SHAPE AND TEXTURE OF AGGREGATES

The shape of an aggregate is an important characteristic since it affects the workability of concrete. Not only the parent rock, but also the type of crusher used will influence the shape of aggregates. Basically the shape of aggregates may be rounded or angular. One of the methods of expressing the angularity qualitatively is by a figure called Angularity Number, as suggested by Shergold. This is based on the percentage of voids in the aggregate after compaction in a specified manner. If the void is 33% the angularity number of such aggregate is considered 0. If the void percentage is 44, the angularity number of such aggregate is considered as 11. An aggregate having angularity number between 0 and 11 is considered is considered suitable for making concrete. Angularity number 0 represents the most practicable rounded aggregates and the angularity 11 indicates the most angular aggregates that could be tolerated for making concrete.

Figure. Different Shape of Aggregate

Murdock suggested a different method for expressing the shape of aggregate by a parameter called Angularity Index ‘f_A’.

Where f_H = Angularity Number 

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 higher strength and durability as a result of interlocking texture of the hardened concrete and higher bond characteristics between aggregate and cement paste. 

Super-imposing plus and minus points in favour and against these two kinds of aggregates it can be summed up as follows: 

For water/cement ratio below 0.4, the use of crushed aggregate has resulted in strength upto 38% higher than the rounded aggregate. With an increase in water/cement ratio the influence of roughness of the surface of the aggregate gets reduced and at a water/cement ratio of 0.65, no difference in strength of concrete made with angular aggregate or rounded aggregate. 

The surface texture is a measure of the smoothness or roughness of the aggregate. 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 roughened it. Hard, dense, fine grained materials will generally have smooth fracture surfaces. 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.