Refinement of the Inverted T-Beam System for Virginia

About the report:

The inverted T-beam system is well suited for short to medium spans, and can be built quickly and efficiently.  The system was developed for Virginia, based on a similar system used in Minnesota, and has been used in two structures in the commonwealth.  Previous research helped to develop the original details, but additional research was recommended to continue to refine the system.  This report presents the results of the research performed to refine the system.

Analysis was performed to develop preliminary design tables for three sizes of the inverted T-beam, utilizing normal weight and lightweight concrete, debonding strands, and specialized construction techniques to extend the usable span length.  It was found that with typical construction methods, the 18-in deep section with a 7½-in topping over the webs can span 50 ft, regardless of concrete unit weight.  Span lengths of up to 70 ft can be achieved with special construction methods, such as staged deck placement or temporary shoring.

Push-off tests were performed to determine the optimum surface roughening technique to ensure full composite action between the precast beams and the cast-in-place topping.  Formed patterns with angular edges and approximately the same area of positive and negative elevation resulted in the highest bond strengths. 

Three sub-assemblage specimens, each having different connection and surface roughening details, were tested to investigate the performance of transverse connections under cyclic loads.  The specimens were subjected to 3.65 million cycles of load, representing about 50 years of service loads on an actual bridge.  Two connection details were deemed adequate for bridges with a high traffic volume.  A non-contact lap splice detail with poor surface roughening did not exhibit an adequate factor of safety against a bond failure.

Work was performed to optimize the cast-in-place topping concrete mixture to minimize the possibility of cracking due to differential shrinkage between the precast beam and the cast-in-place topping.  Performance criteria and prescriptive mixture proportions were developed, which along with fully saturating the beam surfaces before placing the topping and providing a full seven-day moist cure for the topping, should result in a relatively crack-free deck.

Finally, analysis was performed to determine the best orientation of transverse reinforcement in the topping concrete for skewed bridges.  It was found that the stresses in the transverse reinforcement remain very low, even if cracking does occur above the flange-to-flange connection, so orientation is not critical.  However, orienting the reinforcement perpendicular to the axis of the beams is recommended for the non-contact lap splice (no-connection) detail, so the bars in the topping and in the beam are oriented in the same direction.

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