Concrete is a mixture known for its durability and compressive strength. One of the ingredients in concrete is the aggregate, a mix of gravel and crushed rocks that becomes bound together by the other ingredients as the concrete cures. You can divide concrete into two classes, lightweight and heavyweight, based on the type of aggregate it contains.
The key difference between lightweight and heavyweight concrete is the density of the aggregate. Lightweight concrete is made using aggregate whose density is less than 2,100 kilograms per cubic yard (or 131 pounds per per cubic foot), while aggregates with greater densities give rise to so-called heavyweight concrete. Expanded clay, slate and shale are common aggregate materials in lightweight concrete, while heavyweight mixtures use either denser minerals such as barite or manufactured materials such as iron and lead shot. Lightweight concrete has a broader range of applications.
The aggregate material in lightweight concrete typically has a lower density because it is more porous. For example, it may have many microscopic air spaces incorporated into its structure. The result is an aggregate with a high absorption value, which may sometimes necessitate caution in determining the amount of each ingredient to use. Prewetting the aggregate before combining it with the other ingredients may help alleviate the change in consistency that might otherwise accompany absorption.
Heavyweight concrete is made from aggregates with a density between 2,080 and 4,485 kilograms per cubic meter (or between 130 and 280 pounds per cubic foot). These aggregates are much less porous and absorbent, and the resulting concrete has a higher mass-to-volume ratio. The aggregate-to-cement ratio also favors aggregate to a much greater degree than in lightweight concrete, even though the contents of the cement are roughly the same. Mixers and pumps may exhibit greater rates of wear when working with this type of material.
Lightweight concrete can be subdivided into groups such as ultra-lightweight and structural lightweight, depending on the density of the aggregate. Concrete with aggregate density less than 500 kilograms per cubic meter (31 pounds per cubic foot), for example, is considered ultra-lightweight, while structural concrete generally falls in the range of 1,100 to 1,750 kilograms per cubic meter (70 to 110 pounds per cubic foot). Most structural, masonry and insulating concretes are lightweight. Heavyweight concrete, by contrast, is most often used for radiation shielding, although it also is used in the construction of ballasts for offshore pipelines.
Based in San Diego, John Brennan has been writing about science and the environment since 2006. His articles have appeared in "Plenty," "San Diego Reader," "Santa Barbara Independent" and "East Bay Monthly." Brennan holds a Bachelor of Science in biology from the University of California, San Diego.