Cast Underlayment, often referred to generically as “gypcrete,” Maxxon Corp’s registered trademark name, has been used for the leveling of rough and uneven floors, for wood floor systems requiring fire ratings and acoustic control, and as the encapsulation material for in-floor radiant heating. The increased material strengths available today, along with ever-increasing competition, have led to the manufacture of more durable products and an increased use of cast underlayments since the 1990s. The application of cement underlayment is similar to traditional techniques for casting concrete. Cement underlayment (non-structural) can be added over existing concrete or timber floors, both interior and exterior, provided that the substrate is structurally sound.
When concrete underlayment is applied over rough, uneven, and warped surfaces, it creates a smooth, flat floor that is capable of withstanding the point loads found in office and light industrial buildings.
A variety of underlayments are commercially available with many of them based on hydraulic cements containing limestone, certain clay minerals, and gypsum (calcium sulfate dehydrate). These cements harden and set as a result of chemical reactions with water. Materials are designed to come up to required strengths with depths of as little as 1/4" thickness and are typically not used when the total required depth exceeds 4". Most cast underlayment products are not intended to be left exposed, but are to be covered with a finish flooring material.
Methods and Materials
When using cast cement underlayment, the surface to be coated should be thoroughly damp or, if this is not possible, a bonding agent should be applied. The cement is mixed in a slow speed mixer before being deposited into formwork, the side forms of which are coated with a release agent. The underlayment is then spread, consolidated, leveled, finished, and cured. During the curing process, which requires 3-4 hours at 70°F, the cement layer can be covered with wet burlap, or a curing compound can be applied.
Gypsum concrete underlayments produce flat, crack-free layers, are easily installed, and set quickly. Polymer-modified, cement-based underlayment materials are self-leveling and have good flow characteristics, making them attractive for covering unfinished interior floors for subsequent application of tile, marble, wood, or carpet. Materials must comply with ASTM C 317 and ASTM C 956.
For the underlayment to set rapidly, the temperature of the air, substrate, liquids, hardener and powder components should be maintained between 60° and 80°F during mixing, placing and curing. Monolithic substrates should be maintained between 65° and 80°F for 48 hours prior to placing the cement underlayment. The application is not recommended if the temperature is above 80°F or below 35°F. Application in direct sunlight can lead to blisters forming on the surface of the cement underlayment, due to the expansion of trapped air or the presence of water. Substrates that have been exposed to sunshine for a period of time should be shaded for at least 24 hours before applying the cement underlayment.
Most manufacturers have underlayment materials that will work in more extreme weather conditions. However, these usually are at a premium cost.
The design is based on the following:
- The substrate on which the underlayment is to be applied
- The weight that the substrate and its supporting system can carry
- The strength in deflection of the substrate; 1/360 deflection is the recommended maximum
- The point load and impact load requirements for the end use
- The minimum and maximum thickness allowed
- The required fire rating
- The acoustic requirements
- The rate of drying required
- Is in-floor radiant heat required?
The substrate should be structurally sound and its surface free of dirt and debris. Oil and grease stains or other chemical stains must be removed and, if necessary, the surface should be either hydroblasted or sandblasted to remove a thin top layer.
Applications and Installations
Expansion joints in the concrete floor should continue through the underlayment at the same width. Underlayment is applied by pumping the underlayment mix through a hose and applicator valve and nozzle on or near the floor. The placement is from the furthest point to the nearest point with adequate speed to avoid having edges begin to set. Although many underlayments are self-leveling, a screed is used to pull excess material to the open areas. Lasers are often used to assure a consistent elevation.
Surfaces should be protected with high-traffic areas covered with plywood until the finish flooring is applied.
Fast setting underlayments are available with initial set times as low as 1-1/2 hour. Traffic bearing underlayments with compressive strengths exceeding 6,000 psi provide a light traffic bearing surface in as little as 24 hours. Underlayments installed over wood substrates may require reinforcing mesh.
With a moist material being applied to organic substrates or beside materials with organic content the growth of mold and mildew is possible. Concrete Floors and Moisture by PCA addresses these and other concerns and is available at the ACI website.
Standards and Codes
- The International Building Code Chapter 19 Section 1915 by ICC states the minimum design standards for Reinforced Gypsum Concrete.
- Reinforced concrete shall comply with ACI 318 and ACI 318, Section 3.5.
- American Concrete Institute ACI 305 Hot Weather Placement of Concrete.
- ACI 306 Cold Weather Placement of Concrete.