| Prestressed Concrete
Although prestressed concrete was patented by a San
Francisco engineer in 1886, it did not emerge as an accepted building
material until a half-century later. The shortage of steel in Europe
after World War II coupled with technological advancements in high-strength
concrete and steel made prestressed concrete the building material
of choice during European post-war reconstruction. North America's
first prestressed concrete structure, the Walnut Lane Memorial Bridge
in Philadelphia, Pennsylvania, however, was not completed until
conventional reinforced concrete, the high tensile strength of steel
is combined with concrete's great compressive strength to form a
structural material that is strong in both compression and tension.
The principle behind prestressed concrete is that compressive stresses
induced by high-strength steel tendons in a concrete member before
loads are applied will balance the tensile stresses imposed in the
member during service.
Prestressing removes a number of design limitations
conventional concrete places on span and load and permits the building
of roofs, floors, bridges, and walls with longer unsupported spans.
This allows architects and engineers to design and build lighter
and shallower concrete structures without sacrificing strength.
principle behind prestressing is applied when a row of books is
moved from place to place. Instead of stacking the books vertically
and carrying them, the books may be moved in a horizontal position
by applying pressure to the books at the end of the row. When sufficient
pressure is applied, compressive stresses are induced throughout
the entire row, and the whole row can be lifted and carried horizontally
Compressive Strength Added
Compressive stresses are induced in prestressed concrete either
by pretensioning or post-tensioning the steel reinforcement.
In pretensioning, the steel is stretched before the
concrete is placed. High-strength steel tendons are placed between
two abutments and stretched to 70 to 80 percent of their ultimate
strength. Concrete is poured into molds around the tendons and allowed
to cure. Once the concrete reaches the required strength, the stretching
forces are released. As the steel reacts to regain its original
length, the tensile stresses are translated into a compressive stress
in the concrete. Typical products for pretensioned concrete are
roof slabs, piles, poles, bridge girders, wall panels, and railroad
In post-tensioning, the steel is stretched after the
concrete hardens. Concrete is cast around, but not in contact with
unstretched steel. In many cases, ducts are formed in the concrete
unit using thin walled steel forms. Once the concrete has hardened
to the required strength, the steel tendons are inserted and stretched
against the ends of the unit and anchored off externally, placing
the concrete into compression. Post-tensioned concrete is used for
cast-in-place concrete and for bridges, large girders, floor slabs,
shells, roofs, and pavements. More.
concrete has experienced greatest growth in the field of commercial
buildings. For buildings such as shopping centers, prestressed concrete
is an ideal choice because it provides the span length necessary
for flexibility and alteration of the internal structure. Prestressed
concrete is also used in school auditoriums, gymnasiums, and cafeterias
because of its acoustical properties and its ability to provide
long, open spaces. One of the most widespread uses of prestressed
concrete is parking garages.