International Symposium (NDT-CE 2003)Non-Destructive Testing in Civil Engineering 2003
|Start > Contributions >Lectures > Thermography:|
Passive Infrared Thermography as Inspection and Observation Tool in Bridge and Road ConstructionMarcello Stimolo, EMPA Dübendorf, Abt. Strassenbau / Abdichtungen, CH-8600 Dübendorf / Schweiz
TEL.: +41 / 1 / 823 42 13, E-mail: email@example.com
The passive thermography, in its use as a non-destructive thermal investigation in the search of hidden defects or damages in the road or bridge pavement structure, together with information on the degradation mechanism, serves as an early diagnostic tool, which completes the methodologies utilised for the survey of the state of the paving.
In this article the mobile IR test equipment of the EMPA (IRMU) is presented. The damages detectable with IRMU and the limitations of the system are discussed. The effects of the environment on the measurement and some general aspects of bridge and road pavements and of detectable subsurface defects or discontinuities are discussed. Finally, some examples of application of the IR-technology in road and bridge pavements are presented.
The assessment of the condition of road and bridge pavements is a powerful tool to set the bridge management strategy (program of maintenance, rehabilitation, and/or replacement) and this particularly in the predictive and preventive maintenance. IR-technology has dualistic properties as it depicts the current condition and, through periodic inspections, the development and speed of the deterioration.
Besides the construction of new roads, the conservation of the road network becomes increasingly important. Therefore the techniques of survey of the state of the paving play an important role as decision-support tools during the planning of the ways and times of intervention (maintenance strategies) from a technical-economic point of view (resource scenarios). Above all, the best timing facilitates the manager in his choice and formulation of the best program for the infrastructure management.
Persons in charge of this task often make use of processes of structural assessment, which provide some data. As outlined in Table 1, the standards SN offer five indicators for the state of the pavement (meaning the degradation of the surface), the longitudinal and transverse evenness, the anti-skid property and the load capacity.These parameters are based upon the pavement surface status (surface signature) and only some indirectly describe the structural condition. A deterioration that does not manifest some visible symptoms at the surface (surface signature) is not detected or quantified.
The passive thermography, in its use as a non-destructive thermal investigation in the search of hidden defects and/or damages in the road or bridge pavement structure, together with information on the degradation mechanism, serves as an early diagnostic tool, which completes the methodologies utilised for the survey of the state of the paving.
2 Test equipment IRMU
The IR measuring system of the EMPA is a mobile measuring equipment (Infrared mobile unit (IRMU)), which lately underwent numerous improvements (see Figure 1).
The present test equipment IRMU can be divided into four main subsystems.
The first comprises the infrared (IR) camera and a portable video camera (measurement and imaging system). The IR camera is an Agema LW900 fitted with a 40° x 20° lens and measuring in a spectrum field of 8 to 12 mm. The IR camera is connected to a laptop computer running ThermaCam used for data storing and management of the IR camera (data acquisition system). A battery located inside the vehicle supplies power to the IR camera. The video camera records the normally lighted visual scene.
During the analysis phase, the combination of thermal and visual images allows to identifying the false signals (false temperature signature) created from foreign bodies,stain or patching areas (repair of a damage on the road) in the thermal images. In fact, foreign bodies and stains can cause temperature signatures similar to those of defects or damages.
A passive technique is adopted to record the thermal images (i.e., no external heating or cooling is applied; anomalies in the temperature appear under normal environmental or operating conditions). The configuration is done with reflection observation techniques (thermal source and detector are located on the same side of the inspected specimen. The thermal source is the sun). The IR camera records the surface temperature of the road, identifying warmer areas (region of interest, region manifesting a perturbation of the surface temperature distribution). These areas represent possible hidden anomalies or defects in the pavement.
The second major component of IRMU is the global position system GPS 5700 total station from Trimbler, employed as measurement system of the position with the aim to produce a metric grid fothermal map.
The thermal map with co-ordinates can be filed into the building documentation, and would permit a repeated observation of the region of interest and thus an investigation of the development of damages and/or defects.
Up to now polystyrene markers (length 5.0 cm, width 2.2 cm, and height inclusive polystyrene 2.6 cm) are used, which are placed all 50 cm and are fixed firmly on a nylon-rope. All 5 m a longer marker (length 14.4 cm, width 2.2 cm, and height inclusive polystyrene 2.6 cm) is attached to the rope.
The third major component is the supporting system. The IR camera, the video camera and the GPS antenna of the rover are mounted on the supporting system. The height between the pavement and the components mounted on the supporting system can vary from 2.0 m to 3.5 m, allowing to obtain a field of view from 1.40 x 0.70 m2 to 2.60 x 1.30 m2. The dimensions of the field of view and the recording speed of the data acquisition system determine the speed of the vehicle. The maximum speed with IRMU is slightly above 10 kmh-1. When considering the speed of the normal traffic flow, the reduced speed of IRMU represents an obstacle; therefore for security reasons it is preferred to operate closing the measured part of the traffic lane to the traffic.
The fourth major component of the system is the car, where the laptop and the battery are placed.
2.1 Detectable damages and limitations of IRMU
In the main, it can be asserted that those defects and/or damages that generate a disturbance in the thermal flow, perturbing the temperature surface distribution of the pavement, are detected through the IR camera as zones with an elevated temperature compared to the surrounding surface (i.e., a high temperature signature reveals a subsurface scontinuity). Therefore blisters of gas, cavities and delaminations, which lye in the plan perpendicular to the thermal flow, can be detected with the IR-thermography.
The information obtained from the thermal images must be correctly interpreted and a choice of the temperature signatures must be made. In fact, many foreign bodies on the surface (chewing gum, spots of colour or oil, patching areas, shadows) or particular constructions in the paving (heat sealing compound, road joint) can give false signals in the thermograms. In order to obviate to this problem, a comparison of the thermal images with visual images of the investigated pavement is carried out.
2.2 Effects of environment on measurement
An esteem of the effective temperature of the bridge or road pavement serves to establish the optimal observation time to conduct the measurement, specifically if during periods of heating or cooling of the pavement (the outdoor constructions undergo an harmonic heating process, see Figure 2). The optimal observation time is that time in the day when the difference between the temperature signal above a discontinuity and that above sound material reaches its maximum. On this issue literature sources propose two positions. One position limits the optimal observation time from 11 to 18; the other proposes two best measuring times from 8 to 15 and from 20 to 4.
Until today inspections during the cooling period (passive cooling down thermography) of the pavement surface (i.e., during the afternoon) were favoured. A project to characterise the optimal observation time is running at our department with the aim to obtain the optimal probability of detection for the IRMU survey system. In this project a road section, where the presence of blisters has already been established, is being monitored at various moments of the day in different periods of the year (when the environmental conditions allow the execution of the inspection).
Measurements of the temperature path at various depths, carried out on some highway sections in the plateau and on a mountain road [EMPA test report 840325; EMPA report 403210/4] have shown that the temperature changes with a periodical path, as is shown in the diagram in Figure 2. The temperature difference (DT = T32cm - T2cm) between the temperature at a deepness of 2 cm and of 32 cm depth and the temperature of the air (TL) are represented in the diagram.
The bar chart evidences the periodical change of direction undergone by the temperature gradient. During the night until the first hours of the morning there is a negative temperature gradient (blue colour, direction from the lowest layer towards the surface of the pavement). For the rest of the day the temperature gradient is positive (red colour, direction from the surface of the pavement towards the deeper layers). During the summer season the temperature difference between air and pavement can reach up to 20°C.
Empirically it can be asserted that a low solar heating (solar loading) creates a low temperature difference and therefore the typical temperature signature of the subsurface defects or structural defects can not be created. The inspected surface needs to be heated by a sufficient quantity of solar energy to permit a good survey.
2.3 General remarks on road and bridge pavement
Bridge pavements absolve several roles; the first is to avoid the penetration of water and de-icing salt solution (chemical load) up to the concrete deck slabs. Further, they have a conservation function of the construction, a load distribution function, and they ensure a comfortable (through flatness) and safe (through grip) vehicle traffic. The concrete bridge deck is protected through waterproofing (made of gussasphalt or waterproofing membranes). The waterproof is covered with a protective layer (gussasphalt, compacted bituminous mix or cement mortar) and finally with a surface layer. Failures of the waterproofing combined with chemical load can generate optimal conditions for the corrosion process in the upper reinforcement and prestressing steel. The state of the waterproofing of the bridge pavement can condition the service life of any structural member of the bridge (compromising the bearing capacity of the bridge structure). The corrosion damage leads to a reduction of the cross section of the reinforcing and the pre-stressing steels causing a loss of moment capacity in the deck. 
In principle the following corrosion damages for reinforcing and pre-stressing steels can be differentiated:
The pitting and the stress corrosion belong to the category of the local corrosion, whose characteristic is the high speed of the corrosion process. IRMU is heavily dependent on a good solar loading; and this dependency can be overcome through the employment of local heating systems in these sensitive areas of the pavement slab of the bridge. The sensitive areas for the pavement slab are the zones with negative moment line up to zero point of moments and the zones near of the bridge bearing. Thus allowing operating also in winter in special cases.
2.4 General remarks on detectable subsurface defects or discontinuities
A distinction between subsurface defects or structural defects between road and bridge pavements would require a long disgression. However, the many affinities between the two allow not proceeding in such sense in this context. Therefore, the mentioned defects may be found in road or bridge pavements.
On the basis of the manifestations of the damages on the pavement surface (surface signature) two categories are distinguished.
A surface signature at a given point on the surface of an object is a descriptor that encodes the geometric properties measured in a neighbourhood of the point. Curvature is one of the oldest and most basic local descriptors of shape. 
The defects with a surface signature are part of the first category. The local curvature or arching in the pavement surface is their typical descriptor. A relief in the pavement diminishes the riding quality and the safety of the road. Blisters belong to this category. A blister is defined as a locally limited hemispheric elevation of the pavement surface of bituminous covers, caused by the heating up of e.g. hermetically enclosed air, water, oil or solvent. The camber is typical for blisters and illustrates the strip in a layer or the delamination between a layer and the underground. In other words, the camber is a result of the underlying cavities with the gases contained therein. The cavities can appear in the interface to the underground (see Figure 3).
The second category contains damages that do not have a surface signature: "babyblisters", cavities and delaminations (see Figure 3). The "baby-blister" (blister germ) doesn't show a chamber on the pavement surface, it is hidden and on-detectable by eye. Another characteristic is its dimension, which are only some centimetres. A delamination is a bond failure and may occur in the concrete cover of bridge decks as a consequence of rust expansion (volume expansions factor of rusting is 2.5) due to the corrosion of reinforcing steel in concrete. Rusting may occur between the concrete deck and the above layers of the pavement or in-between the layers of the pavement.
The main cause for many bond damages are lasting shear deformations due to shearing through horizontal loads (e.g. starting or braking the car, etc.), which are frequently found together with shear fissures, which can lead to the penetration of water and to underflow. The penetrating water weakens the adhesion between the individual stone aggregates and the binder. This phenomenon may occur predominantly in older pavements, where the binder has hardened due to ageing. The dissolution of an already unsatisfactory layer can affect very unfavourably the further behaviour and lead to surface-like layer separations and to the formation of impact holes.
Blisters can also be differentiated based on their point of origin. In road and bridge deckings blisters are a typical sign of adhesion problems between the layers (e.g. between the pavement and the road base) or within a layer (cohesion error) as a consequence of bad compactability or due to an inhomogeneous temperature distribution during the paving. In the bridge waterproofing systems, blisters appear as steam pressure occurrences beneath the sealing construction in the form of adhesion problems. This phenomenon occurs in all waterproofing systems with sealing membranes such as for example flat roofs or water sealing.
The typology of blisters and the description of the degradation mechanism can be done in three ways:
It is possible to carry out a classification of the types of damages according to their dimensions and their geometrical form and to obtain in this way information on the status of the damaging process (see Table 2). Table 2 presents an overview of the hidden damages detectable through IR-technology.
Through the type of the defect it is possible to assess the condition of the bridge pavement (diagnosis of pavement), i. e. to assess if a given damaging process is completed or is still ongoing.
Due to the peculiarities of baby-blisters (they do not have any surface signature, but the degradation process is still ongoing), their presence allows an early analysis.Thus, IR-thermography permits a preventive and/or a predictive maintenance. The preventive maintenance occurs before the limits of exploitation of the elements are reached (i.e., before baby-blisters have grown to blisters with their curvature). The predictive maintenance can occur based on the documentation of the engineering structure, e.g. based on several thermograms collections at different periods.
3 Some examples of application in road and bridge pavements
The passive IR-thermography can be used for different scopes. It can find employment as a mean of control in all those processes requiring the involvement of thermal energy. Usually in these cases the temperature reached during the construction influences the quality of the result.
But passive infrared thermography also makes it possible to detect the defects or damages hidden in the pavement, which produce variations of its surface temperature by perturbing the heat flux.
In this paper the role of IR-thermography in detecting the damages is discussed.
3.1 Non-destructive defects detection under sealing membranes
The thermograms of a waterproof system are represented in Figure 4, right; the complete thermogram and a detailed view are presented. The evaluation of thermograms is based on the effect of the subsurface discontinuity (defects or damages) on the heat conduction in the investigated specimen. In this case, the real surface temperature of the sealing membrane is not relevant. In fact, the most important task of thermography is the localisation of the possible damages (region of interest) through the temperature contrast or, in other words, through the difference in temperature of sound and damaged areas.
Essentially, five characteristics can be recognised in the thermograms of waterproofing membranes (see Table 3):
Under constant insulation (solar loading) of the entire surface to be measured, no or only minimal variations of the surface temperature should occur in a sound material;temperature contrasts signify structural inconsistencies such as blisters, overlaps or foreign bodies on the surface.
Experiences accumulated to date show that the thermografic detection of blisters is unproblematic in uncovered sealing membranes. However, detection problems arise when the protective layer and the bridge decking cover the waterproofing membrane, as their thickness are an obstacle for the thermografic detection of damages.
Thermography can also be used to identify the most appropriate sites to conduct the peel test and the pull-off test, two destructive tests, which are used to quantify the adhesion of waterproofing membranes on the cement underground. The identification of the most appropriate sites through thermography avoids a random selection.
3.2 Non-destructive defects detection in roads and bridges pavements
The structure of the road is mixed, therefore its surface is nearly never heterogeneous and can present in several regions the following particularities that may leave a temperature signature in the thermal images (see Figure 6 and Figure 8):
The basic principle relies on the comparison between presumed intact specimen zones and presumed defective specimen zones.
Supposing that the investigated specimen has been exposed to a homogeneous sun load and is composed of a uniform structure, the sound area of the specimen possesses a lower temperature than the discontinuity zones. That also allows evaluating zones that possess a particular structure (and perhaps also different optical and thermal properties), and that therefore have a particular temperature signature in the thermal image as do the overlap zones in the waterproof membrane or the inside of the patching areas.
In [2, 3] a description of this principle for the evaluation procedures through three mathematical procedures is exposed. These parameters allow an appraisal of the thermal images excluding the effects of a simple visual and subjective interpretation. For every strip a diagram with the parameter (e.g., s) versus the distance (as shown in Figure 7) is obtained and completes the thermal map (see Figure 6).
The assessment of the condition of road and the bridge pavements is a powerful tool to set the bridge management strategy (program of maintenance, rehabilitation,and/or replacement) and this particularly in the predictive and preventive maintenance. IR-technology has dualistic properties as it depicts the current condition and, through periodic inspections, the development and speed of the deterioration. The owners have to take informed decisions an have four main necessities:
Information on the location and extension (located, extended and/or structural) of the defect zone is important to the owner especially in the decision phase to determine the method of rehabilitation.