Fine aggregates are often made of natural sand or broken stone, with the majority of particles passing through a 3/8-inch screen. Coarse aggregates are any particles greater than 0.19 inches in diameter, but often vary between 3/8 and 1.5 inches. As a result, aggregate selection is a critical step. If you use fine aggregate in your concrete, it should make up at least 30 percent of the total material budget. If you use coarse aggregate, that number drops to 20 percent. The size of the aggregate affects the strength and other properties of the concrete. Concrete mixes usually include several different sizes of coarse and fine aggregates to achieve the desired consistency. The ratio of coarse to fine aggregate affects how much water is required for the mix to reach its full strength. For example, if you double the amount of coarse aggregate, you need only half as much time to pour the concrete. Concrete that contains more coarse aggregate will not flow as easily. This can be an advantage when placing concrete in large projects where smooth surfaces are needed, such as roads or parking lots. Coarse aggregate also makes for stronger concrete. The air inclusions between the grains of coarse aggregate help prevent cracking from occurring inside the slab. Coarse aggregate is available in two main types: natural and manufactured. Natural coarse aggregate comes from rocks such as gravel, cobbles, and boulders that are left over after quarrying for finer materials.