Aggregates influence the workability of the concrete by the amount of aggregate, the relative proportions of fine and coarse aggregate, and the different aggregate properties.

Aggregate Mixture Proportions:

For a constant water/cementious material ratio, an increase in the aggregate/cementious material ratio will decrease workability, also, more cementious material is needed when finer aggregate gradings are used. A fine aggregate deficiency results in a mixture that is harsh, prone to segregation, and difficult to finish. On the contrary, an excess of fine aggregate will lead to some extent more permeable and less economical concrete, although the mixture will be easily workable.

Aggregate Properties:

As a general rule, the more nearly spherical the particles, the more workable the resulting concrete. This happens because the spherical particles will act as "ball bearings" while angular particles will have more mechanical interlock and will therefore need more work to overcome the resulting internal friction. The spherical particles will require less cementious material due to the lower surface-to-volume ratio, leaving more to provide workability. When flat or elongated particles are contained in the coarse aggregate, the quantities of sand, cementious material, and water must be increased.

The porosity of an aggregate may also affect workability of concrete. If the aggregate can absorb a great deal of water, less will be available to provide workability.

The particle size of coarse aggregate influences the water requirement for a given consistency (Table 1). Additionally, very fine or angular sands require more water for a given consistency, which produce harsh and unworkable mixtures at a water content that might have been adequate with a coarser or a well-rounded sand. As a rule of thumb, for similar consistency, concrete needs two to three percent more sand by absolute volume and 6 to 9 kilograms more water per cubic meter when crushed sand is used instead of natural sand.


Table (1), Approximate mixing water requirements for different slumps and nominal maximum size of

Aggregate, Portland Cement Association (PCA)