This graph shows how over-watering lowers gypsum floor underlayment compressive strength. When a specified mix is over-watered by only 2.5 percent, compressive strength decreases by 14.5 percent (Point A to Point B).

This chart shows that the compressive strength of a gypsum floor underlayment increases only slightly until 93 percent of free moisture has been removed from the floor.
Previously I discussed how the growth of poured gypsum underlayment impacted the floor covering industry. For those who did not read that article I would like to reiterate a few important features to remember.

Gypsum concrete is pumped into place over acceptable substrates. It is usually placed over structurally sound concrete or precast at a minimum depth of 1/2 inch or 3/4 inch. It may also be cast over structurally sound plywood.

This type of underlayment is typically used in apartments, condos, or commercial construction as it adds fire resistance and sound resistance, to the structure.

An extremely critical factor in gypsum concrete's performance is the percentage of water used in the formulation. The less water required the compressive strength becomes higher. Over watering is the single most common installation problem. Over watering must be avoided. Over watering not only lowers compressive strength (PSI) it also creates a "soft" surface which can affect the ability of the flooring adhesive to bond tenaciously. Review this graph which shows over watering by as little as 2.5 percent will lower PSI by 15 percent.

These photos illustrate the typical method for installing gypsum poured underlayment.
Another important criteria is water evaporation of the liquid poured gypsum concrete. Review attached graph that shows strength gain is almost level until 80 percent of the mix water has evaporated.

Other factors which significantly affect gypsum's strength are the type of gypsum used (there are several different kinds of crystals) and the percentage of filler in the mix.

For the highest compressive strength with your mix, you must wait an average of 7 to 10 days for drying to complete. This length of time may expand due to climatic conditions in the gypsum concrete areas. Ten to 14 days is not unusual in high humidity environments. Flooring installers who are unfamiliar with gypsum concrete may contact the flooring manufacturers and be told that the manufacturer does not warrant their flooring over gypsum concrete. Alas, they also don't warrant their flooring over plywood or Portland cement underlayments or concrete. No flooring manufacture warrants underlayments of any kind.

If you follow the floor prep you would use on any concrete substrate you should have no problem. Proper floor prep is necessary of any underlayment.

1. TEST FOR DRYNESS. As mentioned the drying time can be longer than normal due to job site conditions. There are a number of tests available. The plastic sheet test, and the rubber mat test. However the quickest and easily documented is to use the Delmhorst pin test. This is quick and accurate. If your readings are below 5 percent (ie: moisture content of the gypsum concrete) you may safety proceed to number 2.

These photos illustrate the typical method for installing gypsum poured underlayment.
2. CHECK POROSITY. First brush off a small area to remove dust. Then place a few drops of water on the surface and observe how quickly it penetrates. If the surface appears to be extremely porous, use an acrylic sealer. This accomplishes two things; one, it prevents too rapid absorption of adhesive into substrate; and two, it makes for a superior bond by the adhesive not skimming over too fast thus allowing more adhesive to be in contact with the floor covering.

3. DUSTY SURFACE. This condition is a result of over watering or excessive construction traffic prior to installation. Sweeping first is encouraged. Addition of an acrylic sealer bonds gypsum concrete particles together and provides a substrate that will produce a more than adequate bond.

NOTES: 1. An anti-facture membrane is recommended when installing ceramic tile or dimensional stone.
2. When installing glue down carpet use a premium carpet adhesive. Do not use low solids cheap adhesive.
3. Wood flooring is best secured by using 100 percent solids urethane.
4. Pay close attention when seaming vinyl as this is a entry point for topical water if a satisfactory seam is not established.
5. Generally, any patching or leveling over a gypsum substrate require the use of a premium gypsum patch.