Implementation of a Precast Inverted T-Beam System in Virginia: Part I: Laboratory Investigations

About the report:

The inverted T-beam system provides an accelerated bridge construction alternative for short-to-medium-span bridges. The system consists of adjacent precast inverted T-beams with a cast-in-place concrete topping. This bridge system is not expected to experience the reflective cracking problems manifested in short-to-medium-span bridges constructed with the traditional adjacent voided slab or adjacent box beam systems. This report presents the results of three phases of a comprehensive research project to develop and implement an inverted T-beam system for Virginia. The three phases are shape and transverse connection design, cast-in-place topping optimization, and composite action.

When concentrated loads are applied to a bridge of this type, the bridge deforms as a two-way flat plate. This phase of testing included an analytical and experimental investigation focused on the first inverted T-beam bridge in Virginia on US 360 over the Chickahominy River to study the relationship between transverse bending and reflective cracking. Transverse bending moment demands were quantified using a finite element model and compared to tested transverse bending moment capacities provided by several sub-assemblage specimens. The tested sub-assemblage specimens featured a combination of various precast inverted T-beam cross-sectional shapes and transverse connections. It was concluded that all tested specimens performed well at service load levels. The detail that features a precast inverted T-beam with tapered webs and no mechanical connection between the adjacent inverted T-beams and cast-in-place topping is the simplest and most economical.

There is a difference in shrinkage properties between the inverted T-beam and the deck because of the sequence of construction. The deck is subject to restrained shrinkage tensile stresses, which may lead to cracking. This phase of testing included an experimental study on the short-term and long-term properties of seven deck mixtures to identify a deck mixture with low shrinkage and high creep. The mixture with saturated lightweight fine aggregates is expected to best alleviate tensile stresses due to differential shrinkage.

The final phase of testing presented in this report investigated the composite action between the unique precast and cast-in-place element shapes. A full-scale composite beam was tested under different loading arrangements with the purpose of simulating the service level design moment, strength level design shear, strength level design moment and nominal flexural strength. To investigate the necessity of extended stirrups, half of the span featured extended stirrups, whereas the other half featured no extended stirrups. In the tests, the system behaved compositely at all loading levels and no slip occurred at the interface. In addition to measuring slip at various interface locations, full composite action was verified by comparing load displacement curves obtained analytically and experimentally. It is concluded that because of the large contact surface between the precast and cast-in-place elements, cohesion alone appears to provide the necessary horizontal shear strength to ensure full composite action.

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