Such concrete units are called as the precast concrete.
The main constituents are the cement, aggregates, and water.
Most commonly used mix design is 1:2:4 which is the normal mix design..
This is a special technique in which the bars or the tendons used in the concrete is stressed before the actual service load application.
During the mixing and the placing of the concrete, these tensioned bars placed firmly and held from each end of the structural unit.
Once the concrete sets and harden, the structural unit will be put in compression.
This phenomenon of prestressing will make the lower section of the concrete member to be stronger against the tension.
The examples of light weight aggregates are the pumice, perlites, and scoria.
The light weight concrete is applied for the protection of the steel structures and are also used for the construction of the long span bridge decks.
These units have the advantage of acquiring speedy construction as only assemblage is necessary.