Structural light weight concrete (SLWC)

Structural light weight concrete (SLWC) has extreme importance to the construction industry. Most of the current concrete research focuses on the high-performance concrete, by which is meant a cost-effective material that satisfied demanding performance requirement including durability. Light weight concrete consist of structural light weight aggregate has been defined in ASTM C30.

The concrete has a minimum 28 days’ compressive strength of 25000 psi (17 MPa), an equilibrium density between 70 to 120 lb/ft3 (1120 to 1929 kg/m³) and consist entirely of light weight aggregate or combination of light weight & normal density aggregate. But that need to be understood that it’s a definition not a specification, hence project specific or raw materials specific and it may vary.

In India light weight concrete is presently being used as filling materials. It has been observed that the density of such type of concrete is around 7-9 kN/m³ but the compressive strength is very poor (around 3-5 MPa). It is mainly used where dead load reduction is the main focus but structural strength is not a concern. The use of LWC is usually predicted on reduction of project cost, improved functionality or combination of both. Using artificial light weight coarse aggregate production of LWSC is a very sound option but may not be cost effective one. Our aim is to produce a type of concrete which will be lighter in density, satisfy the structural requirement & should be made of using locally available resources.

Aggregates with an oven-dry particle density less 2000 kg/m3 or an oven-dry loose bulk density less than 1200 kg/m3 are called as lightweight aggregates according to EN206-1:2000. ASTM C330 also defines a maximum limit for the bulk density, which is 1120 kg/m3 and 880 kg/m3 for fine and coarse lightweight aggregate, respectively.

Lightweight aggregates are divided into two categories according to their sources:

• Natural Lightweight Aggregates.
• Manufactured (Synthetic) Lightweight Aggregates.

Natural lightweight aggregates - They are obtained by processing volcanic rocks. Pumice, scoria, tuff and perlite are some of the examples which fall in this category. Pumice is a light-colored porous glass with elongated voids. Scoria is a dark colored porous glass with spherical voids. Tuff is a porous glass formation of consolidated volcanic ash. Perlite is a porous glass with a high silica content. It generally contains 2-5% water (Mehta & Monteiro, 2006).

Synthetic lightweight aggregates - They are expanded forms of materials such as clay, shale, slate, perlite and vermiculite, produced by heat treatment, generally around 1000 degree Celsius. The materials are either reduced to desired size before calcination or crushed after the calcination process. The expansion results from the entrapment of gases, which are generated during heat treatment, inside the processed material. The use of initially pelletized materials in heat treatment process produces spherical aggregate particles with a semi-impervious coating having 12-30% lower water absorption capacity than the particles produced of un-pelletized material. Therefore, coated particles are preferable from workability point of view, however they are more expensive than uncoated ones (Neville & Brooks, 2010).