PCA Research & Technology
Concrete Technology
Home > PCA Research & Technology
> Application of Infrared Imaging to Fresh Concrete: Monitoring
Internal Vibration
Application of Infrared Imaging to Fresh Concrete:
Monitoring Internal Vibration (SN2806)
 |
| 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.
 |
| 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.
Click here for a printable version.
|