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Using radar direct wave for concrete condition assessment: Correlation with electrical resistivity
Journal of Applied Geophysics 62 (2007) 361 – 374 www.elsevier.com/locate/jappgeo
Using radar direct wave for concrete condition assessment: Correlation with electrical resistivity Z.M. Sbartaï a,b,⁎, S. Laurens a , J. Rhazi b , J.P. Balayssac a , G. Arliguie a a
b
Laboratory Materials and Durability of Constructions (LMDC), Department of Civil Engineering, INSA-Paul Sabatier University, Toulouse, France Research Group on Auscultation and Instrumentation, Department of Civil Engineering, Sherbrooke University, Sherbrooke, Quebec, Canada Received 2 May 2006; accepted 21 February 2007
Abstract This paper demonstrates that the direct wave of a radar ground-coupled antenna may be used for the nondestructive assessment of the physical condition of concrete, which directly influences the corrosion of the reinforcing bars in the structure. The validity of this method was evaluated by a comparison with the electrical resistivity method, which is frequently used for the evaluation of corrosion probability. Both methods were implemented in the laboratory on 72 concrete samples (25 × 25 × 8 cm3) with various degrees of saturation and chloride contamination levels. On-site investigations were also carried out on the concrete slab (1080 m2) of a car-park. The results of the laboratory tests show that the radar direct signal is strongly affected by variations in concrete moisture and chloride contamination level. The tests performed in real conditions confirm the good correlation between radar direct wave attenuation and electrical resistivity and, thus, the aptitude of the radar direct wave to detect concrete conditions leading to reinforcement corrosion. © 2007 Elsevier B.V. All rights reserved. Keywords: Concrete; Radar; Direct wave; Condition assessment; Corrosion risk; Electrical resistivity
1. Introduction Reinforcing bar corrosion is recognized as the most frequent cause of concrete structure deterioration and its early detection is the main problem facing nondestructive testing in civil engineering. Among the various influential factors, water and chloride contents of concrete are predominant in the initiation of rebar ⁎ Corresponding author. Laboratoire Matériaux et Durabilité des Constructions (LMDC), 135 avenue de Rangueil, 31077 Toulouse cedex 4, France. Tel.: +33 5 6155 6711; fax: +33 5 6155 9949. E-mail addresses: [email protected], [email protected] (Z.M. Sbartaï). 0926-9851/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jappgeo.2007.02.003
corrosion. Thus, knowledge of the variations of these physical properties in concrete structures may provide information about the risk of reinforcing bar corrosion at an early stage. The electrical resistivity of concrete is known to be sensitive to the degree of saturation (Lopez and Gonzalez, 1993) and to chloride variations in concrete (Saleem et al., 1996). Since concrete is an artificial rock, Archie's law may be used to describe the dependence of its electrical resistivity on water content, porosity and pore solution salinity: q ¼ a/m S n qw
ð1Þ
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As presented above, Archie's law is an empirical power law involving the electrical resistivity of the concrete ρ, the concrete porosity ϕ, the degree of saturation S, the electrical resistivity of the pore solution ρw and parameters (a, m, n) computed by regression on experimental data. More recently, Laurens et al. modeled the dependence of concrete electrical resistivity on porosity, saturation and chloride content by artificial neural networks (Laurens et al., 2006). Because concrete resistivity is sensitive to water and chloride contents, its measurement is among the most commonly used methods for assessing the probability of rebar corrosion in concrete structures. In addition, a relationship between the rate of reinforcement corrosion and the electrical conductivity of concrete has been demonstrated (Feliu et al., 1989). However, this method has some limits: – a good electrical contact is required between the electrodes and the concrete surface; – the measurement is affected by temperature; – the measurement is time consuming and often requires lane closures. In recent years, radar technology has become more popular for the nondestructive evaluation of concrete structures thanks to its simplicity and rapidity of implementation, which allows large surface areas to be sounded. Regarding corrosion evaluation, radar applications often consist in detecting cracks or delaminations generated by the expansion of the corrosion products (Büyÿköztürk and Rhim, 1996; Halabe et al., 1996; Huston et al., 2000). The disadvantage of this approach is that it is effective only at an advanced stage of deterioration. However, another approach is being developed based on an evaluation of the concrete condition using radar technology. This may allow early assessment of corrosion probability in concrete structures. The rationale of this approach is the sensitivity of radar waves to the changes in concrete moisture and conductivity which are the main causes of concrete deterioration due to corrosion. The electromagnetic (EM) wave propagation is governed by the concrete permittivity, which is influenced by free water (Soutsos et al., 2001) and chlorides (Al-Qadi et al., 1997; Robert, 1998). Based on this approach, changes of the wave reflected by asphalt– concrete or concrete–rebar interfaces have been used to assess corrosion probability (Narayanan et al., 1998; Laurens et al., 2000). However, on-site application of this approach requires the presence of a reflector and the knowledge of its position. This is a major problem for radar sounding because this position is difficult to determine in real conditions. In contrast, the use of the
direct wave that propagates in the first few centimeters of the concrete does not depend on the presence of any reflector and the distance between the transmitter and the receiver is fixed and known. In addition, data processing of this signal is easy and fast. Methods based on the radar direct wave appeared first in the field of geophysical prospecting for the assessment of soil water content. Several authors used multi-offset radar measurements to estimate the velocity of the direct wave and consequently the dielectric constant of the propagation medium (Greaves et al., 1996; Berktold et al., 1998; Huisman et al., 2001). Then, the relation of Topp et al. was used to calculate soil water content from the dielectric constant (Topp et al., 1980). For reinforced concrete structures, multi-offset radar measurements are difficult to process and analyze because of the presence of steel bars in the near subsurface. In this case, the propagation of the direct wave is disturbed by the reinforcements and, moreover, direct and reflected signals may overlap. Thus, in this study, a single off-set of 5.9 cm was used. In this configuration, the receiving dipole is located in the near-field of the source (Roberts and Daniels, 1997), making it impossible to use the plane wave approximation to calculate wave velocity and dielectric constant. In this context, limited information is available in the literature on the use of the near-field direct wave for concrete condition assessment. Some authors have shown that the amplitude of this wave is affected by water content (Laurens et al., 2002; Klysz et al., 2004; Sbartaï et al., 2006) and the possible presence of chlorides (Sbartaï et al., 2006). Moreover, Sbartaï et al. (2006) have reported that the direct wave attenuation correlates well with that of the reflected wave regarding the physical state of the concrete. These results suggest that the direct wave of radar ground-coupled antenna can be applied for the rapid physical characterization of reinforced concrete in real conditions, thus allowing the risk of steel bar corrosion to be assessed. This paper does not deal with the quantitative evaluation of the physical condition of concrete, since this is currently impossible on-site due to the great variety of concretes that may be encountered. For a small number of concretes (made in the laboratory), it is possible to extract the information from radar measurements using statistical models (Sbartaï, 2005). However, it is difficult to verify these results on-site since there are no reliable techniques for evaluating the actual moisture content of real concrete structures. The paper therefore focuses on the aptitude of radar technology to detect physical contrasts in concrete structures. It presents experimental results showing that the analysis of the
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radar direct wave provides useful information on the physical condition of concrete, especially regarding the risk of reinforcement corrosion. The reliability of this information was deduced from a comparison study in the laboratory and on-site between radar and electrical resistivity techniques. The laboratory study was conducted at the Laboratory Materials and Durability of Constructions (LMDC — INSA-Paul Sabatier University), and on-site investigations were performed in the city of Sherbrooke (Qc — Canada) by the Research Group on Auscultation and Instrumentation (Sherbrooke University). The study undertaken in the laboratory consisted in making systematic radar and electrical resistivity measurements on concrete samples with different degrees of saturation and chloride contamination levels. In order to verify the laboratory results in real conditions, on-site investigations were carried out on a car-park slab made of reinforced concrete. 2. Techniques 2.1. Radar The radar system used for this study transmits short pulses of electromagnetic energy, typically at frequencies ranging from 0.1 to 3 GHz using a dipolar bow-tie antenna. The EM waves propagate through the material and are reflected by dielectric interfaces, such as rebar–concrete interfaces (Fig. 1). Another antenna receives the direct and reflected waves, which are recorded as amplitude-time signals (A-scan) by the system. In Fig. 1c, Sd refers to the direct wave signal, which represents the EM energy transmitted directly to the receiving antenna, and Sr is the EM energy reflected from the rebar–concrete interface. Most common radar investigations are based on reflectometric interpretation methods. In the case of bistatic mode with a ground-coupled antenna, the transmitter–receiver pair (T–R) is moved along the concrete surface collecting an A-scan at a fixed sampling rate. The data collected are generally displayed as a time crosssection (Fig. 1b), which corresponds to the juxtaposition of the whole A-scan recorded along a linear profile. 2.2. Electrical resistivity of concrete A commonly used method for the on-site measurement of the electrical resistivity of concrete is the four point method in a Wenner configuration (Fig. 2). A lowfrequency electrical current (I) is applied through the external electrodes and generates an electrical potential field in the material. The potential difference (V ) measured between the internal electrodes allows the
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electrical resistivity to be calculated according to Ohm's theory in a semi-infinite medium (Eq. (2)): q ¼ 2pa
V I
p: V: I: a:
electrical resistivity (Ω m) electrical potential difference (V); electrical current (A); electrode spacing (m).
ð2Þ
3. Laboratory experiments 3.1. Sample casting Four concrete mixtures, referred to as M1, M2, M3, and M4, were made with water-to-cement ratios (w/c ratio) of 0.5, 0.6, 0.7, and 0.78, respectively. All the concretes were designed with Portland cement CEM I 52.5 R. The cement proportions were 400, 350, 300, and 280 kg/m 3 , respectively. Round siliceous aggregates with a nominal size of 10 mm and siliceous sand (0–4 mm) were used. Each fresh concrete mixture was placed in three molds (75 × 50 × 8 cm3) in two layers and compacted by mechanical vibration for 15 s per layer. The concrete slabs were removed from the molds 24 h after casting and stored in water for 1 month. This conservation protocol was implemented to stabilize the hydration process, which strongly affects the permittivity and conductivity of the concrete, and consequently influences the propagation of the EM waves and the electrical resistivity (Moukwa et al., 1991; Robert, 1998). After the curing period, each slab was sawn into six samples of dimensions 25× 25× 8 cm3. Thus, 18 samples were made for each concrete mixture. 3.2. Sample conditioning All the samples were partially saturated in water to degrees varying between 0 and 1. The degrees of saturation were reached by mass control during the drying process. For a homogeneous distribution of water in the samples, the latter were sealed with a plastic film and aluminum adhesive paper and placed in oven at 70 °C for about 15 days. These samples were used first to evaluate the effect of water content on radar and electrical resistivity measurements. For the study of the effect of chlorides, seven samples were arbitrarily selected from each mixture. These seven samples were oven dried and then saturated in aqueous solutions containing NaCl concentrations of 0, 10, 20, 30, 40, 50, 60 g/l. The chloride contamination of the samples was evaluated according to AFPC-AFREM standards (AFPC-AFREM, 1997).
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Fig. 1. Principle of radar measurement on reinforced concrete structure. (a) Schematic representation of reinforced concrete structure, (b) Radar time cross-section of the medium, (c) Typical A-scan.
3.3. Radar measurement The GSSI SIR-2000 radar system was used equipped with the GSSI 5100 ground-coupled antenna, with a central frequency of 1.5 GHz. Data collection was performed in bi-static mode. The transmitting and receiving antennas were located in the same box, which will be referred to as the antenna in the following discussion. The distance between the transmitter and the receiver remained constant at 5.9 cm. First, the antenna was coupled to air in order to record the air wave signal
(Fig. 3). The air wave signal (Sa) was used as a reference for the normalization of further data. Then, the antenna was coupled to the surface of the sample to record the signal (Sd) of the direct wave and the signal (Sr) of the wave reflected by the bottom of the sample. The processing of radar data focused on the signal of the direct wave and consisted in applying a 0.5 to 3 GHz band-pass filter in an attempt to reduce noise and undesirable signals. The signal amplitude was extracted and normalized, according to (Eq. (3)), with respect to the amplitude of the air wave signal. The value obtained
Fig. 2. Schematic representation of electrical resistivity measurement with Wenner probe.
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Fig. 3. Direct wave of radar ground-coupled antenna recorded in air.
is referred to as the attenuation (A) of the direct signal in concrete: Ab A ¼ 20 log ð3Þ Aa Ab Aa
is the peak-to-peak amplitude of the direct wave signal recorded in concrete. is the peak-to-peak amplitude of the air wave signal.
3.4. Resistivity measurement The electrical resistivity of the concrete was measured using the Megger system (DET5–4R) coupled to a Wenner probe. Copper electrodes were used with a spacing of 5 cm. Wet sponges were placed between the electrodes and the concrete to ensure good electrical contact. The nominal applied current was 10 mA with a frequency of 128 Hz. The Megger apparatus enabled electrical resistance to be measured from 0.01 to 19.99 kΩ. The electrical resistivity was deduced from the measured resistance and the electrode spacing using (Eq. (2)). Four measurements were taken after each radar test on each sample (Fig. 4). The resistivity of concretes having saturation degrees lower than 0.4 could not be measured because of the resistance limitation of the Megger apparatus.
for all the mixtures tested, in Fig. 5. An increase in the saturation degree, and therefore an increase in the free water volume in the pores, led to a significant increase in the attenuation of the direct signal. It is worth noting that the attenuation of the direct wave signal decreased by about 60% between saturated and oven-dried concretes. This is explained by the increase in the dielectric constant and loss factor resulting from an increase in the water content (Robert, 1998; Soutsos et al., 2001). A change in the dielectric constant leads to a modification of the radiation pattern of the antenna that governs the energy radiated laterally (Millard et al., 2002; Klysz et al., 2006). An increase in the loss factor expresses an increase in the absorption attenuation. It should also be noted that the four concretes presented the same variation trend. In addition, a small variation of the attenuation versus the w/c ratio can be seen, especially at saturation degrees higher than 0.6.
4. Laboratory results 4.1. Effect of degree of saturation The variation of the average attenuation of the direct wave versus the saturation degree of the pores is shown,
Fig. 4. Resistivity measurement on concrete sample.
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Fig. 5. Variation of Sd attenuation versus degree of saturation.
Fig. 6 shows the variation of the average electrical resistivity with respect to the degree of saturation of the tested concrete. The electrical resistivity of the concrete decreases when the degree of saturation increases, and varies between 54 and 960 Ω m. Similar results have been observed by other researchers (Lopez and Gonzalez, 1993; Saleem et al., 1996). For saturation degrees lower than about 0.6 (depending on the porosity as well as the porometric distribution), the liquid phase of the concrete becomes discontinuous, making ionic conduction more difficult. This explains the nonlinear behavior of electrical resistivity with respect to the variation of the degree of saturation of the concrete.
4.2. Effect of chloride concentration Chemical analysis performed according to (AFPCAFREM) standards indicated that the distribution of the total chloride content inside the samples tested was homogeneous (Sbartaï, 2005). This was because the samples were oven dried before the contamination. Thus, chloride intrusion throughout the material was accelerated by capillary processes. For solutions with chloride concentrations varying between 0 and 60 g/l, total chloride content varied from 0 to approximately 3.8 kg/m3 of concrete. For all the concrete mixtures, the effect of chloride concentration on the direct wave
Fig. 6. Electrical resistivity variation with respect to degree of saturation.
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Fig. 7. Variation of Sd with respect to NaCl concentration levels (saturated concrete).
attenuation in cases of saturated concrete is illustrated in Fig. 7. The attenuation increased by about 3 dB while the NaCl concentration of the solution increased from 0 to 60 g/l. This can be explained by the high mobility of free chlorides in saturated concrete, which increases both ionic conduction and interfacial polarization (Maxwell–Wagner effect). In addition, it appears that the radar direct wave is less affected by chloride contamination than by the degree of saturation. The variation of the electrical resistivity with respect to NaCl concentrations for all the mixtures is shown in Fig. 8. An increase in the chloride content led to a significant decrease in the electrical resistivity. For instance, for mixture M1, the electrical resistivity
decreased from 77 to 21 Ω m as the chloride concentration increased from 0 to 30 g/l. In addition, similar behavior can be seen for all concrete mixtures. It can also be observed that the increase of w/c ratio, which increased concrete porosity, reduced the electrical resistivity at each degree of saturation. 5. Discussion The previous results indicate that both radar direct wave and electrical resistivity of concrete are affected by changes in water and chloride contents. Indeed, an increase in the degree of saturation and/or the chloride content leads to an increase in the electrical conductivity
Fig. 8. Electrical resistivity variation with respect to NaCl concentration levels (saturated concrete).
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Fig. 9. Relationship between attenuation of Sd and apparent conductivity at degrees of saturation varying between 0.4 and 1 (NaCl concentration = 0).
of concrete and in the attenuation of the radar direct wave signal. However, it is currently difficult to distinguish the effects of water and chlorides for a single technique. The distinction could be made by fusing data recorded by different physical sensors. This is currently one of the most important research areas in the field of the nondestructive evaluation in civil engineering. For instance, the French research project SENSO is investigating the fusion of 5 nondestructive techniques to quantify durability indicators of concrete, such as water content, chloride content, porosity, carbonation depth or cracking (SENSO, 2006). Regarding the fusion of the data presented in this paper, research is currently underway and will be presented in a future paper.
As discussed in Section 1, the main objective of the laboratory study was to investigate the potential of the radar direct wave of a ground-coupled antenna to provide information allowing concrete condition to be assessed and thus the probability of reinforcing bar corrosion to be estimated. Bearing in mind that the electrical resistivity is frequently used in this context, a comparison between these two methods was established. Fig. 9 shows the correlation existing between the attenuation of the radar direct wave signal and the apparent conductivity of all samples saturated between 40 and 100%. The apparent conductivity is the inverse of the resistivity and it is used in this graph for easier comparison. A good correlation can be seen between the attenuation of the radar signal
Fig. 10. Relationship between attenuation of Sd and apparent conductivity (saturated concrete, different concentrations of NaCl).
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increase in NaCl concentration of the aqueous solutions from 0 to 60 g/l leads to an increase in the apparent conductivity of the material from 0.015 to 0.14 S/m and an increase of radar direct wave attenuation from 10 to 14 dB, approximately. In conclusion, this laboratory study has demonstrated that the electrical resistivity of concrete and the attenuation of the radar direct wave propagating in the concrete are well correlated, for both concrete saturation variations and chloride content variations. Therefore, it can be inferred that it is possible to use radar technology, and especially the transmitter–receiver direct wave, for the qualitative assessment of physical condition variations in concrete structures. Fig. 11. Area investigated (1080 m2).
6. On-site investigations 6.1. Description of the structure tested
and the apparent conductivity. This correlation is expressed by a coefficient of determination equal to 0.8. For concrete saturated between 40 and 100% and without chlorides, the attenuation of the radar direct wave increases from approximately 7 to 12 dB as the apparent conductivity increases from 0.00125 to 0.02 S/m (S = Ω− 1). Fig. 10 shows similar behavior in saturated concrete with different chloride contamination levels. This figure indicates that the two techniques are correlated at each chloride concentration level. It can be seen that the
On-site investigations were carried out on the reinforced concrete slab of a car-park located in the city of Sherbrooke (Qc—Canada). The car-park comprises a covered first floor and a non-covered upper floor. The area tested is shown in Fig. 11. The visual inspection carried out before the nondestructive tests indicated some moisture areas. Several areas of damage to the concrete surface were observed as well as cracks at the bottom part of the slab. Repaired areas (mortar or asphalt layers) were also observed and some reinforcements
Fig. 12. Statistical distributions of recorded data.
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were visible at the surface of the concrete in the northeastern part.
6.3. On-site relation between radar direct wave attenuation and electrical conductivity
6.2. Tests
The analysis of the radar data showed that, due to the thinness of the concrete cover, the signal of the direct wave could be disturbed by reflections from the rebar. To reduce this effect, only peak-to-peak amplitudes of the direct wave recorded between two reinforcements were taken into account for data interpretation (Sbartaï et al., 2006). Regarding the direct wave, for all the profiles, a strong attenuation was observed, ranging from 9 to 13 dB. Electrical resistivity values lay between 100 and 1000 Ω m, corresponding to apparent conductivity values of 0.01 and 0.001 S/m respectively. According to Polder (Polder, 2001), the probability of corrosion is moderate. In order to make the comparison between attenuation and apparent conductivity easier, both these features were normalized according to their variation ranges. This processing generated data ranging between
The measurements made in May 2004 recorded 9 radar profiles (length: 45 m). The distance between two radar profiles was fixed at 3 m. Radar antennas were connected to a survey wheel system and moved along the surface of the concrete. Along a profile, radar A-scans were recorded at each centimeter with 512 samples per scan, 32 scans per second and a speed of about 1 km/h. Data processing and normalization were carried out according to the protocol described in Section 3.3. Using the Megger system with a Wenner probe as presented in Section 3.4, electrical resistivity measurements were made after the radar tests on the same profiles with 3-meter spacing. Resistivity measurements were then converted into apparent conductivity data.
Fig. 13. Comparison of apparent conductivity map and direct wave attenuation map.
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0 and 1 for both the techniques. Fig. 12 presents the statistical distributions of direct wave attenuation and apparent conductivity normalized values. According to the normalized variation ranges, the median values of apparent conductivity and radar attenuation are about 0.32 and 0.53, respectively. The correlation between data provided by radar direct wave attenuation and apparent conductivity is analyzed on the maps presented in Figs. 13 and 14. In Fig. 13, the variation range of each feature has been broken down into 5 sub-ranges. Similar areas can be seen on both maps. Binary maps were also created and compared to assess the on-site correlation between these nondestructive techniques (Fig. 14). The thresholds necessary for the construction of the binary maps were taken to be equal to the median values identified on the statistical distributions presented in Fig. 12. Therefore, threshold
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values were fixed at 0.32 for apparent conductivity and 0.53 for radar attenuation. The correlation between the maps is thus highlighted. Very similar white areas can be observed, which are located by dotted lines. However, the repaired area, which appears clearly on the radar attenuation map, is not visible on the conductivity map. On the other hand, the area presenting steel reinforcements visible at the concrete surface appears as a very conductive area on the conductivity map, while it is not observed on the radar attenuation map. As a more objective criterion, the ratio of correlating points was also calculated for the binary maps. Each binary map is made of 144 points (16 longitudinal profiles × 9 transverse profiles), and takes values equal to 0 or 1 according to the thresholds discussed above. The correlation ratio calculated from point-to-point comparison between the binary maps was found to be 70%.
Fig. 14. Comparison of binary maps.
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Fig. 15. Comparison of profile variations of radar attenuation and apparent conductivity (Profile 1).
Fig. 15 compares radar direct wave attenuation and apparent conductivity variations along one profile. This profile was chosen because the investigated area did not present any damage or defects, thus limiting noise due to surface condition variations. On the figure, it can be seen that both the curves show the same statistical variation trends. Moreover, the coincidence of maxima and minima locations for both the curves confirms that direct wave attenuation and electrical resistivity are well correlated. It can be noted, for example, that the strong peak of the conuctivity profile located at coordinate x = 27 m corresponds exactly to a significant peak of the attenuation profile. The results presented above show that the correlation found in the laboratory between radar direct wave attenuation and apparent conductivity of concrete was also observed on-site. However, the on-site relationship between these two physical features showed higher statistical dispersion due to the surface quality of the concrete and to the presence of small-depth steel reinforcements that could affect both radar waves and electrical measurements. Nevertheless, radar direct wave attenuation and electrical resistivity/conductivity provided very similar information about the physical contrasts existing in the concrete structures. 7. Summary and conclusions In order to evaluate the effectiveness of the radar direct wave of ground-coupled antenna for the early detection of the probability of corrosion in reinforced concrete structures, a comparison study with electrical
resistivity measurements was performed in the laboratory. The tests consisted in carrying out systematic radar and electrical resistivity measurements on concrete samples with various water and chloride contents. In addition, these techniques were tested in real conditions on a large reinforced concrete slab. The conclusions of this study are as follows: 1. Laboratory tests showed that radar direct wave attenuation and electrical resistivity were strongly affected by the water and chloride contents of the concrete. 2. The electrical resistivity/conductivity and the direct wave attenuation were well correlated regarding water and/or chloride content variations, thus providing similar information about the physical condition of the concrete. 3. On-site tests carried out on a reinforced concrete slab confirmed the correlation observed in the laboratory between the two methods. 4. The aptitude of radar direct wave to detect physical conditions in concrete leading to reinforcement corrosion was demonstrated by comparison with a well known geophysical technique i.e. electrical resistivity. Currently, the radar survey provides only qualitative information but, since radar is quick to implement, it makes it possible to survey a large concrete structure rapidly, mapping physical contrasts and directing complementary investigations (cores, electrochemical techniques, etc.) towards areas that are really relevant as far as the pathological risk is concerned.
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Acknowledgments This work was supported by the CRSNG Industrial Research Chair on Nondestructive Testing of Concrete Structures, Sherbrooke University, Quebec, Canada. The authors thank S. Naar, O. Dous and D. Charbonneau. The GEOLAB enterprise is also thanked. References AFPC-AFREM « Association Française Pour la Construction-Association Française de Recherches et d'Essais sur les Matériaux », 1997. Méthodes recommandées pour la mesure des grandeurs associées à la durabilité — Détermination de la teneur en chlorures. Compte rendu des journées techniques AFPC-AFREM, Durabilité des bétons, Laboratoire Matériaux et Durabilité des Constructions. LMDC INSA-UPS, Toulouse, France, pp. 159–167 (11–12 December). Al-Qadi, I.L., Haddad, R.H., Riad, S.M., 1997. Detection of chlorides in concrete using low radio frequencies. Journal of Materials in Civil Engineering 9 (1), 29–34. Berktold, A., Wollny, K.G., Alstetter, H., 1998. Subsurface moisture determination with the ground wave of GPR. Proceedings of the Seventh International Conference on Ground-Penetrating Radar, GPR 98, Laurence, Kansas, USA, 27–30 May, pp. 675–680. Büyÿköztürk, O., Rhim, H.C., 1996. Detection of delamination in concrete using a wideband radar. In: Harbrower, P.E., Stolarski, P.J. (Eds.), Structural Materials Technology: An NDT Conference, pp. 55–60. Feliu, S., Gonzalez, J.A., Feliu Jr., S., Andrade, C., 1989. Relationship between conductivity of concrete and corrosion of reinforcing bars. British Corrosion Journal 24, 195–198. Greaves, R.J., Lesmes, D.P., Lee, J.M., Toksöz, M.N., 1996. Velocity variation and water content estimated from multi-offset, groundpenetrating radar. Geophysics 61 (3), 683–695 (May–June). Halabe, U.B., Chen, H.L., Bhandarkar, V., Sami, Z., 1996. Laboratory radar evaluation of concrete decks and pavements with and without asphalt overlay. In: Harbrower, P.E., Stolarski, P.J. (Eds.), Structural Materials Technology: An NDT Conference, pp. 373–377. Huisman, J.A., Sperl, C., Bouten, W., Verstraten, J.M., 2001. Soil water content measurements at different scales: accuracy of time domain reflectometry and ground-penetrating radar. Journal of Hydrology 245, 48–58. Huston, D., Hu, J.Q., Maser, K., Weedon, W., Adam, C., 2000. GIMA ground penetrating radar system for monitoring concrete bridge decks. Journal of Applied Geophysics 43 (2–4), 139–146. Klysz, G., Balayssac, J.-P., Laurens, S., 2004. Spectral analysis of radar surface waves for nondestructive evaluation of cover concrete. NDT & E International 37 (3), 221–227. Klysz, G., Ferrieres, X., Balayssac, J.-P., Laurens, S., 2006. Simulation of direct wave propagation by numerical FDTD for a GPR coupled antenna. NDT & E International 39 (4), 338–347. Laurens, S., Rhazi, J., Balayssac, J.-P., Arliguie, G., 2000. Assessment of corrosion in reinforced concrete by Ground Penetrating Radar and half-cell potential tests. RILEM Workshop on Life Prediction and Aging Management of Concrete Structures, Cannes, France. Laurens, S., Balayssac, J.-P., Rhazi, J., Arliguie, G., 2002. Influence of concrete moisture upon radar waveform. RILEM Materials and Structures 35 (248), 198–203. Laurens, S., Sbartaï, Z.M., Kacimi, S., Balayssac, J.P., Arliguie, G., 2006. Prediction of concrete electrical resistivity using artificial neural networks. NDE Conference on Civil Engineering, St. Louis, Mo, USA, 14–18 August 2006.
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Lopez, W., Gonzalez, J.A., 1993. Influence of the degree of pore saturation on the resistivity of concrete and the corrosion rate of steel reinforcement. Cement and Concrete Research 23 (2), 368–376. Millard, S.G., Shaari, A., Bungey, J.H., 2002. Field pattern characteristics of GPR antennas. NDT & E International 35 (7), 473–482. Moukwa, M., Brodwin, M., Christo, S., Chang, J., Shah, S.P., 1991. The influence of hydration process upon microwave properties of cement. Cement and Concrete Research 21, 863–872. Narayanan, R.M., Hudson, S.G., Kumke, C.J., 1998. Detection of rebar corrosion in bridge decks using statistical variance of radar reflected pulses. Proceedings of seventh international conference on Ground-Penetrating Radar, University of Kansas, Lawrence, Kansas, USA. Polder, R.B., 2001. Test methods for on site measurement of resistivity of concrete — a RILEM TC-154 technical recommendation. Construction and Building Materials 15 (2–3), 125–131. Robert, A., 1998. Dielectric permittivity of concrete between 50 MHz and 1 GHz and GPR measurements for building materials evaluation. Journal of Applied Geophysics 40 (1–3), 89–94. Roberts, R.L., Daniels, J.J., 1997. Modeling near-field GPR in three dimensions using the FDTD method. Geophysics 62 (4), 1114–1126 (July–August). Saleem, M., Shameem, M., Hussain, S.E., Maslehuddin, M., 1996. Effect of moisture, chloride and sulphate contamination on the electrical resistivity of Portland cement concrete. Construction and Building Materials 10 (3), 209–214. Sbartaï, 2005, Doctoral thesis, Caractérisation physique des bétons par radar: approche neuromimétique de l’inversion. Paul Sabatier University (France) and Sherbrooke University (Canada). Sbartaï, Z.M., Laurens, S., Balayssac, J.-P., Arliguie, G., Ballivy, G., 2006. Ability of the direct wave of radar ground-coupled antenna for NDT of concrete structures. NDT & E International 39 (5), 400–407. Senso, 2006. French Research Project on Data Fusion for the Nondestructive Assessment of Concrete. information available at http://www-lmdc.insa-toulouse.fr/SENSO/accueilSENSO.htm. Soutsos, M.N., Bungey, J.H., Miljard, S.G., Shaw, M.R., Patterson, A., 2001. Dielectric properties of concrete and their influence on radar testing. NDT & E International 34 (6), 419–425. Topp, G.C., Davis, J.L., Annan, A.P., 1980. Electromagnetic determination of soil water content: measurements in coaxial transmission lines. Water Resources Research 16 (3), 574–582 (June). Dr. Eng. Zoubir Mehdi Sbartaï is an associate professor and researcher at Paul Sabatier University, Toulouse III, France. He received his Ph.D at Sherbrooke University and Paul Sabatier University in 2005. His research interests include durability of concrete, nondestructive evaluation of concrete structures using electrical and electromagnetic methods, and statistical modeling.
Dr. Stéphane Laurens is an assistant professor and researcher at the National Institute of Applied Sciences (INSA), Toulouse, France. His research work deals with the nondestructive evaluation of concrete structures using radar, electrical measurements and statistical modeling.
Dr. Jamal Rhazi is an associate professor at the Department of Civil Engineering of Sherbrooke University. He is a member of the Research Center on Concrete Infrastructures (CRIB) and the American Society for Nondestructive Testing. His research topics include nondestructive testing using electrical, electromagnetic, acoustic and thermal methods.
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He is co-holder of the NSERC Industrial Research Chair on Nondestructive testing of Concrete Structures (2001–2006). This Chair is supported by eleven industrial and governmental partners.
Dr. Eng. Jean-Paul Balayssac is an assistant professor at the Technology Institute of Paul Sabatier University, Toulouse, France. His research topics include nondestructive testing of concrete structures, durability of cover concrete and durability of thin bonded cement-based overlays.
Ginette Arliguie is a professor at Paul Sabatier University, Toulouse, France. She is at present the head of the Laboratory Materials and Durability of Constructions (LMDC), and a member of RILEM committees. Her research works include service life and pathology of concrete, and the physical and chemical behavior of reinforced concrete.
Using radar direct wave for concrete condition assessment: Correlation with electrical resistivity Z.M. Sbartaï a,b,⁎, S. Laurens a , J. Rhazi b , J.P. Balayssac a , G. Arliguie a a
b
Laboratory Materials and Durability of Constructions (LMDC), Department of Civil Engineering, INSA-Paul Sabatier University, Toulouse, France Research Group on Auscultation and Instrumentation, Department of Civil Engineering, Sherbrooke University, Sherbrooke, Quebec, Canada Received 2 May 2006; accepted 21 February 2007
Abstract This paper demonstrates that the direct wave of a radar ground-coupled antenna may be used for the nondestructive assessment of the physical condition of concrete, which directly influences the corrosion of the reinforcing bars in the structure. The validity of this method was evaluated by a comparison with the electrical resistivity method, which is frequently used for the evaluation of corrosion probability. Both methods were implemented in the laboratory on 72 concrete samples (25 × 25 × 8 cm3) with various degrees of saturation and chloride contamination levels. On-site investigations were also carried out on the concrete slab (1080 m2) of a car-park. The results of the laboratory tests show that the radar direct signal is strongly affected by variations in concrete moisture and chloride contamination level. The tests performed in real conditions confirm the good correlation between radar direct wave attenuation and electrical resistivity and, thus, the aptitude of the radar direct wave to detect concrete conditions leading to reinforcement corrosion. © 2007 Elsevier B.V. All rights reserved. Keywords: Concrete; Radar; Direct wave; Condition assessment; Corrosion risk; Electrical resistivity
1. Introduction Reinforcing bar corrosion is recognized as the most frequent cause of concrete structure deterioration and its early detection is the main problem facing nondestructive testing in civil engineering. Among the various influential factors, water and chloride contents of concrete are predominant in the initiation of rebar ⁎ Corresponding author. Laboratoire Matériaux et Durabilité des Constructions (LMDC), 135 avenue de Rangueil, 31077 Toulouse cedex 4, France. Tel.: +33 5 6155 6711; fax: +33 5 6155 9949. E-mail addresses: [email protected], [email protected] (Z.M. Sbartaï). 0926-9851/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jappgeo.2007.02.003
corrosion. Thus, knowledge of the variations of these physical properties in concrete structures may provide information about the risk of reinforcing bar corrosion at an early stage. The electrical resistivity of concrete is known to be sensitive to the degree of saturation (Lopez and Gonzalez, 1993) and to chloride variations in concrete (Saleem et al., 1996). Since concrete is an artificial rock, Archie's law may be used to describe the dependence of its electrical resistivity on water content, porosity and pore solution salinity: q ¼ a/m S n qw
ð1Þ
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As presented above, Archie's law is an empirical power law involving the electrical resistivity of the concrete ρ, the concrete porosity ϕ, the degree of saturation S, the electrical resistivity of the pore solution ρw and parameters (a, m, n) computed by regression on experimental data. More recently, Laurens et al. modeled the dependence of concrete electrical resistivity on porosity, saturation and chloride content by artificial neural networks (Laurens et al., 2006). Because concrete resistivity is sensitive to water and chloride contents, its measurement is among the most commonly used methods for assessing the probability of rebar corrosion in concrete structures. In addition, a relationship between the rate of reinforcement corrosion and the electrical conductivity of concrete has been demonstrated (Feliu et al., 1989). However, this method has some limits: – a good electrical contact is required between the electrodes and the concrete surface; – the measurement is affected by temperature; – the measurement is time consuming and often requires lane closures. In recent years, radar technology has become more popular for the nondestructive evaluation of concrete structures thanks to its simplicity and rapidity of implementation, which allows large surface areas to be sounded. Regarding corrosion evaluation, radar applications often consist in detecting cracks or delaminations generated by the expansion of the corrosion products (Büyÿköztürk and Rhim, 1996; Halabe et al., 1996; Huston et al., 2000). The disadvantage of this approach is that it is effective only at an advanced stage of deterioration. However, another approach is being developed based on an evaluation of the concrete condition using radar technology. This may allow early assessment of corrosion probability in concrete structures. The rationale of this approach is the sensitivity of radar waves to the changes in concrete moisture and conductivity which are the main causes of concrete deterioration due to corrosion. The electromagnetic (EM) wave propagation is governed by the concrete permittivity, which is influenced by free water (Soutsos et al., 2001) and chlorides (Al-Qadi et al., 1997; Robert, 1998). Based on this approach, changes of the wave reflected by asphalt– concrete or concrete–rebar interfaces have been used to assess corrosion probability (Narayanan et al., 1998; Laurens et al., 2000). However, on-site application of this approach requires the presence of a reflector and the knowledge of its position. This is a major problem for radar sounding because this position is difficult to determine in real conditions. In contrast, the use of the
direct wave that propagates in the first few centimeters of the concrete does not depend on the presence of any reflector and the distance between the transmitter and the receiver is fixed and known. In addition, data processing of this signal is easy and fast. Methods based on the radar direct wave appeared first in the field of geophysical prospecting for the assessment of soil water content. Several authors used multi-offset radar measurements to estimate the velocity of the direct wave and consequently the dielectric constant of the propagation medium (Greaves et al., 1996; Berktold et al., 1998; Huisman et al., 2001). Then, the relation of Topp et al. was used to calculate soil water content from the dielectric constant (Topp et al., 1980). For reinforced concrete structures, multi-offset radar measurements are difficult to process and analyze because of the presence of steel bars in the near subsurface. In this case, the propagation of the direct wave is disturbed by the reinforcements and, moreover, direct and reflected signals may overlap. Thus, in this study, a single off-set of 5.9 cm was used. In this configuration, the receiving dipole is located in the near-field of the source (Roberts and Daniels, 1997), making it impossible to use the plane wave approximation to calculate wave velocity and dielectric constant. In this context, limited information is available in the literature on the use of the near-field direct wave for concrete condition assessment. Some authors have shown that the amplitude of this wave is affected by water content (Laurens et al., 2002; Klysz et al., 2004; Sbartaï et al., 2006) and the possible presence of chlorides (Sbartaï et al., 2006). Moreover, Sbartaï et al. (2006) have reported that the direct wave attenuation correlates well with that of the reflected wave regarding the physical state of the concrete. These results suggest that the direct wave of radar ground-coupled antenna can be applied for the rapid physical characterization of reinforced concrete in real conditions, thus allowing the risk of steel bar corrosion to be assessed. This paper does not deal with the quantitative evaluation of the physical condition of concrete, since this is currently impossible on-site due to the great variety of concretes that may be encountered. For a small number of concretes (made in the laboratory), it is possible to extract the information from radar measurements using statistical models (Sbartaï, 2005). However, it is difficult to verify these results on-site since there are no reliable techniques for evaluating the actual moisture content of real concrete structures. The paper therefore focuses on the aptitude of radar technology to detect physical contrasts in concrete structures. It presents experimental results showing that the analysis of the
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radar direct wave provides useful information on the physical condition of concrete, especially regarding the risk of reinforcement corrosion. The reliability of this information was deduced from a comparison study in the laboratory and on-site between radar and electrical resistivity techniques. The laboratory study was conducted at the Laboratory Materials and Durability of Constructions (LMDC — INSA-Paul Sabatier University), and on-site investigations were performed in the city of Sherbrooke (Qc — Canada) by the Research Group on Auscultation and Instrumentation (Sherbrooke University). The study undertaken in the laboratory consisted in making systematic radar and electrical resistivity measurements on concrete samples with different degrees of saturation and chloride contamination levels. In order to verify the laboratory results in real conditions, on-site investigations were carried out on a car-park slab made of reinforced concrete. 2. Techniques 2.1. Radar The radar system used for this study transmits short pulses of electromagnetic energy, typically at frequencies ranging from 0.1 to 3 GHz using a dipolar bow-tie antenna. The EM waves propagate through the material and are reflected by dielectric interfaces, such as rebar–concrete interfaces (Fig. 1). Another antenna receives the direct and reflected waves, which are recorded as amplitude-time signals (A-scan) by the system. In Fig. 1c, Sd refers to the direct wave signal, which represents the EM energy transmitted directly to the receiving antenna, and Sr is the EM energy reflected from the rebar–concrete interface. Most common radar investigations are based on reflectometric interpretation methods. In the case of bistatic mode with a ground-coupled antenna, the transmitter–receiver pair (T–R) is moved along the concrete surface collecting an A-scan at a fixed sampling rate. The data collected are generally displayed as a time crosssection (Fig. 1b), which corresponds to the juxtaposition of the whole A-scan recorded along a linear profile. 2.2. Electrical resistivity of concrete A commonly used method for the on-site measurement of the electrical resistivity of concrete is the four point method in a Wenner configuration (Fig. 2). A lowfrequency electrical current (I) is applied through the external electrodes and generates an electrical potential field in the material. The potential difference (V ) measured between the internal electrodes allows the
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electrical resistivity to be calculated according to Ohm's theory in a semi-infinite medium (Eq. (2)): q ¼ 2pa
V I
p: V: I: a:
electrical resistivity (Ω m) electrical potential difference (V); electrical current (A); electrode spacing (m).
ð2Þ
3. Laboratory experiments 3.1. Sample casting Four concrete mixtures, referred to as M1, M2, M3, and M4, were made with water-to-cement ratios (w/c ratio) of 0.5, 0.6, 0.7, and 0.78, respectively. All the concretes were designed with Portland cement CEM I 52.5 R. The cement proportions were 400, 350, 300, and 280 kg/m 3 , respectively. Round siliceous aggregates with a nominal size of 10 mm and siliceous sand (0–4 mm) were used. Each fresh concrete mixture was placed in three molds (75 × 50 × 8 cm3) in two layers and compacted by mechanical vibration for 15 s per layer. The concrete slabs were removed from the molds 24 h after casting and stored in water for 1 month. This conservation protocol was implemented to stabilize the hydration process, which strongly affects the permittivity and conductivity of the concrete, and consequently influences the propagation of the EM waves and the electrical resistivity (Moukwa et al., 1991; Robert, 1998). After the curing period, each slab was sawn into six samples of dimensions 25× 25× 8 cm3. Thus, 18 samples were made for each concrete mixture. 3.2. Sample conditioning All the samples were partially saturated in water to degrees varying between 0 and 1. The degrees of saturation were reached by mass control during the drying process. For a homogeneous distribution of water in the samples, the latter were sealed with a plastic film and aluminum adhesive paper and placed in oven at 70 °C for about 15 days. These samples were used first to evaluate the effect of water content on radar and electrical resistivity measurements. For the study of the effect of chlorides, seven samples were arbitrarily selected from each mixture. These seven samples were oven dried and then saturated in aqueous solutions containing NaCl concentrations of 0, 10, 20, 30, 40, 50, 60 g/l. The chloride contamination of the samples was evaluated according to AFPC-AFREM standards (AFPC-AFREM, 1997).
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Fig. 1. Principle of radar measurement on reinforced concrete structure. (a) Schematic representation of reinforced concrete structure, (b) Radar time cross-section of the medium, (c) Typical A-scan.
3.3. Radar measurement The GSSI SIR-2000 radar system was used equipped with the GSSI 5100 ground-coupled antenna, with a central frequency of 1.5 GHz. Data collection was performed in bi-static mode. The transmitting and receiving antennas were located in the same box, which will be referred to as the antenna in the following discussion. The distance between the transmitter and the receiver remained constant at 5.9 cm. First, the antenna was coupled to air in order to record the air wave signal
(Fig. 3). The air wave signal (Sa) was used as a reference for the normalization of further data. Then, the antenna was coupled to the surface of the sample to record the signal (Sd) of the direct wave and the signal (Sr) of the wave reflected by the bottom of the sample. The processing of radar data focused on the signal of the direct wave and consisted in applying a 0.5 to 3 GHz band-pass filter in an attempt to reduce noise and undesirable signals. The signal amplitude was extracted and normalized, according to (Eq. (3)), with respect to the amplitude of the air wave signal. The value obtained
Fig. 2. Schematic representation of electrical resistivity measurement with Wenner probe.
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Fig. 3. Direct wave of radar ground-coupled antenna recorded in air.
is referred to as the attenuation (A) of the direct signal in concrete: Ab A ¼ 20 log ð3Þ Aa Ab Aa
is the peak-to-peak amplitude of the direct wave signal recorded in concrete. is the peak-to-peak amplitude of the air wave signal.
3.4. Resistivity measurement The electrical resistivity of the concrete was measured using the Megger system (DET5–4R) coupled to a Wenner probe. Copper electrodes were used with a spacing of 5 cm. Wet sponges were placed between the electrodes and the concrete to ensure good electrical contact. The nominal applied current was 10 mA with a frequency of 128 Hz. The Megger apparatus enabled electrical resistance to be measured from 0.01 to 19.99 kΩ. The electrical resistivity was deduced from the measured resistance and the electrode spacing using (Eq. (2)). Four measurements were taken after each radar test on each sample (Fig. 4). The resistivity of concretes having saturation degrees lower than 0.4 could not be measured because of the resistance limitation of the Megger apparatus.
for all the mixtures tested, in Fig. 5. An increase in the saturation degree, and therefore an increase in the free water volume in the pores, led to a significant increase in the attenuation of the direct signal. It is worth noting that the attenuation of the direct wave signal decreased by about 60% between saturated and oven-dried concretes. This is explained by the increase in the dielectric constant and loss factor resulting from an increase in the water content (Robert, 1998; Soutsos et al., 2001). A change in the dielectric constant leads to a modification of the radiation pattern of the antenna that governs the energy radiated laterally (Millard et al., 2002; Klysz et al., 2006). An increase in the loss factor expresses an increase in the absorption attenuation. It should also be noted that the four concretes presented the same variation trend. In addition, a small variation of the attenuation versus the w/c ratio can be seen, especially at saturation degrees higher than 0.6.
4. Laboratory results 4.1. Effect of degree of saturation The variation of the average attenuation of the direct wave versus the saturation degree of the pores is shown,
Fig. 4. Resistivity measurement on concrete sample.
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Fig. 5. Variation of Sd attenuation versus degree of saturation.
Fig. 6 shows the variation of the average electrical resistivity with respect to the degree of saturation of the tested concrete. The electrical resistivity of the concrete decreases when the degree of saturation increases, and varies between 54 and 960 Ω m. Similar results have been observed by other researchers (Lopez and Gonzalez, 1993; Saleem et al., 1996). For saturation degrees lower than about 0.6 (depending on the porosity as well as the porometric distribution), the liquid phase of the concrete becomes discontinuous, making ionic conduction more difficult. This explains the nonlinear behavior of electrical resistivity with respect to the variation of the degree of saturation of the concrete.
4.2. Effect of chloride concentration Chemical analysis performed according to (AFPCAFREM) standards indicated that the distribution of the total chloride content inside the samples tested was homogeneous (Sbartaï, 2005). This was because the samples were oven dried before the contamination. Thus, chloride intrusion throughout the material was accelerated by capillary processes. For solutions with chloride concentrations varying between 0 and 60 g/l, total chloride content varied from 0 to approximately 3.8 kg/m3 of concrete. For all the concrete mixtures, the effect of chloride concentration on the direct wave
Fig. 6. Electrical resistivity variation with respect to degree of saturation.
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Fig. 7. Variation of Sd with respect to NaCl concentration levels (saturated concrete).
attenuation in cases of saturated concrete is illustrated in Fig. 7. The attenuation increased by about 3 dB while the NaCl concentration of the solution increased from 0 to 60 g/l. This can be explained by the high mobility of free chlorides in saturated concrete, which increases both ionic conduction and interfacial polarization (Maxwell–Wagner effect). In addition, it appears that the radar direct wave is less affected by chloride contamination than by the degree of saturation. The variation of the electrical resistivity with respect to NaCl concentrations for all the mixtures is shown in Fig. 8. An increase in the chloride content led to a significant decrease in the electrical resistivity. For instance, for mixture M1, the electrical resistivity
decreased from 77 to 21 Ω m as the chloride concentration increased from 0 to 30 g/l. In addition, similar behavior can be seen for all concrete mixtures. It can also be observed that the increase of w/c ratio, which increased concrete porosity, reduced the electrical resistivity at each degree of saturation. 5. Discussion The previous results indicate that both radar direct wave and electrical resistivity of concrete are affected by changes in water and chloride contents. Indeed, an increase in the degree of saturation and/or the chloride content leads to an increase in the electrical conductivity
Fig. 8. Electrical resistivity variation with respect to NaCl concentration levels (saturated concrete).
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Fig. 9. Relationship between attenuation of Sd and apparent conductivity at degrees of saturation varying between 0.4 and 1 (NaCl concentration = 0).
of concrete and in the attenuation of the radar direct wave signal. However, it is currently difficult to distinguish the effects of water and chlorides for a single technique. The distinction could be made by fusing data recorded by different physical sensors. This is currently one of the most important research areas in the field of the nondestructive evaluation in civil engineering. For instance, the French research project SENSO is investigating the fusion of 5 nondestructive techniques to quantify durability indicators of concrete, such as water content, chloride content, porosity, carbonation depth or cracking (SENSO, 2006). Regarding the fusion of the data presented in this paper, research is currently underway and will be presented in a future paper.
As discussed in Section 1, the main objective of the laboratory study was to investigate the potential of the radar direct wave of a ground-coupled antenna to provide information allowing concrete condition to be assessed and thus the probability of reinforcing bar corrosion to be estimated. Bearing in mind that the electrical resistivity is frequently used in this context, a comparison between these two methods was established. Fig. 9 shows the correlation existing between the attenuation of the radar direct wave signal and the apparent conductivity of all samples saturated between 40 and 100%. The apparent conductivity is the inverse of the resistivity and it is used in this graph for easier comparison. A good correlation can be seen between the attenuation of the radar signal
Fig. 10. Relationship between attenuation of Sd and apparent conductivity (saturated concrete, different concentrations of NaCl).
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increase in NaCl concentration of the aqueous solutions from 0 to 60 g/l leads to an increase in the apparent conductivity of the material from 0.015 to 0.14 S/m and an increase of radar direct wave attenuation from 10 to 14 dB, approximately. In conclusion, this laboratory study has demonstrated that the electrical resistivity of concrete and the attenuation of the radar direct wave propagating in the concrete are well correlated, for both concrete saturation variations and chloride content variations. Therefore, it can be inferred that it is possible to use radar technology, and especially the transmitter–receiver direct wave, for the qualitative assessment of physical condition variations in concrete structures. Fig. 11. Area investigated (1080 m2).
6. On-site investigations 6.1. Description of the structure tested
and the apparent conductivity. This correlation is expressed by a coefficient of determination equal to 0.8. For concrete saturated between 40 and 100% and without chlorides, the attenuation of the radar direct wave increases from approximately 7 to 12 dB as the apparent conductivity increases from 0.00125 to 0.02 S/m (S = Ω− 1). Fig. 10 shows similar behavior in saturated concrete with different chloride contamination levels. This figure indicates that the two techniques are correlated at each chloride concentration level. It can be seen that the
On-site investigations were carried out on the reinforced concrete slab of a car-park located in the city of Sherbrooke (Qc—Canada). The car-park comprises a covered first floor and a non-covered upper floor. The area tested is shown in Fig. 11. The visual inspection carried out before the nondestructive tests indicated some moisture areas. Several areas of damage to the concrete surface were observed as well as cracks at the bottom part of the slab. Repaired areas (mortar or asphalt layers) were also observed and some reinforcements
Fig. 12. Statistical distributions of recorded data.
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were visible at the surface of the concrete in the northeastern part.
6.3. On-site relation between radar direct wave attenuation and electrical conductivity
6.2. Tests
The analysis of the radar data showed that, due to the thinness of the concrete cover, the signal of the direct wave could be disturbed by reflections from the rebar. To reduce this effect, only peak-to-peak amplitudes of the direct wave recorded between two reinforcements were taken into account for data interpretation (Sbartaï et al., 2006). Regarding the direct wave, for all the profiles, a strong attenuation was observed, ranging from 9 to 13 dB. Electrical resistivity values lay between 100 and 1000 Ω m, corresponding to apparent conductivity values of 0.01 and 0.001 S/m respectively. According to Polder (Polder, 2001), the probability of corrosion is moderate. In order to make the comparison between attenuation and apparent conductivity easier, both these features were normalized according to their variation ranges. This processing generated data ranging between
The measurements made in May 2004 recorded 9 radar profiles (length: 45 m). The distance between two radar profiles was fixed at 3 m. Radar antennas were connected to a survey wheel system and moved along the surface of the concrete. Along a profile, radar A-scans were recorded at each centimeter with 512 samples per scan, 32 scans per second and a speed of about 1 km/h. Data processing and normalization were carried out according to the protocol described in Section 3.3. Using the Megger system with a Wenner probe as presented in Section 3.4, electrical resistivity measurements were made after the radar tests on the same profiles with 3-meter spacing. Resistivity measurements were then converted into apparent conductivity data.
Fig. 13. Comparison of apparent conductivity map and direct wave attenuation map.
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0 and 1 for both the techniques. Fig. 12 presents the statistical distributions of direct wave attenuation and apparent conductivity normalized values. According to the normalized variation ranges, the median values of apparent conductivity and radar attenuation are about 0.32 and 0.53, respectively. The correlation between data provided by radar direct wave attenuation and apparent conductivity is analyzed on the maps presented in Figs. 13 and 14. In Fig. 13, the variation range of each feature has been broken down into 5 sub-ranges. Similar areas can be seen on both maps. Binary maps were also created and compared to assess the on-site correlation between these nondestructive techniques (Fig. 14). The thresholds necessary for the construction of the binary maps were taken to be equal to the median values identified on the statistical distributions presented in Fig. 12. Therefore, threshold
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values were fixed at 0.32 for apparent conductivity and 0.53 for radar attenuation. The correlation between the maps is thus highlighted. Very similar white areas can be observed, which are located by dotted lines. However, the repaired area, which appears clearly on the radar attenuation map, is not visible on the conductivity map. On the other hand, the area presenting steel reinforcements visible at the concrete surface appears as a very conductive area on the conductivity map, while it is not observed on the radar attenuation map. As a more objective criterion, the ratio of correlating points was also calculated for the binary maps. Each binary map is made of 144 points (16 longitudinal profiles × 9 transverse profiles), and takes values equal to 0 or 1 according to the thresholds discussed above. The correlation ratio calculated from point-to-point comparison between the binary maps was found to be 70%.
Fig. 14. Comparison of binary maps.
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Fig. 15. Comparison of profile variations of radar attenuation and apparent conductivity (Profile 1).
Fig. 15 compares radar direct wave attenuation and apparent conductivity variations along one profile. This profile was chosen because the investigated area did not present any damage or defects, thus limiting noise due to surface condition variations. On the figure, it can be seen that both the curves show the same statistical variation trends. Moreover, the coincidence of maxima and minima locations for both the curves confirms that direct wave attenuation and electrical resistivity are well correlated. It can be noted, for example, that the strong peak of the conuctivity profile located at coordinate x = 27 m corresponds exactly to a significant peak of the attenuation profile. The results presented above show that the correlation found in the laboratory between radar direct wave attenuation and apparent conductivity of concrete was also observed on-site. However, the on-site relationship between these two physical features showed higher statistical dispersion due to the surface quality of the concrete and to the presence of small-depth steel reinforcements that could affect both radar waves and electrical measurements. Nevertheless, radar direct wave attenuation and electrical resistivity/conductivity provided very similar information about the physical contrasts existing in the concrete structures. 7. Summary and conclusions In order to evaluate the effectiveness of the radar direct wave of ground-coupled antenna for the early detection of the probability of corrosion in reinforced concrete structures, a comparison study with electrical
resistivity measurements was performed in the laboratory. The tests consisted in carrying out systematic radar and electrical resistivity measurements on concrete samples with various water and chloride contents. In addition, these techniques were tested in real conditions on a large reinforced concrete slab. The conclusions of this study are as follows: 1. Laboratory tests showed that radar direct wave attenuation and electrical resistivity were strongly affected by the water and chloride contents of the concrete. 2. The electrical resistivity/conductivity and the direct wave attenuation were well correlated regarding water and/or chloride content variations, thus providing similar information about the physical condition of the concrete. 3. On-site tests carried out on a reinforced concrete slab confirmed the correlation observed in the laboratory between the two methods. 4. The aptitude of radar direct wave to detect physical conditions in concrete leading to reinforcement corrosion was demonstrated by comparison with a well known geophysical technique i.e. electrical resistivity. Currently, the radar survey provides only qualitative information but, since radar is quick to implement, it makes it possible to survey a large concrete structure rapidly, mapping physical contrasts and directing complementary investigations (cores, electrochemical techniques, etc.) towards areas that are really relevant as far as the pathological risk is concerned.
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Acknowledgments This work was supported by the CRSNG Industrial Research Chair on Nondestructive Testing of Concrete Structures, Sherbrooke University, Quebec, Canada. The authors thank S. Naar, O. Dous and D. Charbonneau. The GEOLAB enterprise is also thanked. References AFPC-AFREM « Association Française Pour la Construction-Association Française de Recherches et d'Essais sur les Matériaux », 1997. Méthodes recommandées pour la mesure des grandeurs associées à la durabilité — Détermination de la teneur en chlorures. Compte rendu des journées techniques AFPC-AFREM, Durabilité des bétons, Laboratoire Matériaux et Durabilité des Constructions. LMDC INSA-UPS, Toulouse, France, pp. 159–167 (11–12 December). Al-Qadi, I.L., Haddad, R.H., Riad, S.M., 1997. Detection of chlorides in concrete using low radio frequencies. Journal of Materials in Civil Engineering 9 (1), 29–34. Berktold, A., Wollny, K.G., Alstetter, H., 1998. Subsurface moisture determination with the ground wave of GPR. Proceedings of the Seventh International Conference on Ground-Penetrating Radar, GPR 98, Laurence, Kansas, USA, 27–30 May, pp. 675–680. Büyÿköztürk, O., Rhim, H.C., 1996. Detection of delamination in concrete using a wideband radar. In: Harbrower, P.E., Stolarski, P.J. (Eds.), Structural Materials Technology: An NDT Conference, pp. 55–60. Feliu, S., Gonzalez, J.A., Feliu Jr., S., Andrade, C., 1989. Relationship between conductivity of concrete and corrosion of reinforcing bars. British Corrosion Journal 24, 195–198. Greaves, R.J., Lesmes, D.P., Lee, J.M., Toksöz, M.N., 1996. Velocity variation and water content estimated from multi-offset, groundpenetrating radar. Geophysics 61 (3), 683–695 (May–June). Halabe, U.B., Chen, H.L., Bhandarkar, V., Sami, Z., 1996. Laboratory radar evaluation of concrete decks and pavements with and without asphalt overlay. In: Harbrower, P.E., Stolarski, P.J. (Eds.), Structural Materials Technology: An NDT Conference, pp. 373–377. Huisman, J.A., Sperl, C., Bouten, W., Verstraten, J.M., 2001. Soil water content measurements at different scales: accuracy of time domain reflectometry and ground-penetrating radar. Journal of Hydrology 245, 48–58. Huston, D., Hu, J.Q., Maser, K., Weedon, W., Adam, C., 2000. GIMA ground penetrating radar system for monitoring concrete bridge decks. Journal of Applied Geophysics 43 (2–4), 139–146. Klysz, G., Balayssac, J.-P., Laurens, S., 2004. Spectral analysis of radar surface waves for nondestructive evaluation of cover concrete. NDT & E International 37 (3), 221–227. Klysz, G., Ferrieres, X., Balayssac, J.-P., Laurens, S., 2006. Simulation of direct wave propagation by numerical FDTD for a GPR coupled antenna. NDT & E International 39 (4), 338–347. Laurens, S., Rhazi, J., Balayssac, J.-P., Arliguie, G., 2000. Assessment of corrosion in reinforced concrete by Ground Penetrating Radar and half-cell potential tests. RILEM Workshop on Life Prediction and Aging Management of Concrete Structures, Cannes, France. Laurens, S., Balayssac, J.-P., Rhazi, J., Arliguie, G., 2002. Influence of concrete moisture upon radar waveform. RILEM Materials and Structures 35 (248), 198–203. Laurens, S., Sbartaï, Z.M., Kacimi, S., Balayssac, J.P., Arliguie, G., 2006. Prediction of concrete electrical resistivity using artificial neural networks. NDE Conference on Civil Engineering, St. Louis, Mo, USA, 14–18 August 2006.
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Lopez, W., Gonzalez, J.A., 1993. Influence of the degree of pore saturation on the resistivity of concrete and the corrosion rate of steel reinforcement. Cement and Concrete Research 23 (2), 368–376. Millard, S.G., Shaari, A., Bungey, J.H., 2002. Field pattern characteristics of GPR antennas. NDT & E International 35 (7), 473–482. Moukwa, M., Brodwin, M., Christo, S., Chang, J., Shah, S.P., 1991. The influence of hydration process upon microwave properties of cement. Cement and Concrete Research 21, 863–872. Narayanan, R.M., Hudson, S.G., Kumke, C.J., 1998. Detection of rebar corrosion in bridge decks using statistical variance of radar reflected pulses. Proceedings of seventh international conference on Ground-Penetrating Radar, University of Kansas, Lawrence, Kansas, USA. Polder, R.B., 2001. Test methods for on site measurement of resistivity of concrete — a RILEM TC-154 technical recommendation. Construction and Building Materials 15 (2–3), 125–131. Robert, A., 1998. Dielectric permittivity of concrete between 50 MHz and 1 GHz and GPR measurements for building materials evaluation. Journal of Applied Geophysics 40 (1–3), 89–94. Roberts, R.L., Daniels, J.J., 1997. Modeling near-field GPR in three dimensions using the FDTD method. Geophysics 62 (4), 1114–1126 (July–August). Saleem, M., Shameem, M., Hussain, S.E., Maslehuddin, M., 1996. Effect of moisture, chloride and sulphate contamination on the electrical resistivity of Portland cement concrete. Construction and Building Materials 10 (3), 209–214. Sbartaï, 2005, Doctoral thesis, Caractérisation physique des bétons par radar: approche neuromimétique de l’inversion. Paul Sabatier University (France) and Sherbrooke University (Canada). Sbartaï, Z.M., Laurens, S., Balayssac, J.-P., Arliguie, G., Ballivy, G., 2006. Ability of the direct wave of radar ground-coupled antenna for NDT of concrete structures. NDT & E International 39 (5), 400–407. Senso, 2006. French Research Project on Data Fusion for the Nondestructive Assessment of Concrete. information available at http://www-lmdc.insa-toulouse.fr/SENSO/accueilSENSO.htm. Soutsos, M.N., Bungey, J.H., Miljard, S.G., Shaw, M.R., Patterson, A., 2001. Dielectric properties of concrete and their influence on radar testing. NDT & E International 34 (6), 419–425. Topp, G.C., Davis, J.L., Annan, A.P., 1980. Electromagnetic determination of soil water content: measurements in coaxial transmission lines. Water Resources Research 16 (3), 574–582 (June). Dr. Eng. Zoubir Mehdi Sbartaï is an associate professor and researcher at Paul Sabatier University, Toulouse III, France. He received his Ph.D at Sherbrooke University and Paul Sabatier University in 2005. His research interests include durability of concrete, nondestructive evaluation of concrete structures using electrical and electromagnetic methods, and statistical modeling.
Dr. Stéphane Laurens is an assistant professor and researcher at the National Institute of Applied Sciences (INSA), Toulouse, France. His research work deals with the nondestructive evaluation of concrete structures using radar, electrical measurements and statistical modeling.
Dr. Jamal Rhazi is an associate professor at the Department of Civil Engineering of Sherbrooke University. He is a member of the Research Center on Concrete Infrastructures (CRIB) and the American Society for Nondestructive Testing. His research topics include nondestructive testing using electrical, electromagnetic, acoustic and thermal methods.
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He is co-holder of the NSERC Industrial Research Chair on Nondestructive testing of Concrete Structures (2001–2006). This Chair is supported by eleven industrial and governmental partners.
Dr. Eng. Jean-Paul Balayssac is an assistant professor at the Technology Institute of Paul Sabatier University, Toulouse, France. His research topics include nondestructive testing of concrete structures, durability of cover concrete and durability of thin bonded cement-based overlays.
Ginette Arliguie is a professor at Paul Sabatier University, Toulouse, France. She is at present the head of the Laboratory Materials and Durability of Constructions (LMDC), and a member of RILEM committees. Her research works include service life and pathology of concrete, and the physical and chemical behavior of reinforced concrete.