This is done by casting concrete around steel rods or bars and most large structures such as bridges need this extra strong concrete. Prestressed concrete is made by casting concrete around steel cables stretched by hydraulic jacks. After the concrete hardens the jacks are released and the cables compress the concrete. Concrete when compressed is the strongest. This type of concrete is used for floors and roofs as well as other things.
Precast concrete is cast and hardened before being used for construction.
Precasting makes it possible to produce a mass number of concrete building materials. Nearly all of prestressed concrete is precast as are concrete blocks. Engineers have designed kinds of concrete for certain uses. Air-entrained concrete is good in harsh weather and is used for roads and airport runways. I needed the density of concrete, and didn't have an engineering text book handy.