Shear and Strengthening Assessment, and Bridge Damage Evolution of Kishwaukee I39-River Bridge

The evaluation of bridge performance involves physical load testing and rating to address the concerns regarding bridge safety and ensure public welfare. Load testing provides valuable insights into the actual behavior of bridges, particularly for aging infrastructure that faces increasing demands. As replacing or rehabilitating bridges is costly and challenging, optimizing the utilization of existing infrastructure is crucial. Proof Load Testing (PLT) has proven to be a reliable non-destructive method, especially for older bridges, in assessing bridge conditions and reflecting their true behavior. This study focuses on the case of the I-39 Kishwaukee bridge, a twin post-tensioned segmental concrete box girder bridge constructed in 1970. Retrofits were conducted on the bridge to address cracks and slippage at the shear key between the pier and adjacent cantilever segments, caused by non-hardened epoxy during construction. In 2006, the Illinois Department of Transportation (IDOT) strengthened the structure by introducing four 12-strand, 15mm external post-tensioning tendons inside the box girders to mitigate shear forces. Proof load testing was conducted on the Kishwaukee bridge using four different trucks with varying weights: 76 tons, 90 tons, 122 tons, and 136 tons. Nine testing scenarios were successfully completed, with a maximum load of approximately 136 tons. The combination of IoT technology, sensors, data acquisition systems, and Finite Element Modeling allowed for the integration of analytical models and field test results, enhancing bridge condition assessment. Instrumentation was installed on half of the bridge, including vibrating wire strain gauges to measure strains near the pier and midspan, crackmeters to measure crack openings near the shear key, and Linear Variable Differential Transducers (LVDTs) at the critical section of Span #5 to measure deflection. The Modified Compression Field Theory (MCFT) was utilized to calculate shear capacity, considering the contributions of reinforcing steel, prestressing Dywidag bars, and the effect of external post-tension tendons. This study presents a comprehensive procedure for load rating the Kishwaukee bridge, encompassing field operations, instrumentation, and interpretation of test results based on the 2018 AASHTO Manual for Bridge Evaluation. The findings indicate no crack slippage across the web-cracked section and confirm the bridge's robust concrete shear capacity, which exceeds the applied shear force by a factor of 1.8. Therefore, the Kishwaukee I-39 bridge remains safe for future traffic loads and heavier truck loads. Moreover, the data obtained from the field test is used to develop a Finite Element Model to analyze the impact of the recently introduced external post-tensioning tendons on the bridge's structural performance. In conjunction with the FEA, this research demonstrated that the rehabilitation of the Kishwaukee I39 bridge using the post-tensioning system reduced the deflection by 88.72 % and minimized the principal tensile strain on the shear key by 80µɛ. Based on these findings, this paper provided a significant allowance for accommodating future traffic load increases on the Kishwaukee I-39 River bridge. Additional data analysis reveals that the nonlinear characteristics of concrete primarily stem from the presence, expansion, and merging of microcracks and/or micro voids. The development and propagation of macrocracks, leading to ultimate failure, can be attributed to the mechanism of microcracks. In the bridge context, Continuum Damage Mechanics (Damage Evolution) has been successfully utilized. This involves the introduction of an internal variable, denoted as "d," which tracks the degradation of the material's elastic properties. The degradation of the Kishwaukee bridge shows that the damage evolution starts first in the concrete tension zone, the prestressed Dywidag bars, and then at the concrete compressive zone. The damage evolution model shows that the concrete depicts a Mazmod and then a linear behavior till it reaches the total collapse.