Concrete surface prior to shot blasting.

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.

Concrete sealer on the underside of the adhesive.

Concrete Sealers, Curing and Parting Compounds.Topical concrete treatments are a major concern to the flooring industry. Concrete sealers are not designed to be a substrate beneath an adhesive, but to keep a concrete walking surface from dusting. Concrete curing compounds are placed on concrete to hold moisture in the concrete during the hydration process. Unfortunately, the curing compounds, which are supposed to either decompose through oxidization or ultraviolet exposure, rarely comes off as intended, especially in areas of low foot traffic.

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.

Surfaces of the concrete before and after a shot blasting shows how clean the concrete is after one pass by the shot blasting equipment.

Another instance I encountered was a curing compound that was applied so heavy by the concrete contractor that there was a residual solvent still present one year later. The residual solvent was attacking the adhesive that was used to install the flooring 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.

Shot blasting equipment in use.

Adhesive Removers.I am not a fan of adhesive removers because most installers do not pay enough attention to detail on the clean-up necessary for use of adhesive removers. Adhesive removers do not know the difference between old adhesive and new adhesives. And when a spot is missed during the clean-up process there will be a failure.

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.