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Ultrasonic evaluation of reinforced concrete reinforcement depth and diameter

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https://www.eduzhai.net International Journal of M aterials Engineering 2013, 3(2): 17-27 DOI: 10.5923/j.ijme.20130302.02 Ultrasonic Evaluation of the Depth and the Diameter of the Rods of Reinforced Concrete Hicham Lotfi*, Ali Moudden, Bouazza Faiz Laboratory of M etrology and Information Processing, Ibn Zohr University, Faculty of Sciences, B. P. 8106, Agadir, M orocco Abstract The work presented in this paper treat the durability of reinforced concrete structures, the experiments performed in the laboratory are carried to determine the depth and the diameter of rod coated by concrete. To study these structures we have developed a non-destructive technique based on ultrasonic signals backscattered from the samples analyzed. The experimental protocol of th is technique is adapted to evaluate the parameters measured. The determination of the group velocity of the wave backscattered by circumferential of cylindrical rods in iron coated with concrete, allows detecting the diameter and depth of these rods in the building structure. Keywords Group Velocity, Rod in Iron, Reinforced Concrete, Depth 1. Introduction Control tests of re inforce ment structures are different, we find the measurement of electrode potential for the reinforcement corrosion rate, the measurement of the speed of corrosion to evaluate the loss of a section of the steel according to the time, the electrical capacitance to measure humid ity and locate the wet surface. The properties of the concrete are evaluated by measuring the co mpressive strength and bending machines presses. Try these methods and are partially or totally destructive, because they leave impacts and holes in the structure and the information obtained is not local. The use of Non-Destructive Testing (NDT) techniques, for emission of ultrasonic waves, has opened new opportunities on the side of quality control and the study of the mechanical p roperties of materials. The study of propagation of elastic wave in solids is a possible way to e xa mine some physical properties of ce mentitious materia ls. Aggelis et al.[1] characterized with two non-destructive methods the surface of the concrete, the first method is to control the thermal variat ion of the temperature, and the second is the use of the propagation of ultrasound to estimate the depth of defects in the concrete. Ismail et al.[2] Have studied the degradation of concrete resulted by to long-term actions by measuring the co mpressive strength by non-destructive and destructive tests. Malagavelli [3] studied the strength and workability of concrete by using different super plasticizers constituents. Corresponding author: lotfi.hicham@yahoo. fr (Hicham Lot fi) Published online at https://www.eduzhai.net Copyright © 2013 Scientific & Academic Publishing. All Rights Reserved To imp rove the control of civ il engineering wo rks, we develop a non-destructive investigation technique (NDT) based on the analysis of the backscattered signal by cylindrical rods of iron used in the reinforcement structures. This paper discusses two main objectives. The first objective is to assess accurately the ultrasonic response of cylindrical rods of different d iameters by determining the group velocity of surface waves backscattered along the circu mference of the rods. The second is to improve the knowledge on the influence of diameters and depths of these rods in the coating structure on the ultrasonic propagation. 2. Samples Preparation The samples are prepared in wooden mo lds parallelepiped with dimension (L × l × h= 100 × 70 × 60mm3 ) . In these mo lds, we p lace an iron rod at a given depth, then we coat with concrete. After a week o f hydration we p roceed to release these molds to obtain samples of reinforced concrete with a single frame. These samples are then emerged in a tank filled with water and excited with a transducer of central frequency is 0,5M Hz. This transducer is put a distance equal to 2cm of the samp le. The duration of the emission of waves should be sufficient to avoid undesired echoes of experimental equip ment. The experimental tank is filled with water of the density ρwater = 1000Kg / m3 , and the velocity of the ultrasonic wave in water is Cwater = 1470 m / s . Iron frames used in the cementitious materials as form of high-strength. Rods are completely surrounded along its length by ribs forming almost spirals in its surface. Theses rods are the main materials used in the formwork. In table 1, we present the mechanical characteristics of cy linders iron (rods) used in this study. In figure 1, we represent the 18 Hicham Lotfi et al.: Ultrasonic Evaluation of the Depth and the Diameter of the Rods of Reinforced Concrete geometry of the reinforced concrete. backscattered by different structures (reinforced concrete) by using application made by the graphical programming language Lab VIEW. The samp les prepared are structures of mortar (concrete) reinforced by cylindrical rods of iron. The circu mference of the rods is of the order of the wavelength λ = Ciron f ( λ ≈ 10−2 m ). Figure 1. Geometry of the sample Table 1. Mechanical propert ies of iron Density Velo cit y Poisson Young Module Module Shear Module 7800Kg/m3 VL=5900m/s VT=3200m/s 0,287 208,3GPa 80,92GPa 3. Principle of the Experiment 3.1. Experi mental Setup Figure 2 shows the experimental setup used for the experiments. This setup consists of an ultrasonic transducer of 0,5 MHz, wh ich function as the transmitter and receiver. This transducer is emerged in a cubic tank filled with water where the sample was placed at 2cm fro m the transducer. The ultrasonic transducer is connected to a pulse generator type (Model 5073 So franel PR, Sofranel Instruments) which sends the electric signal to the digital oscilloscope (Hewlett Packard HP). The dig ital oscilloscope is connected to a microco mputer by a GPIB cable in order to processes the signal not available on the oscilloscope and analyzes the variables determined. The control begin with the acquisition of signals represented on the screen of the oscilloscope and 3.2. Techni que of Control The experimental study of geometric waves, backscattered by solid cylinders, was studied by M.de Billy[4], and has been continued to this day by different researchers work. These studies showed the validity of the theory of geometrical optics. Especially the prediction of the echoes observed during the diffusion of a cylinder fu ll who their elastic properties and geometric are known. Experiences of non-destructive testing realized by ultrasound are performed by the technique of reflection of u ltrasonic waves[5]. The measurements were performed on samples with mass ratios water / cement ( w c = 0, 65 ) and cement / sand (c s = 0,5 ). These samples were prepared with Port land Cement CPJ 45 produced by Agadir factory. The iron rods coated with mortar (concrete) are those sold by vendors of building p ro d u cts . Thanks to this technique, we study the ultrasonic response of the cylindrical rod o f d iameter equal 6mm according to their depths (d = 2, 4 and 6 cm) in the concrete layer. Rods with d iameters 8 and 10mm are not studied resulted by their complicated geometric. They are co mpletely surrounded along its length by ribs forming almost a spiral. Figure 4 shows the type of temporal signals obtained by the oscilloscope in the case of normal incidence 0°. In figure 3 we schematize the path of the backscattered wave surface along the circu mference of a solid cylinder (rod). The series of echoes E2 and En observed in figure 4 is corresponding to successive roundtrip of the wave surface along the circumference of the rod. The echo E1 is the reflection in the perimeter of the cy lindrical rod and water [6]. This signal is obtained in the backscatter geometry, where the ultrasonic transducer functions as transmitter and receiver of ultrasonic signal. The time between two successive echoes may be related to the radius a = d 2 of the cylindrical rod and the group velocity Vg of the surface wave by the follo wing equation: V g = 2π a ∆t (1) International Journal of M aterials Engineering 2013, 3(2): 17-27 19 Figure 2. Experimental setup Figure 3. Circumferential (surface) wave on a solid cylinder 1,6 E2 1,2 Surface wave 0,8 E1 0,4 (Water/Rod) 0,0 Echos successive of surface wave En -0,4 -0,8 ∆t -1,2 -1,6 75 80 85 90 95 100 105 110 115 120 125 130 135 140 145 150 Time (µs) Figure 4. Type of signals backscattered by a cylindrical rod, d=6mm Amplitude (V) 20 Hicham Lotfi et al.: Ultrasonic Evaluation of the Depth and the Diameter of the Rods of Reinforced Concrete 4. Study of the Ultrasonic Response of the Rods Immersed in Water 4.1. Temporal Signal Diffusion of a p lane wave by cylindrical and spherical diffusers has been presented several times in the literature. The most studied is the one presented by Faran[7] for the case of the cylinder and sphere. Long expressions in the case of the cylinder were reformu lated in matrix form by Doolitt le and Überall [8]. Then, the applications were done by Maze et al.[9] and Ripoche et al.[10] on the diffusion of u ltrasound plane waves by a solid cylinder that excited normally on its axis, they showed that these waves are strictly influenced by the propagation of surface waves. Later on, Maze et al.[11] have shown that the velocity of these waves is dependent of the frequency of wave and the radius of the target. In figure 5 we represent the temporal signal backscattered by the iron rod of d iameter equal 6mm, we remark that the first two echoes diffused by the rod are clearly. 4.2. Spectral Amplitude B ackscatterted The theoretical results obtained by Überall et al.[12], and Ugincius and Überall[13] show that the complex structure of the obtained wave is related to the circu mferential wave propagation (Rayleigh wave) wh ich form standing waves on the circu mference of the target (cylinder, sphere ...). To obtain the spectral amplitude and phases of the echo we apply the Fast Fourier Transform (FFT) to the temporal signals. The rods studied are excited normally to their axis by means of a t ransducer of central frequency 0,5 M Hz and bandwidth from 0,2 to 0,8 MHz. The s pectrum backs cattered by the rod of diameter equal 6mm is shown in figures 6. 4.3. Group Vel ocity Vg The velocity of the propagation of ultrasonic waves in solids is an important property that is used, for examp le, in non-destructive evaluation to calculate the rigidity of materia ls. Conventional techniques used to perform velocity measurements in material need to know the thickness of the materia l studied and the time of the reflected echoes between two interfaces limit ing the material. In table 2 we represent the group velocity calculated by the equation (1) for the rod of diameter equal 6mm. The time between two echoes, due to reflection of the surface wave, is determined. V Table 2. Group velocity g of circumferential wave diffused Time of echoes (μs) Echo E1 78,30 Echo E2 89.95 Δt (μs) 11,65 Vg (m/s) 1617,17 Amplitude (V) 1,6 Rod 6mm 1,2 0,8 0,4 0,0 -0,4 -0,8 -1,2 -1,6 75 80 85 90 95 100 105 110 115 120 125 130 135 140 145 150 Time (µs) Figure 5. T emporal signal backscattered by the rod, d=6mm International Journal of M aterials Engineering 2013, 3(2): 17-27 21 0,06 Rod 6mm 0,05 0,04 Amplitude (dB) 0,03 0,02 0,01 0,00 0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0 Frequency (MHz) Fi gure 6. Spect ral amplit ude backscatt ered by the rod,d= 6 mm 5. Application to the Reinforced Concrete Reinforced concrete occupies a large part of their mechanical propert ies and their frames. The first network reinforcement is located a few centimeters deep below the surface (between 2 and 6 cm). This concrete layer, called concrete cover, is contact directly with the external environment and submitted of damages. The main role of this layer is to protect these frames against aggressive agents fro m the environ ment surrounding the structure (air, water, ions ...)[14]. It is therefore important to determine the characteristics of this layer of concrete (ultrasonic velocity, depth...). The methods using the non-destructive mechanical wave propagation in concrete can be used to obtain informat ion on different scales (millimeter, centimeter…) depending on the wavelength used. The acoustic waves can also be used to detect cracks, voids, or measure thicknesses. The informat ion provided by these methods is important. echo EWC corresponding to reflection of the incident wave on the interface between water and the first plane of the concrete. Then secondary echoes, ECW wh ich is the reflection of the wave incident on the interface between the second plane of the concrete and water and the echo EGR corresponds to a roundtrip of the wave in the thickness of the concrete. 5.1. Ultrasonic Velocity of Concrete Figure 7. Signal path in the concrete layer 5.1.1. Geo metry of the Problem The concrete structure is prepared in molds thickness of 6cm. The description of the preparation of these samples is described in paragraph 2. The schema in figure 7 shows the path of different signals backscattered by the concrete. These samples are located at a distance of 2 cm with the transducer. Figure 8 shows a type of ultrasonic signals backscattered by a concrete structure of rectangular geometry and thickness equal 6cm (Figure 7). Th is signal co mprises a main 5.1.2. Calculate of Ultrasonic Velocity of Concrete Control by measuring the velocity of the ultrasonic wave is one of nondestructive testing commonly used to evaluate building materials. Chekroun et al.[15] measured ultrasonic velocity and attenuation to determine the quality and rig idity of the concrete. Then they evaluated the sensitivity of these parameters with the mechanical properties of concrete. Evaluation of concrete, based in aggregate crushed, with ultrasonic velocity was studied by Carcaño and Moreno[16]. 22 Hicham Lotfi et al.: Ultrasonic Evaluation of the Depth and the Diameter of the Rods of Reinforced Concrete 2,5 EWC 2,0 Water/Concrete 1,5 ECW Concrete/Water 1,0 EAR 0,5 Roundtrip in concrete Amplitude (V) 0,0 -0,5 -1,0 -1,5 -2,0 ∆t ∆t -2,5 80 90 100 110 120 130 140 150 160 170 180 Time (µs) Figure 8. Types of signals backscattered by a concrete structure Thanks to the piezoelectric t ransducer the electrical energy emitted is converted into mechanical energy. The ultrasonic transducer detects the time of each echo in each interface of concrete and water, knowing the thickness e of the concrete, it is possible to determine the group velocity of wave in concrete, and it is exp ressed by the formula (2): Vconcrete = 2e ∆t (2) ∆t =34,7µs is the variation t ime between echoes EWC and ECW , and e is the thickness of the concrete e = 6cm . Therefore the ultrasonic velocity in concrete is Vconcrete = 3458,21m / s . 2,5 EWC 2,0 Water/Concrete Depth 2cm 1,5 Amplitude (V) 1,0 0,5 E1 (Concrete/Rod) E2 Surface wave Echos successive of surface wave En 0,0 -0,5 -1,0 -1,5 -2,0 -2,5 80 90 100 110 120 130 140 150 160 170 180 Time (us) Fi gure 9. Types of signals backscatt ered by a reinforced concrete, p = 2cm (6mm) International Journal of M aterials Engineering 2013, 3(2): 17-27 23 5.2. Echoes of Signal B ackscattered by the Concrete Figure 9 represents a type of the signals obtained for the rod coated by concrete. The length of reinforced concrete equals a 10 cm. This signal co mprises a main echo EWC about 8.25 μs, which corresponds to the reflection of the incident wave on the interface between water and the first plane of the concrete. Echo E1 , which corresponds to the reflection between the concrete and a point on the perimeter of the rod. Echo E2 , wh ich is appropriate to a roundtrip of the surface wave around the circu mference of the rod. Finally the echoes En corresponding to successive reflections of the surface wave by the circu mference of rod. Co mparing the position of the echo E1 in figures 9, 10 and 11, we remark that the echo E1 moves to the position of echo E2 , when we varying the depth of rod into the concrete layer. 2,5 2,0 Depth 4cm 1,5 1,0 shifting of E1 0,5 Amplitude (V) 0,0 -0,5 -1,0 -1,5 -2,0 -2,5 80 90 100 110 120 130 140 150 160 170 180 Time (µs) Fi gure 10. Signal backscattered by the reinforced concret e, p = 4cm (6mm) Amplitude (V) 2,5 2,0 Depth 6cm 1,5 1,0 0,5 shifting of E1 0,0 -0,5 -1,0 -1,5 -2,0 -2,5 80 90 100 110 120 130 140 150 160 170 180 Time (µs) Figure 11. Signal backscattered by the reinforced concrete, p=6cm (6mm) 24 Hicham Lotfi et al.: Ultrasonic Evaluation of the Depth and the Diameter of the Rods of Reinforced Concrete 6 Rod 6mm 5 Measured depth (cm) 4 3 2 1 2 3 4 5 6 Selected depth (cm) Figure 12. Comparison of measured and selected depth 5.3. Depth of Rod in Concrete The iron frames used in civil engineering as frames of building whereby we obtained the reinforced concrete. They are introduced into the concrete to imp rove the resistance of slabs, beams and structures of buildings. The diameter and depth of frames in concrete can ensure the rigidity and quality of the coating structures in terms of the nature of the iron used. The use of ultrasound appears as a solution for determining the depth of the rods in the concrete and to evaluate their diameters. The use of ultrasonic waves can also locate the rods iron, identify damaged areas or cracks near the surface of the concrete and evaluating the thickness of a concrete structure. Knowing the u ltrasonic velocity in concrete (paragraph 5.1.2), and fro m the temporal signal backscattered by the concrete, we determine the time between two echoes, respectively due to reflection at the interface water / concrete and concrete / rod. Accordingly, the depth of the rod in the structure is determined by the following relation: p = (Vconcrete × ∆t ) (3) 2 Table 3 shows the results obtained for the different structures of the diameter 6mm and depth chosen. The difference ∆p is the variation between the selected depth and measured depth. In average value, the d ifference is about 0,158 cm fo r the rod of diameter 6mm. For all samp les made the thickness of the layer of concrete cover is fixed and invariab le equal a 10cm. The acquisitions made with a transducer to assess the variation depth of rod coated. The figure 12 shows a comparison between the chosen depth and measured depth. We note that this curve is linear. Therefore the group velocity of ultrasonic wave is a very important for determining the depth of the rod in concrete layers. Table 3. The depth determined of the rod in concrete cover dept h select ed (p) 2cm 4cm 6cm Time of echoes (μs) Écho EWC 85,40 85,40 85,40 Écho E1 96,20 107,55 119,10 Δt (μs) 10,80 22,15 33,70 dept h mesured (cm) 1,867 3,830 5,827 ∆p (cm) 0,133 0,170 0,173 5.4. Spectral Amplitude of the Echo E2 In this paragraph, we will represent the spectral amplitude obtained from filtering the echo E2 that reflected on the circu mference of the cylinder coated with concrete. For this we isolate the echo in its time domain, and we determine the backscattered spectrum. The figure 13 illustrates the variation of the spectral amp litude in function of frequency. The interpretation of these curves allows observing the influence of the depth of the rod in structure of concrete. We also remark that the spectral amp litude is reduced with increasing of depth of the cylindrica l rods, and these spectral amp litudes are almost the same bandwidth and their maximu m correspond to the value of 0,5 MHz, who is exactly the central frequency of transducer used, which indicates that there is no dispersion. In table 4 we represent the values of the maximu m of spectral amplitude for each signal backscattered in each depth. The figure 14 shows the variations of the spectral amplitude in function of the depth of rod 6mm in concrete. Table 4. Maximum of spectral amplitude adjacent of 0,5 MHz, rod 6mm Depth p (cm) 2 4 6 Amplitude (dB) 0,103 0,094 0,084 International Journal of M aterials Engineering 2013, 3(2): 17-27 25 5.5. Diameter of the Rod Coated Table 5 shows the variations of time ∆t between echoes E1 and E2 . The diameter of the rod is determined fro m the relation (4) ddet = Vg × ∆t π (4) In figure 15 we represent the diameter determined in function of diameter of rod 6mm. We remark that the 0,12 0,10 0,08 diameters determined are near to the diameter of the rod 6mm. Therefore, this result allows us to do reverse resolution in order to determine the diameter of the rod coated with concrete. It is therefore clear that we could easily distinguish between the rods used in construction building based on the measurement of group velocity of circu mferential waves backscattered by the rods coated. Comparing the diameters determined for the two cases (coated or insulated rods) shows that the concrete cover does not have a great influence on the circumferential waves backscattered by rods coated. Depth 2cm Depth 4cm Depth 6cm Rod 6mm Amplitude (dB) 0,06 0,04 0,02 0,00 0,0 0,105 0,100 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0 Frequency (MHz) Figure 13. Backscattered spectrum, rod 6 mm coated with concrete Rod 6mm Amplitude adjacent of 0,5 MHz (dB) 0,095 0,090 0,085 0,080 2 3 4 5 6 Depth (cm) Figure 14. Variation of the spectral amplitude in function of the depth of 26 Hicham Lotfi et al.: Ultrasonic Evaluation of the Depth and the Diameter of the Rods of Reinforced Concrete diameter determined ddet (mm) 6,5 6,4 6,3 6,2 6,1 6,0 5,9 5,8 5,7 5,6 5,5 5,5 Depth (p) 2cm 4cm 6cm Depth 2cm Depth 4cm Depth 6cm 5,6 5,7 5,8 5,9 6,0 6,1 6,2 6,3 6,4 6,5 Diameter of rod d (mm) Figure 15. Diameter determined ddet in function of diameter of rod d Table 5. Diameters determined when the rods are coated Time of echoes (μs) Echo E1 Echo E2 ∆t (μs) Diameter mesured (mm) 96,20 107,55 119,10 107,62 119,22 131,23 11,42 11,67 12,13 5,882 6,010 6,247 6. Conclusions structure due to long-term exposure”. Construction and Building M aterials, Volume 24, Issue 6, 898–902 , 2010. For an analysis of reinforced concrete, we have developed [3] Venu M alagavelli and Neelakanteswara Rao Paturu "Strength a technique of non-destructive investigation to study the and Workability Characteristics of Concrete by Using signals backscattered by cement structures reinfo rced by iron Different Super Plasticizers". International Journal of rods. This method is based on the analysis of temporal signal M aterials Engineering (SAP) 2012; 2(1): 7-11. backscattered by various structures of reinforced concrete. [4] G.Quentin, M . de Billy, A.Hayman. “Comparison of Measuring the group velocity of surface waves backscattered backscattering of short pulse solid spheres and cylinders at by the circu mference of the rod allows determining its large ka”. J.Acoust.Sco.Am, 70, 870-878. 1981. diameter and the depth in the layer of concrete. [5] Faiz. B, M aze.G, Decultot. D, Aassif. E and Ezzaidi. M . The results show that the use of ultrasonic waves can “Ultrasonic characterization of the quality of an epoxy resin evaluate the concrete cover and locate rods in concrete, by polymerization”. IEEE Trans. Ultrasonics, Ferroelectrics ,and determining their d iameters (thicknesses) and depth in the frequency control, vol. 46, 188-196. 1999. concrete. [6] Pascal Rembert,” Etude des résonances acoustiques par des méthodes quasi harmoniques et impulsionnelles phase spéculaire ". Doctorat thesis, University of Le Havre, Page 207. 1991. REFERENCES [7] J.J. Faran.” Sound scattering by solid cylinders and spheres”. [1] D.G. Aggelis,E.Z. Kordatos, M . Strantza, D.V. Soulioti, et J. Acoust. Soc. Am. , 23(4) 405–418, 1951. T.E. M atikas. “ NDT approach for characterization of [8] R.D. Doolittle and H. Uberall.” Sound scattering by elastic subsurface cracks in concrete”. Construction and Building M aterials, Volume 25, Issue 7, Pages 3089–3097, 2011. cylindrical shells”. J. Acoust. Soc. Am, 39(2). 272–275, 1966 [9] M aze.G, M arical.S and Lecroq.F. “Diffusion d'une Onde [2] M .Ismail, B.M uhammad, E.Ismail.” Compressive strength loss and reinforcement degradations of reinforced concrete Acoustique Plane Par Des Sphéroïdes”, J. Phys. Colloques 51, C2-419-C2-422. 1990.

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