Gradation Of Aggregates | Types of Aggregate Grading

Gradation of Aggregates

Definition

The particle size distribution of an aggregate as determined by sieve analysis is termed as gradation of aggregates. If all the particles of  an aggregate are of uniform size, the compacted mass will  contain more voids whereas aggregate comprising particles of various sizes will give a mass with lesser voids.The particle size distribution of a mass of aggregate should be such that the smaller particles fill the voids between the larger particles. The proper grading of an aggregate produces dense concrete and needs less quantity of fine aggregate and cement waste, therefore, it is essential that coarse and fine aggregates be well graded to produce quality concrete.

Grading limits and maximum aggregate size are specified because these properties affect the amount of aggregate used as well as cement and water requirements, workability, pumpability, and durability of concrete. In general, if the water-cement ratio is chosen correctly, a wide range in grading can be used without a major effect on strength.

The Grading Curve of Aggregates

The grading of aggregates is represented in the form of a curve or an S-CURVE. The curve showing the cumulative percentages of the material passing the sieves represented on the ordinate  with the sieve openings to the logarithmic scale represented on the abscissa is termed as Grading Curve. The grading curve for a particular sample indicates whether  the grading of a given sample conforms to that specified, or it is too coarse or too fine, or deficient in a particular size.

Types of Grading of Aggregates

1. Dense-or well-graded aggregate – Has gradation close to the FWHA maximum density grading curve.
2. Gap-graded aggregate – Has only a small percentage of particles in the mid-size range.
3. Uniformly graded aggregate – Composed mostly of particles of the same size.
4. Open-graded aggregate – Contains only a small percentage of small-size particles.

UNIFORM GRADED AGGREGATE

It refers to a gradation that contains most of the particles in a very  narrow  size range. In essence, all the particles are the same size. The curve is steep and only occupies the narrow size range specified.

1. Narrow range of sizes.
2. Grain-to-grain contact.
3. High void content.
4. High permeability.
5. Low stability.
6. Difficult to compact.

OPEN GRADED AGGREGATE

In this type of gradation of aggregates, only a small percentage of aggregate particles are in the small range. This results in more air voids because there are not enough small particles to fill in the voids between the larger particles. The curve is near  vertical in the mid-size range, and flat and near-zero in the small-size range.

GAP GRADED AGGREGATE

Gap-graded aggregate contains only a small percentage of aggregate particles in the mid-size range. The curve is flat in the mid-size range. Some PCC mix designs use gap graded aggregate to provide a more economical mix since less sand can be used for a given workability. When gap-graded aggregate are specified, certain particle sizes of aggregate are omitted from the size continuum. Gap-graded aggregate are used to obtain uniform textures in exposed aggregate concrete. Close control of mix proportions is necessary to avoid segregation.

1. Missing middle sizes.
2. No grain-to-grain contact.
3. Moderate void content.
4. Moderate permeability.
5. Low stability.
6. Easy to compact.

DENSE GRADED AGGREGATE

A dense gradation refers to a sample that is approximately of equal amounts of various sizes of aggregate. By having a dense gradation, most of the air voids between the materials are filled with particles. A dense gradation will result in an
even curve on the gradation graph.

1. Wide range of sizes.
2. Grain-to-grain contact.
3. Low void content.
4. Low permeability.
5. High stability.
6. Difficult to compact.