Understanding Moisture in Concrete: Assessing Floors and Slabs

Concrete, a widely-used building material, is susceptible to moisture-related issues that can affect its integrity and appearance. In this exploration, we delve into the significance of concrete moisture and vapor tests, their methodologies, and the essential need for such evaluations.

Concrete Moisture and Vapor Challenges

Moisture within concrete can trigger various problems, such as discoloration, hindering polymerization, and leading to delamination of coatings. Recognizing the presence and quantity of moisture is crucial, prompting the need for comprehensive tests and subsequent corrective actions.

Methods for Assessing Concrete Moisture

Two primary methods for evaluating concrete moisture and vapor are the Plastic Sheet Test (ASTM D 4263) and the Calcium Chloride Test (ASTM F 1869).

1. Plastic Sheet Test (ASTM D 4263)

This test method serves as an indicator of moisture presence in concrete, specifically useful when planning to apply coatings on the concrete surface.

Materials Required:

• Tape (51 mm width)
• Transparent polyethylene sheet (460 mm x 460 mm, 0.10 mm thickness)

Test Frequency:

• One test per 46-square meters for floors, walls, and ceilings.
• Minimum one test for every 3 meters of vertical rise, starting within 300 mm of the floor.

Test Procedure:

1. Securely tape the sheet to the concrete surface, ensuring proper sealing.
2. Leave the sheet in place for at least sixteen hours.
3. Remove the sheet and inspect for moisture presence on both the concrete surface and the underside of the sheet.
4. Check for moisture by wiping a finger on both surfaces.

Limitations:

• Limited reflection of moisture movement.
• Possible moisture condensation rather than moisture flow.

2. Calcium Chloride Test (ASTM F 1869)

This test measures the rate of moisture vapor emitted from various concrete floor types and determines their suitability for receiving resilient floor coverings.

Materials Required:

• Plastic dish (69 mm diameter, 15-20mm height) with anhydrous calcium chloride
• Transparent cover (38mm height) with 12 mm flanges
• Scale (0.1 g accuracy)

Test Frequency:

• Three test locations for areas up to 100 square meters.

Test Procedure:

1. Clean the concrete surface thoroughly.
2. Weigh the dish, calcium chloride, lid, and tape.
3. Record starting weight, time, date, and test location.
4. Place the open dish on the concrete surface and cover it with the transparent lid.
5. After 60 to 72 hours, cut a hole in the plastic cover, remove the dish, and reseal it.
6. Weigh the sealed dish again.

Calculation of Moisture Vapor Emission Rate (MVER)

The formula for calculating MVER is given by: MVER=ΔmassA⋅TMVER=A⋅TΔmass​

Where:

• $MVER$ is the moisture vapor emission rate.
• $\Delta \text{mass}$ is the change in mass (weight gain) of anhydrous CaCl2.
• $A$ is the contact area of the flanged cover on concrete.
• $T$ is the exposure time in hours.

Other Methods

Apart from the discussed methods, various other concrete moisture tests include:

• Relative humidity test (ASTM F 2170 and F 2420)
• Electrical resistance test (No ASTM standard test method)
• Electrical impedance test (No ASTM standard test method)
• Mat test (No ASTM standard test method)
• Qualitative calcium chloride test (No ASTM standard test method)
• Primer or adhesive strip test (No ASTM standard test method)

Understanding and employing these tests provide a holistic approach to managing concrete moisture, ensuring the durability and longevity of structures.