First-Generation ABC System, Evolving Design, and Half a Century of Performance: Michigan Side-by-Side Box-Beam Bridges

Abstract

The side-by-side box-beam bridge is the bridge of choice for short to medium span bridges due to ease of construction, favorable span-to-depth ratios, aesthetic appeal, and high torsional stiffness. The bridge can be constructed in an accelerated fashion, and classified among the systems that qualify accelerated bridge construction (ABC). This bridge is losing favor primarily because of persisting performance issues such as cracking between the beams that reflects to the deck surface. This is in spite of design changes, primarily for resolving the performance issues, since the 1950s. However, performance problems, specifically the longitudinal deck cracking reflecting from the shear keys, still persists. Reflective deck cracking is identified as the leading cause for triggering other distresses that create safety concerns. Significant recommendations in literature to mitigate reflective cracking include: (1) using full-depth grouted shear keys and transverse posttensioning, and (2) incorporating a cast-in-place concrete deck and seven-day moist curing of the deck. The most recent design in Michigan incorporates all these recommendations. Hence, the evolving Michigan Department of Transportation (MDOT) side-by-side box-beam bridge superstructure design procedure was examined, and major design changes were identified. In evaluating the impact of changing design details, numerous in-service bridges were inspected. The list of inspected bridges includes a representative age sample of 15 in-service bridges, a replacement bridge prior to traffic loading, and a demolition project. The distress types, states, and progression were documented from the inspection of in-service bridges. Unfortunately, with full-depth shear keys, high levels of transverse posttensioning, and a 150 mm (6 in.) thick seven-day moist cured cast-in-place concrete deck, Michigan still experiences reflective longitudinal deck cracking. Inspection of a bridge under construction showed that the grout-beam interface cracking develops within a couple of days after grouting and the deck cracking well before the bridge is opened to traffic. The synthesis of all data yielded that the design changes failed to mitigate longitudinal cracking on side-by-side box-beam bridges. The motivation of this article is twofold: first is to analyze and identify the reasons for the reflective cracking. The second motivation is the scientific curiosity of solving a problem that has troubled the bridge engineers for over 60 years.