Pavements - Infrared Thermography (IT)
Target of Investigation
Infrared thermography can be used for multiple pavement applications, including the following:
- Detection of delamination and debonding in existing pavements.(1)
- Identification of thermal segregation defects for asphalt pavement quality control during construction.(2)
Delamination is horizontal cracking that damages the concrete road surfaces; it is caused by corrosion of embedded reinforcing steel.(1) As the steel corrodes, it expands and creates cracks or subsurface fracture planes in the concrete or just above the level of the reinforcement. This cracking can be localized or extensive and is not visible from the surface. Without repair, delamination can progress into open spalls and eventually deteriorate the pavement.(1) Extensive cracking, delamination, and debonding between pavement layers can facilitate intrusion of moisture and chlorides, further promoting deterioration.(1)
The in-place density of asphalt pavement is critical to a pavement’s long-term durability; areas of lower density have a higher susceptibility to premature failure through water ingress, raveling, and cracking.(2) One of the causes of nonuniformity of pavement density is thermal segregation owing to temperature differentials in the asphalt during paving.(3) Some asphalt at the end of truck loads may be colder or coarser than the rest of the load and is, thus, more difficult to compact properly.(2) This difficulty results in portions of the pavement not reaching the required density, which translates to increased air voids, increased permeability, decreased shear strength, and premature pavement distress.(4) In thermal imaging, thermally segregated sections of pavement appear as cold spots relative to the rest of the pavement, and their detection can be used to adjust paving operations accordingly.
Infrared thermography is a nondestructive evaluation method used to detect defects in structures by measuring the variations in surface temperature that result from uneven heating and cooling in the pavement.(5) Areas of defect have different material properties, including density, thermal conductivity, and specific-heat capacity, than the surrounding sound material. These differences result in uneven heating and cooling. The infrared device measures the infrared radiation (wavelengths between 0.7 and 14 µm [0.028 and 0.55 mils]) emitted by the structure and converts it into an electrical signal that is processed to create a map of surface temperatures.(1) Several varieties of infrared devices, including spot radiometers, thermal cameras, and infrared bar sensors, are available for different applications. For pavement evaluation, infrared cameras can be used in stationary positions or mounted on vehicles. For quality control of asphalt paving during construction, infrared cameras can be used from stationary positions, but infrared bar sensors mounted on a paver (figure 1) are more efficient. They provide full coverage of the site and give the paver operator real-time information so that adjustments in paving process can be made to correct problems.(2)
Infrared radiation is based on detecting and measuring radiation in the section of the electromagnetic spectrum with wavelengths between 0.7 and 14 µm (0.028 and 0.55 mils). Thermal radiation of a body is affected by the distribution of heat flow throughout the body based on the following thermal properties: thermal conductivity, specific-heat capacity, and density. Infrared cameras measure emitted thermal radiation and capture the area’s temperature differences.(1)
To examine a structure such as pavement, an infrared camera is used to measure the thermal radiation emitted by the structure; differentials in the emitted radiation are associated with defects. Such an examination generally requires heating the structure and measuring the radiation emitted while heating or cooling. When the structure is heated, its components will emit some of the energy back. For pavement, delaminated and voided areas below the surface are typically filled with water or air, which have different thermal properties than the surrounding material. These areas heat up and cool down more quickly than sound pavement does and register as a different temperature on an infrared image.(1) Active and passive heating are used in infrared thermography. Active heating uses a heater to artificially alter the temperature, but it is ill‑suited for pavement applications. Passive heating uses environmental conditions to heat the structure. To evaluate an existing pavement, the sun generally provides the required thermal radiation to heat the structure, so adequate heating time and good weather conditions are required. For quality control of asphalt paving during construction, the variability in asphalt temperature during placement can be monitored since cold spots in the mat will more likely represent areas of thermal segregation.
Figure 2 illustrates the heat cycle of concrete pavement after sunset and sunrise. Figure 3 illustrates thermal radiation being emitted by a pavement and captured by an infrared camera with active heating sources.
Testing procedures vary based on the type of project, infrared equipment available, and objectives of the survey. Basic procedures and guidelines are outlined in this section, but the infrared equipment manufacturer’s instructions should be followed to ensure proper operation of the equipment.
Survey of Existing Pavements
To survey existing pavements, stationary or vehicle-mounted infrared cameras can be used. ASTM D4788-03, Standard Test Method for Detecting Delaminations in Bridge Decks Using Infrared Thermography, details the use of a vehicle-mounted infrared camera to survey a concrete bridge deck, but some of the basic procedures can be applied to pavements.(6) The following are general procedures for data collection on pavements:
- Remove any debris from the survey area.
- Let the survey area dry for a minimum of 24 h before testing.
- For adequate, uniform heating of the pavement prior to data collection, the weather should be sunny and extreme wind should be avoided. A minimum of 3 h of sunshine should be sufficient to achieve adequate thermal differences between sound and defective pavement areas.
- When examining data, be aware of shadows or other environmental factors that may affect the heating and cooling of the pavement.
- Calibrate the infrared equipment and collect data using the following steps:
- Mark and record the limits of the survey area.
- If using a vehicle-mounted camera, data should be collected from one end of the survey area to the other in a continuous fashion while not exceeding 10 mi/h (16 km/h).
- If using a stationary camera, data should be collected from a position with a clear, unobstructed view of the survey area.
- During daytime surveying, delaminations, debonding, cracks, and other anomalies will appear as hot spots on cooler pavement. During nighttime surveying, they will appear as cold spots on warmer pavement.
- If required, potential defects should be marked for further surveying or coring.
- Perform data collection on the uncompacted mat while the paver is moving.
- Record the first and last station numbers of all thermal profiles. When using a stationary infrared camera, mark the beginning and end of each thermal profile on the pavement.
- Avoid taking measurements within 2 ft (0.6 m) of the edge of the uncompacted mat.
- Collect data with the infrared equipment as follows:
- Stationary camera: Stand at the edge of the uncompacted mat approximately 5 ft (1.5 m) behind the paver or stand on top of the paver screed. Face opposite the direction of paving. Measure the temperature of the mat by pointing the camera and squeezing the trigger. Follow the manufacturer’s guidelines to determine the baseline temperature measurement and record the location of cold spots (figure 4).
- Paver-mounted sensor bar: Install the system to the paver screed. Follow the calibration procedures from the manufacturer’s guidelines. Configure the system to record pavement temperatures at increments of no more than 6 inches (152 mm) of forward movement. Monitor the real-time output to look for cold spots in the asphalt mat and adjust the paving operation accordingly (figure 5).
- Mark the locations of the cold spots in the profiles for further surveying or focused coring.
An infrared camera can take both thermal and visual images of the survey area, and the two can be combined to form a fusion image that helps interpret the thermal profile. Baseline calculations are performed during data collection. A vehicle-mounted camera records a series of infrared images or infrared videos that can be sequenced and stitched together to cover the entire survey area. For paver-mounted infrared sensor bars, the system generates a continuous two‑dimensional (2D) contour plot of the survey area in real time. Contour plots can be inspected by the paver operator or inspectors to adjust the paving process during construction.(2)
The thermal profile generated by the infrared camera can be used to locate potential areas of pavement distress. For pavement evaluation, potential defects such as delaminations appear as hot spots on a cooler background during daytime evaluation and as cold spots on a warmer background during nighttime evaluation. Fusion images that combine thermal and visual images allow for easier correlation between visual and thermal signs of damage (figure 6).
For quality control of asphalt pavement during construction, a 2D contour plot (figure 7) generated by the paver-mounted infrared sensor bar and covering the entire survey area is superior to a thermal camera. The contour plot provides greater testing coverage and requires less effort and operator attendance. Thermal data correlate well with in-place mat density, and real-time monitoring allows rapid recognition of potential thermal segregation so adjustments can be made to the paving process. Once paving is completed, locating potential defects relies on visual surveys and random coring. The thermal profile generated by the infrared thermography survey is able to locate potential defects that the human eye may not see; this allows for more efficient, focused coring that is less likely to miss localized defects. A notable limitation, however, is that thermal uniformity is not always an indicator of sound asphalt pavement. If a thermally uniform mat is not compacted properly, the asphalt will exhibit uniformity and density problems. Other surveying and focused coring should be used to determine if the asphalt mixture is within or outside allowable tolerances.(2)
Advantages of infrared thermography include the following:
- Allows for quick and easy equipment setup.
- Provides rapid data collection.
- Produces accurate results in appropriate conditions.
- Enables real-time monitoring and adjustments during construction when using a paver‑mounted sensor bar.
- Locates potential areas of distress that visual surveying might not detect.
Limitations of infrared thermography include the following:
- Weather and environmental conditions affect accuracy.
- Information about the depth of defects is not provided.
- Flaws deeper than 5 inches (127 mm) in concrete are difficult to detect.
- The presence of a thermally uniform mat does not guarantee good compaction of the asphalt pavement.
- Gucunski, N., Imani, A., Romero, F., Nazarian, S., Yuan, D., Wiggenhauser, H., Shokouhi, P., Taffee, A., and Kutrubes, D. (2013). Nondestructive Testing to Identify Concrete Bridge Deck Deterioration, Report No. S2-R06A-RR-1, Transportation Research Board, Washington, DC.
- Sebesta, S., Scullion, T., and Saarenketo, T. (2013). Using Infrared and High-Speed Ground‑Penetrating Radar for Uniformity Measurements on New HMA Layers, Report No. S2‑R06C-RR-1, Transportation Research Board, Washington, DC.
- Myers, L.A., Mahoney, J., and Stephens, J.E. (2001). Application of Infrared Thermographic Imaging to Bituminous Concrete Pavements – Interim Report, Report No. 2229-1-01-9, Connecticut Department of Transportation, Newington, CT.
- Song, J., Abdelrahman, M., and Asa, E. (2009). Use of a Thermal Camera during Asphalt Pavement Construction, North Dakota Department of Transportation, Bismarck, ND.
- Federal Highway Administration. (2016). “Nondestructive Evaluation Laboratory Overview.” (website) Washington, DC. Available online: https://highways.dot.gov/laboratories/nondestructive-evaluation-laboratory/nondestructive-evaluation-laboratory-overview, last accessed May 1, 2016.
- ASTM D4788-03. (2013). “Standard Test Method for Detecting Delaminations in Bridge Decks Using Infrared Thermography.” Book of Standards 04.03, ASTM International, West Conshohocken, PA.
- Texas Department of Transportation. (2015). Test Procedure for Thermal Profile of Hot Mix Asphalt, Report No. Tex-244-F, TDOT, Austin, TX.