Condition Assessment of Concrete Bridge Elements using Active IR Thermography

Editor's Notes

• #2: Good afternoon, My name is Jason Cattelino and I am an undergrad student at Michigan Technological University in Upper MI. The past year, I have been working under Dr. Tess Ahlborn in conjunction with the MI Dept. of Transportation to Assess the Condition of Concrete Bridge Elements using Active Infrared Thermography. With the population of the U.S. continuously increasing, and our transportation network growing, it has become increasingly important that engineers and government agencies monitor existing infrastructure to ensure the safety of it’s millions of users.
• #3: Highway overpass bridges account for approximately 75% of our nation’s bridges and are commonly part of our every day transportation route. While we often don’t think about the condition of these bridges when we drive over them, we will notice when closures for repair and rehabilitation increase commute time.
• #4: With much of our nation’s infrastructure being built throughout the 50’s and 60’s, it is reaching the end of its design life. In 2013, the average bridge age was 42 years old. Among those bridges, 25% were deficient according to current design standards And 11% were structurally deficient requiring load reductions due to excessive deterioration. With one estimate of over $170 billion to improve current conditions and performance of our nation’s bridges, our infrastructure is in need of an overhaul.
• #5: Several types of deterioration are capable of categorizing bridges as structurally deficient by reducing load capacity of the structure: Spalling - the loss of material due to distress. Cracking - width and patterns are often used as indicators of specific failure modes Rebar Corrosion - often leads to other forms of deterioration Bridge elements such as the concrete pier shown commonly display multiple forms of deterioration.
• #6: Another common type of deterioration is delamination: the separation of concrete layers within the cover above reinforcement due to moisture migration and corrosion of rebar. As a leading cause of cracking and spalling, delaminations pose a concern for agencies affecting the (ridability?) of the top bridge deck and present a hazard of falling debris for traffic underneath a structure. If detected in their earliest stages of formation, delaminations can be used to initiate maintenance extending the service life of bridges. The figure in the bottom left shows a deck replacement. The arrows show a variable depth delamination extending horizontally over the bridge deck which looks to be caused by corrosion of the reinforcement. The figure on the right shows a core from a bridge deck. As you can see, multiple delaminations are present within the sample at different depths from the surface.
• #7: There are many methods that can be used to assess the condition of concrete structures, all of which fall into two broad categories: Destructive and Non-Destructive testing. Because non-destructive testing operates without compromising structural integrity, Michigan Tech has focused on these types of technologies. Non-destructive methods can be further categorized into remote sensing and direct contact. Conventional methods used to quantify delamination include the direct contact methods of hammer sounding and chain dragging but due to accessibility challenges for some bridge elements, especially on the underside, and the desire to reduce traffic disruptions, remote sensing is gaining popularity among inspection agencies. Having investigated several remote sensing technologies at Michigan Tech, including GPR, LiDAR and 3D Photogramatry, results have shown great potential for using infrared thermography for delamination detection.
• #8: Infrared thermography can be conducted using either a passive or active test set up. In a passive test set up, heat from the sun creates a temperature difference between the concrete surface and interior. Heat from the “warmer” surface transfers down through the concrete to “cooler” subsurface areas. As the heat transfers through the concrete, a delamination acts as an air insulator, inhibiting further energy transfer and causing a concentration of heat at the surface. Using a thermal imaging camera, the “warmer” areas above delaminations are distinguishable from areas of sound concrete. Passive infrared thermography has been used to assess the tops of bridge decks, however, several limitations have prevented equally successful applications to the underside of bridges. When dependent on the sun as an energy source, specific time testing windows are necessary and testing is weather dependent. In addition, the sun does not create a sufficient thermal gradient on elements under a bridge.
• #9: Active test set ups are similar to the passive set up but make use of an external heat source to “actively” heat an area of concrete, creating a thermal gradient between the surface and subsurface. As a result, some of the limitations of passive infrared thermography can be overcome which allows for use on the underside of a bridge. Active infrared thermography has been used in other studies but few have focused on concrete applications.
• #10: Two main research objectives were established in this study: 1) Conduct a preliminary lab investigation to validate the application of active infrared thermography to concrete elements and 2) Evaluate active infrared thermography through a proof of concept field application
• #11: Lab testing was conducted in one of Michigan Tech’s research facilities. Several 3ft. By 3ft. concrete test slabs were constructed containing Styrofoam blocks at various depths to simulate delaminations. A tripod mounted patio heater provided a control heat time of 15 minutes. A thermal camera monitored the slab surface temperature during the heat impulse and a cooling period. The camera and heater were positioned at specified distances from the specimen. Experiments investigated the effects of heat time, heater distance and thermal concentrations on delamination detection.
• #12: Data from a thermal camera is represented as a matrix of numbers corresponding to surface temperatures. For visual analysis, a false color map is provided. For numerical analysis, the absolute contrast method was used. A representative area above a delamination and a local reference area were defined and the average temperature within these areas was compared over time to create a figure like the one shown. From the figure, the observation time, or the time at which a delamination appears with maximum contrast to it’s reference area, can be determined. This time generally occurs 10-15 minutes after the heating period where a distinct rise in the graph can be seen. Using the observation time and material properties, the depth of a delamination can also be estimated.
• #13: Lab tests resulted in several key findings: First, a relationship between delamination width and depth was determined. Delaminations are detectable if they are less than or equal to 2 inches below the concrete surface AND if the width of the defect is a least 2 times it’s depth. Second, delamination depth can be successfully estimated using observation time and material properties. Third, heat times less than 15 minutes are acceptable for detecting delaminations.
• #14: To test the lab findings, a proof of concept field application was conducted on Franklin St. Bridge over US-131 in Grand Rapids, MI on June 24th, 2014. MDOT provided ground truth information using conventional hammer sounding techniques. Tests were conducted in 3 locations including the bottom of the bridge deck and the side of a pier cap. 5 and 15 minute heat times were used.
• #15: To conduct the field tests, two thermal imaging cameras were used for comparison, a high resolution FLIR SC640 and a compact lower resolution FLIR Tau 2. The SC640 has an external display providing real time thermal images and temperature output while the Tau 2 records data to an external memory drive and requires manual calibrations for temperature data. The smaller Tau 2 was mounted to the top of the SC640 to capture a similar test area.
• #16: To provide access for testing, MDOT provided a lift platform truck. The top left pictures shows a test conducted on the side of a pier cap.
• #17: Data captured from the SC640 and Tau 2 were compared at each test location. The two left images show data collected from the smaller Tau 2 camera while the images on the right were collected using the SC640. As you can see, there is little difference in resolution between the two cameras. Also noticeable in these images are distinct thermal bands originating from the reflective backing of the heater which was also noticeable in lab tests.
• #18: All data processing from the field demo was conducted using MATLAB. First, the percentage area of delamination was determined from ground truth information based on the camera’s field of view. After aligning the thermal and optical images, the camera field of view was projected to an optical image to define a total area. A polygon was constructed around the paint-marked delamination and the % area was determined. Second, the percentage area of delamination was determined from thermal images. At the time of maximum delamination contrast, a polygon was constructed around the suspected delamination and the % area was determined.
• #19: The total height of the bar for each test represents MDOT’s ground truth information. The blue bar represents the percentage of MDOT’s findings by the high resolution SC640 and the green bar represents findings by the low resolution Tau 2. In all tests, it is apparent that infrared thermography reported less area of delamination than hammer sounding conducted by MDOT with varying degrees of accuracy. Data also shows that the performance of the SC640 and Tau 2 was not necessarily consistent. No single camera displayed a higher delamination percentage for all tests.
• #20: The proof of concept field demonstration resulted in several key findings: First, active infrared thermography is adequate for detecting delaminations on the underside of bridge decks. Second, this technology can be applied to pier caps and vertically oriented elements such as fascia beams. Third, lower cost and resolution cameras are adequate to quantify delamination area. Lastly, infrared thermography can improve inspector accuracy.
• #21: To achieve implementation of infrared thermography by inspectors, Input is necessary with regards to equipment cost and portability as well as testing time. Automated detection algorithms such as threshold analysis, higher order statistics and frequency domain analysis can be used for more objective test results. Additional field applications should also be conducted for procedure refinement.