Implications of using high strength concrete in link slabs of precast, prestressed girders

Abstract

Precast/prestressed concrete girders acting compositely with in-situ concrete deck slabs are often used in bridge construction. The precast girders are rapidly laid out as simply supported and the deck slab cast without false-work by using pre-slabs or by making the top flange of the girder wider. In multi-span construction, continuity is recommended and can be achieved by extending the deck slab over the intermediate supports. The purpose is to reduce the number of joints and eliminate problems that affect the durability, aesthetic and service life of the bridge. The connection portion between the ends of the precast girders over the intermediate supports is labeled as the link-slab. While precast beams generally deflect downward under superimposed loads and live loads and upward under the effect of creep, the link-slab exhibits an elastically continuous flexible behavior between the adjoining spans and will essentially flex in opposite curvatures to the precast beams. This helps avoiding the build-up of large design moments in the link-slab that is usually reinforced to resist long term flexural cracking. Recent advancements in the concrete industry resulted in developing High Performance Concrete (HPC) and Ultra High Performance Concrete (UHPC) with a concrete 28-day compressive strength of up to 200 MPa. Although the use of such concrete mixes can lead to reducing the number and size of girders, they are considered to be expensive and highly specialized and therefore are not widely adopted in the fabrication of precast girders. This paper investigates the implications of using HPC and UHPC in the construction of link-slabs where the volume of concrete required is limited and therefore the cost implication is not that high. The paper mainly examines the redistribution of forces due to the increased strength (stiffness) of the link-slab and its impact on the design of the precast, prestressed girders. A parametric study is conducted for this purpose which considered various concrete 28-day strength (HPC and UHPC), link-slab length and link-slab thickness on flexural moments. Results show that flexural moments in the precast girder can be reduced by up to 20% if the compressive 28-day strength of the link-slab is increased to 200 MPa. Design recommendations are finally made and illustrated by a comprehensive numerical example.