Concrete

Surface Preparation of Concrete – NACE No. 6/SSPC-SP 13

Definitions
Curing (Concrete): Action taken to maintain moisture and temperature conditions in a freshly placed cementitious mixture to allow hydraulic cement hydration so that potential properties of the mixture may develop.
Curing Compound (Membrane Curing Compound): A liquid that can be applied as a coating to the surface of newly placed concrete to retard the loss of water.
Efflorescence: A white crystalline or powdery deposit on the surface of concrete. Efflorescence results from leaching of lime or calcium hydroxide out of a permeable concrete mass over time by water, followed by reaction with carbon dioxide and acidic pollutants.
Fin: A narrow linear projection on a formed concrete surface, resulting from mortar flowing into spaces in the form work.
Honeycomb: Voids left in concrete due to failure of the mortar to effectively fill the spaces among coarse aggregate particles.
Laitance: A thin, weak, brittle layer of cement and aggregate fines on a concrete surface. The amount of laitance is influenced by the type and amount of admixtures, the degree of working, and the amount of water in the concrete.
Soundness: A qualitative measure of the suitability of the concrete to perform as a solid substrate or base for a coating or patching material. Sound concrete substrates usually exhibit strength and cohesiveness without excessive voids or cracks.

Inspection Procedures Prior to Surface Preparation
Concrete shall be inspected prior to surface preparation to determine the condition of the concrete and to determine the appropriate method or combination of methods to be used for surface preparation to meet the requirements of the coating system to be applied. Inherent variations in surface conditions seen in walls and ceilings versus those in floors should be considered when choosing surface preparation methods and techniques. For example, walls and ceilings are much more likely than floors to contain surface air voids, fins, form-release agents, and honeycombs.

Surface Preparation
The objective of surface preparation is to produce a concrete surface that is suitable for application and adhesion of the specified protective coating system.
Protrusions such as from burrs, sharp edges, fins, and concrete spatter shall be removed during surface preparation.
Voids and other defects that are at or near the surface shall be exposed during surface preparation.

All concrete that is not sound shall be removed so that only sound concrete remains.
Concrete damaged by exposure to chemicals shall be removed so that only sound concrete remains.
All contamination, form-release agents, efflorescence, curing compounds, and existing coatings determined to be incompatible with the coating to be applied shall be removed.
The surface preparation method, or combination of methods, should be chosen based on the condition of the concrete and the requirements of the coating system to be applied.
All prepared concrete surfaces shall be repaired to the level required by the coating system in the intended service condition.

Surface Profile
If a specific surface profile is required for the performance of the coating system to be applied, the profile shall be specified in the procurement documents.
The surface profile of prepared concrete surfaces should be evaluated after cleaning and drying but prior to repairs or application of the coating.
The surface profile may be evaluated by comparing the profile of the prepared concrete surface with the profile of graded abrasive paper, by comparing the profile with the ICRI 310.2R (surface profile chips), or by another agreed-upon visual comparison.

Moisture Content
Prepared concrete surfaces should be tested for residual moisture after cleaning and drying but prior to the application of the coating.
ASTM D4263, ASTM F1869, or ASTM F21701 should be used to determine the residual moisture content in concrete.

Standard Guide for Painting Inspectors (Concrete and Masonry Substrates) – ASTM D6237

Preparation for Inspection
The guide describes the duties of the inspector and discusses inspection methods, both visual and instrumental, that can be used to determine that the specification requirements have been met by the painting contractor.
Before the start of the job, the inspector should be provided information by the project engineer from the official plans and specifications as to surface preparation requirements, coating type, thinner to be used, mixing ratios to be used, recommended application thickness, recommended primer, tie coat, topcoat, time between coats, method of application, ambient condition restrictions, and any special precautions to be followed. These details should be recorded in an inspector’s record book to eliminate any misunderstanding between the inspector and the contractor.

Coating Application
Film Defects—All coats should have nearly smooth surfaces relatively free of dry spray, overspray, fisheyes, pinholes, craters, bubbles, or other significant defects. Bleed through, insufficient hiding, skips and misses are not acceptable. Runs and sags should be brushed out during application or removed by sanding if the coating has cured. Abrasive, dirt, or other debris that becomes embedded in the coating film should be removed prior to the application of subsequent coats.
Rate of Application—Properly written specifications require certain minimum and maximum dry-film thicknesses for each coat. The requirements should be augmented with wet-film thickness figures calculated from the composition of the coating so that the proper dry film thickness is obtained. It is useful to check wet film thickness as the work progresses to determine reasonably well that the desired amount of coating is being applied. Later, when the films are dry, the inspector may make spot checks with a dry film measuring gage to ascertain acceptability of the coatings. If a destructive thickness gage is used, the cut must be repaired. Film thickness measurements are more informative than visual inspection, which may show only obvious nonuniform application.
Another technique used on concrete and masonry surfaces is to calculate the spreading rate of the coating for the specified thickness. The number of gallons of paint needed to coat a specific amount of surface area can then be calculated. Spray loss must be included in the calculation if a spray method is used.

Additional Considerations
Film Integrity—Each coat should be applied as a continuous film of uniform thickness free of holidays and pores. Any thin spots or areas missed in the application should be repainted and permitted to dry before the next coat is applied.
Recoat Time—Each coat should be dried throughout the full thickness of the film before application of the next succeeding coat. Coating is considered dry for recoating when the next coat can be applied without the development of any detrimental film irregularities such as wrinkling, lifting, or loss of adhesion of the previous coat. For most coatings the time to dry for recoat, even under optimum conditions varies with their composition and that of the subsequent coat. Some coatings may have maximum recoat times. The coating manufacturer’s written instructions should be consulted for recoat times.

Appendix
(Nonmandatory Information)
Inspection Checklist