Transfer and flexural bond in pretensioned prestressed concrete
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AbstractIn recent times large strands (13, 15 and 16 mm in diameter) have become increasingly popular in the pretensioned prestressed concrete industry and have found wide applications in varying geometries of sections. However use of such elements and their behaviour in several situations have been questioned with respect to anchoring of these strands in concrete; which is accomplished by bond. The experimental results available on bond are limited and information relating to large strands is rare. Laboratory investigations were conducted to determine the influence of some of the inadequately examined parameters on transfer and flexural bond. The principal variables considered were strand size, concrete strength, beam width, clear side cover and bottom cover. The specimens were specifically designed for the purpose of modeling the above parameters. The experimental program consisted of fabricating and testing thirteen full scale beam specimens. Results of these beam tests are presented. The experimental results show that concrete strains in the anchorage region display three different types of prestress-build-up profiles. The curves range from concave, to linear and to convex in the prime portion of the transfer length, exhibiting the significance of the concrete stiffness around the tendon. A definite indication has been observed that concrete strength, strand diameter, cover and effective beam width influence transfer lengths . The information collected during flexural bond tests disclosed the influence of strand size, concrete cover, concrete strength and loading on flexural bond performance in a qualitative sense. Beams that failed in bond-shear and bond-flexure show the dangers associated with bond failures. Beams that failed in shear or flexure indicated the availability of additional bond capacity. During the analysis of the test data the need for a theoretical basis for bond development was realised. Therefore the scope of the research was extended to explore this. The theories hitherto proposed have neither been convincing nor successful in predicting bond development. The absence of a valid theory to complement experiments has stifled the search for solutions to the bond problem. Recognition of the importance of concrete strength and cover properties has strengthened the notion that bond development is primarily a function of the material and geometrical properties of concrete and prestressing steel. A theory has been developed using principles of solid mechanics incorporating these variables. A preliminary analysis using linear elastic theory revealed stresses in concrete which far exceeded its tensile strength. Clearly concrete must crack during the bond development process. The procedure adopted recognises the presence of a disturbed zone around the strand. The concrete in the affected region is analysed as an anisotropic elastic material. The existence of radial cracks has been confirmed by a microscopic examination of a small specimen. A computer program based on the theory put forward is developed to determine the bond stress distribution, concrete strain profiles , end slip, transfer length and the degree of cracking. The important variables have been identified as concrete cover around the tendon, concrete strength, tendon size, initial level of prestress in steel, level of concrete stress and surface form of the tendon. For the purpose of design, an empirical equation has been developed which fits the results calculated using the detailed procedure.
Bibliography: p. 292-297.