Resilient Installation Corner: Concrete Surface Preparation
October 25, 2011
The majority of the failed installations of self-leveling underlayments and moisture mitigation treatments which have I have observed were directly a result of poor substrate preparation. The lack of proper surface preparation causes 90% or more of failures of overlays of underlayments or other bonding issues.
Photos 1–4 – CSP Profiles: For proper bonding of concrete overlays and coatings, it’s important to give surface the correct concrete surface profile, or CSP. To help contractors make this assessment, the International Concrete Repair Institute (ICRI) has developed benchmark guidelines for CSP-a measure of the average distance from the peaks of the surface to the valleys.
They range from CSP 1 (nearly flat) to CSP 9 (very rough). As a general rule, the thicker the overlay or topping, the more aggressive the profile needs to be.
A skim coat, for example, may require a light CSP of 2 to 4. For thicker self-leveling or polymer overlays, acceptable profiles generally range from CSP 4 to 6. Achieving surface profiles in the higher ranges often requires roughening by shotblasting or scarifying.
Photo 5 – Floor sanding or buffing: Most flooring contractors rely on sanding or buffing the concrete surface with a 16 to 24 grit open coat sandpaper. This works well for removing a light laitance, but does little to remove curing compounds or sealers. Curing compounds and concrete sealers are sometimes very hard and difficult to remove. They also do little to open up the pores of the concrete which is necessary for some types of flooring installations, especially those done with epoxies.
Photo 6 – Shotblasting: Most shotblasters use a wheel with paddle-type blades that propel steel shot at the surface at a high velocity using centrifugal force, fracturing off the surface layer of the concrete along with any dirt, coatings, paint or other contaminants. The entire process is confined in an enclosed blast chamber that recovers and separates the dust from the spent steel shot. The removed debris is sent to a separate dust collector while the reusable abrasive is recirculated. The depth of surface removal is controlled by a combination of factors, including the size of the shot and concentration used, the rate of machine travel, shot impact force and whether you need to strip away an existing coating.
Shotblasters are typically walk-behind or ride-on units, with blasting paths ranging from 4 to 32 inches and removal rates from about 200 to over 3,000 square feet per hour. Smaller models are good for working in tight areas, such as around equipment and obstructions and next to walls, and for preparing residential garage floors or balconies for decorative coatings. The most common power options are electric, diesel and gasoline.
Photo 7 - Concrete surface before and after: Shotblasters can perform surface preparation tasks ranging from very light etching to more aggressive removal of surface mortar down to coarse aggregate. They also are a cost-effective method for removing dirt, grime and chemical contaminants from large areas and for prepping substrates for self-leveling or polymer overlays, epoxy toppings and most coatings. Another big advantage of shotblasting is that it produces very little airborne dust or debris, making the method a good choice for floors in sensitive areas, such as food preparation facilities or manufacturing plants.
What shotblasters can’t do effectively is remove rubbery mastics or heavy elastomeric coatings, because the steel shot will just bounce off the surface.
Tips for Best Results When Shotblasting
The hardness of the concrete and the presence of a previous coating can affect production rates and the depth of material removal. When removing thick coatings, you may need to make multiple passes with the shotblaster or consider a more aggressive removal method, such as a scarifier.
Shotblasters can leave “cornrows” where successive passes overlap, and these paths may be visible if you plan to cover the prepared surface with a clear coating or thin overlay. Some larger shotblasting machines have horizontally fed rather than center-fed blast wheel configurations that distribute the shot more evenly and minimize the “cornrow” effect.
Smaller steel shot provides better coverage and higher production rates. Use the smallest steel shot size possible to achieve the desired results.
If the slab has areas of softer and harder concrete, vary the travel speed of the machine to get more consistent results. A slower speed tends to remove more material.
Shotblasting equipment is expensive, but is worthwhile to cut the costs of concrete preparation. Improper concrete preparation can be expensive as well.