Coarse aggregate consists of gravel, crushed stone, or other suitable materials larger than ¼ in. (6.4mm) in diameter. Coarse aggregate that is sound, hard, and durable is best suited for making concrete. Material that is soft or flaky or wears away rapidly is unsatisfactory.
In most concrete, normal-weight coarse aggregate sizes are limited to those between 3∕8 in. (9.5mm)and 11/2 in. (38.1mm). Thin toppings and slabs may be limited to 3∕8-in. (9.5mm) aggregate. Coarse aggregate as large as 6 in. (152.4mm) in diameter is often found in massive concrete structures, such as dams. Lightweight aggregate is usually limited to a maximum of 1 in. (25.4mm). In walls, the largest piece of any type of coarse aggregate should never be more than one-fifth the overall wall thickness. The maximum size in slabs should be about one-third the slab thickness. The largest piece of aggregate should never be larger than three-quarters of the width of the narrowest space through which the concrete will be required to pass during placing. This is usually the space between the reinforcing bars or between the bars and the forms.