Every week, the CPCA publishes a practical technical tip from our community of experienced applicators. This week’s tip is contributed by Mike Theriault, CPS Level 3, Sudbury, ON.
Why Moisture Testing Is Non-Negotiable
Of all the pre-application checks that matter, substrate moisture testing might be the most overlooked — especially by experienced applicators who start to rely on visual assessment. I’ve learned the hard way that visual assessment is not good enough. Concrete that looks and feels dry can still have moisture content that will cause adhesion failures, pinholing, or blistering within weeks of application.
The rule of thumb I’ve always used: trust the instrument, not your eyes.
Testing Methods
There are three main testing methods for concrete substrates:
1. Plastic Sheet Test (ASTM D4263): Tape a 450mm x 450mm plastic sheet to the surface and leave it for 16 hours minimum. Moisture condensation on the underside of the sheet indicates a moisture problem. Simple, cheap, but slow — requires overnight.
2. Calcium Chloride Test (ASTM F1869): Provides a quantitative moisture emission rate in pounds per 1,000 sq ft per 24 hours. Most polyurea manufacturers specify a maximum of 3 lbs/1,000 sq ft/24 hr. More accurate than the plastic sheet test, but also slower (60-72 hours).
3. Relative Humidity Probe (ASTM F2170): My preferred method for project schedule-sensitive jobs. An RH probe is drilled into the concrete at 40% depth and read after a minimum 1-hour equilibration period. Most manufacturers specify a maximum 75-80% RH for direct polyurea application. Fast, accurate, and the results are documented.
When to Test
Always test before surface preparation AND again after surface preparation (especially if abrasive blasting created surface opening that exposes wetter concrete). Also test before application if there has been any rainfall or high humidity event since your prep.
For cistern and tank lining applications, moisture testing is especially critical because the structure may have been holding water until recently.
What If Moisture Is Too High?
Don’t push through. Options include: additional drying time, supplemental heating/dehumidification, or use of a moisture-tolerant primer or moisture-vapour barrier coat before the polyurea. Never skip this step — a failed application costs far more than the time and cost of proper preparation.
For more technical guidance, read our comprehensive article on polyurea chemistry and application requirements. Have a technical tip to share with the community? Submit it to the CPCA.