Inspecting Concrete Block Walls with Thermography

General principles of thermodynamics and heat transfer can be used to explain how voids can be detected by infrared thermography. For most applications, solar energy (sunlight) is permitted to strike the exterior of the wall and heat the surface.

"I have a client who wants me to image a building block wall in search of voids where rebar was incased in concrete as part of the strengthening process. Is there a particular protocol you can point me to that will satisfy the building inspector that no voids exist?"

Solar energy striking the wall is absorbed by the masonry. Hollow or insulated cells create a thermal break or barrier within the cell causing the exterior face shell to heat rapidly. In contrast, the solidly grouted cell acts as a heat sink permitting energy to be conducted through the wall. The emitted energy of the exterior surface is therefore higher in locations of voided cells than that of solid cells. This emitted energy pattern is visible with an infrared camera. This method provides very crisp contrasting data, but is totally dependent on weather conditions and the availability of direct sunlight.

In the daytime, wall sections filled with grout will appear cooler than the other sections. At night, sections with grout will appear warmer than the rest of the wall.

CMU wall section during the day

CMU wall section at night

See these two white papers presented at the InfraMation Conference below:

Use of infrared thermography as a standard in the quality assurance and quality control of grouted masonry construction

Thermographic evaluation of concrete masonry walls: have they been properly reinforced?

Can IR cameras see through walls?

Here is a question recently asked me by a student.

"I recently purchased the FLIR SD camera and took your training course which had a lot of valuable information but I'm still unsure about a couple of things. Since these cameras are supposed to be so sophisticated, shouldn't you be able to see through the wall to the studs and would this be part of thermal tuning, and I know you talked a little bit about it, but how do you get the camera exactly tuned and focused for the best possible picture?"

These cameras are sophisticated but we are dealing with Infrared, not X-Ray. Your question deals with basic physics and has nothing to do with the cameras themselves. Walls are opaque to infrared imaging cameras. Now you may have seen some movies depicting looking through walls and seeing people inside a room with an infrared camera, this is Hollywood creative hogwash.

What we see with the infrared camera is the infrared heat radiation produced by the surface of the wall based on its temperature and emissivity. The reason we can see studs in the wall, under the right temperature conditions, is due to conductive heat transfer changing the surface temperature of the wall differently over the studs, vs the insulated cavities between studs. This requires a temperature difference on each side of the wall.

The better infrared cameras can detect and image this temperature variation with less of a temperature difference on each side of the wall because they have better thermal sensitivity. A less sensitive camera will require a larger temperature difference in order to get a clear picture. This is one of the areas thermal tuning comes into play, adjusting the temperature span and the temperature level of the picture to bring out the details as best as your camera allows. Most of the time, letting the camera do the work for you in automatic mode works pretty well. But when you have low temperature differences, or there is something very hot or cold in the image that is throwing the auto adjustment off, then you have to switch the camera to manual and thermal tune yourself.

Focusing is most easily performed by finding a sharp corner or line on the target and adjusting your lens until it is a sharp as can be. If you have a surface you are viewing that doesn't have any distinct features, then what I often do is warm up a pencil in my hands, place the point against the surface, and then focus on the pencil point so it is sharp.