Lightweight concrete

Lightweight concrete has been successfully used in buildings for over 80 years. However, as reports of moisture related flooring problems have escalated, the drying time of lightweight concrete has attracted much attention.

A key issue is the volume of water in lightweight concrete. It’s a standard practice to pre-soak lightweight aggregates before batching. Because water absorbed into lightweight aggregate is not factored into the water-cement or water cementitious material ratios, lightweight concrete initially contains a greater total volume of water than a normal weight concrete mixture with the same water- cement or water cementitious material ratios.

Concrete slabs made with lightweight concrete generally can take longer to dry than those produced using normal weight concrete.

Normal weight concrete is made by mixing sand, gravel and water with cement. Cement itself is a mixture of lime, iron oxide, magnesium oxide, alumina and silica baked in a kiln and turned into a powder. Once set, concrete is a dense, hard material. Lightweight concrete replaces the sand and gravel with volcanic materials, plastic waste and similar substitutes.

Normal weight and lightweight concrete have different physical characteristics. Normal weight concrete is known for its density, about 150 pounds per square foot. . That translates to about 100 pounds for a 5-gallon bucket full of concrete, or as much as 800 pounds in a standard-sized contractor’s wheelbarrow load. The actual weight will vary depending on the materials and mix proportions, especially the type of coarse aggregate used. The lightweight version can have 120 pounds or less per square foot. Most lightweight concrete is also injected with foam made from polystyrene that thins out the wet concrete, making it easier to pour, though certain types require careful moisture control when wet. As the lightweight concrete dries, the foam disintegrates, leaving hard cement with tiny air holes that help with heat insulation. The minimum requirement for resil- ient flooring over lightweight aggregate concretes must have dry densities greater than 90 lbs per cubic foot and cellular concretes having plastic (wet) densities over l00 lbs per cubic foot (94 lbs dry weight), providing the surface is troweled to a smooth, even finish. Regular concrete, on the other hand, insulates poorly. At the other extreme, high-density concretes used for radiation-shielding walls or bridge counterweights are made with heavyweight aggregates such as hematite. These mixtures can weigh 300 pounds/cubic foot or more.

Concrete can lose some of its structural integrity over time when cracks form. Lightweight concrete is particularly resistant to cracking, due to the flexibility creating by a mix of lightweight materials and cement. The relationship between these two elements is known as the “contact zone,” which is particularly strong in lightweight concrete, contributing to its integrity. Light- weight concrete is also less permeable than its traditional counterpart, also due to the strong contact zone.

Many slabs constructed with either normal weight or lightweight concrete are not drying sufficiently to meet the moisture levels required for installation of flooring materials within the allowable project schedule. So, unless many months of very favorable interior drying conditions are provided, it will be difficult for a concrete slab to reach the moisture levels currently required by many flooring material manufacturers and industry standards

The benefits of lightweight concrete shouldn’t be dismissed solely on the premise that switching

to normal weight concrete will solve the concrete drying issue. Multiple factors influence the time necessary for concrete to reach an acceptable level of dryness, including water/cement ratio, curing methods and duration, and ambient conditions. These factors should be given careful consideration at the design stage and throughout the construction process if either normal weight or lightweight concrete can be expected to dry to the levels currently required by the floor covering industry.

Because lightweight concrete can retain significant amounts of moisture within the slab, it is critical that lightweight floors be tested for moisture in accordance with ASTM F 2170, “Standard Test Method for Determining Relative Humidity in Concrete Floor Slabs Using in situ Probes.” DO NOT TEST LIGHTWEIGHT CONCRETE FLOORS USING ASTM F 1869,

“Standard Test Method for Measuring Moisture Vapor Emission Rate of Concrete Subfloor Using Anhydrous Calcium Chloride.” The Calcium Chloride test method does not indicate the moisture condition deep within a slab and can be especially misleading when used on lightweight concrete slabs.

Special Thanks to JJ Haines for the Info