Field instrumentation and live load testing to evaluate behaviors of three reinforced concrete bridge decks

Abstract

The deterioration of reinforced concrete bridges in Montana has prompted the Montana Department of Transportation (MDT) to investigate alternative bridge deck designs which minimize concrete cracking and subsequent steel corrosion. Construction was completed during the summer of 2003 on three new bridges near Saco, Montana that afford an unusually good opportunity to investigate the performance of different bridge decks. Notably, the bridges are in close proximity, and are of the same geometric design. The only differences between the bridges is the deck construction: one deck has a conventional concrete and conventional reinforcement layout; the second deck has a conventional concrete with a reduced amount of reinforcing steel; and the third deck has a high performance concrete (HPC) with conventional reinforcement layout. MDT contracted with Western Transportation Institute (WTI) at Montana State University to install strain instrumentation within each deck prior to pouring the deck concrete and to subsequently perform analyses on collected data. As part of the project, live load testing was performed on each bridge prior to opening them to traffic. The goal of this study was to evaluate and compare baseline performance of the three new bridge decks under live load demands. Data from the transversely-oriented strain gages cast in the decks were used for this purpose. This analysis found that all three decks performed equally well under vehicle loads, with only minor differences observed between them. None of the decks showed evidence of longitudinal cracking. The position of the neutral axis revealed that in-plane axial tension forces occurred in the decks during testing; internal compression arching was not observed at the load levels used in the tests. Transverse strains over the girders show no indication that cracking has occurred, nor do they suggest future cracking from vehicle loads. Using the geometry of each deck's crosssection and Girder Distribution Factors (GDFs), the relative stiffness of the three decks was compared. As expected, the deck with reduced reinforcing steel was determined to be the least stiff, both longitudinally and transversely. Based upon the applicability of superposition, all three decks were determined to behave linear-elastically. These observations serve as a baseline measure of performance for future analyses on these bridge decks.