RH in concrete stands for Relative Humidity, which refers to the amount of water vapor present within the concrete's internal pore structure, expressed as a percentage of the maximum amount of water vapor it can hold at a specific temperature. It is a critical measurement for determining the readiness of a concrete slab for the application of various flooring systems, adhesives, and coatings.
Understanding Relative Humidity in Concrete
While often mistaken for moisture content, relative humidity and moisture content measure two distinct characteristics of concrete. Moisture content represents the actual amount of water in its liquid form within the concrete, whereas humidity levels describe the amount of water in vapor form. Both are important, but RH is particularly crucial for surface-applied finishes.
Why RH Measurement is Crucial
The internal relative humidity of a concrete slab directly impacts the long-term success of floor coverings and coatings. Many modern flooring adhesives and finishes are sensitive to excessive moisture vapor. If a slab's RH is too high, it can lead to a range of problems:
- Adhesive Failure: High RH can re-emulsify water-based adhesives, preventing them from curing properly or causing them to break down over time.
- Blistering and Delamination: Moisture vapor migrating upwards can create hydrostatic pressure, causing coatings or sheet goods to bubble, blister, or detach from the concrete substrate.
- Mold and Mildew Growth: Elevated RH provides an ideal environment for biological growth beneath impermeable floor coverings.
- Flooring Damage: Wood, laminate, or vinyl plank flooring can swell, cup, or warp due to prolonged exposure to high relative humidity.
Conversely, concrete that dries too quickly or has excessively low RH during its initial curing stages can also experience issues such as increased shrinkage cracking and reduced strength.
Measuring Relative Humidity in Concrete
The most widely accepted and accurate method for measuring RH in concrete is specified by the ASTM F2170 standard: Standard Test Method for Determining Relative Humidity in Concrete Floor Slabs Using in situ Probes. This method involves inserting probes directly into the concrete slab.
The ASTM F2170 Method
- Drilling Holes: Small holes are drilled into the concrete slab to a specified depth, typically 40% of the slab's thickness for slabs drying from one side, or 20% from the top and bottom for slabs drying from two sides.
- Inserting Probes: RH probes, equipped with sensors, are placed into sleeves inserted into the drilled holes.
- Equilibration: The probes are allowed to equilibrate within the concrete for a specified period (usually 24 to 72 hours) to ensure the sensor accurately reflects the internal humidity conditions.
- Taking Readings: After equilibration, a meter is connected to the probes to display the internal relative humidity and temperature.
This method provides a true indication of the moisture vapor conditions within the concrete, which is more predictive of future performance than surface-based tests.
Factors Influencing Concrete RH
Several factors can affect the relative humidity levels within a concrete slab:
- Water-Cement Ratio: A higher water-cement ratio during mixing generally results in more internal water that needs to evaporate, leading to longer drying times.
- Slab Thickness: Thicker slabs retain moisture for much longer periods than thinner slabs. Drying time increases exponentially with thickness.
- Environmental Conditions: The ambient temperature, humidity, and airflow around the concrete slab significantly impact the rate of moisture evaporation.
- Vapor Retarders/Barriers: The presence and effectiveness of a vapor retarder or barrier beneath the slab prevent ground moisture from entering the concrete, which is crucial for controlling RH.
- Curing Methods: Proper curing allows concrete to hydrate efficiently, but can also trap moisture, requiring sufficient drying time afterward.
Practical Insights and Solutions for Managing RH
Effectively managing RH in concrete is essential for project success.
- Plan Ahead: Factor in adequate drying time for concrete slabs during project scheduling. Drying times can range from weeks to many months, depending on slab thickness and environmental conditions.
- Utilize Vapor Retarders: Always install a high-quality, properly specified vapor retarder directly beneath concrete slabs poured on grade to prevent moisture migration from the ground.
- Control the Environment: Maintain consistent temperature and humidity conditions within the building using HVAC systems. Avoid sealing up a structure too early without proper ventilation, which can trap moisture.
- Perform RH Testing: Do not skip RH testing, especially for moisture-sensitive flooring installations. Follow ASTM F2170 guidelines rigorously.
- Consider Mitigation: If RH levels exceed flooring manufacturer's recommendations, consider applying a concrete moisture mitigation system (e.g., epoxy-based coatings) before installing the finish.
| Feature | Relative Humidity (RH) | Moisture Content (MC) |
|---|---|---|
| What it measures | Water vapor in concrete's pores | Amount of liquid water in concrete |
| Form of water | Vapor | Liquid |
| Units | Percentage (%) | Percentage (%) by weight or volume |
| Primary impact | Affects adhesion of floorings and coatings | Affects strength, durability, and overall drying |
| Measurement method | In-situ probes (ASTM F2170) | Oven-drying (ASTM C1152), electrical impedance |
