Effects of deviators and tendon configuration on behavior of externally prestressed beams

Six identical reinforced concrete T-beams were strengthened in flexure using external prestressing tendons (strands). Of these, three beams had identical straight external strands with a different number of deviators along each of thir spans. The other three beams each had a deviator at midspan, and their tendon configurations were varied by either increasing tendon eccentricity, increasing the tendon area (and correspondingly decreasing the prestressing force), or draping the strands. The beams were loaded to failure under third-point loading. Test results indicated that the provision of a deviator at the section of maximum deflection led to satisfactory service load behavior (deflection, cracking, and steel stress) and a higher load-carrying capacity compared to the case where no deviators were provided. An increase in eccentricity of straight tendons with a correspondingly smaller prestressing force led to larger internal steel stresses, crack widths, and service load deflections but higher ductility, while draped tendons resulted in reduced stiffness, greater tendon stress increase, and more ductile behavior near failure. The use of a larger tendon area gave similar service load behavior but a higher ultimate strength and lower ductility. Theoretical calculations based on the concept of bond reduction coefficients and accounting for second-order effects were found to agree with the test results.