For the sake of timely construction and an owner’s speed-to-revenue, developers want to migrate from completing the concrete foundation to laying their flooring as quickly as possible. Though simplistic on the surface, this specific moment in the construction process can be one of the most volatile if certain precautions aren’t taken with care.
Before any flooring can be installed, the moisture levels of the concrete slabs must be thoroughly evaluated to ensure a seamless flooring installation and an elimination of future risk. An installation occurring before the concrete slab achieves proper dryness levels will almost certainly lead to failures such as cracks, curling, blistering, staining, and crowning. This is especially true with hygroscopic flooring materials like wood, which are particularly sensitive to moisture levels in the air and within the concrete slab itself.
Methods to account for moisture levels are varied,and no one method is all-encompassing. Allowing yourself the most data through proper diligence, however, will give you a strong indicator for when to begin the flooring process.
Notes to Consider Before and During Moisture Measurement
The time it takes for concrete to dry varies dramatically on the mix of the concrete used. A standard 4-inch concrete slab can take as long as 120 days to reach an adequate moisture evaporation rate (MVER). Suspended slabs (those above ground level) generally contain more water and require additional time to reach acceptable moisture levels.
Standard moisture tests check for relative humidity (RH), otherwise known as the ratio between the partial pressure of water vapor to the equilibrium vapor pressure at the same temperature (i.e. the pressure of the vapor within the concrete against the pressure of the vapor by itself under the same conditions). RH is measured on a scale of 0-100%. The acceptable industry RH level is 80% or less, but standards might vary for different manufacturers. Installing flooring at critical RH levels increases the risk of damages as high-alkaline moisture (water containing dissolved concrete) rises from the slab. This exposure to high-pH moisture results in the erosion of flooring materials.
The atmospheric environment plays a pivotal role in the timeline of the concrete drying process in addition to effects on moisture measurement. Heating and air conditioning (HVAC) systems affect measurement, as will the humidity levels in a given region. Allowing yourself a control group in advance — a test moisture reading — can give you an indication of how an environment might skew results. If moisture levels are inadequate or otherwise strange on a particular day of testing (especially if atmospheric humidity is a factor), revisiting testing at a later date will give you a better indication of where the drying process stands.
Moisture Detection Methods for Concrete Slabs
- Relative Humidity (RH) Probe — Thought to be one of the most accurate indicators of moisture levels, and now the industry standard, RH Probe testing calculates the relative humidity within a concrete slab. Test holes are drilled to reach 40% of a slab’s total thickness (through which RH probes are inserted), allowing for a more accurate picture of the moisture levels through the entirety of a given slab. It is, therefore, the test most predictive of future moisture-related problems. This test does not calculate moisture moving through concrete slabs.
- Calcium Chloride Test —Calcium chloride testing measures pounds of moisture emitted from a slab over a 24-hour period (MVER). This involves placing a small dish of sealed calcium chloride on the concrete surface beneath a plastic dome. Moisture is absorbed by the salt and the resulting weight, generally taken after a period of three days, allows for a calculation of MVER. This method, though common, only tests the moisture on the surface of the slab.
- Sight/Feel/Smell — Should never be considered in moisture calculations.