Concrete is generally assumed to be the best substrate for any tile installation. However, its assessment and preparation should not be overlooked. Tile installations directly over concrete floors are potentially exposed to cracks and flooring failure. First, any existing cracks should be qualified for their potential to vertical and lateral movements. This will determine when and where to use movement joints or uncoupling membranes. Moisture on the other hand is less obvious to assess with a visual inspection, as the top layer of the slab is never a good indication of its internal moisture level.
During the initial curing of the concrete, capillaries form after excess water evaporates from the slab. These cavities will not only weaken and make the concrete more porous, but they will also channel the moisture, alkaline salts and other contaminants to the surface where they will interact with the bonding mortar. High levels of moisture create higher alkalinity which breaks down the mortar bond and could lead to delamination. This process is further accentuated by the tile layer that acts as a vapor barrier.
Most contractors will allow a minimum curing time of 28 days for new slabs before tilling, but this is arbitrary as most projects do not have proper ventilation, temperature and curing controlled environments. In most situations, minimum curing times should be much longer and without proper moisture testing, anyone’s guest could be the right one. But concrete moisture testing is not only limited to new slabs. Ironically, most of installation failures happen over old concrete. In particular, in cases with humidity exposed slabs without vapor retarders, pumped lightweight concrete mixed with extra water and even slabs drying only from the surface (over metal decking or others vapor barriers). Thus, installation failures due to moisture usually occur a couple of months after the tile installation. As a result, it is important not to gamble on your general contractor’s senses but to properly test the concrete – mortar manufactures will only honor their adhesives if the concrete has been properly scarified, cleaned and moisture tested, prior to the tile installation.