BEHAVIOUR OF STEEL FIBRE REINFORCED PARTIALLY PRESTRESSED CONCRETE BEAM

Abstract

Partially Prestressed structures are those in which certain tensile stresses are permitted in concrete at full service load and this can be achieved by using less prestressed steel and adding some mild steel for reinforcing. This leads to economy i.e. significant reduction in 'overall cost of the structure. There are certain disadvantages that restrict their usefulness. These are early appearance of cracks, greater deflection under overloads and higher principal tensile stresses under working load. In order to make it more useful its design must satisfy the limit state of serviceability criteria. With this reference this study has been done to investigate the behaviour of steel fibre reinforced partially prestressed concrete structures with regard to cracking and deflection. For this purpose five beams were casted, each having same prestressing steel (2 No. ' 7 mm) but different percentage of untensioned reinforcement. In order to study the effect of increase in fibre content, in these beams percentage of steel fibre is increased keeping the untensioned reinforcement same. And the experimental results have been compared with existing literature and codal provision.Besides this different empirical equations given for crackwidth calculation have also been critically examined. I.S. code recommendations for maximum hypothetical flexural tensile stress corresponding to maximum crackwidth has also been compared with the experimental one. From this detailed study it has been found out that addition of steel fibres has significant effect on reducing the maximum crackwidth and deflection. Increase in fibre content for the same percentage of untensioned reinforcement causes significant reduction in maximum crackwidth and deflection. It also restricts the earlier appearance and propagation of cracks. Different empirical equations for crackwidth calculation and IS code recommendation for hypothetical flexural tensile stresses are found to give conservative results. Therefore, it is suggested that higher felxurai tensile stresses may be permitted in the design.