Because of its rigidity and stability, concrete slabs would seem to be the perfect substrate upon which to install ceramic tile. This view is too simplistic, because it does not take into account the dynamic forces acting on a slab. These forces must be taken into consideration when selecting the method of tile installation. If ignored, the installation is prone to fail, especially in structural (above grade) installations.

There are three basic types of installations over concrete slabs:

1. Tile bonded to the slab. In this installation the tile is adhered to the slab with some type of thin-setting mortar, latex thin-set mortar, or organic adhesive. This method works fine as long as the slab is on or below grade. The problem with this installation is that tile bonded directly to the slab inherits all the problems of the slab. Concrete slabs are subject to bending stresses, deflections, sags, creep, and cracking. These forces will transmit directly to the tile resulting in loose or cracked tile. This method should never be used on structural (above grade) slabs where deflection, bending, etc. problems are manifest. When the slab is on or below grade these dynamic forces are significantly reduced permitting the use of this method without problem...providing the slab is acceptable for tiling based on the following criteria:

Slabs should be free of cracks, wax, films, oils, dirt, curing compounds, or anything which may inhibit adhesion. Slope, if required, must be in the slab with maximum variation not exceeding 1/4" per ten lineal feet.

If new, the slab should be steel troweled with a fine broom finish. Although a wooden float finish provides good bonding they have been found to create other problems. Old slabs must be prepared to the extent that a good "bondable" surface is provided. If a surface treatment is needed, consider the types of treatment and the limitations of each.

Acid etching provides a suitable surface, but cannot be used everywhere. Acids have little effect on oily or greasy films. These films would need to be removed before acid can be applied. This method may not prove acceptable in closed or confined areas where acid fumes may damage surrounding surfaces. Extreme care must be utilized since personal injury and damage may result from improper handling of acids. Only qualified professionals should be engaged.

Scarifying is another method that can be performed in several ways. Sandblasting provides a good surface, especially if it exposes the fine aggregate particles in the concrete. Many tile contractors are familiar with coarse grit terrazzo grinders. These have been found to work well. For very large areas, the Tennant Company of Minneapolis, Minnesota can supply economical equipment for rapid scarification. Keep in mind that equipment designed for use in large areas may not be suitable for use in small areas, and vice versa.

Remodeling jobs provide still another problem. Many times concrete slabs have been tiled with vinyl-asbestos or other plastic sheets or tiles. When these are removed, residue from old adhesives remains. That residual adhesive may be of any type, fitting any number of descriptions: soft, gummy, brittle, black, etc. This "residue" is unfit for tile and needs to be removed--not always an easy job. Since it is not easy, many people are tempted to "cut corners" and leave all but the most loose, scaly pieces or sections on the slab.

This decision can be shown to be unwise when one considers the scope of the problem. To demonstrate, let us use conservative figures. An average basement may cover 700 square feet. With a tile at $3.00 per square foot, installation materials at $.50 per square foot, and labor costs at $4.00 per square foot, the total cost is $7.50 per square foot, or $5250 (700 x $7.50). If removing all of that "black gook" costs an extra $500 in labor, the cost percentage to the job is only $500/$5250, roughly 9%. A cheap insurance policy, indeed! Consider if tile or labor costs are higher than this estimate. It would be foolish to risk three, four, or even five thousand dollars and several days on a tile installation to avoid a few hours of removing old tile adhesive. Notice, we haven't included the cost and inconvenience of removing the failed tile installation. After any old adhesive has been removed and the slab cleaned, it is wise to "link" the old floor to the new one by painting the slab with Super-Weld.

2. Tile bonded to a mortar bed on slab. Most of the preparations mentioned above apply to a tile that is bonded to a mortar bed which has been placed over a concrete slab. The difference is mainly that the slab need not be prepared with such diligence. It still must be sound, non-cracking, clean, and properly sloped. But it need not have a trowel finish; a screed finish will suffice since the mortar bed will provide the actual surface to be tiled. Again, it should be linked with Super-Weld. This method, if used on structural (above grade) slabs, will also transmit movement of the slab up through the mortar bed and then through the tile resulting in failure.

3. Isolating the tile with a cleavage membrane. This is the only method applicable in structural (above grade) slabs. It separates the tile from the slab and prevents the tile from cracking and loosening. The drawing below details this method:

Illustration 4.3: