Traditional methods of leveling and repairing concrete floors are both labor intensive and require the use of screeds, trowels and sanders that are less than desirable because of softness, waves, cracks, etc.
Self leveling cements have a variety of advantages over trowelable underlayments:
1. Application is about eight times faster than trowelable underlayments.
2. They do not require the same high degree of expertise as hand troweling.
3. They can be used to repair a variety of substrates.
4. They are fast setting and can be walked on in a few hours.
5. Floors can usually be installed the next day.
6. They can be installed from a featheredge to several inches in one pour with little to no shrinkage.
7. They develop high compressive strengths (4,000 psi or greater).
8. They are water resistant and do not promote the growth of microbial contaminates.
What is self-leveling?
Many think “self-leveling” means that you pour a mixed batch in the center of the room and it will take care of itself. Not so. It means that the mixture of powder and water has a low enough viscosity to allow the material to seek its own level before setting.
The key to success when installing self-leveling products is to achieve a good bond between the substrate and the self-leveling underlayment. Proper preparation of the concrete surface is the most important factor. The surface must be sound, clean and free of such residuals as oil, grease, wax, dirt, sealers, curing compounds and adhesives. Most self-leveling substrates are shot blasted to insure that the substrate is clean and free of contaminants. Remember, taking a shortcut in substrate preparation is an open invitation to failure.
In almost all self-leveling products they recommend the use of a primer to work as a bonding agent. There are two types of primers used. One is for porous and absorbent substrates, while the other type is used when going over a non-porous substrates, such as ceramic, quarry, terrazzo, marble, steel, lead, and cutback adhesive residues.
On special types of substrates an additive may be recommended for special types of substrates such as metal substrates and or cutback adhesive residue. The additive will add additional bonding strength and will allow a little bit of deflection into the mix.
Temperature control is vital to the success of using self-levelers. With self-levelers heat is your worst enemy. When you are dealing with a self-leveling product, you must monitor four temperatures.
1. Ambient temperature
2. Slab temperature
3. Powder temperature
4. Mix water temperature
If any of these temperatures exceed 70°F (21.1°C) it will prove to be detrimental to the application. In warm weather conditions the pour may have to be done early in the morning or late in the evening when temperatures are cooler. Powders can be stored in a cool place and the mix water container can be filled with several blocks of ice to cool the mix water. Heat causes the self-leveling mix to stop flowing prematurely, usually less than five minutes, making it difficult to get it placed on time.
In cold conditions the self-leveler will slow its set time down, but beware of any temperatures below 50°F (10.0C), as it will also have an adverse effect on the mix.
Photo #1 – Mixing: When mixing, it is critical to use the correct water-to-powder mixture. Mixing is done by adding the correct amount of powder to a premeasured amount of water. Once the powder is added to the water it is necessary to power mix with a heavy duty drill at about a 650 rpm for approximately two minutes. Power mixing will break the surface tension in the water, allowing for a smooth mix that will flow easily for about six to eight minutes. Once the mixing is complete, get the mix out of the container and on to the floor without delay. The approximate time from the time the powder hits the water to the stop of the flow time is about 10 minutes at 70°F (21.1°C), and you still need time for the spreading and smoothing process.
Photo #2 – Spreading: Immediately after the mixing process is completed, pour the mix onto the floor and move the mix with a spreader to obtain a uniform thickness. The spreader is a stand-up, hand held device that can be set to apply a desired even thickness of underlayment over the substrate.
Photo #3 – Smoothing: The smoother is a second hand held-device to place the final smoothing by removing the spreader marks, footprints and all irregularities.
The working time varies eight to ten minutes depending upon temperature. While this doesn’t seem like much time it is more than adequate to spread and smooth the area. For example, a crew of three can cover about 1,500 square feet per hour without any difficulty.
Photo #4 – Pumping: For large installations self-leveling underlayments can be pumped. The pump will control the critical water-to-powder mixture and allow for a faster installation. For example, a three-man crew can easily do 7,000 sq. ft. per hour.
The usual drying takes about two hours before you can walk on the newly installed surface, and resilient flooring can be installed the next day. Care must be taken to not allow the underlayment to dry too fast. Underlayments that dry too fast will tend to surface crack.