Q.: We're repairing a bridge deck that requires several partial-depth patches. The engineer recently handed me a computer printout of the bridge deck that showed the delaminated areas. I asked him how the delaminated areas were detected, and he said that one method they use is called infrared thermography. What is this technique and how does it work? Is it accurate?
A.: Infrared thermography is a nondestructive testing technique that has proven to be accurate and efficient in locating voids, delaminations, and other defects in concrete structures. It's based on the principle that heat flows more slowly through voids and delaminations (air) than through solid concrete. These changes in heat flow cause localized differences in surface temperature. By measuring the surface temperature under conditions of heat flow, the location of delaminations can be determined.
During daylight hours, the concrete surface above delaminations is warmer than the surface above sound concrete. That's because heat from the sun takes longer to move deeper into concrete because it moves slower through the delaminated area. The opposite is true at night--the concrete surface over delaminated areas is cooler. Because of this, the best times to perform the inspection are two to three hours after sunrise or sunset. Both are times of rapid heat transfer.
The procedure used to collect the data is outlined in ASTM D 4788, "Test Method for Detecting Delaminations in Bridge Decks Using Infrared Thermography." To measure surface temperatures and determine delaminated areas, engineers use high-resolution infrared thermographic scanners. The scanners' optical systems are transparent only to short- or medium-wave infrared radiation. They can detect temperature variations as small as 0.4° F.
The scanner head and an accompanying video camera are mounted on a vehicle at a height sufficient to allow a minimum image width of 14 feet. Scanning rates as great as 1.2 million square feet per day are possible. The resulting data can be displayed as pictures with areas of differing temperatures designated by differing gray tones in a black and white image, or by various hues on a color image. A wide variety of computer equipment is used to facilitate data recording and interpretation.
1. V. M. Malhotra and N. J. Carino, Handbook on Nondestructive Testing of Concrete, CRC Press Inc., 1991.