Hydroelectric dams are a way of generating electricity by harnessing flowing water. The dams are built to stop the flow of a river, which then creates a reservoir of water behind the dam. This water falls through the dam and spins turbines which in turn spin electric generators. These dams can be made of several different things but the primary building materials are: earth, concrete and steel.
Building a hydroelectric dams starts with a base. The base is the first thing to sit in the river channel and block the flow of water. The base is usually created by pouring tons and tons of rock, sand, gravel and dirt into the channel. It is the largest portion of the dam so an inexpensive material like rock and dirt helps keep the cost of building down while at the same time creating a very effective foundation from which to build the rest of the dam.
The second material used in building a hydroelectric dam is concrete. Concrete is poured around the earthen base to provide shape, structure and strength to the dam. Concrete starts out in a near-liquid form which makes it very effective at modeling to specific shapes and then hardens and holds that shape. It is very strong which helps keep the dam standing.
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Steel plays a critical part in most large-scale construction projects, and a hydroelectric dam is no different. Concrete is very strong in terms of compression strength but it is not very strong when it comes to twisting or pulling. This is where steel comes in. Steel re-bar is inserted into the concrete to provide added dimensional strength. If steel was not included in the concrete, the weight of the water pushing against the dam could easily break the concrete.