Text 10. Prestressed Concrete

Degree of Prestressing. Prestresses in the concrete are designed and induced to counteract the stresses caused by external loads. The designer should aim at a high initial pretension of the steel . A low initial steel stress produces a low and rather uncertain concrete compression, combined with ah uneconomical use of steel. The elastic elongations are relative­ly small and require fine adjustments in the stretching devi­ces. In contrast, a high initial steel stress produces high and reliable concrete compression, obtained with a small amount of steel. The steel elongations are comparatively large, and therefore easier to adjust and maintain. High initial steel stresses are therefore more effective and more economical than low initial steel stresses.

The upper limits of the initial tension should be governed by the creep of the steel and by the crack coefficient.

Cooperation of Steel and Concrete. The working together of the two materials may be secured by bond, or by end anchorages on the prestressing members, or by a combination of both. For steel up to 0.5 in. diameter, the effect of bond is usually sufficient to ensure the transfer and the mainte­nance of the preliminary stresses. For heavier bars, anchorage blocks are required in addition to the bond effect. In bondless structures all prestresses must be induced by anchorage blocks, no matter what diameter the steel.

The strength properties of steel and concrete should be interrelated. The higher the strength of the available steel, the better should be the strength properties of the concrete in bonded structures. Where new types of bonded structures or structural units are to be mass produced, the successful co­operation of steel and concrete by bond should be proved by fatigue tests on prototypes.

Jointing of Pretensioned Steel. Prestressed members should be continuous over their full lengths and joints should be avoided. Connections by overlapping or turnbuckles should not be allowed. Welded connections may only be used when it has been established by preliminary tests that the steel is weldable. The test samples to be welded should be of the same thickness as the steel used in the structure, without any special preparation, and the quality of the weld­ed joint should be tested in the usual manner.

Cables. Where a whole cable is tensioned in one process, all wires of the cable should have the same initial stress. To ensure this, it is adequate to ensure that the wires are as nearly as possible of the same initial straightness. A prac­tical method is to group and maintain the wires in a prear­ranged disposition, so that no wire can diverge from the axis of the cable by an amount sufficient to cause an appreciable variation in length. The spacing of the wires should be ade­quate to permit of grout penetrating through the whole length of the cable. Where sheathed cables are placed in the forms and concrete is cast around them, the sheathing must be completely water-tight.

Non-prestressed Reinforcement. Both prestressed steel as well as non-prestressed reinforcements may be used in the same structure. In fully prestressed structures with eccentric precompression, non-prestressed reinforcements are employed to balance the tensile stresses created in the concrete by the prestressing process before the live loads are applied. They are also placed eccentrically, but at the side opposite to the pretensioned steel. The cross-sectional area of the steel should be designed to cover the full tension in the concrete with a stress not exceeding the values permissible in ordinary construction.

In partially prestressed structures a substantial part of the eccentric main reinforcement is not prestressed. The permis­sible stress in the non-prestressed main steel may be sub­stantially increased by the use of supplementary prestressed wire reinforcements. In this case both the prestressed wires and the non-prestressed bars are placed in the same zone of the structure, and the strengths of the two items are added to each other to form the tensile component of the inner moment.

Concrete Cover and Distances between Prestressing Members.

The cover of concrete measured from the outside of all prestress­ing members, including transverse ties, spirals, stirrups, and all secondary reinforcement, should at all points be at least 0.5 in. or the diameter of the bar, whichever is the great­er. In structures exposed to the weather, the cover should be at least 0.75 in. These relatively low values are justified by .the accurate positioning of the stretched main steel, which in turn determines the position of the secondary rein­forcements.

In girders of large spans, bridges over steam-operated railways, hydraulic structures, and structures exposed to acids, oils, fumes, or other harmful substances, the clear cover should be at least 1.5 in., and protective coating should be applied to the concrete.

The minimum lateral distance between bonded prestress­ing members is mainly dependent on the maximum size of the coarse aggregate used in the concrete, and should be, 0.25 in. greater than this maximum size. Both the lateral and vertical distances between compressor wires or anchored steel bars should be at least 0.75 in. These minimum dimensions are based on the assumption that the concrete is filled and compacted by vibration.

Precautions against Rusting, Adequate cover against rust­ing must be provided on all stressing members and anchor­ages.

Concrete units with bonded compressor wires mass-prod­uced on long stretching beds have their main steel showing at both end faces without any cover. Such units have been used for many years exposed to the weather and no penetration by rust has yet been ascertained. Nevertheless, it is recom­mended that the end faces with the cut steel wires be covered by a layer of gunite 0.5 in. thick.

Fire Precautions. Up to the present, little experience has been gained on the fire safety of prestressed concrete structures. A few British and German test reports indicate that a precompressed, dense concrete offers good protection to the steel. But there is no indication yet how the ultimate strength of the hard-drawn wire is affected, if the protection offered by the concrete should be overcome. Consequently, prestressed structures and units should only be regarded as fire-proof when prototypes have been subjected to, and passed, the specified fire tests. The prototypes must be true copies of the units to be tested, in the sense that they should be made of identical materials, prestressed to the same degree, and have the same shape and dimensions.