Surface Preparation for Concrete and Related Products
Cementatious products are widely used where weight is not a factor, because of cost, strength, and inertness to the effects of weather. However, in industrial environments, they must sometimes be protected against acids, salts and alkali. Further modification is sometimes desired to achieve anti-skid properties or purely decorative effects.
Concrete and related materials consist mainly of hydrated calcium oxide and silicates (cement), along with sand and/or gravel to form a rigid matrix. Hydration occurs during the “cure” or the cement. Literally millions of microvoids per cubic inch develop as the water present reacts with the cement. The result is a porous material through which water vapor can penetrate with relative ease with little or no effect upon the structural integrity. The passage of water vapor goes undetected until a moisture barrier, such as a coating, is present. Then, if there is no other route of escape, the water vapor will cause blisters and loss of adhesion as it collects at the interface between the cement/mortar/concrete and the coating.
Blisters and the loss of adhesion are the major causes of coating failure on masonry surfaces. This is especially true of concrete floors if there is no effective water vapor barrier under the slab. Another major cause of failure is the lack of appreciation for the alkaline nature of cement. Most oil based coatings are attacked by this alkaline environment.
Proper surface preparation prior to coating is as important for concrete and related products as it is for metal. Examples of surface conditions which will interfere with coating performance are:
LAITANCE is partially hydrated cement at the surface, which is generally due to moisture loss prior to full cure. Laitance is the source of dusting seen on new concrete floors. It is very loosely adherent and prevents the coating from adhering to a sound surface underneath the dust.
EFFLORESCENCE is usually a white deposit of salts at the surface. These salts have usually been leached from the cement by water, and are left behind as the water reaches the surface and evaporates. If not removed efflorescence will cause adhesion failure and blisters due to osmotic pressure.
GLAZE is a smooth, glossy surface resulting from hard troweling or machine finishing. Some coatings will develop adhesion to glaze, but it is generally safer to remove it prior coating.
HARDENERS are solutions of calcium/magnesium silicates and lead/zinc fluorosilicates used to make the surface less permeable, and somewhat more chemically resistant. Surfaces so treated will be glossy, have a gray/brown color and be extremely hard. This treatment must be abraded to assure coating adhesion.
OILS AND/OR GREASE are generally present on floors that have been in service for some time. Often these will have penetrated deeply into the concrete and are among the most difficult surface defects to correct. They must be removed to achieve satisfactory adhesion.
FORM RELEASE AGENTS are low-cost coatings applied to forms for the purpose of achieving easy separation from the cured concrete. These agents contain waxes, oils and/or soaps, which contaminate the concrete and prevent adhesion. When it is known that the finished structure is to be coated, lacquer or plastic coated forms should be specified.
CURING COMPOUNDS are waxes or resinous solutions applied to fresh concrete to retard the loss of moisture prior to cure. Other methods of moisture control should be specified if subsequent coating is planned.
SURFACE HOLES AND VOIDS are not usually present on troweled surfaces or gunite, but are always present in poured concrete, despite careful compaction. These holes must be filled prior to coating to assure coating continuity.
Sometimes a “broom” finish is specified in order to attain mechanical adhesion of thick epoxy or polyester coatings. Permite coatings for concrete and masonry surfaces are designed to adhere to properly prepared, but relatively smooth surfaces, because a broom finish requires a thick film. For some service a thick film is necessary; but it is poor economics to require a thick film solely to achieve adhesion.
When the sole function of the coating is cosmetic, obtaining adhesion is the only requirement. Some imperfections due to holes, voids, or the texture of the finish, can be tolerated.
When present, oil and/or grease must be removed before further preparation can be done. Removal of these materials is accomplished through the use of solvents, alkali, detergent, steam cleaning, or some combination of these agents. In case of severe contamination, a specialist such as Oakite or Turco should be consulted.
Abrasive cleaning by Blastrac or other similar means is the preferred preparation. Care must be taken not to remove an excessive amount of surface. This procedure will open holes or voids where air pockets occurred or aggregate has been dislodged. These voids should be filled using a suitable epoxy or cementatious grout.
Acid etching is an acceptable alternate method in certain cases. Prepare a 10-15% solution of muriatic acid by mixing 1 part concentrated acid with 4 parts water.
Muriatic acid is corrosive and has an irritating odor. Goggles, rubber gloves and boots should be used when handling both the concentrated and diluted acid. Adequate ventilation should be provided. The acid vapors are also corrosive to metal parts. When these are present and cannot be protected, other etching solutions should be considered. Alternates are zinc chloride/phosphoric acid and sulfamic acid. A reliable source of information regarding these mixtures should be consulted before use.
To etch the surface, first flush with water, and then apply the acid solution with a stiff brush, scrubbing it into the surface. Allow this solution to remain on the surface until the frothing action ceases. The spent acid should not be allowed to dry on the surface; it should be flushed immediately with large amounts of clean water with scrubbing to assure complete removal of all waste products. This technique is best achieved by cleaning an area no more than 150-200 square feet at a time. A properly etched and rinsed surface will have the texture of medium sandpaper and test alkaline to litmus. If either of these conditions is not reached, the surface must either be treated with more acid, or rinsed more thoroughly.
Concrete treated with chemical hardeners is often encountered. Abrasive blast is the only effective method of preparation. Fill voids as discussed above.
Most preparation procedures will leave the surfaces wet, and valuable time is saved if coatings can be applied before the surface has dried. Most Permite coatings recommened cementatious products can be applied to wet surfaces. However, be sure to follow the manufacturer’s recommendation in all cases.
“Green” or fresh mortar and concrete usually contain an excess of water above that required to hydrate the cement. It has been added to facilitate handling. This excess water escapes into the atmosphere and other contiguous surfaces as the cure develops. If an impermeable coating were applied during this time, the water vapor pressure could cause failure before the coating developed adequate strength. For this reason, most coating manufacturers recommend a curing period of 28 days after pouring, and before coating, to permit the excess water to escape.
Further details pertaining to the preparation and coating of concrete may be obtained from the Portland Cement Association, Skokie, Illinois. Much useful information is contained in Proposed Manual of Coating Work for Nuclear Power Plants by ASTM Committee DOI-43.