Preparing Concrete for Tiling or Top Dressing
Provided courtesy the Tile Professor
Preparation is key!
If you are going to tile over concrete or resurface the concrete with a concrete dressing or self leveling product, it is very important the concrete surface be properly prepared. This could involve anything from a simple vacuuming to remove dust up to physical grinding or blasting of the concrete surface.
If the surface is not clean or solid the tile and/or the resurfacing material will not bond properly. By clean it is meant the surface is free of dust, oil, grease, wax, sealers, paint, curing compounds and latex and gypsum products. The preparation method you use is dependent upon the condition of the surface and the type contaminant on the surface. This is not an area to do the job on the cheap or to take shortcuts.
New concrete requires special treatment...
New concrete must cure a minimum of 28 days before either tiling or putting a dressing onto the new concrete. There are two reasons for this:
1) As the concrete cures, drying and shrinkage cracks
could occur. These cracks may eventually show up in tile or dressing applied
before the 28 day curing period.
Moisture can also be present in aged concrete if the concrete is subjected to constant exposure to water and/or high humidity and poor ventilation does not permit the moisture to evaporate. If you suspect these conditions exist, improve the ventilation by opening doors and windows and blowing air over the concrete area with a fan.
If possible, avoid using heated forced air such as heated air from a torpedo heater. If you do use one, be very sure to ventilate the room... not only for your own safety, but also to prevent the carbon dioxide emissions from the heater from reacting with the calcium hydroxide in fresh concrete. If this piece of chemistry occurs, a layer of calcium carbonate will form on the concrete surface. You must mechanically remove this layer.
Non-chemical Methods for Preparing Concrete Substrates
Mechanical methods are the best way to remove contamination from the surface of concrete. This method removes the contaminants and leaves the surface clean and solid. Mechanical methods include: grinding, sandblasting, shotblasting, chiseling, scarifying and high pressure water blasting.
For large areas, shotblasting is the best method for removing surface contaminants from concrete. Steel shot is blasted from a machine hitting the surface at high pressure and in high concentration. This method quickly removes sealers, coatings, and other methods in an efficient manner. Shotblasting only cleans the surface. It does not smooth the surface or effectively loosen chipped or broken concrete.
Grinding also effectively removes contaminants, but is best used in smaller areas. Grinders are available with diamond tipped blades. These do an excellent job. For homeowners, buying a grinder is probably out of the question. A small hand held grinder or buffer with an abrasive head attachment works well for small areas of a floor and are practical for use by homeowners. Always wear safety glasses and a dust mask.
If you have a large area to do (garage or basement floor, outside patio area), you should contact a local flooring store or professional to do the job for you. The cost is moderate, and the job will be done properly and surely with less hassle on your part. If the grinding is inside, be very sure the professional uses a grinding machine with a vacuum attachment to suck up the enormous amount of dust generated. Also, be sure the area being ground is isolated from other rooms in the building by plastic sheeting.
If the concrete is broken or unsound in some way, chiseling or scarifying is the method of choice to clean up the unsound areas. Hand chisels or hammer drills with chisel blade attachments work well in small areas, but for large areas pneumatic chiselers are the only practical way to go. These upright devices can be rented at most rental supply centers. Small chiseled out areas can be repaired with ready mix concrete patch. Large area should be repaired by mixing together portland cement, sand and water in proper proportions. Unless you want a rough surface, use commercial grade fine sand. Remember that concrete needs 28 days to cure before applying a floor covering or surface dressing over it.
Sandblasting and high pressure water blasting are effective for exterior areas. Do not mix in chemical cleaners with the water. Allow the high pressure of the water do the cleaning. Sandblasting is messy and creates a lot of airborne particles, so check with local codes to see if it is permitted in your area. Also, check with local homeowners to tell them what you are doing. Leave sandblasting up to professionals.
Using Chemical Cleaners? Not always the best choice!
If you are only interested in cleaning the concrete and not going to add a floor covering or surface dressing, then cleaning the concrete with typical acid cleaners is fine. Follow the mixing directions on the manufacturer's package and apply following the directions. It is important that after the cleaners have done their work, you thoroughly rinse off the cleaners. And pre-plan where the rinse will go. If it goes into your lawn, you may find some unsightly brown areas. Always wear safety glasses and gloves. Keep in mind that acid cleaners are good at etching the concrete s surface, but do not effectively remove grease or oil from the floor. Solvents will remove these type contaminants. But as will be discussed in the next paragraph, do not use solvents if you intend to cover the concrete.
Cleaning concrete to prepare it for a topping is difficult. Most failures of concrete coverings are due to the surface not being cleaned or cleaned improperly. How many epoxy painted concrete garage floors have you seen where the paint is coming loose? Many of these are due to cleaning the surface with acid etching cleaners or solvents. But if they clean so well, why shouldn't they be used where the concrete is receiving a covering? First, all types of solvents should be completely avoided. Solvents clean off grease and oil, but they also migrate into the pores of the concrete, bringing the grease and/or oil with them. It is impossible to avoid this. Eventually, the grease and oil will migrate back to the surface and weaken the bond between the concrete and floor covering. Failure of the concrete covering is inevitable. As stated in previous paragraphs, mechanical methods are the only effective ways of removing these contaminants.
Mild chemical cleaners diluted with water can be used if they are thoroughly rinsed off and the concrete allowed to dry for several days in warm to hot weather. Because the cleaners are not strong, the concrete may have to be washed several times to effectively clean them. Once again: rinse very thoroughly and allow to completely dry before proceeding to apply any type topping. Another danger of using strong acid based cleaners is they migrate into the pores of the concrete and can weaken the concrete. After all, they clean by chemically attacking the concrete.
Cutback adhesives are asphalt containing materials that were once very commonly used to adhere vinyl floor coverings. Their removal from concrete is challenging and can be dangerous. But if you want to cover a concrete floor where cut back adhesives were once used, the bulk of the adhesive must be removed. If not, the adhesive can bleed through the new floor dressing or adversely affect the new adhesive being used.
The problem with mechanically removing cutback adhesives is that they may contain asbestos. Do not attempt to grind off or sand these adhesives. The dust can be dangerous. If these adhesives must be removed, check with government agencies to get guidance. It is beyond the scope of this web site to explain the proper procedures to follow to remove these adhesives. It is recommended you read ASTM F710 and contact the Resilient Floor Covering Institute to get guidance. The RFCI s Recommended Work Procedures for Resilient Floor Coverings contains good information on removing cutback adhesive.
Homeowners should leave the removal of cutback adhesive to trained and certified professionals.
For more assistance you can contact the Tile Council of America at 803- 646-4021.
Cracks in the concrete floor should be repaired or the crack may telegraph up into the new floor covering. Small hairline cracks are generally not troublesome. Most of these occurred as the concrete was drying and curing after being poured. But cracks 1/16 or greater should be repaired with a concrete floor patch. Be sure to push the repair compound completely down into the crack. And always allow the patched area to dry thoroughly before proceeding to apply a covering of any kind.
Larger, dormant cracks (1/4 or larger) should be thoroughly cleaned out and loose pieces of concrete removed before repairing. Any cement patching product can be used to repair these larger cracks, although some of the best products are from Ardex, www.ardex.com. They manufacture a wide array of concrete repair products.
If the cracks are still alive, there is a structural problem existing that must be addressed before proceeding to repair the crack(s). Consult a professional engineer or contractor if you find this to be the case. As with removing cutback adhesive, repairs of this kind must be left to a professional.
Crack Isolation Membranes
For extra insurance to minimize the possibility of cracks telescoping up into your tile floor, you can use what are known as crack isolation or anti-fracture membranes. These membranes are thin fabrics that are applied over the length of the crack. They are adhered to the floor by either mortar or a flexible adhesive. The floor covering is then applied as usual over these membranes. These materials are intended to be used over narrow cracks no more than 1/8 wide. What happens is that any minor movement in the cracked area is absorbed by the membrane. Another way of saying it is that concrete movement caused by the crack slides under the membrane. The membrane with the floor covering on top of it stays in place. So the floor covering is isolated from the movement and does not crack. These crack isolation systems work well for small in-plane movements caused by the presence of cracks. They will not prevent cracks in your tile or grout if the overall floor movement is 1/4 or greater. Nor will they prevent cracks caused by vertical movement of the crack; that is, where one side of the crack is higher than the other side of the crack.
Begin as usual by cleaning the area of dust, loose concrete, oil, grease, sealers, waxes and paints. Cut a piece of the membrane 6-8 wide and 6 longer than the cracked area. With a trowel spread the adhesive the length of the crack and 6 on either side of the crack. You do not need a lot of adhesive, so you can use a flat blade on a trowel or scraper to apply the adhesive. Lay the fabric over this area and press it into the adhesive and smooth it out with the flat side of a trowel. Press it down good so the membrane is as flat as possible. Any lumps made at this point will show up in your floor covering. Scrape away the excess adhesive. Let the adhesive dry according to the manufacturer s directions before applying the floor covering.
Some manufacturers supply kits containing both the fabric and the adhesive. Some fabrics are available in narrow widths ready to use while others come in larger rolls which need to be cut to width. The following is a list of some of the suppliers of this product: