Most common type of highway bridge superstructure is slab-on-multi-girder prestressed concrete due to easy construction in less time. In design and construction of these bridges, we need a predefined concrete strength, a mild reinforcement with standard yield strength and a low relaxation prestressing steel.

We usually need a reinforcing steel cage both to minimize concrete cracking and to improve the strength of precast girder. In the design of precast concrete girders, precompression is needed and is provided by jacking prestressing steel to a predetermined force. Cast-in-place deck concrete is placed over girders. Typically precast girder concrete has more strength than deck concrete in AASHTO-LRFD design method. Girder concrete strength at the transfer of prestressing is needed to be around 80% of 28-day concrete compression strength.

Reinforcing steel has usually a have a yield strength of 60ksi (420 MPa). The stress-strain properties need to satisfy the minimum requirements of the AASHTO-LRFD design.
Prestressing steel has typically have low-relaxation properties that allow higher loading capacities for the given AASHTO-LRFD girder design with less losses. Low-relaxation strands can result in savings of material for relatively heavier and long spans.

In prestressed precast units, the mild reinforcement and prestressing steel have been used together. Most of the flexural strength is supplied by the prestressing steel due to its high strength compared to the mild reinforcement

In AASHTO-LRFD design, we need to limit the concrete compressive stress to avoid crushing of concrete and need to limit concrete tension stress to avoid severe cracking of concrete. Severe cracking of concrete will allow water to infiltrate into the concrete that can start corrosion of reinforcement and prestressing steel. In case, the girders crack under high tension, under repetitive loads due to live load, fatigue life of the structure can decrease. The modulus of elasticity of concrete has been used to compute the rigidity of the girder in stiffness analysis.

Reference;
AASHTO LRFD Bridge Construction Specifications, 4th Edition