The Ins and Outs of Concrete Surface Prep

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 bonding seems 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.
Every project that requires leveling or patching has its own set of unique jobsite conditions. Questions to ask:

All of these and other conditions can vary from job to job. Each set of unique conditions has a very specific series of steps required to prepare the substrate prior to installing the patching or leveling compound and then, the finish flooring.
Everyone is looking for an edge. Make note of what is actually being said, and then call the technical department of the product you are using, look up their literature or on their website and verify the claims. You’ll find out that, in fact, you do have to do some prep with any product on the market today.
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.

A large amount of concrete in the United States is cured by using wax-based curing compounds. Patching and leveling compounds, along with flooring adhesives won’t bond to wax any more than they will to grease or oil. These types of materials must be removed from the concrete completely.

There are often chemicals applied to cure concrete slabs that are called “dissipating curing compounds.” Ultra Violet (UV) light and traffic over the surface of the slab will cause these materials to break down and disappear. However, since each one of these materials dissipates at different rates under a variety of jobsite conditions that can occur, you can never be absolutely sure that 100% of the chemical has dissipated, therefore light mechanical prep should be employed to remove the residues that remain.

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.
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.