What's Up With Tilt-Up?
Site cast, tilt -up wall construction has been around for years as a cost-effective means of construction. For those not very familiar with this type of construction, let me give a brief explanation. The main slab is poured and usually allowed to cure for 28-30 days. After the 30 days, forms are placed horizontally on the building floor slab and cement is poured into these forms to create the sections of walls. Prior to pouring the cement into the forms, a Bond Breaker, a solution that enables the de-bonding of the wall section from the horizontal slab, is applied directly to the horizontal slab. Some solutions leave oily residues that do not dry, and some create a thin hard membrane between the slab and poured wall, and are considered curing compounds as well. After the walls are cured 7 to 10 days, they are tilted in place (Photo 1). From the time the floor slab is placed, the typical elapsed time from starting to form the panels until the building shell is completed is 4-5 weeks.
Now this is where it starts to get a bit tricky. Who removes the bond breaker from the horizontal slab? There are ASTM (American Society for Testing and Materials) guidelines for the removal of these products in relation to paint on the walls. Is it the general contractor, the cement contractor or the flooring installer? Many times the general contractor does not even address the removal issue, because it’s another added cost. The cement contractors are responsible for flat work only and not the removal of coatings, and that leaves you, the installer. You are the last person in but the first to be contacted with any problems, so what preventative measures can you take?
First, make sure that the estimator of the project reads up on all the specifications and determines which contractor is responsible for the preparation of the slab. If you are the installer on the job, try to inspect the site prior to the day of installation. This can help minimize down time at the front end of the job.
Make sure a calcium chloride test was conducted to ensure the slab meets the acceptable level of vapor emissions. Many times when a tile job begins to fail, when de-bonding or hollow spots start to occur, usually the first conclusion is that it is moisture related. This is not always the case, and if you can have documentation of a calcium chloride test, this will establish that, at the time of installation, all moisture emission levels were within tolerance. This is where de-bonding products can create failures where they have been applied to the slab. The de-bonding of the tile may not be throughout the entire installation but only where the forms were placed. If you encounter such a situation, look at a wall that has a window or doorway cutout, and you may notice that debonding of the tile has a pattern that resembles the doorway or window, only occurring where the forms were set on the slab.
If not removed, the de-bonding product can be a factor in the overall bonding ability of the thin set mortar to the concrete slab, as bond breaking chemicals are not compatible with thin set mortars (Photo 2). How can you tell if there is a bond breaker type product applied to the concrete surface? Look at the walls. If it is a concrete wall there is a good possibility that it was a site cast tilt up. Knowing this, you can ask the general contractor if the bond breaker was removed, and if it was, what method of removal was used. Many contractors opt to use chemical removers such as D-limonene (a citrus based stripper) and methylene chloride, which can also affect the bond strength, as opposed to mechanical removal systems (shot blast), due to cost.
Some de-bonding products are self-dissipating, but generally require sunlight or UV light for this to occur. These products many times do not have the opportunity to fully dissipate due to the short period between tilt up and roof placement. One simple test to determine if there is a coating of sorts is to apply several drops of water to the slab surface. If the water beads up on the slab surface, this is an indication that there may be a bond breaker or sealer of some kind. Muriatic acid can also be used but care must be taken when using this chemical, as it is an acid. If the acid bubbles, the mortar should bond. If the acid does not bubble, it is an indication that there is a curing agent present.
Knowing the environment that’s around the installation prior to installing can be very beneficial to the success of the installation. Instead of always looking down at the floor, look around at the physical surroundings (Photo 3).