Moisture in concrete. How dry is dry enough?

Most membrane failures on new concrete start before the first coat. The slab looked dry, the membrane went on, and the moisture still in the concrete pushed it off as blisters and pinholes.

What the standards ask for

Internal wet areas (AS 3740)

Measure the moisture before the membrane. It must suit that membrane.

Balconies and roofs (AS 4654.2)

Substrate dry, clean and smooth, with a moisture content that suits the membrane.

Tiling (AS 3958)

Test where moisture could harm the system. The pass mark is each maker's limit.

None of the three sets a single pass mark of its own. Each one points to the data sheet for the product you are using.

The guide figures, where the maker gives none

• In-slab relative humidity: 80% or less, read by a probe set in a drilled hole. Drill at least 24 hours before reading.
• Moisture vapour emission: 15 g/m² per 24 hours or less by the calcium chloride test, where drilling is not practical.
• How many tests: three for the first 100 m², then at least one for each extra 100 m².
• Surface meters: useful to compare one spot with another. Not a pass or fail on their own.

AS 4654.2 also notes 8% or less by a non-invasive meter for external slabs and toppings. That is a meter reading, not relative humidity, so the two figures cannot be swapped.

How long to wait

The tiling standard gives guide drying times for new concrete floors after curing: about six weeks before a bonded screed, and about four weeks before a standard tile adhesive. A site-mixed screed is cured for seven days and then left to air-dry.

Weather, slab thickness and ventilation move all of these, which is why the standards ask for a test, not a date on the calendar.

On site, in order

1. Remove curing compound, laitance and dust.
2. Test and write the readings down, with the date and the spot.
3. Choose the primer from the reading.
4. Keep the record with the job file. It is the first thing asked for if a membrane blisters.