Aggregate shape and surface texture influence the
properties of freshly mixed concrete more than the properties of hardened
concrete. Rough-textured, angular, and elongated particles require
more water to produce workable concrete than smooth, rounded compact aggregate.
Consequently cement content
must also be increased to maintain the water-cement ratio.
Concrete is more workable when smooth and rounded
aggregate is used instead of rough angular or elongated aggregate.
The surface texture of the aggregate can be either
smooth or rough. A smooth surface can improve workability yet a rougher surface
generates a stronger bond between the paste and the aggregate creating a higher
strength.
The grading or size distribution of aggregate is an
important characteristic because it determines the paste requirement for workable
concrete. This paste requirement is the factor controlling the cost, since
cement is the most expensive component. It is therefore desirable to minimize
the amount of paste consistent with the production of concrete that can be
handled, compacted, and finished while providing the necessary strength and durability. The
required amount of cement paste is dependent upon the amount of void space that
must be filled and the total surface area that must be covered. When the particles are of uniform size
the spacing is the greatest, but when a range of sizes is used the void spaces
are filled and the paste requirement is lowered.
