In concrete construction, lightweight concrete has become very popular in recent days. Self weight represents a very large proportion of the total load on the structure and there are clearly considerable advantages in reducing the density of the concrete. The chief of these are the use of smaller sections and the corresponding reduction in the size of foundation. Furthermore with lightweight concrete the form-work need to withstand a lower pressure than would be the case with ordinary concrete and also the total weight of materials to be handled is reduced with a consequent increase in productivity. Lightweight concrete also gives better thermal insulation than ordinary concrete. The practical range of densities of lightweight concrete is between 20 and 120 lb/ft.
Classification of Lightweight Concrete
There are three methods of producing lightweight concrete. First, porous lightweight aggregate whose specific gravity is approximately 2.6. The resultant concrete is generally known by the name lightweight aggregate used.
The second method of producing lightweight concrete relies on introducing large voids within the concrete. These voids are clearly distinguished from the extremely fine voids produced by air entraining. This type of concrete is variously known as aerated cellular, foamed or gas concrete.
The third means of obtaining lightweight concrete is by simply omitting the fine aggregate from the mix so that a large number of interstitial voids is present. Coarse aggregate of ordinary weight is generally used. This concrete is described by the term No fines Concrete.
In essence then, the decrease in density is obtained in each case by the presence of voids, either in the aggregate or in the mortar or in the interstices between the coarse particles. It is clear that the presence of these voids reduces the strength of lightweight concrete compared with ordinary concrete., but in many applications high strength is not essential. Lightweight concrete provides a very good thermal insulation but is not highly resistant to abrasion. In general, lightweight concrete is more expensive than ordinary concrete and mixing, handling and placing also also require considerably more care and attention than ordinary concrete. However, for many purposes the advantages of lightweight concrete outweight its disadvantage and there is a world wide trend towards using more lightweight concrete and also towards using it’s new applications.
Lightweight concrete can also be classified according to the purpose for which it is to be used. Structural lightweight concrete (load bearing) and ordinary or non-structural lightweight concrete(non load bearing such as non-load bearing walls for insulation purposes etc). Structural lightweight concrete should have a compressive strength measured on a standard cylinder specimen at 28 days of not less than 2000 psi. The density of such concrete, determined in dry state, is usually above 60 lb/ft3.
