Concrete normally takes 24 to 48 hours to dry sufficiently for walking or driving on. Concrete, on the other hand, dries in a continuous and fluid process, and it typically achieves its full functional strength after about 28 days. The exact time will depend on the temperature of the concrete as well as its moisture content. Concrete that is used in buildings has a maximum usable life of 100 years. Concrete that is used as aggregate in roads can last much longer than this. The lifetime of concrete structures is largely dependent on how they are maintained after they have been cast. Painted concrete will deteriorate more quickly than unpainted concrete. Concrete that is exposed to sunlight will fade in color and lose strength over time. As far as curing agents are concerned, there are two main types: chemical agents and heat sources. Chemical agents include sodium hydroxide (caustic soda) and calcium chloride (rock salt). These substances react with water to form soluble salts that release hydrogen ions into the concrete, causing it to harden. The amount of time required for this reaction to occur depends on the type of curing agent used. Curing times for common concrete cures range from 24 hours to six months. Curing times for high-strength concretes may be longer than one year. Heating elements are also used as a curing agent for concrete.