Epoxy and other polymers are very useful in concrete construction. They are used as coatings, protective armor, bonding agents, for joinery and for many types of repairs to concrete elements and structures. In many concrete repair jobs, epoxy injection of cracks is the only practical long-term solution. However, when epoxy or epoxy mortar layers have been laminated to concrete in thicknesses of 1/4 inch or more distress sometimes has occurred. Distress is caused by differences in the shrinkage, thermal and mechanical properties of the two materials. These differences are much greater in some combination than in others. A single high thermal shock sometimes can degrade the composite, as can any of various cyclic changes over a period of time.
The change in volume or shape of an epoxy due to changes in temperature, wetting and drying, freezing and thawing, or load is likely to be very different from that of the concrete to which it is attached. These differences can cause high stresses at the bond line that may lead to failure. As each material attempts to accommodate the other, the composite may go into axial tension or compression and/or flexure and may create significant shear forces where they join. The materials may accommodate repeatedly to differences in volume change without any distress. However, if conditions are sufficiently adverse, one single stress may be enough to cause failure.
This article discusses examples of distress (failure of overlay, failure of end cover, and failure of end-block cover) as well as possible contributing causes of the distress suggested by those examples.