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Preparing Concrete for Coating

Due to the unique chemical and physical nature of concrete surfaces, special considerations must be given to surface preparation and coatings application. This process begins with the selecting of coating materials. The coatings must have the desired chemical and physical properties necessary to perform in the anticipated exposure conditions. The coatings must also be compatible with concrete by having a demonstrated resistance to alkalinity. The preparation and treatment of the concrete prior to coating application is dictated by a number of factors, including

  • concrete placement;
  • curing and finishing processes;
  • concrete type and strength;
  • concrete age;
  • previous contamination of the concrete;
  • concrete condition, e.g., bug holes, exposed aggregate, rebar corrosion; and
  • the coatings/systems to be applied.

The methods used for surface preparation and for coating application vary considerably from case to case. Surface preparation methods may range from simple high pressure air blowdown to acid cleaning and abrasive blasting. The application methods may range from simple application by brush, roller, or spray, to complex applications by screed/squeegee or by hand lay-up of fiber-reinforced materials.

This article will concentrate on the preparation of the concrete surface. The nature and properties of concrete and their effects on surface conditions are discussed. Several typical surface conditions and effective methods of surface preparation are also examined. Monitoring the preparation work and selecting appropriate coatings are briefly described.

The Nature and Properties of Concrete

Concrete is mainly a mixture of Portland cement, water, and mineral aggregate, usually sand and gravel. Sometimes additives are used such as fly ash or pozzolans. The mixture cures and hardens by hydration. Water in the mix combines chemically with the cement to bind the aggregate into the rigid mass known as concrete. Although properly formulated and cured concrete is strong and rigid, it can be attacked both physically and chemically. Physical attack usually results in cracking or spalling. Concrete is very strong in compression but relatively weak in tension. It can and often does crack. Concrete is also fairly porous and subject to osmotic and capillary forces that absorb and release water. Absorbed water can freeze within the concrete and cause spalling and cracking.

Chemical attack can occur because concrete is alkaline and chemically reactive. It can be attacked by acids; some alkalies; numerous salt solutions; and organics such as fermenting liquids, sugars, and animal oils, especially if they contain free acids. Seawater will attack concrete. Corrosive solutions penetrating to the steel reinforcing rods may be particularly destructive because the large displacement of the corrosion products of the steel can cause cracking and spalling of the concrete. In addition to the general physical and chemical properties of concrete that make it subject to physical and chemical attack, several other factors influence the makeup of concrete and therefore must be considered before selecting a method of surface preparation. How the concrete will be used (e.g., as structural concrete or for floors), the method used to place the concrete, and the additives that may be present either on the concrete surface or incorporated into it all will affect the strength and the surface condition of the concrete. A discussion of structural concrete, concrete for flooring, and the surface conditions that accompany each follows.

Structural Concrete

Structural concrete for walls, piers, tanks, and machinery pads is usually poured and shaped through the use of wood or steel forms. When vertical pours are made, such as for walls, the concrete must be vibrated to consolidate the mixture and eliminate or reduce the number of voids. While vibration is necessary to consolidate the concrete, it can also cause water and air bubbles to move out to the form face, resulting in tiny voids or holes in the concrete surface. A very high concentration of large rebars in the walls can cause concrete separation and voids because the concrete mix tends to separate as it flows down the rebar. The result can be severe honeycombing.

The vibration causes fine particles to move to the surface (called bleedwater), which often results in a deposit of unreacted cement gels at the concrete-form interface. This material is called laitance, and it is usually in the form of a white powder on the surface and a very weak layer of fines extending into the concrete surface 1 /16 in. or more. Because paints or coating applied over laitance invariably fail, it is essential that this layer be removed during the surface preparation. Most form work is built using commercially available plywood, steel, or even plastic form sections that can be keyed together. Occasionally, however, plywood or planks may be butted together and nailed in place. If the boards do not fit tightly together, or if the joints are loose or warped, concrete can penetrate the cracks and harden, resulting in concrete fins. These fins and other sharp projections must be removed after the forms are stripped and, preferably, before the concrete cures.

Often, when raw board forms are used, particularly without a release agent, the concrete will stick to the board and the forms must be pried off the hardened concrete. When this occurs, wood slivers, sometimes of considerable size, can be left embedded in the concrete. These must be removed, and the area must be repaired and smoothed.

Another prevalent concrete imperfection is called egg shell. Laitance may often form a very thin cover over pits or air pockets in the concrete. When the forms are first removed, the surface may look continuous, with only an occasional small hole. Poking a hole with a rod or even the finger often reveals that it is a sizeable hole almost completely covered with an egg shell. Such areas must be opened up and patched prior to coating, particularly if they are very large or deep.

Because of the weight of the concrete, forms must be braced to resist bowing or distortion. Forms can be internally braced or, as is usually the case, metal tie rods or tie wires can be used to tie the inner and outer forms together. These rods, or snap ties, as some types are called, are designed to be pulled out or broken off after the forms are stripped; but they leave holes or small craters. Resulting holes must be filled. The tie wires protruding from the surface must be cut back and the cavity filled.

Wood, steel, or even plastic forms are usually coated with a release agent to prevent concrete from sticking to the forms. The release agent can wear off after a number of pours, and the forms must be recoated. Form release agents may be organic coatings, waxes, or in some cases, oil (sprayed on steel forms). Release agents that remain on concrete must be removed because most will prevent proper bonding of coatings.

After the forms have been stripped, a sprayed-on concrete curing agent is sometimes applied. The purpose of this is to form a moisture barrier over the concrete to promote curing of the concrete by preventing premature evaporation of the water in the mix. Some curing agents may be compatible with the coatings being used; however, many are not, and a determination must be made whether the curing agent can be allowed to remain or must be removed prior to coating.

Concrete for Floors

Floors are generally poured into limiting forms, and the concrete is spread and trowelled smooth. Poured concrete floors can present different problems. Overtrowelling by steel or magnesium hand trowels or machine trowelling can result in an extremely smooth, hard, slick surface with little or no tooth for a coating. A broom finished surface, where a stiff bristle hand broom is drawn across the partially hardened concrete (Fig. 3), leaves a much better surface for coating, but even this should be checked for laitance. A wood float finish usually has a much better texture for coatings than a steel trowelled finish.

During the pouring of large concrete floors, shrinkage cracking may appear, often in a random pattern over the entire surface. Some cracking is practically inevitable; however, the judicious and planned use of control joints induces a pattern of cracking that can be tolerated by coatings or surfacers.

After they are finished, concrete floors are often treated with clear concrete sealers to control dust, or with concrete hardeners to improve abrasion resistance. Various epoxies and acrylics are used as sealers, and silicate solutions are used as hardeners. Where coatings are applied to the concrete, the specifications should indicate that no sealers or concrete hardeners will be used; however, this does not necessarily assure that they will not be used.

Concrete hardeners are usually sodium silicate solutions or metallic fluorosilicates. Where hardeners have been used, the concrete will usually appear glossy and may be a greyish brown color. The surface generally cannot be scratched with a coin. Hardened concrete cannot be successfully coated unless special methods are used to prepare the surface.

Age of the Concrete

For both structural concrete and concrete for flooring, the age of the concrete, including cure time and the time between cure and coating, also affects surface condition. The likelihood of additional moisture and chemical contamination of the concrete increases with age.

It is generally accepted that concrete should be allowed to cure a minimum of 28 days before any coating materials are applied to it, with the exception of cementitious coatings that water cure together with the concrete. This cure duration is based on the time it normally takes for the concrete to achieve sufficient physical strength to allow various trades to perform their work without damaging the concrete. Although it is a civil/structural requirement, the 28-day cure period has been generally accepted as a limiting factor for the coating of concrete as well. Even after the 28-day waiting period, moisture may still be present in the concrete. All initial water in the concrete mix may not have fully reacted, or the concrete may have absorbed additional water from rain or other sources. This water can migrate to the surface and affect the adhesion of coatings.

Electronic instruments are available to check the moisture content of concrete; however, the method most often used in coatings work is to place a one-foot square piece of visqueen or polyethylene plastic sheeting on the concrete and seal it around the edges with masking or duct tape. Any water evaporating from the concrete will condense on the back side of the sheet and will be clearly visible when the sheet is removed 24 hours later.

The longer concrete remains before coating, the greater the possibility of contamination by grease or oil. Grease and similar products usually remain on the surface and can be removed with minimum difficulty; however, oil may penetrate the concrete to a point where surface cleaning agents are ineffective. Where this is the case, the oil may have to be boiled out by steam cleaning or the contaminated areas may have to be chipped out to the depth of the oil penetration and the area repaired.

Cement spatter is another problem that can increase with time. With other areas being poured, fresh concrete may slop onto the previously poured concrete and harden into rough, irregular masses that must be removed before coating.

Foreign objects can often slip into the form prior to pouring and may not become evident until after the forms are stripped. If fully embedded, the foreign object does not create a coating problem. But a piece of rope on the surface disappearing into the concrete is a problem. It must be removed, usually by chipping, and the concrete must be restored. Wood blocks, hard hats, small animals, electric cords, light bulbs, and tools are often discovered in newly poured concrete after the forms have been stripped.

Efflorescence is more likely to be found on concrete that has been in place for a while. Concrete contains water-soluble salts. As water from the interior of the concrete migrates to the surface and evaporates, salts are deposited on the surface, usually as a white stain. Efflorescence can occur with concrete, brick, or concrete block construction. It can be removed with acid etching. The best way to prevent its recurrence is with adequate waterproofing.

Walls or other structures made from concrete block require only minimal aging prior to coating because the block has been steam-cured when manufactured, and only the mortar requires curing. Other examples of concrete cured during manufacture include most precast concrete items and concrete pipe. The most important requirement when coating the concrete block is that it be clean and dry. Precast concrete may require patching and roughening.

It can be seen from the preceding discussion that concrete presents a much more varied surface than carbon steel when surface preparation for coating has to be considered. Both the physical and chemical characteristics of the concrete must be considered. Both chemical and mechanical means of surface preparation may be required. The following discussion will cover some typical concrete surface conditions encountered as well as the most effective methods of preparing those surfaces.

Typical Surface Conditions and Appropriate Preparation Techniques

Because dust, dirt, cement spatter, oil, grease, form release agents, and possibly, curing compounds must be removed from the concrete prior to.painting, cleaning is the first step to be accomplished. This may be done by a combination of methods, including sweeping, vacuuming, air-blast cleaning, water hosing, steam cleaning, high pressure water jetting, detergent washing, or wire brushing. Depending on the coating or surfacer system to be applied, one or more of these steps may be necessary.

Usually, the repair of concrete imperfections will precede other surface preparation because it is best to repair rock pockets, honeycomb, form tie holes, snap tie cavities, and similar voids while the concrete is still green. In this way, both the patch and the base concrete can cure together. Any patch using cementitious materials must be cured the same as the surrounding concrete.

Heavy build surfacer systems will normally fill holes up to 0.5 in. in any direction and, with some systems, up to 0.75 in. One-hundred-percent solids epoxy grouts are recommended for patching large holes or cracks in cured concrete. Cementitious patching grouts, when used on cured concrete, tend to lose water to the cured concrete, which can result in a very weak patch. Dry pack cement has a tendency to shrink, so repairs using this type of grout should be limited in size, and the cure should be carefully controlled. If chemical cleaners are used, provisions should be made to prevent harmful cleaning solutions and rinse water from entering the plant drainage system. After being cleaned, the surface should be neutralized, if necessary, and then thoroughly flushed with clean water to remove cleaning residues and surface contaminants.

In most cases, surfaces that are washed or rinsed with water must be allowed to dry thoroughly before being coated. Some exceptions to this rule are cementitious paints and water-borne coatings. Good ventilation in the work area will promote proper drying. If torches are used to accelerate drying, they must be used very carefully since a concentration of heat on a small area of concrete may cause sudden expansion of vapor in the concrete and cause it to pop and spall.

Sacking is a term used to describe the application of a thin layer of grout made from Portland cement or a mix of Portland cement and sand in proportions of approximately 1:2. The grout is rubbed vigorously into the green concrete by a cork float or stone to fill small voids. The excess is removed by rubbing with a burlap sack or a glove. The grout must be kept moist or its adhesion will be questionable. This method is generally not recommended where coatings are to be used because of the difficulty of properly controlling the process. There is a chance that coating failure will result from this method of surface preparation since it is difficult to keep the newly applied grout sufficiently moist to assure proper bonding to the existing concrete. However, a properly sacked concrete surface will provide an excellent finish for coating work.

Laitance, glaze, incompatible form release agents, curing compounds, and similar materials that cannot be removed by simple cleaning methods may require removal by other means. Typical methods used to remove such.contaminants may include wet or dry abrasive blasting, scarifying, or occasionally, high pressure water jetting. The most common method is dry abrasive blasting.

Prior to being abrasive blasted, the concrete must be free of oil and grease or other penetrating materials. Oil and grease deposits cannot be removed from concrete by blasting any more than they can be removed from a steel surface by blasting. The contaminants will merely be driven deeper into the concrete by blasting. Because solvents may carry the oil deeper into the concrete, detergents or emulsifying agents are recommended for chemical cleaning prior to abrasive blasting. As in the blasting of steel, the air source should be checked periodically for the presence of oil.

Blasting concrete can produce clouds of dust. For this reason, water-entrained, wet, or vapor blasting is sometimes used. It is more practical to use these methods on concrete surfaces than on steel surfaces since no corrosion inhibitors are needed for concrete surfaces. Sweep blasting is usually sufficient to remove most contaminants; however, since concrete is not homogeneous, care must be taken to avoid overblasting and gouging. As stated earlier, concrete should be cured sufficiently before abrasive blasting is performed, so that the concrete surface is not damaged by the surface preparation.

Another method of abrading concrete surfaces is with the use of scarifying equipment. Scarification may be accomplished by rotary impact, vertical impact, or circular grinding, depending on the type of equipment used. Since each type of machine is capable of producing only a limited range of textures or profiles, it should be determined in advance that the machine to be used will produce an acceptable texture. It is also critical that the equipment not damage the concrete surface, such as can happen with large, blunt needles in needle gunning.

Scarification equipment is capable of removing laitance, glaze, efflorescence, and incompatible curing compounds, as well as fins and sharp projections. (However, scarification is time-consuming, especially in hard-to-get-at places, and caution is needed because overscarification can cause an extremely rough surface.) The machines are often bulky, and although some are self-propelled, their use is normally confined to horizontal surfaces.

Scarification can be effective in restoring old concrete floors that have been severely attacked by corrosive chemicals. Usually, the disintegrated concrete is removed by scarification down to sound concrete. Then, a new surface is poured to bring the floor up to its original grade. Epoxy bonding agents are often used to assure a bond between the old and new concrete. After the concrete is repaired, the usual methods of surface preparation are employed prior to coating.

Acid etching of horizontal concrete surfaces using hydrochloric (muriatic) acid, sulfamic acid, phosphoric acid, or citric acid creates a surface texture.or tooth suitable for the adhesion of coatings to smooth concrete floors or slabs. Acid etching on vertical concrete surfaces is neither practical nor recommended.

A concrete surface to be acid-etched must be free of grease, oil, and similar contaminants. If present, they will insulate the concrete from the acid. The concentration of the acid solution varies, depending on the concrete texture and degree of etching needed. Hardened or very slick steel-trowelled floors, for example, may require a higher concentration of acid to effectively break the surface.

When acid is being used, care must be taken to protect the operator from both the liquid and the fumes. Proper safety precautions must be exercised. Many nuclear power plants are using citric acid to reduce some of the hazard. The area is normally marked off in sections, and the acid solution is applied and allowed to bubble. Areas not showing bubbling of the acid indicate some contaminant on the surface that prevents contact of the concrete by the acid. Some very dense, smooth surfaces may need more than 1 application of acid.

The most important requirement in acid etching is surface cleaning after the acid etch. The spent acid, together with the salts formed by the reaction, must be completely removed by scrubbing with a stiff-bristle broom and copious water rinsing. The final rinse can be checked for pH before the surface is allowed to dry to verify that all acid residues have been removed. A properly etched surface should have the texture of fine to medium grit sandpaper.

Where stainless steel pipe, brackets, or similar items might be exposed to the acid, such as in a nuclear plant, hydrochloric acid cannot be used. Citric, sulfamic, and phosphoric acids contain no chlorides and are permissible. Citric acid has the advantage of being biodegradable and in most locations can be flushed down the drain. Citric acid is available in powder form and is mixed to approximately a 20 percent water solution. Citric, sulfamic, and phosphoric acids react much more slowly than hydrochloric acid.

ASTM has developed a series of standards, listed below, for preparing concrete surfaces.

  • ASTM D 4258, Standard Practice for Surface Cleaning Concrete for Coating
  • ASTM D 4259, Standard Practice for Abrading Concrete
  • ASTM D 4260, Standard Practice for Acid Etching Concrete
  • ASTM D 4261, Standard Practice for Surface Cleaning Concrete Unit Masonry for Coating
  • ASTM D 4262, Test Method for pH of Chemically Cleaned or Etched Concrete Surfaces
  • ASTM D 4263, Test Method for Indicating Moisture by the Plastic Sheet Method.

Monitoring Prep Work

All phases of the repair and preparation of the concrete need to be monitored by knowledgeable coatings personnel. Hold points need to be established to determine what work needs to be done, whether that work has been done, and whether the results are adequate. Typical inspection points could be

  • before starting the work;
  • after initial cleaning of the concrete;
  • after repairing or filling the concrete;
  • after blasting, scarifying, or acid etching; and
  • immediately before applying the coating.

Coating Selection

While the success of any coating applied to a concrete surface is largely dependent upon the thoroughness of the surface preparation, coating material selection and application are equally critical. Proper attention to surface preparation will go a long way to assure a successful coating job only if coatings with a proven compatibility with the alkaline nature of concrete are used. Some examples of coating materials compatible with concrete are epoxies, acrylics, polyvinyl acetate latexes, chlorinated rubbers, and some vinyl esters. Coatings such as alkyds and oil-based paints should not be used directly over concrete. Some polyesters are questionable, whereas some epoxy esters are the work horses of concrete painting. Actual material selection should be made only by persons knowledgeable in the special requirements for concrete.

Conclusion

Each step taken during surface preparation is critical to the success of any coating applied to concrete. The general physical and chemical properties of concrete must be considered first, followed by consideration of the concrete's use, placement, curing and finishing processes, age, and degree of contamination. Techniques for preparing surfaces can include a variety of cleaning and repair methods, often used in combination. Once appropriate surface preparation techniques have been determined and properly implemented, the coating work can be carried out with the confidence that a successful application can be attained.