Development of prestressed concrete beams using controlled detensioning

Abstract

Pretensioned prestressed concrete can be produced in a number of ways. Among them several options can be considered depending on the structure constructed and how prestress is transferred. In the recent times radial and longitudinal cracks have been observed due to high tensile stresses developed in concrete around prestressing steel. In practice to eliminate these harmful conditions modifications are required to ensure serviceability functions of the composite high quality material. Often excessive prestress is reduced by lowering the tensile stress in the prestressing steel or / and the magnitude of the eccentricity towards the end of the member which in vulnerable to this type of effect. In the global context debonding of tendons towards the end of a member, drapping of tendon towards the central portion of the member or controlled detensioning can be applied to achieve the desire outcomes. All these techniques require a sound basis for prestress transfer which is achieved by bond. Some of the practices are prohibitive to developing countries due to the high cost of holding down devices buried in the concrete. Further in third world countries cost of hardware is expensive as opposed to cheep labour encountered in production. This research is aimed at strengtheni~g our understanding of bond mechanism by extending cohesive cracking approach established by experiment and matched by a sound theoretical basis to complement each other. Currently some of the global practices are carried out by relying on.intuition as opposed to theoretical formulations. This study covers a 'comprehensive analysis of bond development for the controlled detensioned process. The experimental verification is not part of the present scope. However the parameters influencing such as strand diameter, initial prestress, concrete strength and cover or half the spacing have been identified as main influences to ascertain bond strength based on available test results exhumed from literature. A parametric study has also been carried out and simplified empirical formulae have been developed to predict normal transfer bond length and the type of bond length required for controlled detensioning. The derived theory is also applicable even to debonded tendons as well as drapped tendons accurately. Further study is required to support these findings by providing experimental evidence