Prestressing Fundamentals

Prestressing offers numerous advantages over ordinary reinforced concrete.

The prestressing and precasting of concrete are inter-related features of the modern building industry.Through the application of imaginative design and quality control, they have, since the 1930’s, made a considerable and growing impact on architectural and construction practices.

Prestressed concrete defined

Prestressing can be defined as the application of compressive stresses to concrete units. Those zones of the unit, in this instance poles, required to carry tensile stresses under working load conditions, are prestressed with steel wire or strand so that the tensile stresses developed during loading are counter-balanced by an induced compressive force.

Advantages of prestressing

The use of prestressed concrete offers distinct advantages over ordinary reinforced concrete.

These advantages can be briefly listed as follows:

  • Prestressing minimises cracking by holding the concrete in a state of compression.
  • Prestressing allows reduced beam depths to be achieved for equivalent design strengths.
  • Prestressed concrete is resilient and will recover from the effects of a greater degree of overload than any other structural material.
  • If the prestressed unit is subjected to overloading, cracks which might develop, will close as soon as the overloading ceases.
  • Prestressing enables both structural elements and entire structures to be formed from several precast units, e.g. segmented and modular construction.
  • Prestressing permits the use of longer spanning members with high strength-to-weight characteristics.
  • The ability to control deflections in prestressed beams and slabs permits the use of longer spans.
  • Prestressing permits a more efficient usage of high-tensile steel and high strength concrete.

Fire-resistance of prestressed concrete

All concrete is incombustible. In a fire, the failure of concrete structures usually occurs due to the progressive loss of strength of the reinforcing steel at high temperatures.

Furthermore, the physical properties of some aggregates used in concrete can change when heated to high temperatures.

Experience and tests have shown, however, that ordinary reinforced concrete has greater fire-resistance than structural steel or timber.

In referring to these materials, current fire codes recognise this. Prestressed concrete has been shown to have at least the same fire-resistance as ordinary reinforced concrete.

Construction applications of prestressing

As with ordinary reinforced concrete, prestressed concrete is an extremely versatile construction medium.

Typical applications of prestressing in building and construction are:

  • Structural components for integration with ordinary reinforced concrete construction, e.g. floor slabs, columns, beams.
  • Structural components for bridges.
  • Water tanks and reservoirs where water tightness (i.e. the absence of cracks) is of paramount importance.
  • Construction components e.g. piles, wall panels, frames,
  • The construction of relatively slender structural frames.
  • Major bridges and other structures.

Conclusions

Prestressed concrete design and construction is precise. The high stresses imposed by prestressing really do occur.

The following points should be carefully considered:

  • Protecting against any prestress losses and ensuring economical material usage requires that the initial stresses during prestressing should be set at the permissable upper limits of the material. This imposes high stresses, which the unit is unlikely to experience again during its working life.
  • As architects and engineers will incorporate the optimum stress capabilities of both concrete and prestressing steel in their design, these materials must be manufactured accordingly. This requires control and responsibility from everyone involved in the use of prestressed concrete, including the designer, the workmen on the site and those responsible for delivery, installation and final inspection.

We have seen that considerable design and strength economies are achieved by the eccentric application of the prestressing force. However, if the design eccentricities are varied only slightly, variation from design stresses could be such that the performance of a shallow unit under full working load could be affected.

Responsible prestressing design and construction should only be undertaken by engineers or manufacturers who are experienced in this field.

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Prestressing Fundamentals