Prestressed concrete slab : analysis and design

Abstract

Consulta en la Biblioteca ETSI Industriales (8138)

[EN] Prestressing is the deliberate creation of permanent internal stresses in a structure or system in order to improve its performance. Such stresses are designed to counteract those induced by external loadings. Prestressing generally involves at least two materials, the stressor and the stressee which, when acting together, perform better than either one taken separately. The principle of prestressing is indeed very convenient and has been widely applied. The application of prestressing to concrete is in a way a natural result. Concrete is strong in compression and weak in tension; its tensile strength varies from 8 to 14 percent of its compressive strength. Prestressing the concrete produces compressive stresses, either uniform or non-uniform, which will counteract tensile stresses induced by external loadings. This force prevents the cracks from developing by eliminating or considerably reducing the tensile stresses at the critical midspan and support sections. The sections are then able to behave elastically, and almost the full capacity of the concrete in compression can be efficiently utilized across the entire depth of the concrete sections when all the loads act on the structure. Prestressing concrete can be classified in several ways; source of prestressing (mechanical, hydraulic, electrical and chemical), external or internal prestressing, pre-tensioning or post-tensioning, linear or circular prestressing. The simplest type of prestressing is the hydraulic, with the use of hydraulic for tensioning the tendons and with calibrated pressure gauges which directly indicate the magnitude of force developed during the tensioning. Prestressed concrete elements improve their properties compared with those obtanied with ordinary reinforced concrete. The advantadges of prestressing are; - Sections remain uncracked under service loads, which leads to a higher moment of inertia, increase of durability and shear capacity and an improved performance. - Higher span-to-depth ratios, that permits larger spans. In slabs typical values are 28:1 for non-prestressed slabs and 45:1 for prestressed ones. - Suitable for precast construction; rapid construction, better quality control, standard shapes, reduced maintenance¿ Although prestresing presents all these advantadges, some other aspects have to be carefully considered, such as; - Skilled technology (is not as common as reinforced concrete) - Use of high strength materials is costly - There is additional costs in auxiliary equipments - Need for quality control and inspection