Prestressed concrete is a when cables are placed in the concrete to combat the tensile forces created when weight is put on concrete. There are two ways in which this is done: pretensioning and post-tensioning. In pretensioning the cables are stretched before the concrete is cast, in post-tensioning, the cables are stretched after the concrete has hardened. The advantages are the elimination of cracking in concrete, reduced cross section of members, reduced weight of member, less concrete is needed, longer spans are possible, and there is greater recovery after overloads. The disadvantages are only the highest quality of materials can be used, closer control is needed, skilled workmen are required, greater capital investment for prestressing equipment is needed and the high cost of end anchorage units (in post-tensioned work).