Two-layer concrete bridge beams as composite elements

Two-layer high-performance RC beams can be used in the traffic (longitudinal) direction of long-span bridges. The beam section consists of two parts: a bottom part made of normal-strength concrete (NSC), which has a U-type section, and an upper one made of steel fibre-reinforced high-strength concrete (HSC), which has a rectangular section. Using two-layer beams is logical because as the beam span becomes longer and the service load increases, so a higher concrete strength is required in the beam's compression zone to withstand rather large bending moments. As concrete in the tension zone of the section contributes little to the beam's loadbearing capacity, this zone is made of NSC. This is an important economical advantage of two-layer beams. Steel fibres are only used in the compression zone made of HSC. The addition of steel fibres results in a higher ductility for the HSC layer, allowing proper design of two-layer bridge beams for dynamic loads. The design parameters for such beams include the HSC class, where the HSC layer depth is known and is equal to the depth of the rectangular section (above the U-section). The bottom NSC part (U-section) of a long-span beam is usually prestressed. Taking into account this circumstance, the total beam section depth should be checked for the serviceability limit state.