Prestressed Concrete Systems

Prestressing boosts the span range of conventionally reinforced floor systems by about 30 to 40%. This is the primary reason for the increase in the use of prestressed concrete. Some of the other reasons are:

1. Prestressed concrete is generally crack-free and is therefore more durable.

2. Prestressing applies forces to members that oppose the service loads. Consequently, there is less net force to cause deflections.

3. Prestressed concrete is resilient. Cracks due to overloading completely close and deformations are recovered soon after removal of the overload.

4. Fatigue strength (though not a design consideration in building design) is considerably more than that of conventionally reinforced concrete because tendons are subjected to smaller variations in stress due to repeated loadings.

Figure 7.23. Waffle slab, unit quantities: (a) reinforcement; (b) concrete.

5. Prestressed concrete members are generally crack-free, and are therefore stiffer than conventional concrete members of the same dimensions.

6. The structural members are self-tested for materials and workmanship during stressing operations, thereby safeguarding against unexpected poor performance in service.

7. Prestress design is more controllable than mild steel design because a predetermined force is introduced in the system; the magnitude, location, and technique of introduction of such an additional force are left to the designer, who can tailor the design according to project requirements.

There are some disadvantages to the use of prestressed concrete, such as fire, the explosion resistance of unbonded systems, and difficulty in making penetrations due to the fear of cutting tendons.

A major motivation for the use of prestressed concrete comes from the reduced structural depth, which translates into lower floor-to-floor height and a reduction in the area of curtain wall and building volume, with a consequent reduction in heating and cooling loads.

In prestressed systems, the savings in mild steel reinforcement quantities resulting from prestress are just about offset by the higher unit cost of prestressing steel. The cost savings come from the reduction in the quantity of concrete combined with indirect nonstructural savings resulting from reduced floor-to-floor height. Although from an initial cost consideration prestressed concrete may be the least expensive, other costs associated with future tenant improvements, such as providing for large openings in floor slabs, must be considered before selecting the final scheme.

Figure 7.24. Flat plate, unit quantities: (a) reinforcement; (b) concrete.

Figure 7.24. Flat plate, unit quantities: (a) reinforcement; (b) concrete.

7.3.1. Prestressing Methods

Current methods of prestressing can be studied under two groups, pretensioning and post-tensioning. In pretensioning, the tendons are stretched and anchored against external bulkheads. Then concrete is placed around the tendons. After the concrete has hardened, the anchors are released, which imparts compression forces in the concrete as the tendon attempts to return to its original length.

In post-tensioning, the tendons are tensioned and anchored against the concrete after it has hardened. The tendons are stressed using hydraulic jacks after the concrete has reached a minimum of about 75% of the design strength. Tendon elongations are measured and compared against the calculated values; if satisfactory, the tendons projecting beyond the concrete are cut off. Form work is removed after post-tensioning. However, the floor is back-shored to support construction loads from the floors above.

Post-tensioning is accomplished using high-strength strands, wires, or bars as tendons. In North America, the use of strands by far leads the other two types. The strands are either bonded or unbonded depending upon the project requirements. In bonded construction, the tendons are installed in ducts that are filled with a mortar grout after stressing the tendons.

In building applications, unbonded construction is the preferred choice because it eliminates the need for grouting. Post-tensioned floor systems in buildings consist of slabs, joists, beams, and girders, with a large number of small tendons. Grouting each of the multitude of tendons is a time-consuming and expensive operation. Therefore, unbonded construction is more popular.



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