INFLUENCE OF CONCRETE PROPERTIES ON THE CALIBRATION OF RADAR, ULTRASONICS AND ACTIVE THERMOGRAPHY

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1 INFLUENCE OF CONCRETE PROPERTIES ON THE CALIBRATION OF RADAR, ULTRASONICS AND ACTIVE THERMOGRAPHY Christiane Maierhofer (1), Martin Krause (1), Jens Wöstmann (1), Mathias Röllig (1), Ralf Arndt (1), Doreen Streicher (1) and Christoph Kohl (2) (1) Bundesanstalt für Materialforschung und -prüfung (BAM), Berlin, Germany (2) Schweizerischer Verein für technische Inspektionen (SVTI), Wallisellen, Switzerland Abstract Non-destructive testing methods like radar, ultrasonics and active thermography are mainly applied for the determination of geometrical parameters like thickness of structural elements and location and depth of inclusions inside structures. The accuracy of the quantitative data is influenced by the error of the determination of the propagation velocity of electromagnetic, elastic or thermal waves, which depend on the material properties, its homogeneity and the procedure of calibration. For the systematic investigation of the influence of material properties of concrete, several concrete test specimens including voids and reinforcement were constructed. The test specimens were varied by the stage of concrete hydration, amount and distribution of pores in the cement matrix, variation of aggregates including porous aggregates and steel fibres, and variation of density of reinforcement. Systematic measurements were performed with radar, ultrasonic echo and active thermography. The propagation velocity, the transfer function and the contrast of voids were determined in relation to the different parameters. The maximum influence of these parameters will be discussed. Keywords Concrete, radar, ultrasonics, active thermography, velocity calibration 1. INTRODUCTION Non-destructive testing (NDT) methods like radar, ultrasonics, and thermography are efficient tools for the assessment of the aging infrastructure as well as for quality assurance during and after construction [1]. Structures consisting of reinforced and prestressed concrete can be investigated efficiently with radar and acoustic methods concerning the localisation of 15

2 reinforcing bars and tendon ducts, and applications of acoustic methods at test specimen have shown their ability for detecting grouting faults and delaminations inside the tendon ducts [2],[3],[4]. For the detection of delaminations, voids, higher moisture content and other deviations of structural and material properties close to the surface of building structures, active thermography is very useful [5]. But up to now the full potential of these techniques has not been achieved. In most cases, only one method is applied to solve a certain problem. For increasing the reliability, for enhancing the accuracy of quantitative results, for unambiguous data interpretation and for taking advantage of different and in some cases complementary physical effects, more basic research is required considering also variations of material properties of complementary methods [6]. For active thermography it is expected that variations of concrete mixture have an influence on the thermal properties which are given by the density, thermal conductivity and heat capacity. Numerical simulations allowing the discrimination of the influence of single material parameters were carried out and their output was compared with the experimental results [7]. Calculations regarding the influence of measurement and material parameters were done and are outlined in [6]. For the application of radar, the detectability of structures beneath the reinforcement decreases with increasing reinforcement density (distance and diameter of reinforcement bars at a depth of approx. 5 cm). Thus, for high reinforcement densities, the recording of data with optimum polarization configuration of the emitted electromagnetic waves as well as data processing concerning reconstruction based on Synthetic Aperture Focusing Techniques (SAFT) are required [2],[8],[9]. The moisture and the salt content of the material are the parameters which have the main influence on propagation velocity as well as on absorption of electromagnetic waves [10],[11]. Here, it has to be considered that only free and physically absorbed water is relevant. For the applications of high frequencies, also parameters like pore size distribution, distribution of aggregates and especially steel fibres have to be considered. This will be demonstrated in the following. It is expected that the transmission of elastic waves is much more effected by the porosity of concrete than by the reinforcement density or content of steel fibres [12]. Systematic investigations of the influence of reinforcement density and bulk material parameters of concrete on the propagation of electromagnetic and elastic waves were performed at BAM and are presented in the following. Several test specimens were constructed and investigated in the laboratory for recording the influence of pore content, pore distribution and steel fibres. For studying the influence of concrete age, measurements were also performed at a fresh concrete specimen with active thermography and were repeated during hydration. 2. EXPERIMENTALS 2.1 Test specimens For the investigation of the influence of bulk material properties, four concrete test specimens (no. 1 to 4) were constructed. A grain size distribution curve of A/B 16 and a water/cement ratio of 0.6 were realized for all mixtures. Further properties like air content in fresh concrete, compressive strength and density determined at 28 days old cubes can be taken from Tables 1 and 2. 16

3 The four test specimens were set-up as shown in Figure 1 (left) containing four voids with a size of 10x10x5 cm 3 at depths of 6 and 10 cm. Two voids were simulated by inclusion of polystyrene cuboids and two by integration of gas concrete parts. Test specimen no. 1 (NC) was made of normal (reference) concrete together with ten cubes and was investigated during hydration (3, 7, 28, 56, 180 and 360 days after concreting) with active thermography. Parallel to each measurement, one of the concrete cubes was tested with conventional destructive investigation methods. In test specimen no. 2, the larger aggregates were replaced by porous aggregates in normal cement (PC). This leads to lower density and compressive strength. Airentraining agents were put into the cement mixture of no. 3 (AC) resulting in low density and compressive strength as shown in Table 1. Test specimen no. 4 (SC) includes steel fibres with a concentration of 0.5 Vol%. For the investigation of reinforcement density, test specimen no. 5 was constructed as displayed in Figure 1 (right). It has three voids with a size of 10x10x5 cm 3 (simulated by polystyrene) at a depth of 6 cm and located behind three different densities of reinforcement (one, two and three layers of reinforcement mats Q188). At the same height and depth, three further reference voids (also simulated by polystyrene and having the same size) without any reinforcement were included. Except for test specimen no. 1, the measurements were performed more than one year after concreting. a) b) Figure 1: a) Plan view and cross section of test specimens no. 1 to 4 (size: 1.0x1.0x0.3 m 3 ). b) Test specimen no. 5 before concreting (size: 1.5x1.5x0.3 m 3 ). 2.2 Active thermography Active thermography is an active approach for a quantitative thermal scanning of the surface of various structures and elements. A thermal impulse is applied to a surface causing a non-stationary heat flow. During the cooling-down process the emitted thermal radiation is observed with an infrared camera. The propagation of the heat into the body depends on material properties like thermal conductivity, heat capacity and density of the inspected 17

4 object. If there are inhomogeneities in the near surface region of the structural element this will result in measurable temperature differences in the local area of the surface [7]. The experimental set-up consisted of a thermal heating unit, an infrared camera (SC 1000, Focal Plane Array, 3 to 5 µm) and a computer system, which allows digital data recording in real time, and is described in detail elsewhere [13]. A thermal heating unit set up with three infrared radiators (each with 2400 W) was used. It was moved along the surface in a distance of about 15 cm. The heating duration was varied between 5 and 30 min. After the heating process, the radiator was switched off and the cooling down process of the surface was observed with the infrared camera. 2.3 Radar Radar is based on the transmission of short electromagnetic impulses by an antenna at frequencies between 300 MHz and 2.5 GHz [8]. These impulses are reflected at interfaces with changing dielectric properties of the materials. Also the propagation velocity depends on the dielectric properties. Since moisture is influencing this parameter, radar can also be applied to detect enhanced moisture content and to determine the moisture distribution. For the presented investigations, radar units and antennas from Geophysical Survey Systems, Inc. (GSSI) were used. At the test specimens, radar measurements in transmission configuration with the 1.5 GHz antennas were performed one year after concreting to ensure only a small amount of free and physically absorbed water. For these investigations, transmitter and receiver antennas were positioned at opposite sides of the test specimen. As there is always a time drift of the signals, for each measurement the propagation time of the signal was calculated as a difference of t 0 and t s. Here, t 0 is the time when the signal is transmitted and t s when the signal is detected. t 0 is calculated from the known propagation time of a signal transmitted to 1 m of air, which is determined for each measurement separately. 2.4 Ultrasonic echo Similar to radar, the ultrasonic echo method works according to the impulse echo principle. Ultrasonic impulses are reflected at the interfaces, where the elastic impedance of the materials changes. In comparison to electromagnetic waves, elastic waves can penetrate through metal ducts. Therefore ultrasonic echo techniques are very promising for investigations of the grouting condition inside metal ducts [3]. On the other side, elastic waves are influenced much more by air inclusions and gaps than electromagnetic waves. Due to the required coupling, ultrasonic echo transducers can only be moved step by step. The elastic sensor must be pressed onto the surface. Here, ultrasonic echo measurements were performed with point contact transducers (24 probes, 50 khz from ACSYS) permitting measurements without any coupling agent. 3. RESULTS 3.1 Active Thermography For quantitative data analysis, transient curves (surface temperature as a function of time for each pixel) from areas above voids and above homogeneous material were compared and difference curves were calculated as shown in Figure 2. These difference curves usually have 18

5 a maximum T max at a distinct time t max, which depends on the depth and size of the voids, on the material parameters and on the heating time [7],[14]. For test specimen no. 1 it is expected that the thermal properties of the bulk material change during hydration (which can be quantified by time or compressive strength) while the free water is transformed into chemically bonded water. Therefore, in Figure 3 left, T max and t max of void 2 (polystyrene at a depth of 6 cm) are displayed as a function of compressive strength (determined at sample cubes) for a heating duration of 30 min. With increasing strength, the contrast decreases while the time of the maximum contrast increases. This is consistent with a decrease of thermal conductivity as outlined in the results of numerical simulation in [6]. T in o C 30 T max reference point void 28 temperature difference T in K cooling time in s Figure 2: Surface temperature T above reference and void position and the respective difference curve with maximum contrast. maximum contrast T max in K 1,80 1,75 1,70 1,65 1,60 1,55 T max in K t max in s time of maximum contrast t max in s T in K 2,5 normale aggregates (no.1) porous aggregates (no.3) 2,0 2) 1,5 1,0 0,5 0,0 a) 1, compression strength of cubes in N/mm b) -0, cooling down time in s Figure 3: a) Maximum temperature contrast T max and time t max at which this contrast appears for void 2 as a function of compression strength of cubes of test specimen no. 1 (heating duration: 30 min). b) Temperature transient curves of void 2 for test specimen no. 1 (NC) and no. 2 (PC) after a heating time of 30 min. The porous aggregates in specimen no. 2 (PC) as well as the air-entraining agents in specimen no. 3 (AC) reduce the density and it is expected that the pores reduce the thermal 19

6 conductivity of the material. As shown in [6], the expected reduction of thermal conductivity should lead to a small decrease of T max and to a larger increase of t max, while the reduction of the density has a larger influence on T max and a smaller influence on t max. Temperature transient difference curves of test specimen no. 1 (one year after concreting) and test specimen no. 2 (PC) are visualised in Figure 3 right. 1,6 1,4 1,2 1,0 low rebar density medium rebar density high rebar density T in K 0,8 0,6 0,4 0,2 0,0-0,2-0,4-0,6 void below rebars reference void 0,0 0,5 1,0 1,5 position in m Figure 4: Horizontal smoothed scans of temperature difference at maximum contrast for voids of test specimen no. 5. The temperature difference was normalized to the reference voids. Table 1: Concrete parameters for the different test specimens and T max and t max of void 2 after a heating time of 30 min. The density was determined at 28 days old cubes. Type of concrete Pore content in fresh concrete in Vol% Density in kg/dm 3 Compressive strength of cubes in N/mm 2 t max in s T max in K Normal concrete (NC), (1) 0.9 ± ± ± ± ± 0.08 Concrete w. porous aggr. (PC),(2) 6.0 ± ± ± ± ± 0.08 Concrete with airentr. agent (AC), (3) 3.9 ± ± ± ± ± 0.08 In Table 1, the properties of the concrete types, which cause the characteristics of these transient difference curves (specified by t max and T max ) of void 2, are listed. For specimen no. 2 (PC), T max as well as t max increase clearly. Here, the effects of increased density and increased thermal conductivity superimpose each other. For specimen no. 3 (AC), T max increases clearly, while t max slightly decreases (within measurement accuracy). Therefore, the reduction of density and not the reduction of thermal conductivity has a main influence on the data here. 20

7 It is expected that different amounts of rebars above a void have an influence on its detectability, too. Therefore, from the data obtained at test specimen no. 5, the thermograms showing the maximum temperature contrast above the voids were selected. Three horizontal line scans were taken at the areas of different reinforcement density. Each of these scans included a void below the reinforcement and a reference void. These temperature scans normalized to the maximum temperature of the reference voids are displayed in Figure 4. Only a small influence of the rebar density could be observed: With increasing rebar density, the temperature contrast of the underneath voids decreases slightly. 3.2 Radar In Figure 5, the radar signals recorded in transmission configuration at positions without voids at the four test specimens are shown. The signals were averaged from ten single shots at ten different positions and were processed with a smoothing algorithm. For the determination of the zero point on time scale, transmission measurements in air along a given distance were performed simultaneously. As shown in the diagram, the larger amount of air pores in no. 2 and no. 3 (PC and AC) results in a decrease of propagation time, while the steel fibres in no. 4 (SC) induce a nearly complete decay of the signal. It is expected that the higher air content in no. 2 leads to an even shorter propagation time as in no. 3, but the opposite can be observed in the diagram. This might be explained by the position of the additional pores, which are included in the cement matrix in no. 3, but only in the aggregates in no. 2. Thus, in no. 2, more waves are travelling though the cement matrix and are scattered with a higher amount resulting also in a reduced intensity of the signal. The resulting propagation velocities are summarised in Table 2. Figure 5: Radar signals after transmission through a 30 cm thick concrete test specimen (NC: normal reference concrete (no. 1); PC: concrete with porous aggregates (no. 2); AC: concrete with air-entraining agents (no. 3); SC: concrete with steel fibres (no. 4)). 3.3 Ultrasonics In Figure 6, the A-scans recorded with ultrasonic echo as well as the respective envelop functions are shown for the four different test specimens. For the propagation velocity of the elastic waves, the concrete density is the parameter with the main influence. Again, the resulting propagation velocities are summarised in Table 2. It is shown that compared to 21

8 normal concrete, the velocity inside the steel fibre reinforced concrete is about 5% higher, while the velocity inside the lightweight concrete types is reduced. The amount of reduction is related to the pore content and is larger for PC (porous aggregates, no. 2) concrete as for AC (air entraining, no. 3) concrete. Especially for PC concrete, the intensity of the reflection from the backside is reduced. In the B-scans which were recorded above the voids and which are not shown here, all voids were detected by a missing backside reflection and except for PC concrete, also direct reflection were displayed. For the PC concrete more scattering is expected at the aggregates due to a larger difference in acoustic impedance at the interfaces. a) b) Figure 6: a) Ultrasonic echo A-scans reflected at the backside of the test specimen. b) Envelop function of the A-scans shown left, calculated by Hilbert transformation. Table 2: Concrete parameters for the different test specimens and the resulting propagation velocities and relative permittivities based on radar measurements with the 1.5 GHz antenna and on ultrasonic echo data (50 khz). Compressive strength and density were determined at 28 days old cubes. Type of concrete Pore content in fresh concrete in Vol% Density in kg/dm 3 Compressive strength of cubes in N/mm 2 Velocity of electromagnetic waves in 10 8 m/s Relative permittivity ε Velocity of elastic waves in m/s Normal concrete (NC), (1) 0.9 ± ± ± ± ±33 Concrete with porous aggr. (PC), (2) Concrete with air-entr. agent (AC), (3) 6.0 ± ± ± ± ± ± ± ± ± ±19 22

9 Type of concrete Pore content in fresh concrete in Vol% Density in kg/dm 3 Compressive strength of cubes in N/mm 2 Velocity of electromagnetic waves in 10 8 m/s Relative permittivity ε Velocity of elastic waves in m/s Steel fibre reinforced concrete (SC), (4) 0.9 ± ± ± ±29 4. SUMMARY AND CONCLUSION In this paper systematic investigations with active thermography, radar and ultrasonic echo were presented. The measurements were performed at test specimens made of different concrete mixtures demonstrating the influence of moisture, pore content, pore content distribution and steel fibres and including different reinforcement densities. The following results were obtained: Active thermography: For test specimens no. 1 to 3, at least the voids at a depth of about 6 cm could be detected. A large influence on experimental data ( T max and t max of transient curves) was observed during hydration, mainly due to changes of thermal conductivity. Void 2 could be detected with best resolution soon after concreting for the normal concrete (no. 1). The pore content of cement matrix and porosity of aggregates have a clear influence on thermal properties (density and thermal conductivity) and thus on the experimental data. The density of reinforcement as realized here for no. 5 has only a slight influence on the detectability of covered voids. Test specimen no. 4 is planned to be investigated in the future. Radar: While an increase of pore content in general leads to an increase of propagation velocity and intensity, an inhomogeneous pore distribution due to porous aggregates as in test specimen no. 2 (PC) yields to an enhanced scattering and thus to a reduction of intensity. In this case, most of the waves still travel though the cement matrix, thus the increase in propagation velocity is less than expected. Nearly most of the intensity of the electromagnetic waves is absorbed if steel fibres are included in concrete. Results about test specimen no. 5 are presented soon. Ultrasonic echo: The velocity inside the steel fibre reinforced concrete is higher, while the velocity inside the lightweight concrete types is reduced as expected. The amount of reduction is related to the pore content and is larger for PC (porous aggregates, no. 2) concrete as for AC (air entraining, no. 3) concrete. In general, nearly all voids could be detected reliably with the above mentioned methods. Only for the radar method, it was not possible to locate the voids inside the steel fibre reinforced concrete due to large signal absorption. For radar and ultrasonic echo, the different mixtures have a maximum influence on the propagation velocity of 10 to 20 %. The temperature contrast recorded with active thermography varies up to 15 %. 23

10 ACKNOWLEDGEMENTS We gratefully acknowledge U. Meinhold from BAM VII.1 for supporting the construction of the test specimen. The presented work has been funded by the German Research Council (DFG) within the projects titled Structure and moisture investigation of buildings and building elements using impulse-thermography (MA ) and Non-destructive structural evaluation of concrete elements with acoustic and electromagnetic echo-methods (FOR 384). REFERENCES [1] Maierhofer, Ch., Krause, M., Niederleithinger, E. and Wiggenhauser, H., Non-destructive testing methods at BAM for damage assessment and quality assurance in civil engineering, DGZfP (Ed.); International Symposium Non-Destructive Testing in Civil Engineering (NDT-CE) in Berlin, Germany, September 16-19, 2003, Proceedings on BB 85-CD, V68, Berlin, [2] Maierhofer, Ch. Non-destructive Evaluation of Civil Infrastructure with Ground Penetrating Radar, Journal of Materials in Civil Engineering (JMCE), ASCE 15 (2003) [3] Krause, M., Milmann, B., Schickert, M. and Mayer, K., Investigation of Tendon Ducts by Means of Ultrasonic Echo Methods: A Comparative Study, Proceedings of the 9th European Conference on NDT, Berlin, September, 2006 (DGZfP, BB 103-CD, Tu.3.2.1). [4] Beutel, R., Reinhardt, H.-W., Grosse, Ch. U., Glaubit, A., Krause, M., Maierhofer, Ch., Algernon, D., Wiggenhauser, H., Schickert, M., Performance Demonstration of Non-Destructive Testing Methods, Proceedings of the 9th European Conference on NDT, Berlin, September, 2006 (DGZfP, BB 103-CD, Tu.3.2.2). [5] Maierhofer, Ch., Arndt, R., Röllig, M., Rieck, C., Walther, A., Scheel, H. and Hillemeier, B., Application of impulse-thermography for non-destructive assessment of concrete structures. Cement & Concrete Composites 28 (2006) [6] Maierhofer, Ch., Arndt, R., and Röllig, M., Influence of concrete properties on the detection of voids with impulse-thermography. Infrared Physics & Technology 49 (2007) [7] Maierhofer, Ch., Brink, A., Röllig, M. and Wiggenhauser, H., Quantitative impulsethermography as non-destructive testing method in civil engineering - Experimental results and numerical simulations, Construction in Building Materials 19 (2005) [8] Daniels, D. J., Ground-Penetrating Radar, 2nd Edition, The Institution of Electrical Engineers, London, [9] Marklein, R., Miao, J., Rahman, M. and Langenberg, K. J., Inverse Scattering and Imaging in NDT: Recent Applications and Advances, Proceedings of the 9th European Conference on NDT, Berlin, September, 2006 (DGZfP, BB 103-CD, Tu.3.3.2). [10] Maierhofer, Ch. and Wöstmann, J., Investigation of dielectric properties of brick materials as a function of moisture and salt content using a microwave impulse technique at very high frequencies, NDT & E International 31 (4) (1998) [11] Tsui, F. and Matthews, S. L., Analytical modelling of the dielectric properties of concrete for subsurface radar applications, Construction and Building Materials 11 (3) (1997) [12] Krause, M., Mielentz, F., Milmann, B., Streicher, D. and Mayer, K., Ultrasonic reflection properties at interfaces between concrete, steel and air: imaging and modelling, Al-Quadi, I. and G. Washer (eds.); Proceedings of the NDE Conference on Civil Engineering, St. Louis, MO, USA, August (2006) [13] Maierhofer, C., Brink, A., Röllig, M. and Wiggenhauser, H., Transient thermography for Structural Investigation of Concrete and Composites in the Surface near Region, Infrared Physics and Technology 43 (2002) [14] Maldague, XPV. Non-destructive evaluation of materials by infrared thermography, London: Springer-Verlag,