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AGGREGATE

Aggregates
  • Aggregates are the materials basically used as filler with binding material in the production of mortar and concrete. 
  • They occupy 70 to 80 percent of the volume of concrete. Therefore, they exert considerable impact on the characteristics and properties of concrete. They are derived from igneous, sedimentary, and metamorphic rocks or manufactured from blast furnace slag, etc.
  • They should be clean, hard, strong, durable, and graded in size to achieve utmost economy from the paste.
  • Earlier aggregates were considered to be chemically inert but the latest research has revealed that some of them are chemically active and also that certain types exhibit chemical bonds at the interface of aggregates and cement paste.
  • To increase the bulk density of concrete aggregates are used in two different sizes—the bigger ones known to be coarse aggregate (grit) and the smaller ones fine aggregate (sand). Coarse aggregate forms the main matrix of concrete and fine aggregate from the filler matrix between the coarse aggregate.

CLASSIFICATION OF AGGREGATE

1. On the Basic of Geological Origin

(a) Natural Aggregates

  • Natural aggregates are obtained by crushing from quarries of igneous, sedimentary, or metamorphic rocks. 
  • The most widely used aggregate is of igneous origin. Aggregates obtained from pits or dredged from rivers most often are not clean enough or well-graded to suit the quality requirement. 
  • They, therefore, require sieving and washing before they can be used in construction.

(b) Artificial Aggregates

  • Broken bricks, blast furnace slag aggregate are obtained from slow cooling of the slag followed by crushing.
  • Blast furnace slag and synthetic aggregates are artificial aggregates. Broken bricks called brick bats are suitable for mass concreting, for example, in foundation bases. They are not used for reinforced concrete works.
  • These dense and strong aggregates as obtained are used for making precast concrete products and have good fire resisting properties but are responsible for corrosion of reinforcement due to the sulfur content of slag.
  • Synthetic aggregates are obtained by thermally processed materials such as expanded clay and shale used for making lightweight concrete.

2. On the Basis of Size

(a) Coarse Aggregate

  • Aggregates ranging from 80mm - 4.75 mm are identified as coarse aggregates.
  •  These are obtained from natural disintegration or artificial crushing of rocks.
Note: Size of coarse aggregates depends on the thickness of section (member), reinforcement spacing, clear cover, mixing handling, and placing methods.
  • For reinforced sections, the maximum size should be at least 5 mm less than the clear spacing between the reinforcement and also at least 5mm less than the clear cover. Aggregates of more than 20 mm sizes are seldom used for reinforced cement concrete structures.

b) Fine Aggregate

  • Aggregate passing through a 4.75 mm sieve is defined as fine. 
  • They may be natural sand deposited by rivers, crushed stone sand-obtained by crushing stones, and crushed gravel sand. 
  • The smallest size of fine aggregate (sand) is 0.6 mm.
  • Depending upon the particle size, fine aggregates are described as fine, medium, and coarse sands.

 3. On the Basis of Shape.

(a) Rounded aggregates (Produce voids about 32%)

  • These are usually obtained from the river or sea shore.
  • These have a minimum ratio of surface area to volume. Thus Cement paste required is minimum.
  • However development of bonds is poor as interlocking between particles is less, thus unsuitable for high strength concrete.

(b) Irregular aggregates (Voids produced 36%)

  • They require more cement paste as compared to rounded aggregate.
  • Due to irregularity in shape, they develop a good bond and are suitable for making ordinary concrete. 

(c) Angular Aggregate (Voids produce up to 40%)

  • These have sharp, angular, and rough particles.
  • Angular aggregate provides a very good bond than the earlier two, is most suitable for high strength concrete and pavements however a requirement of cement paste is relatively more.

(d) Flaky and Elongated Aggregates

  • Both of these influence the concrete properties adversely.
  • The least lateral dimension of flaky aggregate (thickness) should be less than 0.6 times the mean dimension. This test is not applicable to sizes smaller than 6.3 mm.

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