Recommendations for guidelines for the use of GPR in bridge deck surveys

Transcription

1 Recommendations for guidelines for the use of GPR in bridge deck surveys Mara Nord Project in cooperation with: European Commission, Finnish Transport Agency, Swedish Transport Administration, Norwegian Road Administration, ELY center, Lapin Liitto, North Calotte Council, Rovaniemi University of Applied Sciences, Oulu University of Applied Sciences, Road Scanners, Road Consulting, Carement, Ramböll Sweden, Malå Geoscience, Swedish National Road and Transport Research Institute, SINTEF, 3D Radar, NCC Roads

2 Recommendations for guidelines for the use of GPR in bridge deck surveys Contents 1. Introduction Ground penetrating radar (GPR) technology General Electrical Properties aaffecting GPR wave propagation in bridge decks Dielectric value Electrical conductivity Impulse radar principles Impulse radar Stepped frequency radar principles Principles of GPR image Reflection and polarity Depth penetration, resolution and interface depth Ground Penetrating Radar in bridge deck surveys GPR equipment Survey planning and performance Measurement timing, planning and information Bridge GPR measurement implementation Special measurement implementation for a bridge 3D GPR survey Measurement data processing and interpretation Preprocessing Processing Interpretation and analysis Reference data Reporting and reporting standards References... 18

3 PREFACE Ground Penetrating Radar (GPR) is a non-destructive ground survey method that can be used in assessing roads, railways, bridges, airports, tunnels and environmental objects. Its main advantage is the continuous profile it provides. The GPR applications related to bridge surveys can be divided roughly into a) bridge foundations surveys, related to problems such as site investigations and detecting scours around bridge piers, b) bridge decks and bridge beams surveys and c) other surveys such as surveys on approach slabs, bridge abutments etc. These guidelines will focus on the use of GPR in concrete deck surveys, which is the main application of GPR on bridges in Nordic countries. The Nordic countries have reached good quality level of skills and accumulated knowhow of GPR applications on roads over the last 15 years. However Finland, Sweden and Norway have slightly different practices in the use of GPR and that is why there is a need for common procedures as more and more companies are operating across the borders. The level of knowledge, awareness and experience regarding the use of GPR in the Road Administrations vary in all three countries and there is a need to share the knowledge and develop procedures to ensure better quality of GPR services. To respond to these recognized needs, Mara Nord, an international cooperation project financed by Interreg IV A Nord, has been initiated among Finland, Sweden and Norway. In this project one goal was to produce common guidelines that can be used for reference in procurement processes in all three countries. Johan Ullberg from Swedish Transport Administration has been in charge of the guidelines. Mara Nord project lead partner has been Rovaniemi University of Applied Sciences and experts participating have been Anita Narbro and Janne Poikajärvi. Other key experts have been Katri Eskola from Finnish Transport Agency, Kalevi Luiro from ELY-centre of Lapland, Finland, Per Otto Aursand and Leif Bakløkk from Norwegian Public Roads Administration, Rauno Turunen from Oulu University of Applied Sciences, Finland. This recommendation for the guidelines has been written by Timo Saarenketo, Pekka Maijala and Anja Leppälä from Roadscanners Oy, Finland. In Rovaniemi May 16th, 2011 Timo Saarenketo Pekka Maijala Anja Leppälä Roadscanners Oy Roadscanners Oy Roadscanners Oy

4 1. Introduction A bridge deck can be examined in many ways, the following issues can be addressed using GPR: Pavement thickness and thickness of single pavement layer Pavement damage (stripping, debonding etc.) Protective concrete thickness and damages in protective concrete (if there is one) Reinforcement cover depth and spacing between reinforcement bars Position of tendons or tendon ducts Damages in concrete deck Moisture in concrete There are several possible reasons for damages in a bridge deck such as corrosion of the reinforcement, freeze-thaw cycles, traffic loads, poor drainage, damage due to defective planning or construction mistakes and insufficient maintenance. However a non-destructive survey of a bridge deck and mapping of the damages, using only a visual inspection of the surface, is difficult because for several reasons. These include asphalt layer, possible gravel layer or a protecting concrete layer on top of the concrete deck slab. Visual observation does not necessarily give a true picture of the condition of the bridge, because damages usually develop inside the structures before they can be seen on the bridge surfaces. Therefore it is necessary to search and develop new methods for bridge condition inspection, so that possible deterioration could be detected already in the early phase. This recommendation for a guideline has been made due to new procurement policies in Nordic road Administrations which means that bridge surveys will be ordered through open competition. That is why it is essential that current practice is described precisely to make it possible for new entrepreneurs to learn the techniques and quality standards for ground penetrating radar surveys and what the basic quality level for the results is. There is also a need to specify separately the procedures that should be followed in survey projects made in two dimensional (2D) and three dimensional (3D) GPR projects. This recommendation for guidelines should be used when doing data collection with the 2D and 3D GPR systems on a bridge. A description of 2D and 3D antennas suitable for bridge GPR surveys is given as well. In this publication, there is also an exact description of how ground penetrating radar data collection should be done, how the collected data should be processed and interpreted and what the outcome should be. Pertaining to the parts and methods not described in this publication, the other national publications and guidelines related to bridge projects must be followed.

5 2. Ground penetrating radar (GPR) technology 2.1 General Ground penetrating radar method is based on the use of radiofrequency electromagnetic (EM-) waves. The frequency range used is MHz. Inside of this frequency range, it is said that EM- waves can propagate in a low electrical conductivity medium. Physical parameters affecting the wave are the medium s conductivity, dielectricity and magnetic susceptibility. In Nordic countries, magnetic susceptibility does not have a significant effect on ground penetrating radar signal s behavior. On concrete bridge decks a very dense steel reinforcement sometimes causes problems because the GPR signal does not penetrate steel. 2.2 Electrical Properties affecting GPR wave propagation in bridge decks Dielectric value Dielectric value describes substance s ability to charge or polarize from the influence of an electric field. After the electric field s effect ends, the substance returns to its initial state. If the material s structure is such that its initial state does not return completely, its polarization is partly lossed. In such cases, dielectric value can be considered as a complex quantity, where the real part describes reversible polarization and loss is in the imaginary part. The most important element in molecular polarization in bridges decks is the water molecule. The extent of dielectric value depends most on the amount of free water. Therefore an increase in water content increases the dielectric value of the material. A concrete deck is a structure made of a sand and coarse aggregate mixture, cement and water. When these components are mixed reactions occur between the cement and the water, referred to as cement paste, and this compound binds the aggregates together in the form of a hardened concrete. There is a dependence of dielectric properties and moisture content in hardened concrete. The GPR signal reflection technique has been used to monitor the hardening of concrete (Morey and Kovacs 1977). However GPR signal penetration is not good in newly casted concrete due to the high imaginary part of fresh concrete and Lau et al. (1992) reported six times higher values of the imaginary value of Portland Cement Concrete (PCC) compared with asphalt concrete. Normally the dielectric value of concrete bridge decks is about Electrical conductivity A medium s electrical conductivity describes the ability of free charges to move in the medium. External electric fields move charges from place to place. The more free charges, ions and electrons there are, the higher the conductivity of a material and ground penetrating radar signal attenuation. In Nordic countries the electrical conductivity of bridges can sometimes be high due to extensive use of deicing salts and this can also cause high attenuation of a GPR signal.

6 2.3 Impulse radar principles Impulse radar Impulse radar is the most commonly used type of ground penetrating radar. The working principle is described hereafter. A pulse, generated in a transmitter antenna, is sent into the medium. The length of the pulse is from under a nanosecond up to tens of nanoseconds, depending on the frequency. When propagating in bridge structures a part of the pulse energy is reflected from surfaces with different electrical properties; some of them propagate through the interface and are reflected back from the subsequent interfaces. How the signal attenuates is a result of geometric attenuation, signal scattering, reflections and thermal losses. The GPR system records the two way travel time and amplitudes of signal reflections as a function of travel time are presented. When measurements are made rapidly over sequential survey points, it can be compiled as a GPR profile (radar image) of the media (bridge structure) (Figure 1). In Nordic ground penetrating radar projects, the gray scale tone and a reflector polarity in the printouts should appear as they do in Figure 1. Figure 1. An example of a GPR data profile from a bridge. Individual reinforcement bars can be seen as hyperbolas Stepped frequency radar principles With a steppedfrequency radar, the radar waveform consists of a series of sine waves with stepwise increasing frequency. The radar measures the phase and amplitude of the reflected signal on each frequency and uses an inverse Fourier Transform of this data to build a time domain profile. Thus, the step-frequency radar collects data in the frequency domain and converts the data to time-domain data through computer processing. The resulting radargram is similar to impulse radar data, and can be

7 processed and interpreted the same way as impulse radar data. However, since stepped-frequency data is collected in the frequency domain, it allows advanced filtering and signal processing to be applied directly to the raw frequency domain data. Figure 2 presents a time slice example of bridge deck data collected using stepped frequency radar. Figure 2. An example of a stepped frequency GPR data time slice taken from a depth of 0,18 m from a concrete bridge. In addition to moisture anomalies the time slice also shows the position of drainage pipes (small circles at level of 3 m and 9 m). 2.4 Principles of GPR image Reflection and polarity When GPR signal propagates from medium 1 to medium 2 and medium dielectricity values are E 1 and E 2, the reflection amplitude will be: R On the basis of the formula, the polarity of a reflection changes if E 1 is smaller than E 2, which is the most common situation, for instance when a signal proceeds from asphalt of dielectric value of 6 into concrete with dielectric value of 9. If E 1 is bigger than E 2, then the polarity of the reflected wave remains the same as the progressive wave s polarity at the interface. In radar measurements, however, it is common practice that the surface reflection is recorded as positive, even though the reflection coefficient is negative. Similarly, the other layers, where ε (upper) < ε (lower), are recorded as positive reflections. In bridge surveys, polarity is easy to check from reinforcement reflections where the white reflection should be in the middle (see Figure 1). Correspondingly, if the dielectricity of the lower layer is smaller than the upper, for instance in the deck bottom where signal proceeds from concrete to air, with dielectric value of 1, then the reflection should be negative in which case the black reflection will be in the middle. Ground penetrating radar signal polarity, leaving the antenna and progressing in the medium, can easily be changed 180 degrees by reversing the positions of the transmitter and receiver antenna or when doing GPR data post processing by multiplying the signal with a factor -1.

8 2.4.2 Depth penetration, resolution and interface depth Achievable depth penetration with ground penetrating radar depends on what antenna frequency is used and therefore the wave length of signal. The attenuation increases when GPR central frequency increases. A highly conductive medium results in an increase in the amount of energy scattering objects, when the length of wave gets shorter. Similarly, the penetration depth becomes smaller as the frequency gets higher. On the other hand the resolution improves at the same time. The resolution also improves as dielectric value increases. In bridge decks surveys optimum central frequency range is from 1,5 GHz to 2,5 GHz but it also should be noted that 1,0 GHz air coupled horn antennas especially have been used successfully. Resolution refers to how close interfaces can be to one another and still be identified as separate interfaces. This applies to both directions, horizontally and vertically. The vertical dimension s resolution of the pulse can be calculated from the following formula: c h 2 r, where c = the speed of light in vacuum (0.3 m/ns) = the pulse length (ns) ε r = medium s relative dielectricity Observed interface in the depth can be calculated from this formula: s v t 0.5 twt c r, where twt = two way travel time of the wave. 2.5 Ground Penetrating Radar in bridge deck surveys The first tests to use ground penetrating radar (GPR) for bridge surveys were in USA and Canada in the 1980s (Cantor and Kneeter 1982, Manning and Holt 1983, Clemena 1983). Sweden tested the use of a multichannel 400 MHz system for bridge decks in the late 1980s and early 1990s. In Finland, the first tests were done in the late 1980s but at that time, even though the results were quite good, the cost of the survey was too high. Since these initial tests, the development of GPR equipment and antennas has led to major improvements in the accuracy of the method and the speed of the surveys. Processing and interpretation software development has made visualization of the results better. Ground Penetrating radar can be used in several bridge deck applications such as determining the thickness of the pavement, pavement damage, debonding, reinforcement cover depth, damage in the concrete deck slab and condition of the water insulation layer and thickness of the concrete deck slab. However, in most cases, GPR alone cannot provide reliable enough information of the damages and their nature, but it is an excellent tool for the initial mapping and specifying of locations where other non-destructive evaluation methods and limited ground truth testing can be used to verify the problems (Saarenketo, 2006).

9 3. GPR equipment A description of GPR technique and different GPR antennas is published in: The Use of GPR in Road Rehabilitation Projects, a document published by the MaraNord project. This recommendation for specifications describes the characteristics of 2D and 3D GPR systems that have been used in bridge deck surveys in Nordic countries. Both high frequency ground coupled and air coupled antennas can be used in bridge deck surveys (Figure 3). The 2D ground-coupled antenna systems can provide very detailed information about the bridge deck s structures and reinforcement bars. However, the ground coupled systems are slow in speed and require lane closures during data collection. Therefore the high-frequency 2D air-coupled antenna systems, which can perform data collection without causing major traffic problems, are normally used for bridge GPR surveys on high traffic volume roads. (Saarenketo, 2006). Figure 3. Different GPR systems used in bridge surveys in Nordic countries. Upper left: GSSI 1,5 GHz ground coupled system, Upper right: IDS Hi-Britt 2,0 GHz ground coupled system, Lower left.3d Radar 100 2,0 GHz stepped frequency system and lower right Malå MIRA 3D ground coupled system.

10 4. Survey planning and performance 4.1 Measurement timing, planning and information, restrictions Late spring, summer and early autumn are the best seasons for bridge GPR surveys in Nordic countries. Winter measurements are not recommended because, in frozen concrete, moisture anomalies do not show well in the GPR data and because deicing salt on the asphalt surface causes high signal attenuation. The surface of the bridge should not be wet during the data collection, because then the moisture in the concrete cannot be detected reliably. However, water in cracks can be seen in the data and thus the optimum time for bridge deck data collection is about 1-2 days after rain. There are also some restrictions with GPR for certain bridges. There are, for instance, some special bridge materials, like steel fibers in protective concrete, that can cause so much GPR signal attenuation, that it makes the use of GPR almost impossible. This kind of structural information from bridge data bases, such as BaTMan, or bridge design drawings should be analyzed before the work is done. This information can also be used later when the interpretation is being done. The measurement speed with a ground-coupled system is walking speed, so a detailed survey with 2D or a 3D GPR can take from 2-8 hours depending on the size of the bridge. Longitudinal sampling rate is recommended to be 100 scans/m and the distance between parallel profiles 50 cm with the 2D system and a maximum of 12 cm with the 3D system. Ground coupled antenna systems should be in direct contact with asphalt during data collection. Before the surveys, a plan should be sent to the client s contact person, describing when and how the measurements are intended to be done (longitudinal sections and cross sections), measurement equipment and personnel. The note can be sent via mail, , or fax or if this is not possible, then handled over the phone. On high traffic bridges, lane closures should be organized to ensure safety. Further safety considerations for the ground penetrating radar measurements are described in a road works safety plan required by each Nordic road agency. In general, survey crews must possess the appropriate national road work safety course documents (Finland: Tieturva I, Sweden: Arbete på Väg, Norway: Kurs i arbeidsvarsling) and each country requires traffic safety plans and advance notice for GPR work especially on high volume roads. Nordic government authorities may also require radio license for the use of each GPR sysytem in the country. These national guidelines may also define special geographical areas, such as airports, astronomical survey stations, hospitals, prisons or defense force areas, where the use of GPR is prohibited or a special license is required. The GPR equipment, if working properly, will not cause interference but broken equipment can potentially cause false alarms.

11 4.2 Bridge GPR measurement implementation It is recommended that two persons should do a bridge GPR survey, one focusing on data collection and the other taking care of safety, making notes, taking photos and helping with positioning of the measurement profiles. Additionally one or two persons may be needed for guiding the traffic in areas where lane closures are needed. The following recommendations for GPR data collection on bridge decks have proven to ensure good quality data for both 2D and 3D GPR systems. In order to measure straight GPR profiles, good markers for each GPR profile should be made in advance at the start and end locations and at regular intervals along the survey line (with chalk or water-soluble paint). Distance from the 0-point is marked at each of the survey line start and end points (Figure 4). The starting point line and the end point line are marked with reflective tape across the entire width of the bridge (Figure 4). This helps afterwards when adjusting the start and end locations of parallel survey profiles to be exactly the same scale. Tape should be used because the reflections from the bridge joints are not always clear enough to be used for adjusting parallel profiles. The reflective tape is normally placed about 1 meter before the joint. If the joint is diagonal the reflection tape is put straight in an adequate distance. Each profile is started so, that the front side of the antenna is exactly on top of the edge of the tape. Figure 4. Example of use of aluminum tape and panted markings used to control the positioning of the survey. The length of the bridge from tape to tape should be measured accurately. The width of the bridge is also measured. Other measures are taken if the joints are diagonal or if the bridge deck is curved etc.

12 Photos should also be taken from underneath the bridge (if possible) and from the sides. Photographs should also be taken of damaged areas on the deck surface. The photographs are then used as a reference for the GPR data. They are labeled according to the position from which each photo was taken. Notes are taken by hand during the measurement. The notes should be delivered to the interpreter as clearly as possible to avoid any misunderstandings (Figure 5). Bridge GPR survey results are always shown as east-west or south-north positions. Figure 5. Example of notes from a bridge measured with 3D GPR. The sampling rate with the ground-coupled 2D and 3D systems should be 100 samples/m. If this is not possible, as many samples as close to 100 should be taken. With horn antenna systems the sampling rate should be at least 10 scans/m. The measurement time depends of the thickness of the bridge, but usually the maximum is 10 ns. Gain, if it is used, should be flat gain and signal clipping is not allowed. No filters should be used during the data collection. After the survey all tapes used for marking the profile start and end positions are removed as carefully as possible. Only water soluble paints can be used. 4.3 Special measurement implementation for a bridge 3D GPR survey In bridge surveys all 3D GPR profiles are normally measured in the same direction. This is due the fact that it is difficult to reverse 3D data afterwards in GPR data processing and interpretation software and there are many sources of errors. The measurement should be continuous, since stopping while measuring can cause noise in the data of some GPR systems.

13 5. Measurement data processing and interpretation Detailed instructions for processing and interpretation of GPR data are described in GPR processing and interpretation software manuals. The major goal for the bridge GPR surveys is to provide information about the condition of the bridge for rehabilitation design. In this case the most important thing is to produce precise information about pavement thickness, the condition of the water proofing layer and the condition of the concrete deck. In addition, the interpreter should produce other information needed in the rehabilitation design, such as the reinforcement cover depth, condition of water pipes and possible reasons for damages. 5.1 Preprocessing In bridge survey cases, preprocessing means GPR data editing and combining other bridge reference data. Data editing includes operations which do not change the original information content of the data. This means distance scaling, joining and splitting of different lines and reversing directions (if needed). At first, reversing is done to the profiles if needed. Secondly, profile lengths must be adjusted according the true length of the bridge or measured length between reflecting tape. The positioning of the parallel profiles must also be checked. The reflection from the tape is a good marker. It should be used to aid positioning of parallel profiles. 5.2 Processing Often the quality of a high frequency ground coupled 2D GPR raw data from a bridge is so good that no special processing is needed. Vertical or horizontal filters are applied, if the data needs some cleaning or dc-level removal. The 3D GPR data and air-coupled antenna data needs processing. Normally, at least vertical time domain filtering, bouncing removal and horizontal high pass filtering are applied. If there is a need to visualize the iron enforcement mesh better, migration can be used to focus the hyperbolas. Time variable gain and especially automatic gain should not be used, because they will ruin the amplitude information. 5.3 Interpretation and analysis The interpretation of bridge GPR data includes tracking of the major structural features and defining their extent, depth and thickness. The qualitative interpretation and analysis includes detection of the areas of abnormal behavior of the signal amplitude or frequency content of the signal. Both are important for locating and understanding deterioration detected in the bridge deck. If asphalt pavement covers the bridge deck, its thickness must always be interpreted. The thickness of intermediate protective concrete course and gravel layer, and the level of the first reinforcement bars

14 should also be interpreted from the bridge GPR data. If possible and visible, the total thickness of the slab should be interpreted. The waterproofing layer is normally too thin to be interpreted as a separate layer, but if possible, it should be done (Figure 6). Often the reflection from the top of the concrete deck is merged with the reflection from the asphalt bottom. Figure 6. 3D GPR bridge data. The first interpreted line is the asphalt bottom and the second is the bridge deck slab top. Between these two interpreted lines is the waterproofing layer. In addition to layer thicknesses, deterioration in the deck slab can be analysed through a) the signal amplitude strength, b) attenuation and c) dispersion calculated from the GPR data. These so called deterioration maps or time slices should be calculated and presented from different depths in the concrete deck slab, in order discover the extent of deterioration. If the bridge is asphalt covered, the maps should be defined in relation to the surface of the concrete deck, since the asphalt protective concrete thickness varies and may cause false anomalies if not taken in to account. Blueprints, photos and reference core data (if available) should be used before making analyses of what kind of deterioration is present and what is causing it. Deterioration maps can be calculated from both 2D and 3D GPR data, but 3D GPR data allows calculation of more detailed deterioration maps. An example of a bridge deck deterioration map based on signal attenuation is shown in figure 7. The signal attenuates strongly (red areas) because of possible corrosion in reinforcement, disintegration of the structures or increased salt content. Blue color indicates areas with no damage in the structure.

15 Figure 7. An example of deterioration maps calculated from two different depths in the bridge concrete deck. 5.4 Reference data It is necessary to have some reference data in addition to the bridge GPR data, when analyzing the results (Figure 8). Core data (samples) is seldom available but blueprints are usually provided by the client when requested and photographs can be taken during the GPR survey. Other methods such as thermal camera survey and 3D laser scanning can provide additional information about the surface conditions of the bridge and help to understand how deterioration in the concrete deck has developed. Figure 8. An example of deterioration map presented together with drill core information.

16 6. Reporting and reporting standards The ground penetrating radar survey results and printouts are published on a case by case basis, but, by the minimum standard, structural and deterioration maps, described below, should always be made. In addition, the client's instructions should be followed. Bridge GPR survey results should not be delivered only as raw data or bitmaps of the data. It is recommended that results are presented as a) structural surface maps of the layer thicknesses (see Figure 9.) and b) deterioration maps of the bridge concrete deck slab. The layer depth data can also be given as tables if needed. Next, structural maps should be presented as a basic standard of a report 1. The thickness of the asphalt pavement (if asphalt covered the bridge deck) (Figure 9) 2. The depth of the surface of the concrete deck, (if not the same as the bottom of asphalt). 3. The concrete thickness to the top reinforcement bars (reinforcement cover depth) Figure 9. Asphalt thickness on top of a bridge presented as a surface map. The following deterioration maps should at least be presented: 1. The amplitude/attenuation at the level of concrete deck surface (0-2,5 cm) (figure 7). 2. The amplitude/attenuation at the level of reinforcement 3. The amplitude/attenuation below the reinforcement In addition, amplitude/attenuation maps calculated at 2,5 cm from the concrete deck surface have proven to be very useful when evaluating the extent to which deteriorated concrete has to be removed. The dispersion/frequency content decay maps should be presented from the same levels if possible. The maps can be very useful in detecting areas with horizontal cracks in the concrete deck (delamination).

17 Using the same dispersion/frequency data, maps of moisture problems can also be produced (Figure 10). Figure 10. Fourier dispersion map from a 250 m long motorway bridge calculated from 1.0 GHz horn antenna data. In addition to the surface and deterioration maps and possible tables, the GPR survey consultant will release a report to the client, where the bridge GPR survey is described and results presented and discussed.

18 References Azevedo S.G et al., HERMES, A High-Speed Radar Imaging System for Inspection of Bridge Decks. Nondestructive Evaluation Techniques for Aging Infrastructure and Manufacturing, SPIE Vol. 2946: Cantor T & Kneeter C, Radar and Acoustic Emission Applied to Study of Bridge Deck, Suspension Cables and Masonry Tunnel. Transportation Research Record 676, Washington D.C, Clemena G, Nondestructive Inspection of Overlaid Bridge Deckswith Ground Penetrating Radar. Transportation Research Record, 899: Lau, C-L., Scullion, T. and Chan, P Using Ground Penetrating Radar Technology for Pavement Evaluations in Texas, USA. Geological Survey of Finland, Special Paper 16, p Manning DG & Holt FB, Detecting Deterioration in Asphalt Covered Bridge Decks. Transportation Research Record, 899: Morey, R.M. and Kovacs, A Detection of Moisture in Construction Materials. CRREL (Cold Region Research and Engineering Laboratory). Report Hannover, New Hampshire, 9 p Saarenketo T, Electrical properties of road materials and subgrade soils and the use of ground penetrating radar in traffic infrastructure surveys. Acta Universitas Ouluensis, A 471.