Size of Aggregate

The largest maximum size of aggregate practicable to handle under a given set of conditions should be used. Perhaps, 80 mm size is the maximum size that could be conveniently used for concrete making. Using the largest possible maximum size will result in :-

(i) reduction of the cement content

(ii) reduction in water requirement

(iii) reduction of drying shrinkage

However, the maximum size of aggregate that can be used in any given condition may be limited by the following conditions:-

(i ) Thickness of section

(ii ) Spacing of reinforcement

(iii ) Clear cover

(iv ) Mixing, handling and placing techniques

Generally, the maximum size of aggregate should be as large as possible with in the limits specified, but in any case not greater than one-fourth of the minimum thickness of the member.

Aggregates are divided into two categories from the consideration of size :-

(i) Coarse aggregate and

(ii) Fine aggregate.

The size of aggregate bigger than 4.75 mm is considered as coarse aggregate and aggregate whose size is 4.75 mm and less is considered as fine aggregate

Shape of Aggregate

The shape of aggregates is an important characteristic since it affects the workability of concrete. It is difficult to really measure the shape of irregular body like concrete aggregate which are derived from various rocks. Not only the characteristic of the parent rock, but also the type of crusher used will influence the shape of aggregates, e.g., the rocks available round about Pune region are found to yield slightly flaky aggregates, whereas, good granite rock as found in Banglore will yield cubical aggregate. The shape of the aggregate is very much influenced by the type of crusher and the reduction ratio.

Rounded shape aggregate

Crushed shape Aggregate

From the standpoint of economy in cement requirement for a given water/cement ratio, rounded aggregates are preferable to angular aggregates. On the other hand, the additional cement required for angular aggregate is offset to some extent by the higher strengths and sometimes by greater durability as a result of the interlocking texture of the hardened concrete and higher bond characteristic between aggregate and cement paste.

Flat particles in concrete aggregates will have particularly objectionable influence on the workability, cement requirement, strength and durability. In general, excessively flaky aggregate makes very poor concrete.

Classification of particles on the basis of shape of the aggregate :-

Classification Description

Rounded ---- Fully water worn or completely shaped by attrition

Irregular or Partly rounded ---- Naturally irregular or partly shaped by attrition,having rounded edges

Angular ---- Possessing well-defined edges formed at the intersection of roughly planar faces.

Flaky ---- Material, usually angular, of which the thickness is small relative to the width and/or length (Laminated rocks)

Angular aggregates are superior to rounded aggregates from the following points of view:-

(a) Angular aggregates exhibit a better interlocking effect in concrete, which property makes it superior in concrete used for roads and pavements.

(b) The total surface area of rough textured angular aggregate is more than smooth rounded aggregate for the given volume. By having greater surface area, the angular aggregate may show higher bond strength than rounded aggregates.

The shape of the aggregates becomes all the more important in case of high strength and high performance concrete where very low water/cement ratio is required to be used.

Texture of Aggregate

Surface texture is the property, the measure of which depends upon the relative degree to which particle surfaces are polished or dull, smooth or rough. Surface texture depends on hardness, grain size, pore structure, structure of the rock, and the degree to which forces acting on the particle surface have smoothed or roughend.

Hard, dense, fine-grained materials will generally have smooth fracture surfaces. Experience and laboratory experiments have shown that the adhesion between cement paste and aggregate is influenced by several complex factors in addition to the physical and mechanical properties.

Aggregate crushing apparatus

As surface smoothness increases, contact area decreases, hence a highly polished particle will have less bonding area with the matrix than a rough particle of the same volume. A smooth particle, however, will require a thinner layer of paste to lubricate its movements with respect to other aggregate particles. It will, therefore, permit denser packing for equal workability and hence, will require lower paste content than rough particles. It has been also shown by experiments that rough textured aggregate develops higher bond strength in tension than smooth textured aggregate.

Measurement of Surface Texture :-

Measurement of Surface texture is broadly divided into direct and indirect methods.

Direct methods include:-

(i) making a cast of the surface and magnifying a section .

(ii) Tracing the irregularities by drawing a fine point over the surface and drawing a trace magnified by mechanical, optical, or electrical.

(iii) getting a section through the aggregates and examining a magnified image.

Indirect methods include:-

(i) measurement of the degree of dispersion of light falling on the surface.

(ii) determining the weight of a fine powder required to fill up the interstices of the surface to a truly smooth surface.

(iii) the rock surface is held against rubber surface at a standard pressure and the resistance to the flow of air between the two surfaces is measured.

Related Topics :-

Source of Aggregates

Size of Aggregate | Shape of Aggregate-Classification | Texture | Measurement of Surface Texture |