BETOSCAN A ROBOTIC SYSTEM FOR SIMULTANEOUS DIAGNOSES OF REINFORCED CONCRETE STRUCTURES

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1 BETOSCAN A ROBOTIC SYSTEM FOR SIMULTANEOUS DIAGNOSES OF REINFORCED CONCRETE STRUCTURES Kenji Reichling (1), Michael Raupach (1), Herbert Wiggenhauser (2), Markus Stoppel (2), Gerd Dobmann (3), Jochen Kurz (3) (1) Institute for Building Materials Research, RWTH University Aachen, Germany (2) Federal Institute for Materials Research and Testing, Germany (3) Fraunhofer Institut für zerstörungsfreie Prüfverfahren, Germany Abstract A reliable prognosis of the condition and behavior of a structure is an important basis for an effective service life management. In order to determine the most economical point in time for repair measures in the life-time of a structure, the knowledge about the deterioration process at exposed regions as well as a detailed knowledge about the current condition of the whole structure is essential. To record the condition of a structure, different non destructive diagnostic tools are available. When the concrete cover is known for the whole surface and reliable estimations for the speed of carbonation and ingress of chlorides are worked out based on the data from diagnosis, the time to a specific limit state (e.g. corrosion initiation) can be estimated by means of established models. To assess the condition of the components an investigation of the whole concrete surface is recommended by recording useful key parameters (electrochemical potentials, concrete cover, carbonation depth, chloride profiles, cracks, delaminations, condition of the reinforcement, etc.). The derived information of the potential mapping results combined with additional information from other investigations lead to significant conclusions concerning the current condition of a structure with regard to suitable repair methods. To accelerate these intricate measurement procedures a research and development project has been initiated with several partners from science and industry to develop a robotic system called BETOSCAN which is able to navigate autonomous over large concrete surfaces and to measure and record all relevant data automatically in one step. With the presented system the results of all used test methods can be evaluated simultaneously for the investigated surface. Detailed maps can be created to locate all critical areas. By means of periodical measurements the life-time prognoses of structures can continuously be updated. A prototype of the system is under development. This paper focuses on the basic functions and the possible fields of application of the BETOSCAN system. 321

2 1. INTRODUCTION A large number of reinforced concrete structures world-wide are damaged due to reinforcement corrosion e.g. parking structures or bridges. Primarily due to the ingress of deicing salts and the insufficient concrete quality of elder structures the steel reinforcement can corrode causing cracking, spalling and losses in cross section leading finally to structural problems. To evaluate the condition of such structures adequately usually extensive investigations are necessary. Frequently the costs of a complete investigation exceed the available funding, so that only abbreviated investigation programs are carried out. These results are usually not sufficient as a basis for an adequate design of repair and protection measures. Accordingly extensive measures are applied leading finally to higher costs compared to the measures based on a reliable database. This situation has been the basis for the research and development project BETOSCAN with the objective to develop a robotic system which is able to drive over large floors and to measure all relevant parameters simultaneously. The collected data is stored for each investigated point of the structure allowing for evaluations regarding condition assessment, prognoses of the future state, design of measures for protection and repair as well as quality controls. A prototype of the BETOSCAN-system is actually under development. For this purpose three research institutes syndicated with seven small- and medium-sized enterprises (SME) and one industry partner (see Table 1). Table 1: Institutions involved in the BETOSCAN project Category Institution ibac Institute of Building Materials research of Aachen University research institutes BAM Federal Institute for Materials Research and Testing, Berlin IZfP Fraunhofer Institute for Non-Destructive Testing, Saarbrücken GPS GPS GmbH, Department Robitics, Neobotix, Stuttgart IGF Engineering company for repair of buidings, Gieler-Breßmer & Fahrenkamp GmbH, Süßen SKP Specht, Kalleja + Partner GmbH, engineering company, Berlin SME CITeC Concrete Inprovement Technologies GmbH, Dresden Germann In-Situ Test Systems for Concrete and Reinforced Instruments Concrete Structures, Copenhagen (DK) ACSys Acoustic Control Systems, Ltd, Moscow (RU) Arxes Information Design Berlin GmbH industry partner Sika Supplier of specialty chemicals, AG, Stuttgart 322

3 2. THE BETOSCAN SYSTEM The BETOSCAN-system consists of a mobile robot platform which is able to drive autonomously over horizontal areas (e.g. multi storey car parks, bridge decks) equipped with different sensors for non-destructive measurements (see Fig. 1). With its 270 Scanner the platform is able to generate a digital environmental map in order to navigate properly. The ultrasonic sensors on the front side allow the robot to detect even movable obstacles. On the back side of the platform the sensors are fixed. With the special attachments developed within the project, the sensors can be positioned in different ways. With this modular setup it is possible to upgrade the system with further sensors. Fig. 1: BETOSCAN robot system The sensors collect the data at a driving speed up to 17 cm/s /1/. So the system is able to investigate surfaces of some thousand square metres a day. The accumulator pack at the top of the platform (Fig.1, black boxes) provide enough energy for a minimum of 8 hours. All measured data is stored in a defined format along with the individual local positioning information of the platform /2/. With these information it is possible to generate a map for each measuring value. A computer in the platform stores all relevant data, which can be downloaded via WLAN to the notebook of the operator in order to generate the maps on site. 3. THE SENSORS To investigate reinforced concrete structures different commercially available sensors have been chosen to be integrated in the robotic system (see Fig. 2). The integrated measuring instruments can be used further on as standard handhelds, so that the manual collection of further data is still possible. Especially for concrete structures exposed to de-icing salts the first step of a condition survey should in many cases be a potential mapping of the whole concrete surface. During the recent years affordable instruments to measure, store and plot the electrochemical potentials are commercially available. Within the project Proceq s instrument Canin + is 323

4 used in combination with copper/coppersulfate reference electrodes. In order to generate concrete cover depth maps of the whole surface, Proceq s Profometer 5+ based on the eddy current method and Mala s GPR ProEX system based on the radar method are attached to the system. The knowledge of the concrete cover depths can be very helpful in cases of assessing the condition state of structures in order to design an adequate repair measure or to control the application quality after replacement of the concrete cover by a repair mortar. optical analysis microwaves potential mapping position of cracks and damaged zones corrosion probability Humidity distribution Voids, thickness crack depth ultrasonic eddy current method + radar reinforcement: location and cover - air temperature -concrete temperature concrete resistance rel. humidity Fig. 2: Investigation methods of the BETOSCAN-system By means of the ultrasonic system A1220 Monolith from Acsys the structure thickness can be determined over the whole surface. Additionally it is possible to detect and to map voids in the concrete if additional information of e.g. the biggest aggregate diameters and the surface quality are available. One advantage of the use of this specific instrument is the possibility to connect the ultrasonic sensor heads on concrete without a coupling gel, which simplifies the automation of the measurement. As mentioned above, the ultrasonic sensor needs a direct connection to the concrete surface. Therefore the robot platform has to stop while doing the measurement. A pneumatic pump in the frame of the attachment module presses the sensor onto the floor while the whole measuring procedure is carried out. To investigate the relative moisture distribution of the areas near to the concrete surface, the microwave sensors Moist PP and Moist RP from HF-Sensor are used. These sensors do not need a direct connection to the concrete surface. 324

5 4. THE DRIVING MODES Considering the automation of the non-destructive methods two driving modes are required. The first mode implies a continuous driving for the contact-less measurements and the potential mapping with wheel electrodes. The second, discontinuous mode, considers the measurements of the contact sensors. While the ultrasonic sensors are measuring the robot platform switches into the discontinuous mode. In this mode the robot stops driving while the measurements are accomplished. Therefore this mode is more time consuming but still faster than a measurement by hand. 5. FEATURES OF THE BETOSCAN-SYSTEM The most outstanding advantage of the BETOSCAN-system is the simultaneous measurement of all key parameters over the whole surface. Each parameter can be investigated with a corresponding sensor before it is saved with the appropriate coordinates in a database where all relevant information is collected. The operator can track the gained data at the same time via WLAN and can generate first maps on site. With these information further investigations can be planned, prepared and carried out without time delay. As shown in Fig. 3 for every measurement point all relevant parameters are known. Combining these information results in a more explicit knowledge of the current state of the structure. Sensor1 Sensor2 Sensor3 Fig. 3: Evaluation of the data by single and combined plots of the data. Source: CITEC/BAM The BETOSCAN-system opens several new possibilities for the management of concrete structures. In the following the different fields of application are presented. 1.1 Diagnoses of RC structures The process of a diagnosis by means of the BETOSCAN-system can be divided into 4 steps which are shown in Fig

6 Step 1 Mapping of the relevant parameters, e.g. electrochemical potentials, concrete cover, cracks Step 2 Evaluation of the whole surface using combined sets of limit values Classification of the state of damages, identification of critical areas Step 3 Additional measurements (ph-value chloride profiles, etc.) in defined areas (local analyses) Step 4 Integration of the additional data into the database new maps Complete documentation of the condition Fig. 4: Schematic description of the analysis procedure As a result the operator gets all measurement values over the whole surface of the investigated area and a complete assessment can be carried out. One major advantage of the BETOSCAN-system is the possibility to divide the whole surface into zones with defined damage classes. Such classes could be: - No damage due to high concrete cover and good concrete quality - Corrosion of the reinforcement bars due to carbonation - Chloride induced corrosion of the reinforcement - Cracking due to corrosion - Spalling due to corrosion - Concrete damages due to frost, etc. 1.2 Prognoses of the service life of RC structures Some of the measurement data can be used as basis for a detailed evaluation of the future state of the structure. Suitable models on corrosion initiation (chloride ingress and carbonation of the concrete) are available and models for the propagation phase (corrosion of the reinforcement) are actually investigated in an extensive research project / When the concrete cover is known for the whole surface and reliable estimations for the speed of carbonation or the ingress of chlorides is worked out based on the data from the diagnosis, the time to corrosion or the time to reach a critical limit value can be calculated for each position of the surface. This allows again to divide the surface into zones with defined classes regarding the prognosis. A possible classification of the zones could be done as follows: - No actions required for the next 20 years - No actions required for the next 10 years - No actions required for the next 5 years - Immediate actions required, etc. 326

7 These maps could be a useful basis for the service life management of reinforced concrete structures. 1.3 Service life management A reliable prognosis of the condition and behaviour of a structure is an important basis for an effective service life management. After finishing the building process an initial diagnosis should be carried out with the BETOSCAN-system to document the initial state of the structure over the whole surface as basis for a first prognosis (birth certificate). From time to time additional diagnoses allow an update of the prognosis leading to an increased reliability of the prognosis with time. By means of these repeated or cyclic diagnoses an update and sharpening of the prognosis are possible. With the BETOSCAN-system the costs for a complete survey can be reduced and the data base for a service life management can be improved economically. 1.4 Planning of measures Based on the knowledge of all measurement data over the whole surface, a selection of suitable measures for protection and repair can be carried out adequately. Zones where the same repair methods could be applied can be carried out. As a result detailed maps for the design of suitable measures for protection and repair will be available. 1.5 Quality control During and after the application of protection and repair measures different key parameters can be determined for quality control purposes using the BETOSCAN-system (e.g. concrete cover depths after the replacement by a repair mortar). Scanning the whole surface area allows a reliable statistical evaluation of the data and a detailed localisation of critical zones. 6. SUMMARY AND OUTLOOK Within the BETOSCAN project the project partners are developing a measurement system which is able to move autonomously over large horizontal areas of RC structures and to carry out non-destructive measurements automatically. Various non-destructive testing instruments are integrated in which every single measurement result forms a jigsaw piece of the current condition of the investigated area of the structure. By combining the different methods within only one measurement operation, an analysis can be accomplished faster and more accurate. Due to the knowledge of all measurement data over the whole surface various damage zones can be worked out which can be individually repaired. In order to manage the service life of a component or to control the quality of a repair achievement, the operator can gain accordant data over the whole surface (statistical evaluation) more economically than it would be possible with a manual measurement. REFERENCES /1/ Eckhold, T.: Eignungstest ausgewählter zerstörungsfreier Messverfahren für die großflächige automatisierte Untersuchung von Parkdecks. Berlin, Fachhochschule für Technik und Wissenschaft, Fachbereich 2 Ingenieurwissenschaften II, Diplomarbeit, 2008 /2/ Stoppel, M. 2008: BetoScan - Selbstfahrendes Scannersystem für Betonoberflächen. Ostfildern : Technische Akademie Esslingen - In: Verkehrsbauten Schwerpunkt Parkhäuser/Brücken. 3. Kolloquium, Ostfildern, S