Infrared Thermographic, - termowizja
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15
Infrared Thermographic
Techniques
Gary J. Weil
Entech Engineering, Inc.
15.1
15.2
15.3
15.4
15.5
15.6
15.7
15.8
Infrared thermography, a nondestructive, remote sensing technique, has proved to be an effective, con-
venient, and economical method of testing concrete. It can detect internal voids, delaminations, and
cracks in concrete structures such as bridge decks, highway pavements, garage floors, parking lot pave-
ments, and building walls. As a testing technique, some of its most important qualities are that (1) it is
accurate; (2) it is repeatable; (3) it need not inconvenience the public; and (4) it is economical. This
chapter provides a summary of the historical development of this technique, discusses the underlying
theory, describes the test equipment, and gives example case histories.
15.1Introduction
Concrete is one of the world’s most useful building materials. It is used in almost every phase of society’s
infrastructure: from the buildings that house people to the roads and bridges that allow us to travel from
place to place; from the dams that help control nature’s forces to the launchpads that help us explore the
heavens. This building material has strength and rigidity along with versatility, but it does have its limits.
Most concrete structures have a design life of 20 to 25 years, and when they begin to deteriorate they do
so slowly at first and then gradually progress to failure. This failure can be expensive in terms of both
dollars and lives, but this scenario can be avoided. Planned restoration can extend the life of concrete
structures almost indefinitely, and testing of concrete structures to establish the existing conditions is the
basis of economically viable restoration. For any testing technique to be widespread, it must have the
following qualities:
1.It must be accurate.
2.It must be repeatable.
3. It must be nondestructive.
4. It must be able to inspect large areas as well as localized areas.
5. It must be efficient in terms of both labor and equipment.
6. It must be economical.
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7. It must not be obtrusive to the surrounding environment.
8. It must not inconvenience the structure’s users.
One technique for testing in-place concrete has emerged during the past 30 years that fulfills all of
these requirements. That technique is called infrared thermographic testing. During its gestation period,
it has been used to test concrete on bridge decks, highways, dams, garages, airport taxiways, and buildings.
It has shown itself to be both accurate and efficient in locating subsurface voids, delaminations, as well
as poor binding, moisture entrapment, and other anomalies in concrete structures.
15.2 Historical Background
Infrared thermographic investigation techniques are based on the fundamental principle that materials
with subsurface anomalies, such as voids caused by corrosion on reinforcing steel, or voids caused by
poor concrete consolidation called honeycombing, or pooling fluids such as water infiltration, in a
material affect heat flow through that material. These changes in heat flow cause localized differences in
surface temperature. Thus, by measuring surface temperatures under conditions of heat flow into or out
of the material, one can determine the presence and location of any subsurface anomalies.
The first documented experimental paper on using infrared thermography to detect concrete subsur-
face delaminations was published by the Ontario Ministry of Transportation and Communication in
1973. It illustrated effective methods, although they depended on relatively crude, inefficient techniques.
1
Using these basic techniques, additional research was performed.
2
These later studies were performed on
concrete bridge decks, again located in Canada. They were based on the use of a simple infrared imager
to measure surface temperatures, without the use of computer enhancements. They were carried out
using a variety of techniques, such as both daytime and nighttime data collection. They proved that
infrared thermographic techniques could be used to detect concrete subsurface delaminations on bridge
decks.
During the next 10 years, the Ontario Ministry of Transportation and Communications was a strong
advocate of research on these infrared thermographic techniques. At the same time, research was pro-
gressing in the United States,
3,4
and continued into the late 1980s.
5<10
An early study was performed for
the Wisconsin Department of Transportation along a four-lane, 16-mi (27-km) portion of Interstate 90/
94. In this study, videotape was used to record both visible and infrared images of the highway. These
tests used manual methods to transfer the delamination data to scaled plan drawings.
In 1983, major concrete bridge deck delamination analysis was performed on the Dan Ryan Expressway
located in Chicago. This investigation was significant because it showed that infrared thermography could
be used efficiently on congested highways. The fieldwork was performed from a mobile van with traffic
control provided by two signboard vehicles behind the data collection van. Permanent lane closure was
not required, thereby reducing costs and inconvenience, particularly for the motorists using the express-
way. Field data on the 11-mi (17.6-km), eight-lane expressway in Chicago was collected in 14 h during
five separate days, significantly less time than would have been needed for other inspection techniques
such as chain dragging, deflectometer, sounding, or coring.
In 1985, concrete pavement delamination inspections were performed on the Poplar Street Bridge
entrance and exit ramps and bridge decks spanning the Mississippi River at St. Louis, Missouri for the
Illinois Department of Transportation. The bridges are a major part of the highway system on Interstate
55-70 and include approximately 40 lane-mi (65 km) of bridge deck roadways. These were crucial
structures because more than 90% of the traffic between Missouri and Illinois, near St. Louis, crossed
these bridges. Traffic stoppages had to be kept to a minimum. Five techniques were evaluated: (1) visual
inspections, (2) infrared thermography, (3) ground penetrating radar, (4) corings, and (5) chloride
measurements. The various tests were performed by separate firms, and the results were analyzed by an
independent engineering firm. All data were recorded on a scaled computer-aided design (CAD) system
to allow overlaying of the data and comparisons of the results of the various techniques at individual
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locations as well as overall statistics. infrared thermography proved to be the most accurate nondestructive
method as well as the most efficient and economical to perform.
One of the largest individual infrared thermographic inspections occurred in 1987 at the Lambert
St. Louis International Airport. This involved testing concrete taxiways. The concrete slabs ranged
from 14 to 18 in. (360 to 460 mm) in thickness. The rules set up by the airport engineering department
dictated that the testing had to be performed during low air traffic periods (11:00
P
.
M
.to5:00
A
.
M
.)
and no loading gates could be blocked. The field inspection was completed in five working nights.
Approximately 2,000,000 ft
2
(186,000 m
2
) of concrete was inspected with production rates approaching
1,000,000 ft
2
(93,000 m
2
) per night. In addition to determining individual slab conditions, the use of
an infrared thermography<based system with computer enhancements allowed the determination of
damage caused by traffic patterns and underground erosion caused by soil migration and subsurface
moisture problems.
15.3Theoretical Considerations
Aninfrared thermographic scanning system measures surface temperatures only, but the surface tem-
peratures of a concrete mass depend on three factors: (1) the subsurface configuration, (2) the surface
conditions, and (3) the environment.
The subsurface configuration effects are based on the principle that heat cannot be stopped from
flowing from warmer to cooler areas; it can only be moved at different rates by the insulating effects
of the materials through which it is flowing. Various types of construction materials have different
insulating abilities or thermal conductivities. In addition, differing types of concrete defects have
different thermal conductivity values. For example, a dead air void caused by “honeycombing” or
corrosion-related “delaminations” has a lower thermal conductivity than its surrounding solid concrete.
There are three ways of transferring thermal energy from a warmer to a cooler region: (1) conduction,
(2) convection, and (3) radiation. Sound concrete should have the least resistance to conduction of heat,
and the internal convection and radiation effects should be negligible. However, the various types of
anomalies associated with poor concrete, namely, voids and low density, decrease the thermal conductivity
of the concrete by reducing the energy conduction properties, without substantially increasing the
convection effects because dead air spaces do not allow the formation of convection currents.
For heat energy to flow, there must be a heat source. Because concrete testing can involve large areas,
the heat source should be both low cost and capable of giving the concrete surface an even distribution
of heat. The sun fulfills both these requirements. Allowing the sun to warm the surface of the concrete
areas under test will normally supply the required energy. During nighttime hours, the process may be
reversed with the warm concrete acting as the heat source and the clear night sky acting as the heat sink.
For concrete areas not accessible to sunlight, an alternative is to use the heat storage ability of Earth
to draw heat from the concrete under test. The important point is that to use infrared thermography,
heat must be flowing through the concrete. It does not matter in which direction it flows.
The second important factor to consider when using infrared thermography to measure temperature
differentials due to anomalies is the surface condition of the test area. As noted above, there are three
ways to transfer energy. Radiation is the process that has the most profound effect on the ability of the
surface to transfer energy. The ability of a material to radiate energy is measured by the emissivity of the
material. This is defined as the ability of the material to radiate energy compared with a perfect blackbody
*
radiator. This is strictly a surface property. The emissivity value is higher for dark, rough surfaces and
lower for smooth, shiny surfaces. For example, rough concrete may have an emissivity of 0.95 whereas
shiny copper metal may have an emissivity of only 0.05. In practical terms, this means that when using
thermographic methods to collect temperature values on large areas of concrete, the engineer must be
*
A blackbody is a hypothetical radiation source that radiates the maximum energy theoretically possible at a given
temperature. The emissivity of a blackbody equals 1.0.
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aware of differing surface textures caused by such things as broom-textured spots, rubber tire tracks, oil
spots, or loose sand and dirt on the surface.
The final factor that affects the temperature measurement of a concrete surface is the environmental
system that surrounds that surface. Various parameters affect the surface temperature measurements:
1.
Solar Radiation:
Testing should be performed during times of the day or night when the solar
radiation or lack of solar radiation would produce the most rapid heating or cooling of the concrete
surface.
2.
Cloud Cover:
Clouds will reflect infrared radiation, thereby slowing the heat transfer process to
the sky. Therefore, nighttime testing should be performed during times of little or no cloud cover
toallow the most efficient transfer of energy from the concrete.
3.
Ambient Temperature:
This should have a negligible effect on the accuracy of the testing because
the important consideration is the rapid heating or cooling of the concrete surface. This parameter
will affect the length of time (i.e., the window) during which high-contrast temperature measure-
ments can be made. It is also important to consider if water is present. Testing while ground
temperatures are lower than 32$F (0$C) should be avoided, as ice can form, thereby filling sub-
surface voids.
4.
Wind Speed:
Highgusts of wind have a definite cooling effect and reduce surface temperatures.
Measurements should be taken at wind speeds lower than 15 mph (25 km/h).
5.
Surface Moisture:
Moisture tends to disperse the surface heat and mask the temperature differences
and thus the subsurface anomalies. Tests should not be performed while the concrete surface is
covered with standing water or snow.
Once the proper conditions are established for thermal data collection, a relatively large area should
be selected for calibration purposes. This should encompass concrete areas both good and bad (i.e., areas
with voids, delaminations, cracks, or powdery concrete). Each type of anomaly will display a unique
temperature pattern depending on the conditions present. If, for example, the data collection process is
performed at night, most anomalies will be between 0.01$C and 5$C cooler than the surrounding solid
concrete depending on configuration (
.
A daylight survey will show reversed results; i.e.,
concrete surfaces above damaged areas will be warmer than the surrounding sound concrete.
15.4Testing Equipment
Inprinciple, to test concrete for subsurface anomalies, all that is really needed is a sensitive contact
thermometer. However, even for a small test area, thousands of readings would have to be made simul-
taneously to outline the anomaly precisely. Because this is not practical, high-resolution infrared ther-
mographic radiometers are used (
to inspect large areas of concrete efficiently and quickly.
This type of equipment allows large areas tobe scanned, and 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 auxiliary equipment can be used to facilitate
data recording and interpretation.
A complete thermographic data collection and analysis system can be divided into four main sub-
systems. The first is the infrared sensor head that normally can be used with interchangeable lenses. It
is similar in appearance to a portable video camera. The scanner’s optical system, however, is transparent
only to short-wave infrared radiation with wavelengths in the range of 3 to 5.6 +m, or to medium-wave
infrared radiation with wavelengths in the range of 8 to 12 +m. Normally the infrared radiometer’s highly
sensitive detector is cooled by liquid nitrogen to a temperature of –196$C, and it can detect temperature
variations as small as 0.1$C. Alternative methods of cooling the infrared detectors are available that use
either compressed gases or electric cooling. These last two cooling methods may not give the same
resolution, because they cannot bring the detector temperatures as low as liquid nitrogen. In addition,
compressed gas cylinders may present safety problems during storage or handling. New types of cooling
include mechanical Stirling coolers that are capable of bringing temperatures as low as liquid nitrogen.
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FIGURE 15.1 Color figure follows p. 15-10.)
Visual and thermal images of powdery concrete on the Martin Luther
King Bridge in St. Louis, Missouri. Red areas on thermal image represent powdery concrete.
FIGURE 15.2
Infrared thermographic radiometer.
Several manufacturers have developed detectors capable of detecting infrared wavelengths at normal
room temperatures. These uncooled sensors, coupled with new array-type sensors hold promise for the
future of lower-cost radiometers.
The second major component of the infrared scanning system is a real-time microprocessor coupled
toa black-and-white or color display monitor. With this component, cooler items being scanned are
normally represented by darker gray tones, and warmer areas are represented by lighter gray tones. To
make the images easier to interpret for those unfamiliar with interpreting gray-tone images, a color
monitor may also be installed. The microprocessor will quantize the continuous gray-tone energy images
into two, three, or more “buckets” of energy levels and assign them contrasting visual colors representing
relative temperatures. Thus, the color monitor displays the different temperature levels as contrasting
colors and patterns, which are easier to decipher.
The third major component of the infrared data collection system is the data acquisition and analysis
equipment. It is composed of an analog-to-digital converter for use with analog sensors, a computer with
a high-resolution color monitor, and data storage and analysis software. The computer allows the transfer
of instrumentation videotape or live images of infrared scenes to single-frame computer images. The
images can then be stored individually and later retrieved for enhancement and individual analysis. The
use of the computer allows the engineer in charge of testing to set specific analysis standards based on
invasive sample tests, such as corings, and apply them uniformly to the entire pavement. Standard, off-
the-shelf image analysis programs may be used or custom-written software may be developed.
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