Source: PCA, Concrete Technology E-Newsletter, CTEC 2006 No. 3, Portland Cement Association, June 2006
Also available at www.cement.org/tech/cct_research_infrared.asp

Application of Infrared Imaging to Fresh Concrete: Monitoring Internal Vibration (SN2806)

Vibrating concrete
Proper vibration ensures adequate consolidation in concrete.
Careful and thorough consolidation of fresh concrete is necessary to ensure proper placement and uniformity. Internal vibration is the most commonly used method of consolidation. It is ideal to have a way to determine if an area of concrete has been vibrated properly. Currently, a device to measure the adequacy of consolidation of concrete in-situ does not exist and the judgment of adequacy is normally left to the vibrator operator. Judging consolidation adequacy may be one of the most difficult jobs in concrete construction as the vibrator operator can only see the surface of the concrete during vibration. Ultimate assessment of vibration adequacy does not come until the forms are stripped and the surface inspected, or when deterioration or strength problems arise. Unfortunately, at this stage it is too late to repair the concrete by additional vibration and costly repairs or retrofits may result.

Recently, thermal imaging technology has been shown to be a possible solution to determining the adequacy of vibration (Burlingame 2004). The ability to assess consolidation adequacy via thermal imagery allows visual confirmation of proper placement and technique.

What is Thermal Imaging?

All objects emit electromagnetic radiation as a function of their temperature and the nature of their surfaces. The wavelength of this radiation depends on an object’s temperature. Many of these wavelengths extend into the infrared range and thus are invisible to the human eye. These infrared waves can be detected by a variety of infrared sensors that compute and display the apparent surface temperature of an object based on incoming radiation and environmental factors. Through the use of digital conversion techniques, a two-dimensional array of infrared sensors can produce a picture (thermograph) by assigning colors based on the apparent temperature of each picture element (pixel) (Figure 2).

Infrared imaging is a non-contact method of measuring the temperature of an object. Thermal imaging systems have been used for a wide range of industries, including medical, environmental and electrical applications. In the medical profession, local deviations in temperature may indicate health problems ranging from a fever to the presence of cancer cells, while in industrial settings, local temperature differences may indicate design flaws, poor workmanship, or damaged components.

Infrared imaging technology has also been used in the concrete industry to assess the health of hardened concrete—finding defects and areas of localized delamination. Infrared imaging has also been used to check for areas of voids and honeycombing in fresh concrete by viewing the outside of the formwork during concrete placement.

Typical thermograph showing vibrator insertion points
Fig. 2. Typical thermograph of nine internal vibrator insertion points.

A hot vibrator can provide local heating to the concrete it touches, leaving a persistent “thermal signature” as shown in Fig. 2. This thermal signature can be detected using infrared imaging after the vibration operation is completed. This allows an inspector to return to an area of fresh concrete and observe the remaining heat signature up to 20 minutes after vibration was completed.

Limitations of Infrared Imagery as a Quantitative Quality Control Tool

Along with the possibilities for use of infrared imagery the researchers also found limitations that must be kept in mind when using this technology. There may be an inherent variability, or lack of repeatability, with the same internal vibrator under similar operating conditions that could limit the usefulness of infrared imagery as a qualitative tool for indicating consolidation effectiveness. For example, it may be possible to observe vibration insertion points with an infrared imager; however, one should not specify that the vibrator operator vibrate concrete until a certain temperature or temperature difference is achieved unless the variability was controlled or included within the tolerance.

It was also observed during laboratory testing and through field observations that the heat signature of the vibrator could be made less distinct or completely removed by floating and finishing operations. As the concrete is screeded and finished, the concrete on the surface is mixed, and locally heated zones may be displaced. These processes tend to average out the concrete surface temperature. Also, temperature variations within the fresh concrete due to delivery from more than one source could show localized hot spots that could be mistaken for vibrator insertion points.

Summary

Infrared imagery can be effectively used as a quality control tool. The use of infrared imaging would allow a concrete inspector to determine the location of the vibrator insertion points in a section of concrete. The spacing of these insertions could then be checked against specifications and recommendations for a given mixture. Previous research has established recommendations for optimal vibrator insertion spacing under various conditions and thermal imaging could be a useful tool in ensuring that proper insertion spacing is performed.

Reference

Burlingame, S. E., Application of Infrared Imaging to Fresh Concrete: Monitoring Internal Vibration, MS Thesis, Cornell University, Ithaca, NY, 2004. Available as PCA Serial Number Report 2806.