Reasons for prestressing

The development of reinforced-concrete construction has been very rapid. Its application in engineering structures and buildings began toward the end of nineteenth century. Within the period of about 50 years, it has reached a high qualitative level and applied to the most important engineering structures.

Up to present, steel construction, rather than reinforced concrete, has succeeded in holding the field of long-span bridges and other long-span engineering structures. The main reasons why reinforced concrete has been lagged behind steel structures may be found in the following:

a) The dead load of structure increases with the length of the span. This causes a sharp drop in economic efficiency of the structure when a certain length of span is exceeded.

b) Materials cannot be used efficiently in reinforced-concrete members subjected to bending. The use of high-strength reinforcing steel in unprestressed reinforced concrete is limited because of the considerable elongation of the steel under high tensile stresses. No economy can be achieved by applying high- strength steel in conventional reinforced concrete.

c) One of the fundamental principles of conventional reinforced-concrete theory is the proper transmission of stresses from concrete to steel so that the steel may be considered as an incorporated part of the concrete section.

In view of the consideration outlined above, steel structures are in a privileged position in the field of long-span construction, and reinforced-concrete structures of conventional design are unable to compete with them. To place reinforced-concrete construction in a wider competition with steel construction, it has been necessary to develop new ideas and systems for the application of reinforced concrete which are more economical and technically more refined than those previously used. One of these ideas was prestressing. Although not very new idea (the first attempts to apply prestressing were made in 1886), prestressing was not applied successfully in early days of its development. First of all, as the physical properties of concrete, such as shrinkage, plastic flow, etc. were unknown and compression strength was then less than 3,000 psi.

Principles of prestressing

The basic principle of prestressing is the induction of stresses in a concrete member before the dead and live loads are applied, so that these stresses act in the opposite direction to those developed by loading (dead, live, temperature). When the loads are applied, the resulting stresses from loading will be superposed on the prestresses. In this manner a more economical stress distribution over the cross section is obtained, and cracking in the tension face of the member can be better controlled.

Prestressing converts a concrete structure into a more homogeneous state with improved elastic capacity; the stresses and deformations caused by load can be computed quantitatively and qualitatively with satisfactory accuracy.

Another important advantage of prestressing is that, if the design load on the prestressed member is exceeded and cracks occur, the latter will be closed when the excess load is removed. Thus, as compared to conventional reinforced-concrete structures, maintenance costs are considerably reduced and the life of the structure is increased. So it is obvious that prestressed reinforced-concrete structures are now able to compete with steel structures in wide range of spans and applications.