Q.: For an electronics company operation we recently designed a slab on grade built on a moisture barrier over a gravel base to receive a special antistatic vinyl flooring. Warranty requirements for the vinyl were that the rate of emission of water vapor from the floor be not more than 3 pounds per 1000 square feet per 24 hours. When the slab was approximately 6 months old (the building having been enclosed for 3 months and heated for about 2 weeks) tests gave a value of 6 pounds per 1000 square feet per 24 hours.

The test consisted of placing a petri dish containing anhydrous calcium chloride on the slab and covering it with a 0.5-square-foot plastic cover. The cover was sealed to the surface to prevent moisture from being taken up from the air of the room. The dish of calcium chloride was removed after 24 hours and weighed to determine how much moisture had come out of this area of the floor.

The owner, contractor and design team believe that the moisture criteria may be excessively stringent. What kind of emission of moisture vapor can be expected from a curing concrete slab at 6 months? Do the test-measured moisture values correspond in any way to these values?

A.: H. W. Brewer studied the rates of moisture emission from slabs on grade under a considerable variety of conditions and reported his results in "Moisture Migration--Concrete Slab-on-Ground Construction," Portland Cement Association, Development Department Bulletin D89, published in 1965. (Available for $2.00 from Order Processing Department, Portland Cement Association, 5420 Old Orchard Road, Skokie, Illinois 60077.) All of his specimens were stored in a room at a constant temperature of 73 degrees F and a constant relative humidity of 30 percent. Some specimens were cast in a container on a 4-mil polyethylene vapor barrier. During this particular test, water was kept in contact with the lower side of this vapor barrier. At an age of 180 days, according to Brewer's Figure 16, the rate of moisture outflow was about 16 grains per square foot per hour, which is equivalent to 2.33 pounds per 1000 square feet per 24 hours. It should be noted that these slabs were 4 inches thick, so the rate of outflow from them would be smaller than from thicker slabs. You may wish to study the rates of outflow under various conditions and at various ages as given in Brewer's report.

It should also be noted that your method of test may have tended to make the results high. Anhydrous calcium chloride is a strong desiccant. It attempts to maintain a relative humidity that is close to zero in the space immediately surrounding the crystals. This would mean that concrete that had reached almost a steady rate of moisture loss at the ambient relative humidity of your building would suddenly be exposed to a lower relative humidity. This new difference in partial vapor pressure between the concrete and the air under the plastic cover would immediately speed up the rate of moisture outflow. This would produce a higher apparent rate than the floor actually experiences at ambient relative humidity.