Structural Concrete - Hassoun

CHAPTER19
INTRODUCTION
TO
PRESTRESSED
CONCRETE
Library building, South Dakota State University, Brookings, South
Dakota.
19.1 PRESTRESSED CONCRETE
19.1.1 Principles of Prestressing
To prestress a structural member is to induce internal, permanent stresses that counteract the tensile
stresses in the concrete resulting from external loads; this extends the range of stress that the
member can safely withstand. Prestressing force may be applied either before or at the same time
as the application of the external loads. Stresses in the structural member must remain, everywhere
and for all states of loading, within the limits of stress that the material can sustain indefinitely. The
induced stresses, primarily compressive, are usually created by means of high tensile steel tendons,
which are tensioned and anchored to the concrete member. Stresses are transferred to the concrete
either by the bond along the surface of the tendon or by anchorages at the ends of the tendon.
To explain this discussion, consider a beam made of plain concrete, which has to resist the
external gravity load shown in Fig. 19.1. The beam section is chosen with the tensile flexural stress
as the critical criterion for design; therefore, an uneconomical section results. This is because
concrete is considerably stronger in compression than in tension. The maximum flexural tensile
strength of concrete, the modulus of rupture, f r , is equal to 7.5λ √ f ′ c (Fig. 19.1).
In normal reinforced concrete design, the tensile strength of concrete is ignored and steel bars
are placed in the tension zone of the beam to resist the tensile stresses, whereas the concrete resists
the compressive stresses (Fig. 19.1).
In prestressed concrete design, an initial compressive stress is introduced to the beam to offset
or counteract the tensile stresses produced by the external loads (Fig. 19.1). If the induced compressive
stress is equal to the tensile stress at the bottom fibers, then both stresses cancel themselves,
whereas the compressive stress in the top fibers is doubled; in this case, the whole section is in
compression. If the induced compressive stress is less than the tensile stress at the bottom fibers,
these fibers will be in tension, whereas the top fibers are in compression.
In practice, a concrete member may be prestressed in one of the following methods.
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1. Posttensioning: In posttensioning, the steel tendons are tensioned after the concrete has been
cast and hardened. Posttensioning is performed by two main operations: tensioning the steel
wires or strands by hydraulic jacks that stretch the strands while bearing against the ends of
the member and then replacing the jacks by permanent anchorages that bear on the member
and maintain the steel strands in tension. A tendon is generally made of wires, strands, or bars.