Structural Concrete - Hassoun

726 Chapter 19 Introduction to Prestressed Concrete
Wires and strands can be tensioned in groups, whereas bars are tensioned one at a time. In the
posttensioning process, the steel tendons are placed in the formwork before the concrete is cast
and the tendons are prevented from bonding to the concrete by waterproof paper wrapping or
a metal duct (sheath). Tendons bonded to the concrete are called bonded tendons. Unbonded
tendons, left without grout or coated with grease, have no bond throughout the length of the
tendon.
2. Pretensioning: In pretensioning, the steel tendons are tensioned before the concrete is cast.
The tendons are temporarily anchored against some abutments and then cut or released after
the concrete has been placed and hardened. The prestressing force is transferred to the concrete
by the bond along the length of the tendon. Pretensioning is generally done in precasting
plants in permanent beds, which are used to produce pretensioned precast concrete elements
for the building industry.
3. External prestressing: In external prestressing, the prestressing force is applied by flat jacks
placed between the concrete member ends and permanent rigid abutments. The member
does not contain prestressing tendons, as in the previous two methods (also called internal
prestressing). External prestressing is not easy in practice because shrinkage and creep in
concrete tend to reduce the induced compressive stresses unless the prestressing force can be
adjusted.
The profile of the tendons may be straight, curved (bent), or circular, depending on the design
of the structural member. Straight tendons are generally used in solid and hollow-cored slabs,
whereas bent tendons are used in beams and most structural members. Circular tendons are used in
circular structures such as tanks, silos, and pipes. The prestressing force may be applied in one or
more stages, either to avoid overstressing concrete or in cases when the loads are applied in stages.
In this case, part of the tendons are fully prestressed at each stage.
A considerable number of prestressing systems have been devised, among them Freyssinet,
Magnel Blaton, B.B.R.V., Dywidag, CCL, Morandi, VSL, Western Concrete, Prescon, and
INRYCO. The choice of the prestressing system for a particular job can sometimes be a problem.
The engineer should consider three main factors that govern the choice of the system:
1. The magnitude of the prestressing force required.
2. The geometry of the section and the space available for the tendons.
3. Cost of the prestressing system (materials and labor).
The following example illustrates some of the features of prestressed concrete.
Example 19.1
For the simply supported beam shown in Fig. 19.2, determine the maximum stresses at midspan section
due to its own weight and the following cases of loading and prestressing:
1. A uniform live load of 900 lb/ft.
2. A uniform live load of 900 lb/ft and an axial centroidal longitudinal compressive force of
P = 259.2 K.
3. A uniform live load of 2100 lb/ft and an eccentric longitudinal compressive force P = 259.2 K
acting at an eccentricity e = 4in.
4. A uniform live load of 2733 lb/ft and an eccentric longitudinal compressive force P = 259.2 K
acting at the maximum practical eccentricity for this section (e = 6in.).
5. The maximum live load when P = 259.2 K acting at e = 6in.