy seams are far less apparent than seams filled with flexible, sanded tile caulk.

Typically epoxy is the adhesive of choice in the granite industry because of its reliability, availability and the option of selecting a thick-bodied or flowable viscosity. However, epoxy is also a very powerful adhesive, and problems can arise if it’s used to fill a seam located in an area with significant movement, such as near a sink. While the epoxy itself is very strong, the concrete it is bonded to is not. Concrete has very low tensile strength. If there is movement or flexing, a crack will occur in the concrete, often just outside the epoxy seam. Essentially the crack is a new isolation joint that formed where the flexural stresses were highest. Although this doesn’t compromise the strength or integrity of the countertop, clients will often object to the appearance.

The key to eliminating these kinds of cracks is to locate flexible isolation joints in those areas, such as filling the seam with flexible tile caulk, rather than an epoxy.