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 determined, and he said that one method they used was called infrared thermography. What is this technique and how does it work? Is it accurate?
Infrared thermography is a nondestructive testing technique that has proven to be both accurate and efficient in locating voids, delaminations, and other defects in concrete structures. It is based on the principle that heat flows slower 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 a longer time to move deeper into the 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 either 2 to 3 hours after sunrise or 2 to 3 hours after sunset. Both are times of rapid heat transfer. The procedure used to collect the data is outlined in ASTM D 4788. To measure surface temperatures and determine delaminated areas, engineers use high-resolution infrared thermographic scanners. The scanners' optical systems are transparent only to short-wave or medium-wave infrared radiation. The scanners 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. Production rates as great as 1.2 million square feet per day have been attained. 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 colors on a color image. A wide variety of computer equipment is used to facilitate data recording and interpretation. References 1. V. M. Malhotra and N. J. Carino, Handbook on Nondestructive Testing of Concrete, CRC Press Inc., 1991. 2. 1992 Annual Book of ASTM Standards, Section 4, Construction, Volume 4.03, Road and Paving Materials; Pavement Management Technologies, ASTM.