Department of Civil Engineering, N-W.F.P UET, Peshawar
Prestressed Concrete
1. Introduction: Concrete is basically a compressive material, with its strength in tension being relatively low. Prestressing applies a precompression to the member that reduces or eliminates undesirable tensile stresses that would otherwise be present, fig 1a and 1b. Cracking under service loads can be minimized or even avoided entirely. Deflections may be limited to an acceptable value; in fact, members can be designed to have zero deflection under the combined effects of service load and prestress force. Deflection and crack control, achieved through prestressing, permit the engineer to make use of efficient and economical high-strength steels in the form of strands, wires, or bars, in conjunction with concretes of much higher strength than normal. Thus, prestressing results in the overall improvement in performance of structural concrete used for ordinary loads and spans and extends the range of application far beyond the limits for ordinary reinforced concrete, leading not only to much longer spans than previously thought possible, but permitting innovative new structural forms to be employed.
2. Principle of prestressing: Many important features of prestressed concrete can be demonstrated by simple examples. Consider first the plain, unreinforced concrete beam with a rectangular cross section shown in Fig. 1a. It carries a single concentrated load at the center of its span. (The self-weight of the member will be neglected here). As the load W is gradually applied, longitudinal flexural stresses are induced. If the concrete is stressed only within its elastic range, the flexural stress distribution at mid span will be linear (Fig 1a). At a relatively low load, the tensile stress in the concrete at the bottom of the beam will reach the tensile strength of the concrete fr, and a crack will form. Because no restraint is provided against upward extension of the crack, the beam will collapse without further increase of load. Now consider an otherwise identical beam, shown in Fig. 1b, in which a longitudinal axial force P is introduced prior to the vertical loading. The longitudinal prestressing force will produce a uniform axial compression fc = P/Ac, where Ac is the cross-sectional area of the concrete. The force can be adjusted in magnitude so that, when the transverse load Q is applied, the Prof Dr. Qaisar Ali (http://www.eec.edu.pk)
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