Project Update — BridgeGuard® validates infrared technology on concrete substructure.
Client: Florida Department of Transportation - District 2; facilitated by Kisinger Campo & Associates Corp. (Tampa, FL)
Service: Bridge Substructure Delamination and Spall Survey using the BridgeGuard® 300 thermal imaging and mapping system.
In coastal regions of Florida, bridge substructure is exposed to a steady onslaught of saltwater. This leads to corrosion of the reinforcing steel and subsequent delamination and spalling of the concrete. Repairing these defects is an expensive and time-consuming process, even more so when quantity overruns are encountered due to missed defects. Some overruns may be as much as 50%. In an effort to mitigate these overruns, FDOT contracted with Kisinger Campo & Associates, Corp. and BridgeGuard® to develop and employ an infrared defect survey methodology. The results were then compared to the actual repair quantities at a later date.
“Thermo-imaging spall detection has benefits when traditional sounding techniques have limitations due to the volume of access. Determining the right temperature differential is important in Florida for efficiency and accuracy.” said Will Watts, P.E. Bridge Management System
At the time the project commenced, infrared defect collection was limited to primarily desk top inspections. This was tot take advantage of the direct solar energy used to generate the necessary thermal gradients to produce defect signatures. Underneath the bridge, however, the solar exposure is limited and the main thermal gradient driver is convection. There had been research that indicated early success, but questions remained as to the adaptability in practical applications.
Some basic questions were developed for the study:
•Is there enough convection in the ambient environment to create the necessary gradients (thermal contrast) to show delaminations?
•What are the optimum times of the day for data collection?
•How far away can these defects be identified and quantified?
•Will the quantified areas be accurate enough to minimize overruns?
The first part of the experiment was to find a delamination by traditional methods (hammer sounding), then observe it for twenty-four hours with the infrared sensor to determine thermal contrast throughout that period. A variety of sensors were evaluated and application-specific software was developed in-situ to collect the necessary data. During the scanning, temperatures ranged from 60° F during the day to 40° F at night, through the resultant temperature contrasts in the concrete defects were far less. Wind speed and water temperature were also recorded and considered as part of the test. Software programs were written on-site to analyze the collected data. upon completion of the diurnal study, the rest of the experiment involved data collection at established optimal times of all remaining pile caps, analyzing and entering the defect data into a summary report for review.
Construction plans were created based on the BridgeGuard® defect maps and a contrac was let for the rehabilitation. During the rehabilitation, as-built quantity sketches were recorded and then overlaid onto the original infrared data set in CAD for comparison.
The as-built item comparison reflected a 10.7% quantity overrun from plan. This was due to three main factors including:
1. Fracture plan propagation. Duration from survey to rehabilitation was 1.5 years.
2. Careful evaluation of the limits of deficiencies must be performed when large fracture planes extend over an entire face of an element, leaving little area to contrast with.
3. Perimeters of delaminations have regions that are polluted, or compromised, extending into the conforming concrete. Yet these regions still hammer-sounded solid or did not appear delaminated with infrared scanning. More research needs to be conducted to determine the extent of these regions.
In answering the four questions stated at the beginning of the study;
1. Ambient thermal gradients with a slope of 2°F/hr. are minimum requirements to develop the necessary defect-to-solid contrast so they can be identified.
2. Optimal imaging times are generally in the early afternoon and early morning hours. They are dependent upon, and affected by, the slope of the gradients.
3. Defects as small as 4 in. x 4 in. can be identified and quantified to 100 ft. from the observer.
4. A 10.7% overrun is a substantial improvement over the 50% that has often been observed with less accurate methods. Ongoing research will hone a more efficient software and equipment suite for reducing this amount even further.
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