BAM Federal Institute for Materials Research and Testing, Berlin, Germany; 2

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1 Monitoring of cracks in historic concrete structures using optical, thermal and acoustical methods Christiane Maierhofer 1 ; Rainer Krankenhagen 1 ; Philipp Myrach 1 ; Jeannine Meinhardt 2 ; Uwe Kalisch 2 ; Christiane Hennen 2 ; Rüdiger Mecke 3 ; Thomas Seidl 3 ; Michael Schiller 3 1 BAM Federal Institute for Materials Research and Testing, Berlin, Germany; 2 IDK Institute for Diagnosis and Conservation on Monuments in Saxony and Saxony-Anhalt, Halle, Germany; 3 Fraunhofer Institute for Factory Operation and Automation IFF, Magdeburg, Germany 1. Introduction An efficient and repeatable 3D mapping of damage is urgently required for preservation as well as for assuring the safety and reliability of historic buildings and structures. This enables visualization, monitoring and assessment of temporal changes of damage and deformation. Currently, there exists no systematic and standardized best practice. Conventional 2D methods are based on photos or drawings with the disadvantages of shifts of 2D projections and the limitations to fi xed views, thus in several cases important areas are not visible due to masking. Crack documentation is mostly limited to manual techniques and for monitoring, only local crack sensors allow a manual or remote recording of crack movement. For the investigation of crack depth, no sufficient state-of-the-art solutions are available. In this paper, new developments of an efficient 3D mapping and monitoring of cracks based on a tracking system, active thermography and ultrasonics are presented. With tracking, a continuous positioning of a target is performed. For the recording of cracks, a probe tip was developed. A commercial tracking system is illuminating the probe with infrared radiation and is recording the reflected radiation [Seidl, 2012, 87-96]. Active thermography is usually applied to masonry structures for investigating plaster detachments, moisture and masonry structure below plaster [Maierhofer, 2010, ]. In the following it is shown that cracks oriented perpendicular to the surface as well as tilted cracks can be visualized with simple heating of the surface. Image processing using edge fi lters can increase the contrast. By analysing the thermal contrast systematically, information about crack angle and crack depth might be gained. A more accurate determination of crack depth can be achieved from the travel time of ultrasonic impulses recorded in a configuration of two transducers positioned on opposite sides of the crack [EN 14579:2004]. In the following, the development of these methods is described more detailed. The application of these methods is demonstrated on a case study. Here, two sculptures on a historic concrete bridge located in Halle, Germany, have been investigated. 2. Historic concrete bridge: Giebichensteinbrücke in Halle From 1926 to 1928, a new bridge was built over the Saale river, right below the castle ruins Giebichenstein in Halle, see photo in figure 1 left. On the accompanying icebreakers on both sides of the bridge, two animal sculptures - a cow symbolizing the rural side and a horse on the city side were mounted. 625

2 Fig.1 - Giebichensteinbrücke in Halle (Saale), Germany. Left: Photo of the bridge; Right: Manual crack mapping at the south side of the cow The precise manufacturing procedure of the concrete sculptures is not known. Each consists of a compressed solid body of Portland cement as a binder and an additive mixture of quartz sand, porphyry gravel and copper slag in different size fractions [Müller-Gerberding, 1994, 57-60]. First damage to both the sculptures and the bridge were already detected shortly after completion as crack pattern on the surface. Over the decades, these cracks have widened and deepened to several millimeters. From 2011 to 2012, the sculptures were restored as part of a research project on conservation practice of concrete funded by the Deutsche Bundesstiftung Umwelt. The objective of the restoration project was the reduction of moisture incorporation into the sculptures by filling and closure of the numerous cracks and honeycombs. Within the project and in the process of restoration, 3D mapping of the cracks with the tracking method and with active thermography was applied at selected areas. The results of these studies were compared with manual crack mapping. Such a manual crack map of the cow is shown in figure 1, right. An overall crack length of 222 m was detected. About 50% of these cracks have been intended for grouting. 3. Development of methods for crack detection and characterization D mapping of cracks using a tracking system For 3D mapping of cracks directly on the object, a method was developed in which the user runs a measuring tool along the crack, see figure 2, middle. The location of the tool is determined by means of an optical tracking system. Tracking means a continuous determination of the position of a recognizable object (here: known arrangement of optical markers). With the information of the tracking systems the pose, i.e. position and orientation of the tip of the measuring tool in space can be determined. The recording of data can be done either for individual discrete points or continuously (or in defined range intervals). The recorded data serve as a basis for mapping the cracks. In addition to the tool and the tracking system also software is used, which was developed specifically for the purpose of convenient data recording directly on the object. Thus, the whole system consists of three components: tracking system, measurement tool and software module. For motion detection of the measurement tool, an infrared (IR) tracking system from NaturalPoint was 626

3 Fig.2 - Set-up of the 3D tracking system. Left: Principle set-up in laboratory. Middle: Tracking along the surface of a crack with the measuring tool. Right: On-site application used, see fi gure 2 left. The usual application of this system is the recording of human movements for MotionCapturing applications. However, in these applications variations in the positioning of the markers can be tolerated in the centimeter range, as in general the data is reworked and corrected afterwards. For mapping of cracks, signifi cantly higher demands are made; i.e. only deviations in the millimeter range can be accepted. To obtain a reliable indication of the actual achievable accuracy of the position, detailed investigations were carried out on the tracking system concerning accuracy. The different measurements to fi nd out the tracking accuracy have shown that a determination of the positioning of single markers is possible with a precision of ± 0.5 mm. This accuracy can be achieved with a camera set-up as illustrated in figure 2 right. However, a good calibration of the camera system is essential Determination of crack depth using ultrasonics With an electro-acoustic transducer, an ultrasonic pulse is generated, thereby various types of ultrasonic waves are generated. The most important parameter for ultrasonic measurement is the propagation velocity (vl) of the longitudinal wave (p-wave). Normally the velocity of the longitudinal wave is determined in the transmission configuration. In comparison to other acoustic wave modes, the longitudinal wave has the highest propagation velocity. Therefore, the fi rst arrival of the wave signal is evaluated. The denser the structure of the material, the better the signal is transferred from grain to grain. Air-filled cavities such as cracks and pores cannot be passed. Instead, the sound is transmitted in a roundabout way, thereby increasing the signal path. The resultant ultrasonic travel time is enhanced. The knowledge of this fact is used for the determination of crack depths. An accurate determination of crack depth using ultrasonics requires a very precise work and a good coupling of the transducers onto the material surface as the crack tip signals are relatively weak. For crack depth measurement transmitter and receiver must be positioned in approximately the same distance from the crack, see fi gure 4 a. The geometry of transmitter, receiver, 627

4 and crack tip should correspond approximately to an isosceles triangle. The hypotenuse of the triangle should be greater than the suspected crack depth to receive an appropriate signal. As the crack depth is generally unknown, the distance between the transmitter and receiver must be varied until a usable signal appears on the oscilloscope. If a dense network of cracks is present on the object under investigation, the recommended separation distance between transmitter and receiver cannot always be considered. In other cases, it might be possible that the measurement is disturbed by the sphere of another crack. Thus, a compromise in the quality of the signal has to be received. As coupling agent, commercially available fine chamotte clay is recommended, which can be easily removed from the surface after the measurement. The coupling agent increases the signal strength by a factor of 100 with respect to a dry coupling. Furthermore, also silicone based putty can be used for optimum coupling. Since the coupling takes place on a small surface area and it is a surface measurement, first the position of the real coupling point has to be calculated. For that purpose, the determination of the propagation velocity occurs fi rst in a transmission configuration. Thereafter, a surface measurement along undisturbed material is performed at a defined distance of both transducers. The measurement of this distance occurs between the outer edge of the transmitter and the tip of the receiver. From the recorded transit time and the determined propagation velocity, the relevant distance can be calculated. Now this value has to be subtracted from the previously manually measured distance and the correction value of the measuring transducers is the result. The distance between transmitter and receiver can always be accurately measured from the outer edge of the transmitter to the probe tip of the receiver. Then the correction value of the ultrasonic transducer set has to be added. For crack depth measurements, first the crack must be identified and understood in its course. In accordance to the length, structure and texture of the crack or a crack polygon, suffi cient distances for measurement positions have to be selected. In fi gure 3 right, the distribution of measuring points at a crack polygon on the back of the cow is shown exemplarily. Fig.3 - Determination of crack depth using ultrasonics. Left: Principle of method; Right: Crack 628 polygon with marked ultrasonic measurement positions

5 3.3. Crack characterization with active thermography With thermography methods, in principal open surface cracks as well as hidden cracks can be located and characterized. Here, only the open surface cracks were examined. Various studies on sandstone specimens in laboratory with sawed notches and real cracks have shown that these can be easily detected by a transient heating of the surface of 1 min using IR radiators, see figure 4 a,c. Directly after heating, in particular the notches appear as cooler (dark) lines. During cooling down, there is a contrast reversal (fi gure 4 b,d). This afterglow can be explained by the scheme shown in figure 4 e,f. Shortly after the excitation only a thin layer of the material parallel to the surface appears warmer. Thus, the surface has a higher temperature than the bottom of the crack. Therefore, the edges and the bottom of the crack appear cold (dark). In the further course of time, the heat diffuses into the specimen heating up also the crack edges. During and after external heating, heat losses due to convection and radiation occur at the surface. Both loss processes are significantly reduced at the crack edges. Therefore, the crack edges remain warmer for longer times than the surface. However, this effect can be observed at cracks which are nearly perpendicular to the surface, but almost vanishes in the case of tilted cracks. Here, the heat conduction into the specimen is reduced by the tilted gap. Thus, the area Fig.4, a) to d) - Thermograms of Cottaer sandstone specimens with notches (a,b) and with a real crack (c,d) after 1 min of heating with an IR radiator (a,c) and after 4 min of cooling down after heating (b,d). During cooling down, there is a contrast reversal: the notches and cracks appear warmer in relation to the environment. e) and f) - Schematic representation of the thermal contrast development of perpendicular cracks during and after heating. Shortly after heating the surface, the cold notches appear colder, as one looks into the still cool interior (e). The edges of the crack cool down more slowly than the outer surface, as convection and radiation losses are significantly reduced (f). Now the notches appear warmer 629

6 Fig.5 - a) Manual mapped cracks on the cow of the Giebichensteinbrücke; b) 3D mapping of the cracks with the tracking system; c) Phase image of a thermal sequence on the surface above the crack appears warmer. A localization of the cracks as well as an estimation of the crack angle and crack depth is possible by analyzing the temperature evolution. Depending on the crack angle, different temperature profi les over the crack are formed. 4. Results Figure 5 a shows a section of the manual crack mapping in the chest area of the cow. The manual mapping was used as a template for recording the tracking-based 3D crack mapping. With the tracking system 41 cracks with a total of 4451 measurement points were taken in this area. Besides the cracks several reference points were also included in the survey, which are marked by colored tiles on the sculpture, so that they are easily visible in photos. The result of the 3D mapping is a 3D data set as shown in figure 5 b. The crack structure is in good agreement with the manually recorded but shows some more details. The required heat for active thermography was introduced with an IR radiator (2.4 kw). This was done by moving it in a distance of 5 to 15 cm above the surface to be examined. A homogeneous heating was controlled by simultaneous observation with an IR camera. Due to the highly curved surface, this was difficult to achieve. Therefore, the cooling sequence was analyzed with the pulse-phase thermography, which typically is less sensitive to global inhomogeneities. In fi gure 5 c, a phase image of the measurement area is shown. Here, the cracks can be identifi ed much better as in the raw thermograms (not shown here). The cracks can be represented by a similar resolution as the manual mapping, but provide only 2D images. In summary, the most accurate 3D acquisition of the cracks can be performed using the tracking method. At the back of the cow, a further crack area was selected, which was already shown in fi gure 3, right. Also this area was heated up with an infrared radiator for 1 min. A thermogram recorded 4 min after heating is shown in figure 6 a. Here, the crack structure is already visible, therefore cracks might be tilted. By analyzing the thermograms using spatial edge filters (Sobel operator), the cracks appear even more clear. In figure 6 b and c, this is performed for a thermogram after 4 min and after 60 min of cooling down. Even after 60 min, 630

7 Fig.6 - Investigated area on the Built back Heritage of the cow a) Thermogram, Monitoring Conservation recorded 4 min Management after heating; b) Application of a Sobel fi lter to a thermogram recorded 4 min after heating; c) Application of a Sobel fi lter to a thermogram recorded 60 min after heating the cracks are still visible. Further on, the cracks depths along the crack polygon of this area was measured with ultrasonics. The recorded values are listed in table 1 for the different positions shown in fi gure 3 right. Crack depths between approx. 4 and 40 cm have been measured, the deepest cracks appear along position 6 to position 10, which is close to the thinner part of the back (neck of the cow sculpture). In the thermogram in figure 6 a, at this position the temperatures have a maximum, which might be related to the distortion of heat conduction by these deep cracks. But an inhomogeneous heating could not be excluded. 5. Summary The following methods have been developed and optimized: new measurement method for three dimensional mapping of cracks and crack structures based on a 3D tracking method. Extensive tests concerning Table 1 - Determination of crack depth with ultrasonics along a crack polygon at the back of the cow 631

8 accuracy and precision have been performed and new measurement tools have been developed and optimized. active thermography for the effective crack mapping, which can distinguish between cracks which are either oriented perpendicular to the surface or tilted. For homogeneous materials, the crack angle can also be estimated. Together with crack depth determination with ultrasonics, these methods have been applied on-site for characterising the crack structure of sculptures made of historic concrete. It has been shown that with the 3D tracking method, an efficient and very accurate 3D crack mapping is possible, where the cracks are recorded with a higher geometric resolution than manually in 2D. Active thermography enables a fast detection of cracks, even if these are not directly visible on the surface. As the geometric resolution depends on the detector size of the IR camera and is infl uenced by thermal diffusion processes, it is less than for the 3D tracking method. Only with ultrasonics, a determination of the crack depth is possible. Although the experimental effort using all three methods is high, together they gave a comprehensive crack characterisation of the structure. Acknowledgements The research project was funded by the Research Initiative Zukunft Bau of the Federal Institute for Building, Urban Affairs and Spatial Development (Reference: SF / II 3 - F ). Special thanks to Asmus Schriewer, who accompanied the project on behalf of the BBR very constructive, and to the members of the Support Working Group: Matthias Hemmleb, Ralf Lindemann and Johannes Vielhaber. References EN 14579:2004 Natural stone test methods - Determination of sound speed propagation. Maierhofer, Ch., Röllig, M., Krankenhagen, R., 2010, Integration of active thermography into the assessment of cultural heritage buildings, «Journal of modern optics», Vol. 57, No. 18, pp Müller-Gerberding, Ralf, 1994, Instandsetzung der Giebichensteinbrücke in Halle, in «TU Dresden (Hg.)», 4, Dresdner Brückenbausymposium - Tagungsband, Dresden 1994, S