Flexural behavior and serviceability evaluation of staged prestressed concrete composite beams: Experimental and theoretical study

Prestressed concrete composite beams enable sup¬port-free construction, accelerating the construction process. However, when structures use large-span beams that bear heavy loads, reducing the cross-sec¬tional dimensions of composite beams while working with limited prestressing strands in precast concrete beams is difficult. This study suggests the use of pre-embedded ducts in precast, prestressed concrete beams. After postcast (cast-in-place) concrete is placed to complete the composite beam, prestressing strands are inserted in the ducts and secondary tensioning is performed, which forms the staged prestressed composite beam (SPCB). Four SPCB specimens were tested to simulate construction and service conditions to evaluate flexural performance. The crack distribu¬tion patterns, deflection evolution, and reinforcement strain development during loading were then observed. The experimental results revealed that cracking during the precast concrete beam–bearing stage significant¬ly reduced the overall crack resistance of the SPCB, consequently affecting subsequent deflection and crack development. This paper also presents proposed design methods for determining the cracking moment, midspan deflection, reinforcement effective stress, and maximum crack width of SPCBs. The internal force arm coefficient for the SPCB was derived through regression analysis based on experimental data.