Aggregates are inert granular materials such as sand, gravel, or crushed stone that are used in concrete together with water and portland cement. Coarse aggregates are any particles greater than 0.19 inch in diameter, but most commonly vary between 3/8 and 1.5 inch in diameter. Fine aggregates are defined as particles smaller than 0.19 inch, and usually range from 0.075 to 0.125 inch in diameter. The three main ingredients of concrete are water, cement, and aggregate. Concrete is a hydrophilic material that attracts water molecules inside its structure where they dissolve the cementing material. As the water evaporates, it leaves voids inside the concrete that lead to deterioration. This process is called "carbonation." Concrete also absorbs oxygen from the air which causes oxidation. The oxidized surface layer peels off and falls away when the concrete is placed beneath a roof or covered with another material. This protective layer is called an "epoxy" skin. The ratio of cement to water should be approximately $0.45$ per $100$ gallons ($17.1$ L) for a general-purpose concrete. Higher ratios of cement to water increase the strength and durability of the final product. Lower ratios may produce a concrete that sets up more quickly, but this may be desirable if you need the concrete to act as a binder instead of a structural element.