Class A concrete shall be used for reinforced and non-reinforced concrete structures, as well as concrete pavements. Curbs, gutters, and sidewalks may all be built with Class B concrete. Class C concrete can be used for thrust blocks, encasements, fill, or over-excavation, among other things. The grade of a road surface can be changed from Class A to Class G by adding sand and gravel. Class A concrete has a minimum strength of 30 minutes after casting. Class A concrete must meet certain requirements regarding its ingredients and processing method. It should consist of fine aggregate (gravel) with a maximum size of #8 mesh (or less) and coarse aggregate (sand) with a maximum size of #120 mesh (or less). The ratio of coarse to fine aggregate must be at least 3:1. Coarse aggregates should account for no more than 15 percent of the total weight of the concrete. The main ingredient in Class A concrete is portland cement. This concrete also uses pure water as a medium for transporting oxygen into the concrete while allowing carbon dioxide to escape. This makes it important that the water used in mixing Class A concrete be free of chlorine or other chemicals that could affect the setting time of the concrete. Class A concrete must also be mixed thoroughly before pouring. This ensures an even distribution of air bubbles and allows any additives such as plasticizers or colorants to work their way into the mixture.