Another typical phone call. An owner wants to know how long it will take for the lightweight concrete in the elevated slabs of his new building to dry before he can place the floor covering. The slabs were placed 4 months ago, but tests still show a moisture-vapor-emission rate of 8 pounds per 1000 square feet in 24 hours. The floor-covering manufacturer requires the concrete to be at 3 lbs/1000 sf/24 hrs. Delaying floorcovering installation will delay building occupancy. The owner has never had this problem in other buildings he has constructed. Why won’t this concrete dry? Concrete is concrete, right?

Well, unfortunately it’s not. Many owners and contractors have told us they’ve experienced project delays while waiting for lightweight concrete to dry. Though we couldn’t find any data regarding the drying time of lightweight concrete, field experience tells us that lightweight concrete takes longer to dry than normal-weight concrete. To help fill this information gap, CONCRETE CONSTRUCTION devised a testing program to find out how long it takes lightweight concrete to dry.

The test program 

Three normal-weight concretes with water-cement ratios of 0.31, 0.37, and 0.40 were delivered to a testing lab in 1-cubic-yard loads. A lightweight concrete mix with a water cement ratio of 0.40 was also delivered to the testing lab. The proprietary mixes were supplied by the ready-mix division of CAMAS Colorado, Denver. The normal-weight concrete moisture-vapor-emission test results were reported in THE CONCRETE PRODUCER (Ref. 1). Here we compare the lightweight concrete moisture-vapor-emission test results with those for normal-weight concrete having the same water-cement ratio. The producer tightly controlled water content and water-cement ratio by closely monitoring aggregate moisture and water left in the drum. The fresh and hardened properties of the lightweight concrete were as follows:

■ 3-inch slump
■ 4.5% air content
■ 124-pound-per-cubic-foot unit weight
■ 48° F temperature
■ 6850-psi 28-day compressive strength

Workers placed and vibrated the concrete in 3-foot-square test slabs 2, 4, 6, and 8 inches thick. After striking off the surface with a 2x4 and floating it by hand, they covered the slabs with plastic sheeting for 3 days. Calciumchloride moisture-emission testing began after the sheeting was removed.

Moisture-emission tests were conducted in accordance with the test manufacturer’s instructions. Technicians ran one test on each slab at the 3-day age but thereafter ran two tests on each test slab and reported an average test value. 

The calcium-chloride test kits were left in place for 72 hours on slabs stored inside the test lab at a 70±3° F and a relative humidity of 28±5%. This is the normal indoor environment during the winter in the Rocky Mountain region, where the tests were conducted, and represents the environment found in many buildings without relative-humidity controls. 

Drying in months instead of weeks 

moisture-vapor emission rates for the test slabs after drying for up to 183 days.
The table shows moisture-vapor emission rates for the test slabs after drying for up to 183 days. Drying times are measured from the end of the 3-day curing period to the end of the 72-hour moisture-emission test. Thus the 3-day measurement was taken 6 days after concrete placement. The test data support the following conclusions.

Lightweight concrete dries slower. Regardless of the test slab thickness, the lightweight concrete took about 6 months to dry to a moisture-vapor emission rate of 3 lbs/1000 sf/24 hrs. Normal-weight concrete of the same water-cement ratio took only 6 weeks of drying in laboratory air to reach the same level (Ref. 1). From previous work (Ref. 2), we know that laboratory drying represents the fastest drying time. So field conditions that include wet-dry cycles will increase the actual time for the slab to reach the specified moisture-vapor-emission limits. 

Thickness effects

As with normal-weight concrete, the moisture-vapor-emission rates were unaffected by slab thickness. 

Don’t blame the contractor

Lightweight concrete offers many advantages. However, any owner using lightweight concrete that’s to be covered by a moisture-sensitive floor covering or any architect/engineer specifying this combination should consider the slower drying time as an important part of the construction schedule. Once lightweight concrete is specified, the contractor can’t change its drying characteristics. If owners want the benefits of lightweight concrete and a fast-track schedule, they may need to consider applying a polymer coating or sheet product to reduce moisture emissions.


1. Bruce A. Suprenant and Ward R. Malisch, “Quick-Dry Concrete: A New Market for Ready-Mix Producers,” THE CONCRETE PRODUCER, May 1998.
2. Bruce A. Suprenant and Ward R. Malisch, “Are Your Slabs Dry Enough for Floor Coverings?” CONCRETE CONSTRUCTION, August 1998.

By Bruce A. Suprenant and Ward Malisch

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