How prestressed concrete works

What happens when any beam carries a load? It bends and its center sags lower than its ends. Thus the bottom fibers are stretched while the top fibers are compressed. Since concrete resists compression well, the designer puts enough of it in the top to absorb all the compression safely. On the other hand, since the concrete has very little tensile strength – but steel has a lot – he inserts steel bars to take care of tensile stresses.

The trouble is that concrete shrinks as it hardens. The reinforcing bars, however, do not shorten much and consequently offer resistance to the concrete shrinkage, actually putting the bars in compression. When the concrete is loaded, the load causes considerable tension in the reinforcement. Since this reinforcement started out with a slight compression, and then in turn is subjected to considerable tension, it is obvious that its change in lengths is of such magnitude that the concrete cannot usually follow; it cracks.

In prestressing, concrete’s virtue of high compressive strength is used to compensate for its lack of tensile strength through a very different concept in the use of reinforcing steel.

Steel wires are strung through a concrete beam, for example, are stretched and then anchored at the ends of the beam when the concrete is hard, to put a “squeeze” on the beam. The wires either are strung through a hole in the beam provided by a mold, and are tensioned against the end of the beam (we shall call this process post-tensioning), or else they are stretched first and held by some anchorage, after which the concrete is poured around them. When the concrete is hard, the wires are cut and the ends of the wires return to their original shape outside the beam – because the stress is relieved there – and act as wedges to help hold the wires bonded to the concrete in tension.

In prestressing, the concrete in the beam is squeezed so that it is always in compression, and any tensile stresses that might appear due to loading, and cause cracks, are automatically canceled out. The application of the stresses before the beam is loaded is the basis for the name “prestressed concrete”.

The advantages of prestressed concrete are:

a) it is economical of materials due to the use of higher steel and concrete stresses;

b) it eliminates cracks because the concrete is always in compression;

c) it permits less depth of beam as related to the span, and hence gives more headroom (this is especially important with bridges and airplane hangars);

d) beams do not have to be cast at the site in one form, but may be cast in small sections or blocks at the factory with reinforcing wires threaded through them. When the wires are stressed, the small units are brought together like one large beam;

e) it develops remarkable resistance to shear stresses. In one case its resistance to this shearing action was 800 psi.

The items which contribute most to the higher cost of making prestressed concrete in comparison with regular reinforced concrete are the special form-work and devices required to anchor the prestressing steel on the ends of the beam, and the cost of the actual prestressing operation in the field.