The concrete industry has evolved in recent years with a significant shift from normal weight to structural lightweight aggregates. This has saved on installation costs and has helped meet green build requirements. However, there are some specific considerations when using this type of material in a roof deck system.
Structural concrete is a mix of aggregates, cement and water that works with steel to withstand different levels of pressure over long span conditions. This provides the strength to the structure and allows it to withstand movement from earthquakes, wind, etc. Structural lightweight concrete is a mixture of shale, slate, slag and clay that is used to achieve the same strength as traditional structural concrete but at a much lower weight.
This new mix is achieved by soaking the aggregates prior to mixing with the wet concrete and reducing the normal water to cement ratio. The structural concrete can still have a high strength but the aggregate can be up to 20-25% lighter than typical concrete. The aggregates can be recycled and repurposed as well.
Lightweight insulating concrete (LWIC) is batched on-site and placed over a variety of structural deck systems that are designed to carry loads. The LWIC is often encapsulated with a foam insulation such as expanded polystyrene and a slope-to-drain arrangement.
In a roof deck application, the LWIC is typically capable of supporting foot traffic two to three days after it has been cast. The roofing contractor can then easily install the built-up or single-ply roof membrane. This is in contrast to the time required to erect and remove pre-tapered board stock insulation systems.