#61 Course Aggregate being Stored

Description:  This may look like a simple pile of gravel, but in reality it is a key ingredient in one of the most widely used building materials in the world; concrete.  Concrete is a mixture of cement, water, and aggregate.  Aggregate is a mixture of both sand and gravel, or fine and course aggregate respectively. Aggregate composition is very controlled in terms of the size and quantity of sand and gravel particles to be used.  This gradation of aggregate particles ensures that a proper force transfer between particles and cement occurs to maintain strength of the overall concrete element in a structure.  

Special note: If you look carefully, this pile of gravel is being stored under a bridge out of the path of precipitation.  Keeping the aggregate dry allows concrete mixers to assume no water is being added to the mix because the aggregate is wet on the surface.