Concrete moisture testing and mitigation

Moisture is by far the most common issue that can compromise a concrete asset and the coating that protects it.

Concrete is prone to moisture issues due to a channel of capillaries formed during the hydration phase (hardening phase). The number of channels can be dramatically increased or decreased depending on the water / cement ratio used in the mix. The permeability of concrete can be reduced by using mix designs with lower water / cement ratios, various admixtures or by applying protective coatings. When applying protective coatings to concrete, moisture-related issues happen quite frequently.

Those sources of moisture include, but are not limited to:

• Water left over from concrete mixing, known as bleed water
• Naturally occurring sources like precipitation, groundwater, condensation or infiltration and inflow
• Water escaping from nearby man-made sources such as broken plumbing pipes

Concrete moisture testing methods

Because moisture issues are so common, an array of concrete moisture testing methods have been developed to address them.

ASTM D4263 – Indicating Moisture in Concrete by the Plastic Sheet Method. This test involves taping an 18” x 18” polyethylene sheet to a concrete surface and then waiting at least 16 hours. Visible condensation on the sheet or darkening of the concrete indicates excessive moisture. Any coating applied to the surface may be at risk of failure. It’s a useful surface-level test, but it sheds little light on the severity of the moisture problem within the concrete slab.

ASTM F1869 – Measuring Moisture Vapor Emission Rate of Concrete Subfloor Using Anhydrous Calcium Chloride. In this test, a dish containing calcium chloride is set on a concrete surface and covered with an airtight dome. After waiting 60 – 72 hours, the dish is weighed to record pounds of water emitted per 1,000 square feet. Failure is identified if the calcium chloride tablet is above 3 pounds in weight. Note that this test can give false readings because 90% of MVE measured comes from top ½” of concrete. This kit does not detect moisture below ¾”.

ASTM F2170 – Determining Relative Humidity in Concrete Floor Slabs Using in situ Probes. In this test, a sleeve and probe are inserted into holes drilled into the concrete. This test is more precise because it can show humidity readings at various depths in concrete. Probes can also be left in concrete to deliver readings over time.

ASTM F2659 – Preliminary Evaluation of Comparative Moisture Condition of Concrete, Gypsum Cement and Other Floor Slabs and Screeds Using a Non-Destructive Electronic Moisture Meter. A concrete moisture meter measures concrete water percentage in a non-destructive manner. The meter is ideal for determining the moisture conditions of concrete.

We recommend an all-of-the-above approach to concrete moisture testing to get the best picture of how moisture impacts your concrete surfaces. You should also follow coating manufacturer recommendations regarding proper application of coatings based on the results from moisture testing.

Mitigating concrete moisture issues

Once concrete moisture testing is complete, owners have a wide array of methods at their disposal to combat moisture problems. Available moisture mitigation options include:

• Waiting– For starters, coating manufacturers usually recommend you allow at least 28 days for curing before coatings are applied (although some new “green” coating formulations can be applied after 14 days). Waiting for concrete to dry out on its own is the easiest and cheapest mitigation option. It only requires patience, and it’s the preferred method provided it doesn’t delay or disrupt other ongoing work.
• Dehumidifying– Equipment supplementing a building’s HVAC system is commonly used to help lower the relative humidity or increase the temperature of new concrete to accelerate drying. This technique can also add to project timelines and requires care and constant monitoring of moisture levels.
• Employing vapor barriers or retarders – These systems either slow or completely stop the migration of moisture from the ground to a slab on grade. Choose a material that isn’t easily damaged during installation, as any advantage gained by using a barrier is lost if it tears.
• Moisture mitigation systems– These systems are applied as topical treatments to concrete and have been successful in reducing moisture vapor emission rates. They include liquid membranes, reactive penetrants, modified cementitious overlays, dispersive membranes, assembly systems and combination systems.
• Design against moisture – Some naturally-occurring moisture is unavoidable, but other problems can be eliminated through design. For instance, flat expanses, dimensional changes, or voids can all harbor water. During design, these should be avoided if possible. If these features are critical to the function of a facility, designs should include channels designed to divert water away from problem areas.
• Use green concrete coating systems – These systems start with the application of a high-solids, no-VOC epoxy primer applied to a new concrete surface well before concrete’s 28-day cure window has closed. These formulations penetrate and then form strong bonds with the concrete surface. It significantly reduces the impact of vapor transmission and allows for mid- and top coating of the surface sooner, leading to shortened project timelines.