GRADING AGGREGATE

GRADING AGGREGATE

The grading of aggregate can be obtained by carrying out the sieve analysis test, and from this analysis, grading curves can be drawn. But, firstly, the quantity of material to be represented by the bulk sample has to carry out and this can be done by:-
a) Quartering by hand
b) Riffle box
c) Sample splitter

· The grading of an aggregate can have considerable effect on the workability and stability of concrete mix and is also an important factor in concrete mixed design. If all the particles of an aggregate are uniform size, the compacted mass will contain more voids and vice verse.

· The particle size distribution of a mass of aggregate should be such that the smaller particles fill the voids between the larger particles in order to produce dense concrete with needs less quantity of fine aggregate and cement paste. Therefore, it is essential that the coarse and fine aggregates be well graded to produce quality concrete.

· The grading of an aggregate is expressed in terms of percentage by weight retained or passing through a series of sieves taken in order. The sieves are arranged in such an order that the square opening is half for such smaller size.
From the grading curve, it indicates whether the grading of a given sample conforms to that specified, or is too coarse or too fine or deficient in particular size.

The reading of the grading curve will indicate the followings:-

1. If the actual grading curve is lower than the specified grading curve, the aggregate is coarser and segregation of mix might take place.
2. If the actual grading curve lies well above the specified curve, the aggregate is finer and more water will be required, thus increasing the quantity of cement also for a constant water cement ratio. Therefore, this is uneconomical.
3. If the actual grading curve is steeper than the specified, it indicates an excess of middle-size particles and leads to harsh mix.
4. If the actual grading curve is flatter than the specified grading curve, the aggregate will be deficient in middle size particles.

Surface Area of Aggregate

The surface area is affected by the maximum size of aggregate. The surface area will vary with the shape but is inversely proportional to the particle size. The mortar consisting of fine aggregate and cement should be slightly in excess of that just required to fill the voids in the coarse aggregate. Too coarse the aggregate results in harshness; too fine the aggregate required too large a water cement ratio for adequate workability. The surface area of aggregate also influence the amount f mixing water cement required. Generally, the water-cement ratio is fixed from strength consideration.
Maximum Size of Aggregate

In general, the larger the maximum size of aggregate, the smaller is the cement requirement for a particular water-cement ratio. This is due to the fact that the workability of concrete increase with the increase in the maximum size of aggregate.

· In the mass concrete, the use of a larger size aggregate is beneficial due to the lesser consumption of cement.
· The smaller the surface area of larger size aggregate, the water-cement ratio can be decrease which increase the strength
· The stress concentration in the mortar aggregate interface increasing with the maximum size of aggregate.