# Development of a Low-Cost Passive Strain Sensor for Bridge Structural Health Monitoring

## TL;DR

A low-cost strain sensor made from traffic sign material is developed to monitor bridge health and improve safety.

## Contribution

A novel passive strain sensor using retroreflective sheeting material is developed for cost-effective bridge monitoring.

## Key findings

- Retroreflectivity decreases linearly with strain after an initial plateau of ~1000 × 10−6 m/m.
- The sensor can estimate maximum strain changes using periodic retroreflectivity measurements.
- The sensor is practical, low-cost, and easily implementable for typical bridge monitoring.

## Abstract

Complex structural health monitoring (SHM) systems are rarely installed on typical bridges, likely because of an expected low return on investment; however, low-cost, passive sensors made from a retroreflective sheeting material (RRSM) offer an economical alternative for SHM of typical bridges. Most departments of transportation (DOTs) fabricate and maintain traffic signs made from RRSMs. By using a material familiar to DOTs, the technology transfer from signs to strain sensing is streamlined. This paper focuses on the development of a passive strain sensor made from an RRSM. A standard Type XI fluorescent yellow-green RRSM is tested in tension to establish the relationship between retroreflectivity (RR) and induced strain. Results show RR decreases linearly with increasing strain after an initial plateau of ~1000 × 10−6 m/m. To function as a strain sensor, the RRSM is pre-strained beyond the plateau. A production sensor is designed to attach to the tension face of a structural element for monitoring. Periodic RR measurements are used to estimate the likely maximum strain change at the sensor location. The sensor has the potential to provide a practical, low-cost, and easily implementable solution to improve the monitoring of typical bridges, enhancing their safety and longevity.