Fine aggregates are made of sand with particle size of 10 mm or less. Coarse aggregates are made of gravel or crushed rock with particle size of 150 mm or less. The maximum particle size is usually 20 mm. Aggregates should be strong and should be able to withstand exposure to the elements, and not be composed of materials that can cause deterioration of the concrete.

All of the aggregates should be held in suspension in wet concrete and be of plastic consistency. The ingredients in wet concrete should not segregate during travel and should form a homogeneous mixture when the concrete hardens. The concrete's slump is a measure of the freshly mixed concrete's consistency and is determined by the slump test. A high slump means the concrete is stiff and a low slump means it is watery.