If a plain concrete member made from concrete with a compressive strength of 5000 psi (35 MPa) were subjected?to axial tension, the concrete would crack when the average applied stress (axial tension divided by cross-sectional area ) reaches about 300 psi(2 MPa). As well as being low, the concrete tensile strength is rather unpredictable and is associated with very small deformations prior to failure.

If longitudinal reinforcing bars were cast into the concrete member, its tensile performance would be much improved. Instead of failing when the first crack forms, the member can now continue to resist loads until the reinforcement crossing the cracks yields. Because it now requires a substantial energy input to fail the member, we can classify it as a tough, ductile member. There is, however, a substantial loss in stiffness after cracking.

Prestressed concrete is a type of reinforced concrete in which the steel reinforcement has been tensioned against the concrete. This tensioning operation results in a self-equilibrating system of internal stresses (tensile stresses in the steel and compressive stresses in the concrete) which improves the response of the concrete to external loads. While concrete is strong and ductile in compression it is weak and brittle in tension, and hence its response to external loads is improved by applying a precompression.?

The originator of practical prestressed concrete was Eugene?Freyssinet of France, who in 1928 began to use high-strength steel?wire for prestressing concrete.Earlier attempts at producing?prestressed concrete using normal-strength reinforcement had been unsuccessful.

After being precompressed, concrete continues to shorten with time in a process called creep. Loss of moisture with time also causes shortening of the concrete by shrinkage. Creep and shrinkage together may cause a total shortening of the concrete of nearly 1 part per thousand. With normal-strength reinforcement it was not possible to elongate the steel in the prestressing operation by more than about 1.5 parts per thousand. Thus in the early attempts to prestress concrete about two-thirds of the prestressing in the reinforcement was lost due to creep and shrinkage. High-strength steel wire, on the other hand, can be elongated about 7 parts per thousand in the prestressing operation. Even with a loss of 1 part per thousand, six-sevenths of the prestressing would remain.

To reduce losses due to creep and shrinkage and to make possible much higher levels of precompression, Freyssinet recommended the use not only of higher-strength steel but also of higher-strength concrete. In a dramatic demonstration of the potential of prestressed concrete, Freyssinet commenced his 1936 lecture in London by applying an internal pressure of 2000 psi (14 MPa), equivalent to a head of water of 4600 ft (1400 m), to a water-filled prestressed concrete pipe. The pipe had been prestressed " to induce in the reinforcement a permanent tension of about 170, 000 to 185, 000 psi, counteracted by a permanent compression of the concrete amounting to something like 7000 psi. " The pipe could withstand "before leaking, internal pressure ten times greater than that which caused the above-mentioned pipe of ordinary reinforced concrete of highest quality and of same dimensions to give way. " Freyssinet was justified in titling his?lecture " A Revolution in the Technique of the Utilization of Concrete."

After Freyssinet's original work, prestressed concrete was increasingly utilized in both Europe and North America. By the time of the First United States Conference on Prestressed Concrete in 1951, it was reported that about 175 prestressed concrete bridges and 50 prestressed concrete building frames had been constructed in Europe and about 700 prestressed concrete water tanks had been built in North America.

Since these early developments, prestressed concrete has grown to be a multibillion-dollar industry in North America. Currently, about 200000 tons of prestressing steel is used in North America each year, which is about one-quarter of the total world consumption. In North America, about two-thirds of the prestressing steel is used to manufacture precast, pretensioned products, with the remaining one-third being used for post-tensioning. About 65% of precast products are structural elements for buildings and bridges, with the rest being architectural precast elements.

The basic concept of reinforced concrete, for both prestressed and non-prestressed construction; is that steel reinforcement is placed in those locations of a structure where tensile stresses will occur. In prestressed concrete construction, high-strength reinforcement is used, and this reinforcement is tensioned prior to the application of external loads.?This initial tensioning of the reinforcement?precompresses the surrounding concrete, giving it the ability to resist higher loads prior to cracking.

The non-prestressed reinforcement undergoes strain only when the surrounding concrete is strained. Thus it can develop high tensile strains only when the surrounding concrete is severely cracked. On the?other hand, the strain in prestressed reinforcement is considerably higher than the strain in the surrounding concrete. Because of this, the prestressed reinforcement can have high tensile stresses even before the concrete has cracked. Non-prestressed reinforcement can be considered as passively accepting imposed strains. By prestressing the reinforcement the engineer can actively control the stress in the reinforcement and the deformations of the structure.