Moisture Problems Overhead Lightweight concrete in roofing and waterproofing applications by Anthony Nicastro, PE, and Kenneth A. Klein, PE Image © BigStockPhoto.com ONE SOURCE OF ROOFING FAILURES CAN BE THE USE OF CAST-IN-PLACE LIGHTWEIGHT STRUCTURAL CONCRETE AS THE SUBSTRATE FOR THE MEMBRANE. THE MATERIAL CAN ADVERSELY AFFECT WATERPROOFING PERFORMANCE WHEN ITS PROPERTIES ARE NOT CAREFULLY CONSIDERED. Lightweight structural concrete retains much more moisture and releases it far more slowly than regular-weight concrete. To date, the design/construction industry has not fully understood how this additional moisture affects roofi ng and waterproofi ng systems and their components. Lightweight structural concrete is used primarily to achieve cost savings—due to decreased dead load—that are realized despite its higher cost per cubic foot. Specifically, lightweight structural concrete is approximately 25 percent lighter, but just as strong as regular concrete.1 To achieve an overall reduction in weight, the material uses aggregate more porous than conventional types. These lightweight aggregates consist of both natural materials (e.g. pumice and scoria) and manufactured substances that are produced by heating and plastically expanding shale, slate, clay, or slag. Porous materials absorb water and may adversely affect the concrete mix if not controlled by pre-wetting the aggregate.2 Pre-wetting introduces moisture that the aggregate retains after the mixing of concrete and placement at the jobsite. In addition to absorbing 10 to 20 percent of water by weight, the lightweight concrete can deceptively continue to absorb water for several weeks.3 Even advanced building science computer modeling programs cannot easily predict the manner in which lightweight structural concrete will store 34 the construction specifier | september 2009 CS_Sept09.indd 34 8/12/09 1:23:58 PM