Concrete is composed of four main ingredients: cement, water, fine aggregate (sand), and coarse aggregate (rock). Sometimes recycled cementitious materials known as slag and fly ash are substituted for cement, causing varying effects on the concrete’s plastic and hardened properties. Admixtures can also be supplemented to the mix in order to reduce curing time, increase workability, increase strength, or to change the material properties. Varying proportions of these main ingredients are what create the many types of concrete.
Weather plays a major role in the placement, finish, and lasting durability of concrete. Interior concrete is typically not exposed to natural changes of the environment and often does not require any air entrainment as it has less of a tendency to crack than exterior concrete. The major difference remains in the finish of the concrete. Finishing exterior concrete too early (before the excess water has time to rise to the top) can trap the bleed water, setting the stage for other surface problems. Exterior concrete usually has a “rough broom” finish to provide traction for vehicles and pedestrians, reducing the chances of any slipping or falling.