POSITIONING OF STEEL RODS INCLUSIONS IN REINFORCED CONCRETE SIMULANT BY COMPTON BACKSCATTERING

Transcription

1 2011 International Nuclear Atlantic Conference - INAC 2011 Belo Horizonte,MG, Brazil, October 24-28, 2011 ASSOCIAÇÃO BRASILEIRA DE ENERGIA NUCLEAR - ABEN ISBN: POSITIONING OF STEEL RODS INCLUSIONS IN REINFORCED CONCRETE SIMULANT BY COMPTON BACKSCATTERING Emerson M. Boldo, Ana A. P. Prestes and Carlos R. Appoloni Laboratório de Física Nuclear Aplicada Departamento de Física - CCE Universidade Estadual de Londrina Rodovia Celso Garcia Cid Pr 445 Km Londrina PR, Brasil eboldo@gmail.com ABSTRACT Reinforced concrete is susceptible to a range of environmental degradation factors that can limit its service life. There has always been a need for test methods to measure, in situ, the properties of concrete for quality assurance and to evaluate the condition of existing structures. Compton scattering of gamma radiation is a nondestructive technique used for the detection of defects and inclusions in materials and it can be employed on reinforced concrete. The methodology allows for one-side inspection of large structures and can be implemented with a relatively inexpensive, portable apparatus. In this work, we used the Compton backscattering technique to measure both the size and depth of steel rod inclusions in plaster block samples. The samples were irradiated with gamma rays from a Ø2 mm collimated 241 Am (100 mci) source, and the inelastically scattered photons were collected at an angle of 135⁰ by a high-resolution CdTe semiconductor detector. Scanning was achieved by lateral movement of the sample blocks across the field of view of the source and detector in steps of 1 mm. The tests on plaster blocks with steel rod inclusions suggest that, for a low-energy and low-activity gamma source, beam attenuation has greater effects on the scattered intensity than does increased material density. Density contrast analysis allows determination of the size and depth of steel rods. Furthermore, the experimental results agree with theoretical data obtained through Monte Carlo simulation. 1. INTRODUCTION Reinforced concrete is one of the most commonly used engineering materials in civil construction. Ordinary reinforced concrete is a combination of two materials: concrete and reinforcement bars (rebar) made of steel. Although reinforcement bars added to concrete provide great structural resistance, there is also the unavoidable problem of corrosion. Concrete is permeable, and thus the steel will eventually suffer from contact with the environment in which the structure is located. This process is accelerated if the concrete is too porous or if there are cracks in the structure. However, compared with the development of non destructive tests for other materials, the development of reinforced concrete inspection methodology has progressed slowly on account of its inherent complexity. Concrete is highly heterogeneous on a macroscopic scale, does not conduct electricity, and can be made with varying proportions of its components depending on its purpose. This is only exacerbated by the fact that reinforced concrete is a composite material that is generally found in structures of high thickness. Both ultrasound and eddy current techniques have been applied to evaluation of the state of a reinforced

2 concrete structure; however, both of these techniques present serious limitations [1]. In this work, we propose the use of Compton scattering instead for evaluating reinforced concrete. This is a non destructive technique that allows for one-side inspection of large structures without previous surface treatment, offers good spatial resolution, and can determine simultaneously the size and position of the reinforcement bars and defects. The methodology is based on the fact that Compton interaction is highly dependent on the electronic density of the scattering medium and, consequently, on its physical density. It is possible to identify density changes by monitoring photon scattering of a well-defined inspection volume (VOL) within the material. However, there are other factors that affect scattered intensity as well. Changes in the scattering angle and the collimation can modify the inspection volume size and the path length of the beam within the material, and source energy may have an effect on scattering probability and differentiation of materials on the basis of density contrast. Therefore, several variables must be carefully considered in order to optimize the system accuracy. In this work, a study was conducted to optimize a compact portable system that uses the Compton scattering technique for detecting and positioning reinforcement bars in concrete. The best geometry configuration, source energy, and scattering angle were evaluated using a Monte Carlo simulation. Experimental results agree with theoretical data obtained in the simulation. 2. THEORETICAL BACKGROUND The shared energy between the recoil electron and the scattered gamma photon in a Compton interaction depends exclusively on the scattering angle. The scattered gamma photon energy E s is given in terms of scattering angle Ø according to the following relation: Ei ES 2 (1) 1 E / m c 1 cos i 0 where m 0 c 2 is the electron rest mass energy (511 kev). The number of photons scattered by the Compton effect depends on the electron density of the material analyzed. If N A is the Avogadro s number, Z the target atomic number and A its atomic mass, the number of electrons/cm 3 is given by: Z e N A (2) A where represents physical density of the material (in g/cm 3 ). For non-hydrogenous materials Z/A 1/2. For a collimated gamma ray (I 0 ), the number of photons that are subjected to scattering inside a well-defined volume (VOL) and reach the detector can be given by [2]: I C CI VOL exp Ei X i exp ES X S M C 0 e (3)

3 where C is a proportionality constant that includes solid angle, detector efficiency and the differential cross section per electron, I 0 is the incident intensity and e is the average electron density within the inspection volume. The exponential functions represent attenuation of the both incident and scattered beam along the paths X i and X s within the material, (E i ) and (E S ) are the respective attenuation coefficients. Lastly, M C is the contribution value of multiple scatterings due to the arriving photons in the detector after them suffering scattering outside the inspecting volume. An effective way to locate defects and inclusions within a sample is to monitor the intensity of the scattered radiation while performing a scan on the region of interest. In this way, the information about inspection volume (VOL), defined as the intersection of the incident beam with the detector's field of view, can be gathered during the scan. Density variations produce differences in scattering counts. The greater the contrast between densities the bigger the difference among intensities scattered by different materials. In this work, we have defined density contrast (DC) mathematically as: C bulk Cinclusion DC 100% (4) Cbulk where C bulk is the mean count values when the incident beam is only on the sample material and C inclusion represents mean count values registered by the detector when the incident beam intercepts an inclusion inside the sample. Contrast defines the ability of the system in distinguishing amongst materials of different densities. 3. MONTE CARLO SIMULATION In this work, the FLUKA code was used to perform the Monte Carlo simulation. This code is a completely integrated simulation package that uses sophisticated programming to deal with particle transport through matter, and it is particularly useful for scattering problems [3,4]. In the work of Randeniya et al. [5], the FLUKA code proved four times faster than MCNPX and 14 times faster than GEANT4, two simulation codes that also are often recommended for particle transport problems. FLUKA can simulate with high accuracy the interaction and propagation of photons in matter with incident energies starting from 1 kev. In the specific case of the Compton effect, it takes into account the atomic bonds in the calculation of the scattering probability using the Hartree Fock inelastic form factor. The code can handle complex geometries using improved versions of the packages of combinatorial geometry and has several tools for viewing and debugging. The initial purpose of the simulation was investigate the influence of parameters such as geometry, scattering angle, and source energy on measurement results with a view to achieving greater scattered photon count, better contrast among sample materials, and higher spatial resolution in detecting and positioning reinforcement bars in concrete. Scattering of gamma rays in an ordinary concrete block (10% humidity; = 2.34 g/cm 3 ) with dimensions of cm was simulated. A cylindrical steel bar measuring Ø10 mm and 1.5 cm deep from the surface was inserted in the block (Fig. 1). To detect scattered photons, a CdTe detector was set with brass cylindrical collimation measuring Ø7 30 mm.

4 Net Counts 3.1. Geometry Test Figure 2 (left) shows two source and detector configuration sets studied in this work. One of them is such that the incident beam is perpendicular to the sample (Geometry 1). In the other (Geometry 2), the detector and source are changed positions such that the backscattered photons are captured in a direction perpendicular to the sample. The geometries were adjusted such that the centre of the inspection volume was located at a depth of 1.5 cm from the surface. The net counts of the backscattered photons for both geometries are shown in Fig. 2 (right). In Geometry 2, a larger portion of the inspection volume (VOL) is closer to the surface; hence the path traveled by the scattered photons within the material toward the detector is shorter. This explains the higher photon count in the case of Geometry 2. Greater counts imply in shorter experimental counting time intervals for obtaining statistically significant values during the experiment. Figure 1 - Top view of the concrete block ( cm) used in the simulation containing a Ø10 mm reinforcing steel bar located at a depth of 1.5 cm from the surface Simulation Experimental Geometry 01 Geometry 02 Figure 2. Two geometries of the experimental setup (left). Net counts obtained in the two geometries (right). Scattering angle: 135.

5 Even though Geometry 2 had the abovementioned advantages, it was not chosen because of certain practical considerations. As the inspection volume is oblique to the sample surface and its size along this direction depends mainly on detector collimation, the inspection volume may reach a region of interest without the detector being exactly above it. Consequently, an external reference in the region of interest is not guaranteed, regardless of whether the region has an inclusion or a defect. The beam direction in Geometry 1 may be viewed as an external reference. When the perpendicular beam crosses a region in the sample where the density varies the detector immediately registers a change in the scattered photon count Backscatter Angle Test In the backscattering setup, small angles close to 90 are not adequate because there is considerable increase in distance traveled by the scattered beam inside the sample, which implies increased attenuation. For a low energy incident radiation, as is used in this study, wider angles also present increased scattering probability. In addition, a small and portable device requires that source and detector be close to each other in order to obtain a compact assembly. Adequate shielding around the gamma ray source and the size of the detector and collimation system must also be taken into account when testing various positions of the Compton device. In this way, two backscattering angles were tested: 135 and 150. These two configurations benefit from a high scattering probability and also provide sufficient space for positioning the equipment components. Figure 3 shows the scattered intensity as a function of concrete block position. Each dot corresponds to the integrated net count of Compton s peak, set to two different scattering angles according to Eq. 1. When the inspection volume intercepts the steel bar, there is a decrease in count rate, indicating that the attenuation effect of the incident and scattered beam within the denser material (steel) is predominant in this interaction. This is attributable to the low energy produced by the 241 Am source (59.5 kev). The greatest count values obtained at 150 are easily understood, as Klein Nishina s differential cross section predicts larger scattering probability when this angle is used at this incident energy [6]. The angle of 135 presents the best contrast (DC 135 = 34.6%) and leads to the best estimate for the size of the rebar via Gaussian fit. This result is attributable to an increase in inspection volume for the angle of 150. Larger the inspection volumes correspond to worse resolution and density contrast, and this directly affects the technique s ability to distinguish different materials inside the sample.

6 Net Counts mm rebar o - DC = 34.6% 150 o - DC = 26.9% Position (mm) Figure 3. Net counts of backscattered photons due to the position of the concrete sample with a Ø10 mm steel rebar. Scan performed at the centre of the bar. Incident beam (59.54 kev) with 1.2x10 6 primary photons, 1 mm step Energy Test Two energies were evaluated in this work: kev (main 241 Am emission) and kev (main 57 Co emission). Figure 4 shows the net counts for the two energies obtained as a function of the concrete block position. Now, the two charts were set aside for better viewing. Even though contrast values obtained using the 59.4 kev energy are higher, the Gaussian fit for both energies overestimates, within statistical error, the same values for the reinforcement bar diameters. This indicates that both sources are equivalent in this case, with the advantage of higher penetration provided by the 57 Co energy.

7 Net counts E = kev GaussFit FWHM = R 2 = 0.93 DC = 33.5% E = kev GaussFit FWHM = R 2 = 0.90 DC = 19.5% Position (mm) Position (mm) Figure 4. Net counts with a Gaussian fit (red line) of backscattered photons due to the position of the concrete sample with kev (left) and kev (right) incident energy. Scan performed at the centre of the bar. 1.2x10 6 primary photons, 1 mm step. 4. MATERIALS AND METHODS The experimental setup used in this study consisted of an 241 Am (100 mci) collimated source (Ø2 mm) shielded by a lead capsule positioned at a distance of 100 mm from the sample. A high-resolution CdTe detector (model: Amptek X-123) also positioned at 100 mm distance from the sample measures the energy spectrum of scattered radiation. The detector collimation is composed of a brass cylinder measuring Ø7 30 mm. These components were chosen aiming at the construction of a lightweight, portable and compact device. The inspection volume within the sample is defined by the source and detector collimators and scattering geometry. For the inclusion positioning tests, a plaster block ( = 0.81 g/cm 3 ) with a steel bar ( = 7.68 g/cm 3 ) that had diameter of Ø10 mm and was embedded 15 mm deep from the sample surface was used. For the analysis of density contrast as a function of depth, was used a block of plaster with steel bars (Ø5 mm) positioned at 8 mm, 12 mm, 18 mm and 24 mm from the surface. Samples were mounted on a mobile support, which allows for lateral movement perpendicular to the incident gamma beam in steps of 1 mm. The detector, which had a builtin preamplifier, digital pulse processor, and multichannel analyzer, was connected to a notebook for data collection and analysis.

8 Net Counts 5. EXPERIMENTAL RESULTS Figure 5 represents the net counts obtained from backscattered photons in two different targets, namely plaster and concrete. Counts were made with four cylindrical collimators of different sizes positioned in front of the detector, set to the two angles studied in his work. Even though the two materials have different densities, the scattering profiles of both are much similar. This result indicates that the plaster may be used as concrete simulant in Compton backscattering measurements o Plaster - = 0.81 g/cm 3 Concrete - = 2.34 g/cm Ø7X30mm Ø5X15mm Ø3X15mm Ø1.5X15mm Ø7X30mm Collimators Ø5X15mm Ø3X15mm Ø1.5X15mm Figure 5. Net counts of backscatter photons from two targets - plaster and concrete - as a function of size collimation and backscatter angle. The scattered intensities are plotted as a function of the position of the plaster sample (with the Ø10 mm rebar) in Fig. 6. Because of the backscattered intensity drop due to beam attenuation, it is possible to infer the position of the reinforcement bar inside the material. Count fluctuations of adjacent points are due to the non-homogeneity inside the plaster. The scan performed with the scattering geometry corresponding to an angle of 135 resulted in better contrast (DC = 49.8%) than that performed with the geometry corresponding to an angle of 150 (DC = 32.8%). This agrees with the theoretical results obtained through Monte Carlo simulation. The approximate diameter of the steel rod can be directly obtained from experimental data through the Gaussian fit FWHM value. The result (FWHM 135 = 8.21 mm) is slightly lower than the nominal value of the reinforcement bar diameter. The distance between two adjacent minima may give the distance between two rebars inside the reinforced concrete.

9 Contrast (%) Net Counts mm steel bar 135 o - DC = 49,8% 150 o - DC = 32,8% Position (mm) Figure 6. Net counts of backscattered photons due to the position of the plaster sample with a Ø10 mm steel bar. Scan performed at the centre of the bar, 1000 s measurement time at each point, 1 mm step, 135 backscatter angle. The decrease in the density contrast toward the inner portions of the sample are analyzed, may be used to determine the depth of an inclusion, as illustrated in Fig. 7. The points in this figure represent the density contrast as a function of depth for a plaster block with Ø5 mm steel bars positioned up to depths of 8 mm, 12 mm, 18 mm, and 24 mm from the surface. Contrast decreases with depth until the saturation thickness. Even though this curve has been obtained by a single preliminary analysis, it may be used to calibrate the equipment with the used configuration Depth (mm) Figure 7. Contrast as a function of depth to a sample of plaster containing inclusions of steel bars of Ø5 mm in four different depths: 8 mm, 12 mm, 18 mm, 24 mm (left). Schematic drawing (top view) of the sample (right).

10 6. CONCLUSIONS In this work we were able to confirm, theoretically and experimentally, the applicability of the Compton scattering technique to inspecting and positioning rebars in reinforced concrete, developing a compact, small and portable system for this kind of inspection. The Monte Carlo simulation indicates that, amongst the variables studied, the best scattering angle to detecting Ø10 mm steel bars is 135 in geometry with the gamma-ray source perpendicular to the sample. Experimentally, the system presented good spatial resolution slightly underestimating the diameter of reinforcement bar. Density contrast measures may provide the depths of inclusions in materials. The same methodology shown here may be used to detecting and dimensioning defects (cracks, voids, etc) in reinforced concrete. The technique has proven to be promising to the non-destructive testing of this type of structure. ACKNOWLEDGMENTS The authors of this work would like to thank CNPq for monetary support provided through edict MCT/CNPq 14/2008. REFERENCES 1. H. Irie, Y. Yoshida, Y. Sakurada and T. Ito, Non-destructive-testing Methods for Concrete Structures, NTT Technical Review, 6, pp.1-8 (2008). 2. M. J. Anjos, R. T. Lopes and J. C. Borges, Scattering of Gamma-Rays as Surface Inspection Technique, Nucl. Instr. and Meth. A, 280, pp (1989). 3. A. Fasso, A. Ferrari, J. Ranft and P. R. Sala, FLUKA: a multi-particle transport code CERN , INFN/TC_05/11, SLAC-R-773, (2005). 4. G. Battistoni, S. Muraro, P. R. Sala, F. Cerutti, A. Ferrari, S. Roesler, A. Fasso, and J. Ranft. "The FLUKA code: description and benchmarking Proceedings of the Hadronic Shower Simulation Workshop 2006, Fermilab, 6-8 September 2006, AIP Conference Proceeding, Vol. 896, pp (2007). 5. S. D. Randeniya, P. J. Taddei, W. D. Newhauser and P. Yepes, Intercomparision of Monte Carlo Radiation Transport Codes MCNPX, GEANT4 and FLUKA for simulating proton radiotherapy of the eye, Nuclear Technology, 168, pp (2009). 6. M. J. Cooper, P. E. Mijnarends, N. Shiotani, N. Sakai and A. Bansil, X-Ray Compton Scattering, Oxford University Press, New York, USA (2004).