Should You Calculate or Estimate the Time for Concrete To Dry?
Disclaimer: The Swedish Concrete Association method is a guideline only and should not be used as part of any construction document. Actual field concrete installation drying time calculations must be conducted by qualified personnel, based on actual jobsite conditions. Reference to the method here is for information purposes only.
Strong, durable concrete requires careful and accurate slab moisture content (MC) management. Yet, stakeholders too often assume that dry concrete surfaces equal concrete dryness. Beware the moisture content inside the slab, which may not have yet reached the surface to evaporate according to relative humidity (RH) conditions. Cover the slab prematurely, and concrete moisture content could result in all sorts of costly and unwanted trouble in the future.
As with many things in life, truth lies in the fundamentals. Concrete moisture refers to the composite of three moisture quantities: the total amount of water used to produce a concrete batch, plus curing water, minus the water which is bound in the hydration process. Consider the amount of water content in concrete used in a high-rise, mall or car-park, and you get a sense of how much moisture content is involved. However, the industry rule-of-thumb applies to every slab: Allow 28 days of drying time for each inch of concrete thickness, once the environment is conducive for drying [i]. Installers are encouraged to install tests earlier and monitor in order to ensure the concrete is actually drying “according to plan”.
A concrete moisture meter can indicate moisture content levels at the slab surface, but it does not assess the amount of internal water content in concrete because time is not the only factor in the drying equation.
Ambient relative humidity (RH) exerts the greatest influence on slab moisture content levels. When ambient RH (the water vapor in the surrounding air) rises, the rate of drying slows due to slab moisture retention. When ambient relative humidity decreases, the rate of drying increases as MC evaporates more quickly from the surface of the slab.
While historically, surface-based test methods were used to try to assess moisture content and slab readiness, at the dawn of the 21st Century, industry professionals researched moisture content assessment in order to improve concrete drying time estimation. It was methodically determined: Relative humidity (RH) testing was the most precise and thorough means of MC assessment.
The Swedish Way: RH
The Swedish Concrete Association has developed a way to estimate minimum concrete slab drying times at the planning stage, rather than fretting over the poured slab itself while project times swell.
The calculation for “standard drying time” requires several important environmental assumptions, including:
- 7.2-inch thick (180 mm) slab of Portland cement concrete
- drying temperature: 64° F (Fahrenheit); 18° Celsius (C)
- 60% ambient (air) relative humidity
- elevated dual-side drying
- curing conditions: two weeks of rain and two weeks of high relative humidity (prior to commencing drying)
- estimation parameter: water-to-cement ratio
- RH which characterizes moisture conditions after drying: 85% and 95% [ii]
From this “standard drying time” they have also estimated certain correction factors that can be used for additional variables, such as varying slab thickness, slabs only drying from one side, varying W/C ratios, etc.
Although the estimation method above is not scientifically corroborated at this time, results of testing did provide an informative variance of slab drying time estimations – enough to have drawn recognition from the Portland Cement Association (PCA). At the very least, the Swedish way proves to be a cautious advancement in “rule of thumb” calculations.
The Wagner Meters Rapid RH®
Wagner Meters, of Rogue River, OR, has developed a thorough and precise moisture test based on relative humidity methodology: the Rapid RH®. The Wagner Meters Rapid RH® 4.0 EX system uses state of the art in situ probes, which can be inserted into concrete floor moisture test holes for timely relative humidty (RH) measurement. The American Society for Testing and Materials (ASTM) also established the standard ASTM F2170, which recognizes using relative humidity as a methodically-sound way to assess slab moisture conditions.
With the Rapid RH® from Wagner Meters, RH assessors can discern overall slab moisture conditions by measuring RH at 40 percent of slab depth. In as little as one hour after installing RH Smart Sensors, the Wagner Meters Rapid RH® provides moisture data to within three percent of the final RH result.
When the slab looks dry, appearances can be deceiving. Consider what the Swedes and Wagner Meters’ Rapid RH® have in common: relative humidity methodology.
[i] “Drying of Concrete“: Portland Cement Association.
[ii] Laticrete: Drying of Concrete TDS 183.