What is Infrared Thermography (IRT)?
Infrared Thermography (IRT)
Infrared thermography is a non-destructive test method that uses an infrared camera to map surface temperature differences and find defects below the surface.
How it works
An infrared camera converts the infrared radiation that a surface emits into an image called a thermogram. A defect such as a delamination changes how heat moves in the material. Its surface then heats up and cools down at a different rate from the sound material around it. The analysis is mostly qualitative.
In a passive survey the sun supplies the heat. After sunrise a delamination can read several degrees Celsius warmer than sound concrete and appear as a hot spot. During cooling it appears as a cold spot. An active survey uses a heater instead, mainly for smaller elements.
Limits
Water, oil stains and debris on the surface can corrupt the data. The method gives no information about the depth of a flaw, and flaws deeper than about 4 to 5 in. are hard to detect. Cloud and precipitation reduce the contrast.
Use on pavement
FHWA lists thermography for delaminations and voids in concrete decks and tunnels, delaminations in concrete pavements, and quality control during asphalt paving. The FAA names it in AC 150/5320-6G as one of the nondestructive methods for the evaluation of airport pavement. The standard test method, ASTM D4788, covers only bridge decks.
Frequently Asked Questions
- When should a thermography survey of a concrete surface take place?
- A passive survey needs a surface that the sun has heated or cooled enough to show a contrast. FHWA gives the best window as about 5 to 9 hours after sunrise or sunset. Clouds and precipitation reduce the contrast.
- What is the difference between passive and active thermography?
- Passive thermography uses the heat of the sun, and active thermography uses a heater. Passive surveys are the more common type. FHWA says active surveys suit mainly smaller elements.
- Which standard covers thermography?
- ASTM D4788 covers delaminations in portland-cement concrete bridge decks. It applies to exposed decks and to decks with an asphalt or concrete overlay up to 100 mm (4 in.) thick. It does not cover airfield pavement.
References
- fhwa-infotech-ir
- Bridge - Infrared Thermography (IT) FHWA
- faa-ac-150-5320-6g
- AC 150/5320-6G, Airport Pavement Design and Evaluation FAA
- astm-d4788
- ASTM D4788-03(2013), Standard Test Method for Detecting Delaminations in Bridge Decks Using Infrared Thermography ASTM International
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