Two tests decide whether a waterproofing job starts right and ends right. Before coating, you need to know whether the concrete is dry enough to bond to; after a horizontal membrane is installed, you need proof it does not leak before anything covers it. This guide explains the in-situ relative humidity test (ASTM F2170), the calcium chloride test (ASTM F1869), quick screening methods, flood testing under ASTM D5957 and electronic leak detection, with a calculator for calcium chloride results.

Why moisture matters to coatings and membranes

Concrete holds water long after it looks dry. When a bonded coating seals the surface, moisture that would have evaporated collects at the bond line. The results include blisters (sometimes filled with liquid under osmotic pressure), loss of adhesion and, for fluid-applied systems, pinholes from vapor and air escaping while the coating cures. Spray polyurea is especially sensitive because its isocyanate component reacts with water to form carbon dioxide, which shows up as bubbles in the film. That is why coating, primer and flooring manufacturers publish moisture limits, and why a few days of testing is cheap insurance.

Before you test: conditions and sampling

Both major test methods expect the slab and the air above it to be at the temperature and humidity of normal service for at least 48 hours before the test begins. Testing a cold, unheated shell will not predict what happens once the building is conditioned. Both methods also call for at least three tests in the first 1,000 sq ft and at least one more for each additional 1,000 sq ft, and F2170 calls for locations near exterior walls. Place tests where the slab is most likely to be wet: near exterior walls, at low spots, over areas of known groundwater, and in any zone that will be coated first.

ASTM F2170: in-situ relative humidity probes

The RH method measures the moisture condition inside the slab at a depth that represents what the surface will equilibrate to once it is sealed. The key steps, summarized from the standard and the Wagner Meters F2170 guidelines:

1. Drill the holes. Drill to 40% of slab thickness for a slab drying only from the top (such as a slab on grade over a vapor retarder), or 20% for a slab drying from both top and bottom, such as an elevated deck.
2. Clean and line. Vacuum dust out of the hole and insert the liner or sensor sleeve so the reading comes only from the concrete at the bottom of the hole. Cap it.
3. Wait for equilibrium. The current standard requires at least 24 hours of equilibration before a reading counts (older editions required 72 hours).
4. Read and record. Use a calibrated sensor, record RH and temperature, and confirm the reading is stable.
5. Report. Include locations on a plan, slab thickness, hole depths, dates, times, ambient conditions and each reading.

Results are percent RH, and acceptance limits come from the coating, primer or flooring manufacturer. Limits differ widely between products, so a reading that is fine for one system may need a moisture-mitigation primer under another.

ASTM F1869: calcium chloride test

The calcium chloride test measures how much moisture vapor leaves the surface. A pre-weighed dish of anhydrous calcium chloride is sealed under a plastic dome on a cleaned area of bare concrete for 60 to 72 hours. The dish is then reweighed, and the weight gain is converted to a moisture vapor emission rate (MVER) in pounds per 1,000 sq ft per 24 hours. One test-kit manual from Gilson lists ambient conditions of 65–85°F and 40–60% RH for at least 48 hours before testing, and the area must be free of coatings, sealers, curing compounds and adhesive residue.

The calculation is simple: MVER = 52.91 × weight gain in grams ÷ (test area in sq ft × exposure hours). The calculator below does the math and tells you how many tests your slab needs.

F1869 has real limitations. It reflects mostly the upper portion of the slab, and it is sensitive to the temperature and humidity of the room during the test, so poorly controlled conditions skew results. Many flooring and coating manufacturers now prefer or require the RH method, while others still publish MVER limits. Use whichever method the product you are installing names.

Quick screens and what they cannot tell you

The plastic sheet test (ASTM D4263) tapes an 18 in. by 18 in. sheet of clear plastic to the concrete for at least 16 hours and looks for condensation underneath. It is useful for spotting gross moisture and separating condensation from rising moisture, but it is qualitative. Electronic surface meters (covered as a preliminary method by ASTM F2659) are great for mapping relative wet and dry areas to decide where to place real tests. Neither should be used as acceptance testing.

When the slab is too wet

If results exceed the product's limit, the options are to wait and dry (with conditioning and ventilation, then retest), use a moisture-mitigation primer or membrane rated for the measured RH or MVER, or choose a more moisture-tolerant system. Do not average a high reading away; a single wet area will blister regardless of the slab-wide average. On below-grade walls, moisture problems often trace back to drainage, covered in positive vs negative side waterproofing.

Flood testing horizontal waterproofing (ASTM D5957)

A flood test proves a completed horizontal membrane is watertight before pavers, topping slabs, soil or tile cover it. ASTM D5957 is a guide for flood testing horizontal waterproofing over habitable or usable space at slopes up to about 2%. It is not intended for building roofing systems. Typical parameters include:

• Plugging or isolating drains and building temporary dams at edges and around penetrations.
• Ponding water from a minimum of 1 inch to a maximum of 4 inches deep.
• Holding the water for 24 to 72 hours while monitoring below for drips, wet spots or moisture-sensor alarms.
• Using a small reference container of water to account for evaporation.
• Draining immediately and stopping the test if a leak appears.

Water is heavy: 4 inches adds about 21 pounds per square foot, so confirm the structure, drains and temporary dams can handle the load. Flood tests are pass or fail and cannot pinpoint a small leak's location, and absorbent substrates can hide slow leaks during a short test. They are standard practice on plazas, terraces and balconies and over occupied space in parking structures and stadiums.

Electronic leak detection (ASTM D7877)

Electronic leak detection uses the fact that most membranes are electrical insulators while concrete and water conduct. Low-voltage methods, such as electric field vector mapping, wet the membrane surface and trace current flowing through breaches to the grounded deck. High-voltage methods sweep a charged brush over a dry membrane and spark at pinholes, which lets them check vertical upturns and penetrations too. Both need a conductive substrate directly beneath the membrane, and they cannot test conductive membranes such as standard black EPDM. The IIBEC paper The Leak Stops Here compares the methods in depth.

Testing only proves what was installed. Getting thickness right in the first place starts with the material quantity calculator.