Passive strain sensing for structural health monitoring of civil infrastructure using retroreflective sheeting materials
Date
2025
Authors
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Publisher
University of Delaware
Abstract
Structural health monitoring (SHM) is the implementation of sensors on a structure that allow engineers to evaluate the condition of the structure in a non-destructive, quantitative manner. SHM systems consist of sensing networks and data acquisition that are designed to provide information early on, before problems become too serious, and therefore reduce maintenance and repair costs and extend the life of structures. For bridges, SHM systems are not required, and are typically only implemented on very large and important structures where the return on investment is clear; however, smaller bridges could also benefit if structural health monitoring systems were implemented on them. Therefore, it is desirable to create a cost-effective solution so that SHM of smaller bridges is attainable. This can be achieved by creating a new passive sensor that utilizes materials and technologies that bridge owners are already familiar with. ☐ Retroreflective sheeting materials (RRSM) are used to make highly reflective traffic signs, and state Departments of Transportation (DOTs) are required to demonstrate that they meet minimum levels of retroreflectivity (RR), which can be measured using a handheld retroreflectometer. Many of these agencies also fabricate their own traffic signs. Thus, DOTs have familiarity with RRSMs and RR measurements. State DOTs also own a large percentage of the nation’s bridge infrastructure. Therefore, if RRSMs could be used to monitor strain in bridges, it would be an effective, straightforward and innovative way to encourage more frequent SHM of typical bridges. ☐ For this work, a total of 15 unique RRSM materials were evaluated for potential use as a passive strain sensor for structural health monitoring. They included 5 different ASTM standard types, 2 manufacturers and a variety of colors. The materials were subjected to various mechanical tests. First, all materials were subjected to tension testing, and it was determined that as strain is induced in RRSMs, their retroreflectivity decreases. Many materials have a linearly decreasing relationship between RR and strain. The sensitivity of each material’s RR to strain is a key parameter in determining viability for strain sensing. Materials with higher sensitivities were down selected for further testing, which included mounting the materials to steel and testing them in tension. Results showed that materials’ RR was not as sensitive to strain when it was mounted to a substrate as when it was tested on its own. Exploration into remedies for this were studied and ultimately a single material was chosen to be the best candidate for passive strain sensing of civil infrastructure. This material must be strained prior to use as a sensor. To accommodate this pre-straining, a production sensor was developed and tested. Ultimately, the RRSM sensor was able to be used to accurately predict strain in a beam loaded in four-point bending. ☐ Results show that RRSMs are affected by changes in temperature and an RR-Temperature relationship was developed for the final down selected material. When measuring RR of an in-place sensor to calculate structure strain, the RR value needs to be adjusted for the change in temperature between the time of installation and the time of RR reading. The process for fabricating, installing and reading an RR sensor was developed and is laid out in a stepwise manner for simplicity. In addition to this, various materials were microscopically evaluated to determine the fundamental mechanism by which RRSMs change when loaded. Finite element models were developed to provide additional insight into the material behavior under load. Finally, ASTM Type XI materials were subjected to exposure testing to determine how RR and adhesion change with continued exposure to the environment. ☐ This research led to a better understanding of RRSMs and how they respond to strain. This understanding can be used to create and implement a low-cost and accessible passive strain sensor that can be used on bridges to passively monitor their strain. RRSM sensors are thought to be an initial detector of a problematic structure, and not a solution to a deficient bridge. Bridge owners have access to and familiarity with RRSMs and retroreflectometers to measure their RR, which should lower the burden for transfer of technology and make the use of these sensors simple which ideally will encourage quantitative analysis of more bridges.
Description
Keywords
Bridge, Passive strain sensing, Retroreflectivity, Finite element models