It is not uncommon to see failures of materials bonded to concrete that are related to poor concrete preparation. There are many possible reasons for this occurring, such as taking shortcuts in preparation, trying to save money in the preparation effort, or inexperience. The bottom line is that adhesives need to achieve a firm bond to the substrate, one that will be able to withstand the rigors of stress exerted.
There are three areas where I find bonding to be a problem: existing adhesives; concrete sealers, curing and parting compounds; and a hard trowel finished (burnished) concrete.
Existing adhesives create all types of concerns ranging from discoloration of materials, to compatibility issues with the new adhesive, to the new adhesives ability to bond to the old adhesive. Cutback adhesive is an adhesive that is notorious for discoloration and is difficult to bond to. Existing multi-purpose adhesives can create problems with compatibility, and some of the old multi-purpose adhesives can cause the new adhesives to crystallize and turn to powder. A glue-down carpet adhesive might be left on the concrete, scraped down to a thin residue. The bond between this and the new adhesive will be too weak to withstand the traffic requirements for the material.
All too often the failure does not occur immediately and when it does, the blame is pointed at the adhesive manufacturer, flooring manufacturer and/or the installer. In fact, the failure has most likely been caused by the concrete treatment. I once saw a flooring material that was removed to reveal that the concrete sealer had de-bonded from the concrete surface about two years after the installation. This mess represented several thousand yards of material.
Hard Trowel Finished Concrete (burnished). Concrete is not supposed to be smooth and shiny. The ideal concrete surface should have a texture similar to fine sandpaper. When applying an adhesive over burnished concrete, make sure to exercise extreme caution. Urethane and epoxy adhesives need to achieve a good mechanical bond to work well over a burnished concrete. You’ll need to mechanically abrade the concrete surface to have success.
It is not uncommon to find bonding issues over a burnished concrete, especially if a topical concrete coating has been used. Pressure sensitive adhesives will work better over a burnished concrete, but a texture to the concrete will allow the adhesive to perform much better.
Flooring contractors who are completing an installation where an adhesive remover has been used must proceed with extreme caution. This is because it is difficult to know how well the clean-up process has been done. It is assumed, “If you start the installation you own it.” This is one area where many flooring contractors have been bitten. I know of a vct installation where the new tile adhesive contained 14 percent adhesive remover.
Mechanical removal of adhesives. The best method is to mechanically remove the top surface of the concrete. Shot blasting is the most acceptable method. The shot blasting will remove the existing adhesive and any residuals that might create a problem. Concrete Surface Profiles above a CSP-2 will require a skim coat of a cementitious underlayment or a self-leveling cementitious underlayment.