Department of Civil Engineering, N-W.F.P UET, Peshawar
Prestressed Concrete
shortening of the concrete is reduced. Creep losses, which are roughly proportioned to elastic losses, are lower also. High bearing stresses in the vicinity of tendon anchorages for post tensioned members are more easily accommodated, and the size of expensive anchorage hardware can be reduced. In the case of pretensioned elements, higher bond strength results in a reduction in the development length required to transfer prestress force from the cables to the concrete. Finally, concrete of higher compressive strength also has a higher tensile strength so that the formation of flexural and diagonal tension crack is delayed. Figure 4 shows typical set of compressive stress strain curve for normal density concrete, obtained from uniaxial compressive test performed at normal, moderate testing speeds on concretes that are 28 days old. In present practice, compressive strength between 4, 000 and 8, 000 psi (28 and 55 MPa) is commonly specified for prestressed concrete members, although strengths as high as 12,000 psi (83 MPa) have been used. It should be emphasized, however, that the concrete strength assumed in the design calculations and specified must be attained with certainty, because the calculated high stresses resulting from prestress force really do occur. In recent years there has been a rapid growth of interest in high-strength concrete. Although the exact definition is arbitrary, the term generally refers to concrete having uniaxial compressive strength in the range of about 8000 to 15,000 psi or higher. Such concretes can be made using carefully selected but widely available cements, sands, and stone; certain admixtures including high-range water-reducing super plasticizers, fly ash, and silica fume; plus very careful quality control during production. In addition to higher strength in compression, most other engineering properties are improved, leading to use of the alternative term high-performance concrete. For bridges, too, smaller cross sections bring significant advantages, and the resulting reduction in dead load permits longer spans. The higher elastic modulus and lower creep coefficient result in reduced initial and long-term deflections, and in the case of prestressed concrete bridges, initial and time-dependent losses of prestress force are less. Other recent applications of high-strength concrete include offshore oil structures, parking garages, bridge deck overlays, dam spillways, warehouses, and heavy industrial slabs. Prof Dr. Qaisar Ali (http://www.eec.edu.pk)
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