Peter Craig

Q: Under resilient floors, what is less likely to telegraph joints after moisture equilibration: semirigid epoxy or polyurea?

A: First let’s briefly describe the issue. When a concrete slab on ground dries out, it becomes drier near the top surface, which then shrinks more than the bottom causing the slab to curl upward at the joints. When an impermeable resilient floor is installed over the top of the slab, the moisture in the slab equilibrates so that it is the same from the top to the bottom. This causes the top to expand, taking out the curl and slightly closing the joints. If the joints in the concrete were filled prior to installation of the flooring, the closing up of the joint can cause the filler to bulge and push up a line in the surface of the resilient floor.

We asked a couple of experts in flooring and joint fillers for their thoughts on this. Scott Tarr, Concrete Engineering Specialists, replied that it is dependent on the properties of the specific filler materials. “Polyureas and epoxies are not necessarily two distinct groups in terms of hardness and elongation, and there is likely overlap depending on what two products you’re comparing. In general, though, polyurea is a little softer (still semirigid but just a little less rigid) than epoxy. It typically has more elongation but I’m not sure what is more easily bulged or extruded to ‘telegraph’ though the flooring.”

Scott Metzger, of Metzger/McGuire, agrees that the answer depends upon the properties of the individual products and the installation. “In our experience, we have seen polyurea joint fillers protrude or bulge slightly more than epoxy fillers, but the sample size for this experience is limited and our expectation would be that both product types would show the tendency comparably in a given application. Because there is such a wide range of formulations and cured properties for both product types, as well as multiple variables in installation conditions, which would be more or less likely to lead to protrusion—such as installation timing, joint spacing, and moisture in the slab at the time of covering—I don’t think it would be fair to circumstantially conclude that a given polyurea or epoxy is more likely to exhibit protrusion when installed under floor coverings.”