Sometimes there's a need to alter the surface of concrete even though it's basically structurally sound. Contractors may be asked to remove surface irregularities, high spots, or edge curling, either for aesthetic reasons or to facilitate vehicular and foot traffic. An owner may want to remove and replace deteriorated or unwanted coatings or finish floor coverings. It's often necessary to remove surface damage, scaling, or spalled concrete in order to do more extensive repair work. The goal may be to produce a uniform surface to accept a new coating or overlay.
In any of these instances, a wide variety of equipment is available to do the job, but the project's success will depend on making the right choice. To make this decision, you need to consider several factors.
- What is the depth and makeup of material to be removed? Is there tile or adhesive to contend with? Is there a coating or just plain concrete? If a coating exists, what kind is it?
- If you're working with plain concrete, what is its condition? If it's mostly sound, what is its estimated compressive strength? Is there rebar located near the surface?
- What is the new surface to be applied and to what thickness?
You also need to consider factors related to safety and environmental concerns. Is the work to be done inside a building? If so, will the building be occupied while the work is going on? What provision will be needed to control dust and noise? Is vibration an issue? Is it feasible to use a wet system, and if so, how will you dispose of waste? Answering these questions can help you decide what types of machines to use and estimate your rate of production.
In 1997, the International Concrete Repair Institute (ICRI), Des Plaines, Ill., first published “Guideline Number 03732: Selecting and Specifying Concrete Surface Preparation for Sealers, Coatings, and Polymer Overlays,” which was reapproved in 2002. The publication describes in detail various methods of surface preparation and when they should be used. It also recommends procedures to help contractors make the decisions. A hard copy and electronic version of the guidelines are available through the ICRI at www.icri.org.
Key to the guideline is ICRI's Concrete Surface Profile (CSP) classifications—a system of nine distinct textures ranging from CSP1 (nearly flat) to CSP9 (very rough) that are represented photographically in the printed version and also available in molded replicas. The guideline designates each CSP classification as a suitable base for specific coating types and thicknesses, and also describes the method(s) or equipment typically used to achieve the texture. These benchmark profiles are meant to be referenced in specifications, material data sheets, application guidelines, and contract documents to effectively communicate surface preparation requirements.
The coatings and thicknesses considered include sealers (0 to 3 mils), thin-film (4 to 10 mils), high-build (10 to 40 mils), self-leveling (50 mils to 1/8 inch), and polymer overlay (1/8 to ¼ inch). The surface preparation methods described include (from gentlest to most aggressive) detergent scrubbing, low-pressure water cleaning, acid etching, grinding, abrasive blasting, steel shotblasting, scarifying, needle scaling, high/ultra-high-pressure water jetting, scabbling, flame blasting, and milling/ rotomilling. Three of the more common methods are discussed in more detail in this article.
In most cases, there is some overlap in the methods used to achieve a particular surface profile. The ICRI guideline provides a series of checklists to help you evaluate the options. The general goal is to prioritize the performance criteria, then select the method that provides the optimum balance of performance, risk, and cost factors.
Grinding is the rotation of one or more abrading stones or discs applied under pressure at right angles to the concrete surface. It is used to reduce or smooth slight surface irregularities and to remove mineral deposits and thin coatings. It generally produces a smooth surface, but other methods may be combined with grinding to produce textured profiles of CSP 1 to 3.
Grinding equipment ranges from handheld grinders for smoothing isolated irregularities and slab edges to walk-behind machines weighing thousands of pounds. Grinders are available in electric, pneumatic, and gas models, with rotation speeds that vary from 1000 to 9000 rpm.
The grinding stone or disc is the consumable material, and these come in a wide range of sizes, configurations, and compositions, depending on the grinder used and the material removed.