Excessive water in a gypsum underlayment mix will compromise the strength of an underlayment pour, resulting in inferior quality and a poor ﬁnish. Correctly executed, slump tests provide a simple, on-site method to ensure that your gypsum underlayment mix contains an appropriate amount of water.
• 2” diameter PVC cut to 4” in length
• 12” square piece of plexiglass
• Tape measure (not require if using FM Slump test kit)
• Pitcher or container • Mixing stick
Note: For most Formulated products and mixes, an 8 ½” to 9 ½” slump will provide an ideal pour.
Step 1 – Take a Clean Sample
Fill pitcher or other clean container with current underlayment mix as needed from end of hose. (Do not take sample from any other sources)
Step 2 – Keep Sample Agitated
Use a clean mixing stick to keep sample agitated until it can be poured. This will prevent the aggregate from settling and providing an inaccurate test result.
Step 3 – Fill Sample Tube
Place the 4” long, 2” diameter piece of PVC pipe on center of plexiglass and ﬁll to top of tube. Maintain downward pressure on pipe to ensure sample stays contained.
Step 4 – Release the Sample
Pull tube up vertically in one smooth, clean motion allowing qypsum to escape from bottom of tube. Keeping the tube centered will help to ensure an accurate measurement
Step 5 – Determine Slump
After the sample has stopped spreading, measure the diameter of spread of the gypsum. This measurement is referred to as the slump (i.e. – “a 9” slump”)