Concrete Drying Times for Normal Weight and Lightweight Concrete in Real World Applications

By Bill Wolfe, Norlite Corporation

Since the 1980’s there have been several high profile cases of sick building syndrome.The Environmental Protection Agency (EPA) has identified indoor air pollution as one of the four greatest risks to human health. The World Health Organization estimates that 30% of new and remodeled buildings worldwide experience indoor air quality problems.In an effort to improve indoor air quality the EPA mandated that volatile organic compounds (VOC’s) be limited in flooring adhesives.This September 1999 action sent shock waves through the construction industry which prompted development of many new flooring adhesives that were low in VOC’s.

These low VOC adhesives are water based as compared to the previous solvent based adhesives.The interaction between substrate, adhesive, and flooring had little room for error and failures on all types of substrates and flooring types increased.Adhesives were much more susceptible to degradation from alkalinity.Flooring adhesive manufacturers came out with specific recommendations for substrate moisture and pH levels prior to placing the flooring down.

Recommendations were established and published concerning the recommended moisture vapor emission rates (MVER) and relative humidity in concrete floor slabs1. These rates were established at 3 pounds per 1,000 square feet per 24 hours as measured by ASTM method 1869 Standard Test Method for Measuring Moisture Vapor Emission Rate of Concrete Subfloor Using Anhydrous Calcium Chloride, and 75% relative humidity as measured by ASTM method F 2170 Standard Test Method for Determining Relative Humidity in Concrete Floor Slabs Using in situ Probes.

A study was designed to evaluate the interaction between the concrete substrate, flooring adhesive, and the flooring. The rate at which the concrete dried was a key component evaluated. To achieve the real word conditions, the assemblies used in this study were large, 12 feet by 12 feet, and use construction methods and materials found on jobsites. The assemblies used commercial corrugated decking and were constructed with a minimum of 10” of air space below the assemblies to allow air to circulate.

The concrete was supplied by a local ready mix supplier and the concrete was finished using a commercial concrete contractor. All of the concrete was designed, batched, and finished as though it was a typical job. Fire rated assemblies complying with Underwriters Laboratories Design No. D916 for a two hour fire rated assembly were constructed to compare normal weight and lightweight concrete2.

Slab Finishing Sequence

Drying Times

Slab drying times were monitored using two methods.Test method ASTM F1869, Standard Test Method for Measuring Vapor Emission Rate of Concrete Subfloor Using Anhydrous Calcium Chloride, was used to measure the Measuring Vapor Emission Rate (MVER) on the surface of the slabs. Although this test method only measures the top surface of the concrete, it is a useful tool when used in conjunction with other moisture monitoring methods.Test method ASTM F2170 Standard Test Method for Determining Relative Humidity in Concrete Floor Slabs using in situ Probes, was used to measure the internal relative humidity (RH) of the test slabs.

The testing assemblies were constructed in a warehouse located in Dalton, GA.There were no environmental controls so the assemblies were exposed to ambient temperatures and humidities. The overhead doors adjacent to the testing area were opened on a daily basis as well as the doors on the opposite side of the warehouse.This allowed air flow over and around the slabs.The slabs were protected from any precipitation so no rewetting of the testing assemblies ever occurred throughout the duration of the study.The ambient conditions found in the test area ranged in temperature between 47.6OF and 98.9 OF.The relative humidities ranged between 21.4% and 83.8%.

The MVER tests were conducted throughout the testing period to study the drying characteristics of the real world assemblies. The tests were run over concrete that was lightly ground prior to the testing domes being placed. The results shown in the chart below show that the lightweight concrete (slab 1) and the lightweight concrete with slots in the metal decking (slab 3) had very similar MVER drying curves.The slots in the metal decking had no effect.The normal weight concrete MVER data always was slightly less than that of the LWC assemblies but was very similar.The different types of concrete showed MVER results very similar.This shows much different results from testing that was conducted in previous studies3.The MVER’s only dipped below the ASTM F710 recommended emission rate of 3 pounds per 1,000 square feet per 24 hours during the winter months when the ambient RH was low enough to get the concrete that dry.When the RH climbed back up, the MVER went back up above recommended levels for flooring placement1.After the concrete reached equilibrium with its surroundings, the MVER’s were a function of the ambient conditions.

Moisture Vapor Emission Rates for Test Assemblies

In situ relative humidity probes were placed near the center of the testing assemblies to monitor the internal RH of the concrete as it dried. The probes were placed in sleeves in the deepest part of the deck corregations, 40% below the slab surface. The sleeves were drilled and the probes placed in the concrete 30 days after the completion of concrete curing. The initial probe on slab 1 was knocked out of calibration at the beginning of the testing period. This was most likely caused by the very high internal RH found in the concrete during the early stages of drying. The probe was exchanged in mid-September and the values fell into a more realistic range. Both the lightweight and normal weight concrete performed very similar always within a few percentage points of each other. None of the testing assemblies made it to the 75% recommended internal RH for flooring application1.

Internal Relative Humidities for Test Assemblies

The similar drying profiles of the NWC and LWC assemblies can be explained by looking at the totals water in the concrete for the assemblies.The water that is not consumed by the hydration of the cementitious materials must either evaporate of or be included in the internal moisture of the slab at equilibrium.The total water in the NWC and LWC assemblies is shown in the table below.Even though the lightweight aggregate was saturated to a point that was suitable for pumping, there was only 11.1% more water in the LWC assembly.Since evaporation can take place from the top surface of the testing assemblies, the water in the NWC assembly has to travel 23.8% further from the deepest part of the slab to reach the surface.This extra distance may account why the slabs took an equal amount of time to reach the 3 pounds per 1,000 square feet per 24 hours mark.

Water in Pounds Found in Test Assemblies


Concretein Assembly

Mix Water per cubic yard

Sandper cubicyard


WaterfromSandper cubicyard

HWStoneper cubic yard


Water from HWStoneper cubic yard

LWAper cubic yard


Waterfrom LWAper cubicyard

TotalWaterper Assembly
























In an effort to better understand the drying characteristics of real world concrete flooring assemblies, three comparable test assemblies were constructed. The assemblies were constructed as specified by commonly used 2 hour fire rated deck designs. These assemblies indicated that both lightweight and normal weight concrete assemblies dry at very similar rates. The study also indicated that the drying characteristics are heavily dependent upon the ambient conditions surrounding the slabs. All of the slabs fell below accepted moisture vapor emission rates for only a short period of time. This only occurred when the ambient conditions were suitable to allow the concrete, which was at equilibrium with its surroundings, to achieve the industry accepted values. Once the ambient conditions changed the concrete followed and raised above the accepted levels.

The internal relative humidities that were measured in the normal weight and lightweight slabs never achieved the recommended values for floor covering placement. The readings followed the same trend lines mirroring the ambient conditions. The values reached their lowest point when the ambient conditions were the driest. After the ambient relative humidity increased, all of the concrete slabs followed.

The normal weight and lightweight slabs dried at the same rate and mirrored the ambient conditions in the study area. Unless the environment is controlled after concrete has reached equilibrium with its surroundings, the slabs may never reach the currently accepted values for flooring placement.

More buildings are facing fast track construction schedules to get early occupancy in the structure. We need to understand the drying characteristics of all flooring systems to make informed decisions concerning construction and occupancy schedules.


  • ASTM F710 – 08 Standard Practice for Preparing Concrete Floors to Receive Resilient Flooring, ASTM International, West Conshohocken, Pennsylvania, 2008.
  • Fire Resistance Directory, Volume 1, Underwriters Laboratories, Inc., Northbrook, Illinois, 2009, pp 219-222.
  • Suprenant, Bruce A. and Malisch, Ward R., “Long Wait for Lightweight”, Concrete Construction Magazine, Hanley Wood, LLC., November 16, 2000.