By Ray Thompson, Jr.
The flooring industry has always placed strong emphasis on a smooth, level concrete surface to install resilient floor covering. With today’s large amount of commercial remodeling/renovation projects, coupled with attempts to reduce cost in the new construction market and the declining availability of competent cement finishers, you need a product designed to solve most problems related to floor leveling and repair. Fortunately, modern self-leveling cements provide you with solutions that are technically sound and cost-effective.
Traditional methods of leveling and repairing concrete floors are both labor-intensive and require the use of screed trowels and sanders there are less than desirable because of factors including the softness, waves, cracks, etc.
Self-leveling cements have advantages over trowelable underlayments:
- Application is about eight times faster than trowelable underlayments.
- They do not require the same high degree of expertise as hand troweling.
- They can be used to repair a variety of substrates.
- They are fast-setting and can be walked on in a few hours.
- Floors can usually be installed the next day.
- They can be installed from a featheredge to several inches in one pour with little or no shrinkage.
- They develop high compressive strength (4000 PSI or greater).
- They are water-resistant and do not promote the growth of microbial contaminants.
What is Self-Leveling?
Many think self-leveling means that you simply pour the mixed batch at the center of the room, and it will take care of itself. Not so. What it actually means is 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 successfully installing self-leveling products is to achieve a good bond between a 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 residuals such as oil, grease, wax, dirt, sealers, curing compounds and adhesives. Most self-leveling substrates are shot-blasted to ensure the substrate is clean and free of contaminants. Remember, taking a shortcut and failing to complete all required steps of your substrate preparation is an open invitation to failure.
Almost all self-leveling products recommend the use of primers to work as a bonding agent. Make sure to use either a porous (absorbent substrate) or a non-porous product for ceramic, quarry tile, terrazzo, marble, steel, lead, and cut-back adhesive residues.
On special types of substrates, an additive may be recommended for metal and or cutback adhesive residue. The additive will provide additional bonding strength and will allow a little bit of deflection into the mix.
Temperature control is vital to successfully using self-levelers. With self-levelers, heat is your worst enemy. When you are dealing with a self-leveling product make sure to monitor four temperatures:
- Ambient temperature
- Slab temperature
- Powder temperature
- Mix water temperature
If any of these temperatures exceed 70 degrees Fahrenheit (21.1 degrees Celsius) it will prove to be detrimental to the application. In warm weather, 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 blocks of ice to cool the mix water. Heat causes the self-leveling mix to prematurely stop flowing, usually giving a set time of less than five minutes and making it difficult to get the product in place.
In cooler conditions, the self-leveler will have a slower set time. Beware of temperatures below 50 degrees Fahrenheit (10 degrees Celsius), as it will have an adverse effect on the mix.
Drying usually takes about two hours before you can walk on the newly installed surface. Resilient floors can be installed the next day. Care must be taken to not allow the underlayment to dry too fast. Underlayments that dry too fast tend to exhibit surface cracks. The way to prevent this is to monitor the four temperatures listed above. The key to self-leveling is to keep the temperatures around 70 degrees Fahrenheit (21.1 degrees Celsius).
When priming, it is necessary to know if the surface is porous or nonporous as the type of primer is different. Using a primer ensures a good bond to the substate and can prevent future problems.
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 pre-measured amount of water. Once the powder is added to the water, power mix with a heavy-duty drill at about 650 RPM for approximately two minutes. Power mixing will break 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, pour the mix out the container onto the floor without delay. The approximate time from when the powder hits the water to the stop of the flow is about 10 minutes at 70 degrees Fahrenheit (21.1 degrees Celsius). You must allow time for spreading and smoothing.
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, handheld device that can be set to apply the desired, even thickness of the underlayment over the substrate.
This smoother is a hand-held device used in the final smoothing by removing the spreader marks, footprints and all irregularities.
The working time varies from eight to 10 minutes depending on temperature. While this does not seem like much time, it is more than adequate to spread and smooth the area. For example, a three-person crew can cover about 1.500 sq. ft. per hour without any difficulty, while hand mixing.
For large installations, self-leveling underlayments can be pumped. The pump will control the critical water-to-powder mixture and allow for faster installation. A three-person crew can easily do 7,000 sq. ft. per hour.