Classification of Aggregates According to Size

Classification of Aggregates as per Size

 

What is an Aggregate?

Aggregates are the important constituents of the concrete which give body to the concrete and also reduce shrinkage. Aggregates occupy 70 to 80 % of total volume of concrete. So, we can say that one should know definitely about the aggregates in depth to study more about concrete. The size of aggregates used in concrete range from few or more down to a few microns. The maximum size of the aggregate may vary, but in each case, it is to be so graded that the particles of different size fractions are incorporated in the mix in appropriate proportions. The particle size distribution is called the grading of the aggregate according to size the aggregate is classified as: fine aggregate, coarse aggregate and all in aggregate.

aggregate

Fine Aggregate :-

It is the aggregate most of which passes through a 4.75 mm IS and contains only that much coarser material as is permitted by the specification sand is generally considered to have a lower size limit of about 0.07 mm. Material between 0.06 mm and 0.002 mm is classified as silt, and still smaller particles are called clay. The soft Deposit consisting of sand silt, clay in about equal proportions is termed loam. The fine aggregate may be one of the following types: 

1) natural sand that is the fine aggregate resulting from natural disintegration of Rock and for that which has been deposited by stream and glacial agencies, 

2) crushed stone sand that is the fine aggregate produced by crushing hard stone or 

3) craft Travels and that is the fine aggregate produced by crushing natural gravel. 

According to size, the aggregate may be described as coarse, medium and fine sands. Depending upon the particle size distribution IS: 383-1970 has divided the fine aggregate into four grading zones. The grading zones become progressively finer from grading zone 1 grading zone 4.

fine aggregate

Fine aggregate	Size variation
Coarse Sand	2.0mm – 0.5mm
Medium sand	0.5mm – 0.25mm
Fine sand	0.25mm – 0.06mm
Silt	0.06mm – 0.002mm
Clay	<0.002

 Coarse Aggregate:- 

Aggregates most of which are written on the 4.75 mm IS sieve and content only that much of fine materials as is permitted by the specification are termed coarse aggregates. The coarse aggregate may be one of the following types;

1) crushed gravel for stone obtained by the crushing of gravel for hard stone, 

2) uncrushed gravel or stone resulting from the natural disintegration of rock, or

3) partially crossed gravel or stone obtained as a product of the bleeding of the above two types. 

The graded coarse aggregate is described by its nominal size, i.e. 40 mm, 20 mm,16 mm, and 12.5 mm, etc. For example, a graded aggregate of nominal size 12.5 mm means and aggregate most of which passes the 12.5 mm IS sieve. Since the aggregates are formed due to natural disintegration of rocks or by the artificial crushing of rock
or gravel, they derive many of their properties from the parent rocks. These properties are chemical and mineral composition, petrographic description, specific gravity, hardness, strength, physical and chemical stability, pore structure, and color. Some other properties of the aggregates not possessed by the parent rock are particle shape and size, surface structure, absorption, etc. All these properties may have a considerable effect on the quality of concrete in fresh and hardened states.

coarse aggregate

Coarse aggregate	Size
Fine gravel	4mm – 8mm
Medium gravel	8mm – 16mm
Coarse gravel	16mm – 64mm
Cobbles	64mm – 256mm
Boulders	>256mm

  All-in-aggregate:-

Sometimes combined aggregates are available in nature comprising different fraction of fine and coarse aggregates, which are known as all-in-aggregate. In such cases adjustments often become necessary to supplement the grading by addition of respective size fraction which may be deficient in the aggregate. Like coarse aggregate, the all-in-aggregate is also described by its nominal size. The all-in-aggregates are not generally used for making high quality concrete.

Single-Size-Aggregate:-

Aggregates comprising particles falling essentially within a narrow limit of size fraction are called single-size-aggregates. For example, a 20 mm single- size-aggregate means in aggregate most of which passes through a 20 mm IS sieve and the major portion of which is retained in a 10 mm IS sieve.