Structural Lightweight Concrete Floor Drying Study
Author: Peter Craig
January 31, 2011
A recent study sponsored by the Expanded Shale, Clay and Slate Institute has shed new light on the issue of how long it takes for elevated slabs using structural lightweight concrete to dry sufficiently for the successful application of flooring materials. The study is available here in pdf format and a summarized version was also published in the January 2012 Concrete International (see the link below.)
The lead investigator for the study was Peter Craig, an independent Concrete Floor Consultant. He is a Past National President of the International Concrete Repair Institute (ICRI), a member of ACI Committee 302, Construction of Concrete Floors, and a Lead Instructor for the ICRI Moisture Testing Certification Program.
Our synopsis: Over the past decade moisture-related flooring and coating problems have become one of the most serious and costly of construction problems. Over the years, there have been several concrete drying studies performed. These studies have measured the drying time of both normal weight and lightweight structural concrete mixtures of various water-to-cement ratios (w/c). Some studies reported significant extension in drying time for structural lightweight concrete.
To answer questions raised by previous studies, the Expanded Shale, Clay and Slate Institute (ESCSI) in cooperation with members of the flooring and concrete industry conducted two concrete drying studies that compared the drying times of normal and lightweight concrete in both laboratory and field conditions. The first study was a non-climate controlled drying study of both normal and lightweight concrete placed and maintained in a warehouse in Dalton, Georgia. The second study was a climate controlled drying study of both normal and lightweight concrete placed and maintained at the W.R. Grace Laboratories in Cambridge, Massachusetts.
Based on the results of these two studies, and observations from scores of projects around the country, the drying of both normal weight and lightweight concrete, to the levels currently being required by the flooring industry, can be a very difficult challenge within the construction schedule. But in general, the difference observed between lightweight and normal weight concrete was at most several percent relative humidity (RH), and delays to reach equal RH or moisture vapor emission rate (MVER) were a few weeks, not multiple months.
The design benefits of lightweight concrete should not be discarded with the belief that switching to normal weight concrete alone will solve the slab drying problem. Concrete floor drying is a complex issue with many variables. You should refer to the actual report data for your use and analysis.
January 2012 Concrete International article on Floor Drying