Sphericity of aggregate

Sphericity is the degree to which a particle approaches the shape of a sphere. It is the ratio of the surface area of a sphere having the same volume as the particle to the actual surface area of the particle:

$$\text{Sphericity} = \frac{\text{surface area of equal-volume sphere}}{\text{actual surface area of particle}}$$

Its value lies between 0 and 1; a perfect sphere has sphericity = 1. In practice it is also estimated as the ratio of the particle's smallest to largest dimension, or as the ratio of the diameter of the equal-volume sphere to the diameter of the smallest circumscribing sphere.

Example

• A rounded river gravel or a cube-shaped crushed stone has a sphericity of about 0.8-0.9.
• A flaky (slab-like) stone or a needle-like elongated particle has a low sphericity of about 0.3-0.5.
• A glass bead or a marble (a sphere) has sphericity = 1.

Effect of particles with higher sphericity

• Lower surface area per unit volume, so less water and paste are needed.
• Better workability and easier compaction; lower tendency to segregate.
• Better packing, so a denser concrete with higher compressive strength at the same cement content.
• Less stress concentration and cracking under load, since there are no thin flaky pieces to split.
• Smooth, rounded particles may reduce bond (interlock), so angular but equidimensional particles (high sphericity with rough texture) are best for high strength.