Prestressed concrete joins steel and concrete, using the best attributes of each. But this is not the 50-50 arrangement of conventional reinforced concrete. Rather, this is a special combination where cold drawn or alloy steels with strengths ranging to about 270,000 psi dominate the "marriage" by putting the concrete (usually 5000 to 10,000 psi concrete) into compression when the two are joined. In reinforced concrete, the steel bars are placed wherever tensile stresses are expected. In prestressed concrete, the reinforcing steel first is stretched or tensioned. The force in the stretched steel then is used to precompress the concrete. This precompression or prestressing counteracts the tensile stresses from the applied service loads.
Since reinforced concrete and bonded prestressed concrete are essentially the same at the ultimate load stage, it is clear that the major advantage of prestressing occurs at the service load stage. The structural designer controls the stress distribution on the cross section as well as the deflection. Applying the prestressing force below the center of gravity of the beam counteracts the usual dead and live load stresses and permits shallow sections. This saves on building height. But prestressed concrete also has a toughness not found in other materials. With bonded prestressed concrete it is virtually impossible to create the buckling that sometimes occurs in steel structures. And since the concrete is precompressed, cracking is reduced and durability improved.
The ability to recover after overload cracking makes it a preferred material for pressure vessels and nuclear containment structures. But the basic use of prestressed concrete has been and remains for buildings and bridges where the long-span advantages bring flexibility of interior layout to buildings and competition for steel bridges. Prestressed concrete has shouldered its way into the marketplace to make concrete a major competitor to structural steel over the full range of spans.