Concrete Tips: How to Measure the Moisture Content of Concrete

Water plays an important part in the formation of concrete; it is one of the three main ingredients: water, cement and aggregate, and, is responsible for hardening the concrete through a process called hydration.

As the concrete drys, excess moisture will escape to the surfaces of the slab through capillaries that are created during the curing process. (Concrete poured for indoor floors usually have a moisture barrier underneath so, in those cases, all of it goes to the top surface.)

Generally, the surface of a slab is drier than its core; the deeper the slab, the greater the moisture content disparity between the slab’s surface and centre.

According to the ASTM (American Society of Tests and Materials) International—one of the oldest and most influential international standards societies, the RH (relative humidity) level of concrete should be no more than 75 percent unless otherwise specified. As the concrete drys, it’s possible for water to enter the concrete from the environment (either from rain, humidity or wet soil).

The ratio of water to cement in your concrete remains one of the most influential factors regarding how long a concrete slab needs to cure and dry, so removing excess water is important to meeting curing schedules. The rule of thumb for how long it takes concrete to dry is 30 days per inch of slab depth.

You should control the evaporation process during the first stage of curing. The use of products or processes to hasten the drying process are becoming more popular. Reducing the water-to-cement ratio when batching, using self-desiccant products, controlling the ambient temperature, and running fans, heaters, or air conditioning are common ways of controlling the evaporation process.

Measuring concrete moisture content

Properly measure the moisture condition of a slab. You can perform a quick measure of a slab’s moisture concentration by laying a small (one-metre by-one-metre) piece of clear polythene or rubber over the floor and taping down all its edges. If there is condensation under the plastic after 24 hours, the slab is wet. If it’s dry, that doesn’t mean the concrete is dry; this method will accurately tell you when a slab is very moist but more accurate tools are needed when the moisture content of the slab approaches the specified moisture content.

For more accurate test results, there are two standard tests.

The anhydrous calcium chloride test (CaCl test) is conducted by placing a drying agent on top of the slab and sealing the agent with a cover, so environmental conditions don’t interfere with the results. After 60-72 hours, weight the material. Any increase in weight of the CaCl crystals is considered to be a direct correlation to the slab’s moisture vapor emission rate ((MVER), which is the rate at which water vapor leaves the surface of a concrete slab.

The in-situ RH test uses probes to measure the RH at a specific depth within the concrete (40 per cent of the slab’s thickness if the slab drys from just one side or 20 per cent of the slab’s thickness for a slab drying from two sides. At these depths, the readings will reliably predict the slab’s point of equilibrium (the point at which the moisture content in the slab is equivalent to the moisture content of the environment.