In a perfect world, movement would not take place in a concrete slab, and life would be good. Unfortunately, this is not a perfect world, and concrete does move and crack. Therefore, products such as crack isolation membranes, which fall under ANSI A118.12, are many times more of a necessity rather than a luxury.
Given that all structures move (some significantly more than others), it is wise that all involved in a tile project understand these dynamics, ask questions and seek solutions. This may also require the services of an expert who can assist in properly assessing the situation.
In order to chart the course, questions must be asked. Is the slab stable and no longer moving? Were the cracks the result of shrinkage in the curing process and they now are stable? These two questions are related and may be treated in a similar way. Most times, cracks of this type experience an “in and out” movement similar to clapping hands, as seen in Image #1, while others shift back and forth, parallel to each other, like two hands being rubbed together, as seen in Image #2. Most crack isolation, or crack I products as they are popularly known, function well in these situations and solve the problem; keeping the crack from migrating up through the tile causing an unsightly crack.
Is the crack the result of a structural fault or a component failure of the building and does it continue to move? The cause possibilities are endless. For instance, it could be an on-ground slab where the soil and crushed stone base were not compacted properly, resulting in the concrete shifting and cracking from nonsupport. As you can see in Image #3, the slabs on either side of the crack are offset, meaning that one side has sunken, one side has been pushed up or the combination of the two. Either way, the slab is unstable and an architect or structural engineer should be consulted to determine the appropriate remedy. In this case, no crack isolation manufacturer would want, nor recommend, their products for this use.
If it is a moving crack, the only way to treat it effectively is to walk away. That may sound like a flip answer, but it is better than funding a failed installation. This type of crack should never be covered without the appropriate correction made to the structure and an approval from the engineer. But be aware, using a grinder to even out or flatten the adjacent side is never a good solution. In most cases, the design professional is responsible to specify a category or product which will overcome the movement issues that are characteristic in almost all buildings. In addition to the design professional, there are others who need to participate, namely the manufacturer, the distributor and the installer. Here, the crack isolation membrane manufacturer is the key. They develop, test and market their products that solve movement issues. Without this research and development of new products, problems continue. Manufacturers have done a great job in this regard and stand ready to assist.
The distributor’s role is to not only get the products into the marketplace, but to help educate the installers on the proper product selection, preparation and installation of crack isolation materials. To further this process, many distributors host the manufacturer’s reps that can go one on one with the installers and school them in the installation techniques that will ensure a high-quality and long-lasting system. Armed with this information and experience, the installer can properly execute the work.
All concrete surfaces or substrates must be inspected and prepared to receive the membrane. This surface must be clean, dry and free of curing compounds, sealers or coatings. This procedure is necessary for any of the crack I products regardless of their form. Once the contaminants are removed, be certain to check the manufacturer’s recommendations to determine if a primer is required before installing the membrane.
Crack isolation products come in three varieties: sheet, liquid-applied and trowel-applied. Sheet membranes come on a roll in two variations: dry back and self-adhesive. The dry back is installed using the recommended thin-bed mortar or the manufacturer’s approved adhesive. The self-adhesive membrane is applied to the concrete by removing the temporary paper cover and pressing it onto the surface. Generally, the tile may be installed immediately. Liquid applied membranes may be applied using a roller, notched trowel or brush to a prescribed thickness, which is monitored by the use of a wet film thickness gauge. Depending on the manufacturer, a second coat may be required. Many of the liquid-applied products change color when dry, indicating that the surface is ready for the tile to be installed. Trowel-applied membranes are actually thin-bed mortars that are engineered with crack isolation properties, which allow the tile to be installed in the conventional way without the application of a separate membrane.
The TCNA Handbook provides two details for the installation of crack isolation membrane over concrete. The F125-Partial method calls for the installation of the membrane over the crack at the manufacturer’s prescribed width. Following the crack in this way is commonly referred to as crack chasing. The F125-Full method differs in that it has the membrane being installed over the entire area. This method is especially wise for protection from cracking that may occur in the future. But no matter which method is employed, the installer should always check with the manufacturer to determine how many soft or movement joints need to surround the crack.
There are two aspects that are critical to installation excellence. Asking questions prior to the start of the job is a good course of action which can play a huge role in determining the difference between an industry black-eyed failure and a quality installation destined to a long life. Secondly, utilizing qualified labor installers who have been certified in their craft will propel the tile industry to the next level of satisfied clients who will buy more tile in the future. Working offensively here is far superior to doing so defensively.