The American Society for Testing and Materials (ASTM) sets various standards across various industries. One such standard, utilized to check moisture levels prior to installing finished floor products, is ASTM F2170. It currently states: “Allow 72 hours to achieve moisture equilibrium within the hole before making relative humidity measurements.” Installers measure concrete slab moisture content (MC) by inserting probes to measure water vapor levels.
However, not all concrete RH probes are alike. In fact, there are two distinct types: single-use and multi-use. In a recycled world, one might consider multi-use moisture content probes to be a viable way to save on the expense of their single-use cousins. Why buy a new one if the old one will still do?
The concrete slab moisture content world has a surprising answer.
Understanding Concrete Slab Equilibrium
Equilibrium is when the moisture content of a concrete slab (usually expressed as the relative humidity or RH of the concrete) reaches balance with the relative humidity (RH) of its surrounding environment. This is not achieved when concrete is poured, or even when it has cured. EMC occurs after slab moisture content rises from the bottom of the concrete slab to its surface in order to evaporate. Moisture levels vary within a slab at different depths. This why concrete contractors measure moisture content with probes placed at 40% of the slab depth. Studies have shown that the relative humidity measurement at 40% of the slab depth most accurately represents the final RH of the fully equilibrated slab.
Multi-use probes contain sensors which are inserted into a sleeve that has been installed in the concrete slab. The American Society for Testing and Materials (ASTM) requires wait time so that moisture content can equilibrate around the sensor itself. Although multi-use probes are removable, they must reach EMC themselves each time they are deployed in a concrete slab. The practical drawback is clear. Time. The larger the concrete slab, the greater the need to monitor moisture content with multiple assessments – each one requiring at least three days to conduct.
Multi-use functioning is a concern over time. Each multi-use probe must be verified for calibration within 30 days of each use. That means time to either verify calibration with each probe with your own equipment, or it means time for your probe set to be out of circulation for factory re-calibration.
Single-use sensors can be placed inside a concrete slab and left in place to avoid repeated equilibration time. When the sensor stays within the concrete, additional equilibration time is not necessary between repeated readings. In time costs alone, this represents a huge cost savings. And single-use sensors are so innocuous that they can be left inside without any adverse effect on equilibrated concrete. That`s the practical advantage. Place single-use probes across the slab`s breadth according to the ASTM standards, and you can cost-effectively assess overall concrete slab MC.
Wagner Meters produces digital single-use moisture content probes in compliance with ASTM Standard F2170. In fact, the Rapid RH 4.0 empowers installers to measure MC in one hour to within 3 percent of the final MC reading. If accuracy and cost-efficiency decided the battle, single-use concrete moisture probes undoubtedly trump their multi-use partners from the past.