Serviceability of Partially Prestressed High Strength Concrete T-section Beams

Abstract

In recent years, high strength concrete (HSC) has been widely used. With advent of superplasticizers and micro silica; HSC with strength higher than 100 MPa became within reach. This helped in many fields of construction; one of these fields is High Strength Concrete Pretensioned Bridge Girders, which has become an accepted practice in many countries. This high strength permitted longer spans, larger spacing between girders thus reducing total bridge cost. Since having fully prestressed sections, free of cracks, are not always a serviceability requirement, designers could adopt partially prestressed members. This allows the tensile stress to exceed concrete tensile strength, thus leading to concrete cracking. This is done to decrease the prestressing steel which eventually decreases the total cost of the structural elements, in addition to better functional planning of the structure and its surrounding area. Use of HSC partially prestressed girders is very promising, however HSC was found to be more brittle on the material level. Accordingly, the girder may exhibit brittle/less ductile behavior and less deformability. There is lack of knowledge in the effect of HSC on the serviceability requirements of such partially prestressed members, so a need exists for reassessment of the design provisions for the analysis of these girders. The primary objective of this research was to investigate the serviceability deflection and cracking for HSC partially prestressed members experimentally and analytically. Results were compared with the current provisions of Egyptian Code of Practice ECP 203-2007 to expand its applicability. The study also considered the

iv provisions of international codes (ACI & Euro Code). The research aimed to find the most adequate method for calculating these deformations. The experimental program was planned to evaluate the structural behavior of HSC partially prestressed beams under service loads. A total of seven concrete beams were experimented under static increasing loads till failure. Mid- span deflection, crack spacing and crack widths were measured at different load levels. In addition strains in the concrete and conventional steel were also measured. The anchoring force was measured by a donut load cell. The main variables in the study included the concrete compressive strength ranging from 45 to 100 MPa, prestressing steel ratio and non-prestressed steel ratio.