The concrete masonry units used in structures built with medium weight concrete (LC) have much higher compressive strengths than normal weight concrete, but they do not reach the same high levels as those of lightweight aggregate concrete or autoclaved aerated concrete. The increased strength is mainly due to the admixture of different types of air-entraining material in the mix.

The structural properties of LC are comparable to those of normal concrete (NC). In addition, it has good insulation characteristics. To satisfy the insulating requirements, the concrete must have a low bulk density. The concrete is therefore usually manufactured with different types of alternative aggregates. These may include more porous rock like pumice, industrial byproducts like fly ash or slag, or aerated natural raw materials such as clay, slate or shale that have been thermally expanded. The most common material is expanded clay from a single producer, which has a loose bulk density of up to 1120 kg/m3.

The quality of LC depends on the type of aggregates and the mixing process. Structural LC with coarse and fine aggregates made of standard NC is called conventional lightweight concrete (CLC) while CLC with only fine aggregates and no coarse aggregates is known as all-lightweight aggregate concrete or sand lightweight aggregate concrete (SLAC). For the sake of clarity, in this article we will refer to structural LC with an oven dry density between 1360 and 1920 kg/m3 and a minimum compressive strength of 17 MPa.