Installers pump MAPEI Ultraplan M20 Plus self-leveling underlayment in the Mansueto Library at the University of Chicago. Photo courtesy MAPEI.
The industry has always placed a strong emphasis on smooth and level concrete surfaces for a successful resilient flooring installation. With today’s ever-increasing amount of commercial remodeling/renovation projects, attempts to reduce costs in the new construction market and the declining availability of competent cement finishers, installers need self-leveling products to solve most problems related to floor leveling and repair.
These new self-leveling products are technically sound and cost-effective. Traditional methods of leveling and repairing concrete floors are 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. 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; and they are fast-setting and can be walked on in a few hours.
Other advantages include: Flooring materials can usually be installed the next day; the self-levelers can be installed from a featheredge to several inches in one pour with little to no shrinkage; they develop high compressive strengths (4,000 psi or greater); and they are water-resistant and do not promote the growth of microbial contaminants.
What is self-leveling? Many think self-leveling means 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.
Substrate preparation.The key to success when installing self-leveling products is to achieve a good bond between the substrate and the self-leveling underlayment. Proper concrete surface preparation 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 ensure the substrate is clean and free of contaminants. Remember, taking a shortcut in substrate preparation is an open invitation to failure.
Priming.Almost all self-leveling products recommend the use of a primer to work as a bonding agent. There are two types of primers used. One for porous and absorbent substrates, while the other type is used when going over non-porous substrates, such as ceramic, quarry, terrazzo, marble, steel, lead, and cutback adhesive residues.
Additives.On special types of substrates (such as metal or over cutback adhesive residue) an additive may be recommended. The additive will add bonding strength and allow a little bit of flexural strength to the mix.
Temperatures.Temperature control is vital to successfully using self-levelers. Heat is your worst enemy. You must monitor four temperatures: ambient, slab, powder and mix water.
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. These cooler conditions will increase flowability and smoothness of the finished floor.
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.
Mixing.When mixing, use the correct water to powder mixture. This is a concern with the self-leveling manufacturers and overwatering is an ongoing problem. Mixing is done by adding the correct amount 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 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.
Spreading.When the mixing is complete, don’t delay in getting the mix out of the container and onto the floor. The approximate time from the time the powder hits the water to the stop of flow time is about 10 minutes at 70°F (21.1°C).
Make sure to allow for the spreading and smoothing process. Move the mix with a spreader to obtain a uniform thickness. The spreader, a stand-up, handheld device, can be set to apply a desired thickness of underlayment over the substrate.
Smoothing.The smoother is a handheld device to place the final smoothing by removing spreader marks, footprints and all irregularities. The working time varies between eight to 10 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 or more sq. ft. per hour without any difficulty.
Pumping.Large self-leveling underlayment installations can be pumped. A properly set pump can control the critical water-to-powder mixture and allow for a faster installation; a three-man crew can easily do 7,000 sq. ft. per hour.
Drying usually 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 exhibit surface cracks and checks.
While the initial cost for self-leveling materials is more expensive than trowelable underlayments, the benefits are much greater. I have observed installations of self-leveling underlayments that have achieved a FF 90+, which is more than twice as flat as the American Concrete Institute (ACI) requirement for on-grade slab, (FF 35) and much flatter than the traditional 3/16” in 10’. In my opinion, that’s an investment well worth making.
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