Grading Of Aggregate And Grading Curves

This Article Covers What Is “Grading Of Aggregate” Maximum Size Of Aggregate, Sieve Analysis And Curves Etc.

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Grading Of Aggregate

Grading is the distribution of particles of a granular material among various size ranges, usually expressed in terms of cumulative percentage larger or smaller than each of a series of sizes of sieve openings, or the percentage between certain range of sieve openings.
ASTM C-33 grading requirements for coarse and fine aggregates.
There are several reasons for specifying grading limits and maximum aggregate size; the most important is their influence on workability and cost.
Different grading ranges has been specified for economical design.
Aggregates that do not have large deficiency or excess of any particular size produce most workable and economical concrete mixtures.
The effect of aggregate packing with particles of varying sizes is demonstrated.
Low void contents achieved by using smoothly graded coarse aggregates with suitable proportions of graded sand.

If there is not enough fine aggregate to fill the voids between coarse aggregate particles, the space must be filled with cement paste.
Such under-sanded mixtures also tend to be harsh and difficult to finish.
On the other hand, aggregate combinations with excessive amounts of fine aggregate or excessively fine sands may produce uneconomical concretes because of the larger surface area of finer particles, which requires additional cement.

The maximum size of aggregate is conventionally designated by the sieve size on which 15 percent or more particles are retained.
Larger the maximum aggregate size, the smaller will be the surface area per unit volume which has to be covered by the cement paste of a given water-cement ratio.
Price of cement may be 10 to 15 times as much as the price of aggregate, so any action that saves cement without reducing the strength and workability of concrete can result in significant economic benefit.
Maximum size of an aggregate is the smallest sieve that all of a particular aggregate must pass through. Nominal maximum size of an aggregate is the smallest sieve size through which the major portion of the aggregate must pass.
The nominal maximum-size sieve may retain 5% to 15% of the aggregate depending on the size number.
Amount of cement required decreases as the maximum size of coarse aggregate increases.

The maximum size of aggregate particles generally should not exceed:

One-fifth the narrowest dimension of a concrete member.
Three-quarters the clear spacing between reinforcing bars and between the reinforcing bars and forms.
One-third the depth of slabs.

Grading Of Aggregate

Charts are often used to show the results of a sieve analysis graphically.
The percent passing is usually plotted on the vertical axis, while the sieve sizes are plotted on the horizontal axis.
Upper and lower limits specified for the allowable percentage of material passing each sieve may also be included on the grading chart.

Aggregate sometimes analyzed using the combined grading of fine and coarse aggregate together, as they exist in a concrete mixture.
This provides a more thorough analysis of how the aggregates will perform in concrete.
Sometimes mid-sized aggregate, around the 9.5 mm (3⁄8 in.) size, is lacking in an aggregate supply, resulting in a concrete with high shrinkage properties, high water demand, poor workability, poor pumpability, and poor place ability.
Strength and durability may also affected.

In gap-graded aggregates certain particle sizes intentionally omitted.
For cast-in-place concrete, typical gap-graded aggregates consist of only one size of coarse aggregate with all the particles of fine aggregate able to pass through the voids in the compacted coarse aggregate.
Gap-graded mixes used in architectural concrete to obtain uniform textures in exposed-aggregate finishes.
They can also used in normal structural concrete because of possible improvements in some concrete properties, and to permit the use of local aggregate gradations.

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