CLIMBING ROBOT FOR CORROSION INSPECTION AND MONITORING OF REINFORCED AND POST-TENSIONED CONCRETE STRUCTURES

This paper presents a climbing robot for inspection, condition assessment and monitoring of reinforced and post-tensioned concrete structures such as cooling towers, dams, parking-decks, bridges or buildings. The robot combines a vortex adhesion mechanism with a wheel electrode sensor for half-cell potential mapping of the concrete surface. Thus chloride-induced corrosion of the reinforcement can be detected during regular inspections at a very early stage, long before it manifests at the surface. The climbing robot is a lightweight device, well suited for rough surfaces, which can climb on vertical surfaces or move upside-down. The robot can reach any part of a structure, even those usually not accessible. Regular inspections of any structures can be performed with the climbing robot for corrosion detection resulting in improved quality of the inspection, allowing the change from a reactive to a pro-active maintenance strategy. This robot will therefore provide engineers in charge of infrastructure maintenance with the means to do their job much better than they can today. It offers them a way to circumvent all present barriers and brings a radical innovation in this area. INTRODUCTION Chloride induced reinforcement corrosion is the main cause of damage and premature failure of reinforced and post-tensioned concrete structures. It adversely affects the durability and safety of our infrastructure (bridges, power plants, tunnels or buildings). Corrosion is mainly due to the ingress of chloride ions from sea-water or de-icing salts. Chloride ions destroy the protective oxide films on the reinforcement and in presence of humidity and oxygen localized corrosion attacks [1] develop, leading to a dangerous loss of cross-section (figure 1). Due to acidification of the pit electrolyte and the formation of soluble iron compounds [2] rust formation will occur only in a very late stage – thus these severe localized corrosion attacks will manifest only at a very late stage at the concrete surface and will not be detected by visual inspection. This might be one of the reasons for the very high repair costs of reinforced concrete structures. Figure 1: Severe localized corrosion attack of the reinforcement in a bridge deck in the Swiss Alps Figure 2: Cost of Corrosion for industry categories in US dollars (1998) and in percent [3] Cost of corrosion A detailed study on the cost of corrosion has been conducted by NACE in 1998 for the USA [3]. It was stated that the aging infrastructure is one of the most serious problems faced by society today. In past decades, corrosion professionals focused primarily on new construction—specifying materials and designing corrosion prevention and control systems for buildings bridges, roads, plants, pipelines, tanks, and other key elements of the infrastructure. Infrastructure in the NACE study was divided into the following sectors: highway bridges, gas and liquid transmission pipelines, waterways and ports, hazardous materials storage, airports, and railroads. The total annual direct cost in this category was estimated to be $22.6 billion (figure 2). A large part of the infrastructure in reinforced concrete (cooling towers, bridges, pipelines, parking-decks, buildings etc.) was built before concrete technology had matured. All these structures are aging and – as experience shows – have to be repaired mainly due to corrosion of the reinforcing steel. From the approximately 583,000 highway bridges in the U.S. about 15% are structurally deficient because of corroded steel and steel reinforcement. The cost of these repair works has been estimated [3] to a total $8.3 billion, including $3.8 billion to replace deficient bridges over the next 10 years, $2 billion for maintenance and capital costs for concrete bridge decks and $2 billion for their concrete substructures, and $0.5 billion for maintenance painting of steel bridges. Indirect costs to the user, such as traffic delays and lost productivity, were estimated to be as high as 10 times that of direct corrosion costs. Today, it is estimated that the numbers in the NACE corrosion study in 1998 approximately can be doubled [4]. Also Europe faces the same problems with similar costs for bridge repair and replacement [5]. It is further well known (“the rule of five”) that repair costs drastically increase with the severity of damage or “major repair can be expected to cost roughly five times what routine maintenance would have cost if you'd done any” [6]. Thus a method to detect and locate corrosion of the reinforcement in a very early stage, preferably at the time of the regular inspections, could save much money (both of private owners and of the tax-payer). Robotic inspection In recent years more and more research has been done on developing climbing robots that would perform specific tasks in dangerous environments or in difficult to access areas. These robots may use different mechanisms in order to generate the adhesion force needed to move on vertical or upside-down surfaces. For non-ferromagnetic surfaces, a vacuum system can be used [7]. This suction system needs a proper sealing between the adhesion generator apparatus and the wall and is not appropriate for rough surfaces such as concrete. A very versatile climbing robot [8] for cleaning, monitoring, decontamination and inspection was developed. The technique that was preferred in the current work is the “vortex” one. Here an impeller generates a low-pressure region between the surface and the robot and with that an adhesion force. This solution leaves a significant clearance below the robot and facilitates the locomotion. It was successfully implemented in robots such as the Alicia VTX [9] or the City-Climber [10]. Climbing robots can be found in several applications such as surveillance, maintenance, cleaning or rescue operations [8]. In particular two research projects have been focused on condition assessment of concrete structures. An example is the Bridge Inspection Robot Development Interface (BIRDI), intended to inspect autonomously with a camera [11]. The RoboSense project used a robot for the instrumentation of dams and the inspection of bridges with NDT-sensors. It used a camera for visual inspection and other sensors such as a covermeter were planned to be added to the system [7]. Figure 3: Principle of half-cell potential mapping [14] Figure 4: Multiple wheel electrodes for half-cell potential mapping [12] Half-cell potential mapping Half-cell potential mapping [12, 13] is state of the art and very reliable in detecting and locating corrosion of the reinforcement in concrete structures. The method is described in several international and national standards or recommendations [14, 15]. The principle (figure 3) is straight-forward: corroding steel in concrete shows a potential of about 0.5 V more negative than the non-corroding (passive) steel. The resulting potential field can be measured at the concrete surface with suitable reference electrodes that can be single rods, single wheels or multiple wheels (figure 4). Areas with more negative potentials locate the corroding zones of the reinforcement long before visible signs of distress appear at the surface. On the other hand, half-cell potential mapping requires a direct electrolytic contact between the reference electrode(s) and the concrete surface, the electrodes have to touch the surface with a slight pressure. The technique thus faces problems of accessibility. Thus the technique is – despite its huge advantages not used in the regular inspection cycle of reinforced concrete structures. These rely on visual inspection only.