Concrete is used for all types of construction projects. Concrete is often used for foundations, basements, sidewalks, driveways, fences, poles and so much more. Concrete is a construction material composed of cement as well as other materials such as fly ash and slag cement, aggregate (generally a coarse aggregate such as gravel limestone or granite, plus a fine aggregate such as sand and water) and chemical admixtures. Concrete is used primarily outdoors but can also be used indoors. Concrete is very strong and durable, however it is not the best for tension. Often times concrete is reinforced with metal bars, fiber, plastic or fiberglass. More concrete is used than any other man-made material in the world. As of 2006, about seven billion cubic meters of concrete are made each year - more than one cubic meter for every person on Earth. Concrete powers a $35-billion industry which employs more than two million workers in the United States alone. More than 55,000 miles of highways in America are paved with this material.