Introduction and Motivation

• Over 200 million trips across nations structurally deficient bridges daily
• 1 out of every 9 of bridges are structurally deficient
• Average age of bridges – 42 years
Estimated investment required – over $20 billion annually

I-35W Bridge Collapse, Minnesota
• 13 people died, economic impact of over 500 million dollars
• The NTSB states lack of inspection guidance and inadequate use of technologies for assessing the bridge condition as one of the main reasons for the collapse

Background:

CARBON NANOTUBE-BASED TESTING
• Carbon nanotubes (CNTs) can form a nerve-like electrically conductive network
• The resistance of this network changes under deformation and due to formation of cracks
• Conventional sensors such as strain gages provide only point
measurements
• Carbon nanotube-based sensors enable distributed sensing capability, allowing damage detection over large areas

STRUCTURAL REHABILITATION WITH CARBON NANOTUBE-BASED SENSING COMPOSITES
• Fiber composites can rehabilitate deteriorated structures by carrying load in the structure
• Integrating distributed sensing with structural reinforcement allows for continuous structural health monitoring (SHM) capabilities

Sensor Fabrication, Characterization, and Integration with Structural Layer

RESEARCH METHODOLOGY
• CNTs are deposited on nonwoven aramid using different processing methods, forming a uniform coating on the fibers
• Sensors are infused with epoxy resin and tested in tension to determine sensitivity
• The gage factors of these sensors are strongly influenced by the processing technique varying from ~1.5-10
• Gage factors are equal to or higher than traditional strain gages
Carbon nanotube-based sensor is integrated with unidirectional carbon fiber prepreg
• Integrated ‘structural sensing’ layer is then bonded to a steel test coupon using an epoxy adhesive

Steel Coupon Preparation

Resistance - Strain Behavior

• Resistance curve closely follows the strain behavior
• Changes in slopes of resistance behavior can be used to identify damage

Improvement of Mechanical Properties

• The specimen with the structural layer improves the stiffness by 30%

Sensor Sensitivity

• Both specimens have a linear resistance-strain response and similar sensitivity

Conclusions

• A novel method for simultaneous structural health monitoring and strengthening of steel structures has been established
• The processes used to manufacture CNT sensors are inherently scalable
These sensors can cover large areas and are highly effective for distributed sensing
• The gage factor of these sensors is higher than most commercially available strain gages
• Fiber composites can be used to improve the mechanical properties of deteriorated steel

Future Work

• Optimizing the manufacturing process and characterizing the sensor under different loading conditions
• ‘On-field’ testing using large scale sensors

Acknowledgements

This research is supported by the US Federal Highway Administration, Grant No. DTFH31-13-H00010