The surface area per unit weight of the material is termed as specific surface. This is an indirect measure of the aggregate grading. Specific surface increases with the reduction in the size of aggregate particle. The specific surface area of the fine aggregate is very much more than that of coarse aggregate.
Importance of Specific Surface
The quantity of water required to produce a given workability depends to a large extent on the surface area of the aggregate.
Greater surface area requires more water for lubricating the mix to give the desired workability. Workability of a mix is therefore influenced more by finer fraction than the coarser particles in a sample of aggregate.
For a given quantity of water, the presence of smaller particles reduces the workability. Very fine particles i.e. for fine aggregate (within 300 micron to 150 micron) contribute more towards workability by acting like ball bearings to reduce the internal friction between coarse particles.