Our questions this time will all be addressing the issues involved with tiling over concrete.
How long does concrete need to cure before tile can be installed?
While this seems like a simple question, in fact many issues must be considered.
We understand from the PCA (Portland Cement Association) and ACI (American Concrete Institute) that many variables affect the length of time a slab needs to cure and the length of time during which curling can be expected. Some of the more common variables are:
1) Cement - water mix ratio
2) Cement - sand ratio
3) Particle size distribution
4) Presence of accelerators
5) Curing compounds
6) Environmental conditions
7) Location of vapor membrane
8) Exposure to water during curing
You can see that it is not possible to definitively state how long the concrete should cure before tiling.
Although it may not be possible to say for sure, what is a typical period for the concrete to cure?
Many people suggest a minimum cure of 28 days under normal conditions - although, some thinset manufacturers say you may get away with 14 days using a premium latex modified thinset. There are risks though to tiling too soon - as the slab continues to cure, it will continue to shrink as it hydrates and the excess moisture evaporates. This places the tile and thinset under compressive stress. A premium thinset can better accommodate this stress and compressive force, as long as movement joints are used in the tile installation.
What are the benefits to allowing a sufficient cure time for the concrete?
Clearly, the longer you can wait before tiling, the less stress that will be applied to the tile by the concrete. Also, should the concrete crack, the crack can be treated (with an anti-fracture membrane) before it is tiled. If curling occurs (and it often does), the concrete can be ground before it is tiled. Tiling before the curling occurs can cause undesirable debonding of the tile and lippage between adjoining tiles.
What if I can't wait 14 to 28 days for the concrete to cure?
If you really can't wait 14 to 28 days, you may want to consider a vapor equalization membrane. Reportedly, this membrane isolates the tile layer from the concrete layer but still allows for the vapor to leave the system. This is different from a crack isolation layer that may not allow the "green" concrete to breathe.
Here is your opportunity to ask the Tile Council of North America (TCA) about tile specifications and installation challenges. Content for future columns will be drawn from questions submitted to firstname.lastname@example.org. Our objective is to prepare a column that addresses the challenges you face.