Neutral Axis-Based Health Monitoring and Condition Assessment Techniques for Concrete Box Girder Bridges

The neutral axis position is considered as a potential parameter of early abnormal and long term performance for flexural members of structures. This paper is to propose the Neutral Axis Indicator (NA) based on strain measurements, to evaluate the behavior of NA, and to investigate challenges associated with the neutral axis for structural health monitoring purpose. A numerical example of a simply supported reinforced concrete (RC) beam was first developed and analyzed followed by an experimental study for further verification. With the aim of application to real bridges, a case study was conducted on Gantao River Bridge, a typical concrete box girder bridge, for detailed analysis using field data. The numerical and experimental examples, as well as field tests, aim to figure out: (1) the sensitivity of NA to structural damages, especially at early age; (2) stability and uncertainty of NA using field data; (3) capability of NA on detecting abrupt and slowly changing structural degradation. The results show that: NA takes on good index accuracy and damage sensitivity on numerical and experimental cases; NA tends to keep constant during linear elastic phase, while responses quickly and sharply to nonlinear behaviors such as concrete cracking and hardening; and NA is a potentially reliable indicator both to capture nonlinear performance and to detect minute damage. Additionally, the uncertainty of multiple disturbing effects can be mostly eliminated through statistical analysis. Thus, it can be employed effectively in condition assessment of concrete box girder bridges for long-term health monitoring.