Field Load Test on the New Maryland Modular Slab Bridge System

slab-bridgeMaryland intends to revise its current practice of using tie-rods for the transverse post-tensioning in slab bridge design. The new design of using high strength rods will provide a more tightly integrated modular slab bridge system with higher post-tensioning forces and also provide integral abutments to reduce deck joints. Integral abutments may provide a smooth approach to the bridge with no deck joints. Integral abutment bridges cost less to construct and require less maintenance than equivalent bridges with expansion joints. In addition to reducing first costs and future maintenance costs, integral abutments also provide for additional efficiencies in the overall structure design.

image004The function of the re-designed shear key and transverse post-tensioning for a precast deck bridge is to make a smooth deck surface and a continuous unit under permanent and transient loads. Shear-flexure transfer joints are formed by transverse post-tensioning, cast-in-place closure joints, a structural overlay, or a combination thereof (AASHTO LRFD Transverse post-tensioning is the main reason that decks with shear-flexure transfer joints can be considered as continuous plates.

However, discrepancies were found among the current Maryland practice, the AASHTO LRFD Bridge Specifications and the PCI Bridge Design Manual for the post-tensioning force along the transverse direction for the precast, prestressed concrete slab bridge. This prompted the Maryland State Highway Administration (MDSHA) to ask the BEST Center to conduct an early investigation of this system. In the study, the following tasks were conducted:

  1. Reviewing existing AASHTO code requirements and PCI recommendations.
  2. Reviewing practices of other States.
  3. Providing a report transmitting findings and recommending a post tensioning force.

In summary of the study, in order to minimize potential for cracking in the overlay of the slab bridges and to meet current industry recommendations for post-tensioning force, revisions were recommended with a high post-tensioning force to replace the current MD SHA practices.

Since adjacent precast, prestressed box girder bridges have nowadays been used more often for short-spans, standardization of this modular bridge is highly desired. Quick construction and cost effectiveness are the most frequently mentioned benefits. With the new design, Maryland State Highway Administration is highly interested in the performance of the new design, especially compared with the old design. A research plan is made to serve this purpose.