You can frequently trace the failure of a protective system on a concrete floor to a surface that was not suitable to accept the applied material in the first place. So to avoid premature failure, assess concrete surfaces for contamination or other problems before applying a coating.
Inspection Procedures Before Surface Preparation
Visual Inspection: Look for signs of concrete defects, physical damage, chemical damage, contamination, and excess moisture. Remove and replace the nondurable concrete to return the surface to the specified condition required for the coating application.
Concrete Cure: Check that the concrete was cured in the procedures described in ACI 308. Test the slab for surface tensile strength before beginning work. Surface preparation performed on insufficiently cured or low-strength concrete may create an excessively coarse surface profile or remove an excessive amount of concrete.
Concrete Defects: Create a drawing showing honeycombs, spalling, and any other surface damage from impact, abrasion, or corrosion of reinforcement. Also use the procedures described in NACE Standard RP03902 or ICRI 037303 to ensure that the concrete surface and/or subsurface is sound.
Contamination: Remove all materials that may affect the adhesion and performance of the coating, including dirt, oil, grease, chemicals, and existing incompatible coatings. You can use the methods described in NACE Publication 6G1915. These methods include visual examination, water drop (contact angle) measurement, pH testing, petrographic examination, and various instrumental analytical methods.
Moisture: Determine the rate of moisture migration through the concrete slab. Excess moisture will greatly reduce coating performance and cause problems such as blistering or adhesion loss. Measure the moisture vapor emission rate (MVER) in pounds per 1000 square feet per 24-hour period with the calcium chloride test (ASTM F 18696) or by using an electronic moisture meter that measures the percent of static moisture within the concrete substrate. Use the plastic sheet method (ASTM D 4263-837) to measure the moisture.
Proper surface preparation creates a concrete surface that is suitable for application and adhesion of the specified protective coating system. Contractors should:
- To remove all contamination, form-release agents, efflorescence, curing compounds, and existing coatings that are incompatible with the coating.
- Use vacuum cleaning, air blast cleaning, and water cleaning (ASTM D 42588) to remove dirt, loose materials, and/or dust. Use detergent water cleaning and steam cleaning to remove oils and grease.
- To remove protrusions such as from burrs, sharp edges, fins, and concrete spatter, existing coatings, laitance, and weak concrete. Use impact tool and power tool methods, including scarifying, planing, scrabbling, and rotary peening. Power tool methods include circular grinding, sanding, and wire brushing (ASTM D 4259.)
- But be careful, as impact tool methods may fracture concrete surfaces or cause microcracking. Power tool methods may not produce the required surface profile. They also may need to be followed by abrasive blasting or high-pressure waterjetting to produce a surface with adequate profile and porosity.
- To remove all unsound concrete, laitance, and concrete damaged by chemical exposure, and to produce a sound surface with adequate profile and porosity, use either dry abrasive blasting, wet abrasive blasting, vacuum-assisted abrasive blasting, or centrifugal shot blasting (ASTM D 4259); or use high-pressure water cleaning or waterjetting (NACE No. 5/SSPC-SP 1210 and ASTM D 4259).
- To extract organic contaminants, use flame cleaning often employing a propane torch or other heat source. Remember, high temperatures reduce concrete's strength or causes damage. Test surfaces prepared using flame methods for soundness and surface tensile strength. Repair surfaces that are unsound or low in tensile strength.
- To remove laitance and weak concrete and to provide a surface profile on horizontal concrete surfaces, use acid etching (ASTM D 426011 and NACE Standard RP089212). This method requires completely removing all reaction products and pH testing to ensure neutralization of the acid.
- Acid etching is not recommended where curing compounds or sealers have been used. Use it only where procedures for handling, containment, and disposal of the hazardous materials are in place. Do not use acid etching with hydrochloric acid where corrosion of metal in the concrete (rebar or metal fibers) is likely to occur.