Project Title: Nondestructive Evaluation (NDE) for corrosion detection in reinforced concrete structures incorporating time-resolved thermography combined with three-dimensional (3-D) microwave imaging, a benchmark approach cosponsored by NRC and Federal Highway Administration (FHWA) Exploratory Advanced Research.
Project Abstract: Corrosion of steel reinforcements is the main cause of damage and early failure of reinforced concrete structures in civil engineering, leading to enormous costs for inspection, maintenance, restoration, and replacement of the infrastructure worldwide. A recent study commissioned by FHWA states that the cost of corrosion in the United States consumes about $286 billion per year (published at www.corrosioncost.com). In this study the average costs for corrosion control methods and services, alone, are estimated to be $121.41 billion per year, of which less than 0.1 percent is spent for research and development. The human consequences of not dealing with corrosion in time are shown by the loss of five lives in the recent collapse of a concrete highway bridge in Laval, Canada, because of rebar corrosion.
An objective, spatially resolved, and rapid corrosion inspection method could lead to cost savings of billions of dollars worldwide by the detection of corroded reinforcement in concrete at an early stage. Damaged areas could then be targeted for strengthening or repair at the appropriate stage of the lifecycle of the investigated structure. Conventional methods for detecting corrosion are based on electrochemical techniques such as half-cell potential and linear polarization. These methods can be affected by a number of factors and require direct contact with the concrete. To overcome difficulties with conventional inspection techniques a benchmark project using and evaluating state-of-the-art NDE methods is set up at FHWA’s Turner Fairbank Highway Research Center. The benchmark project incorporates the latest developments in NDE inspection methodologies. A special focus lies on the promising technique of time-resolved thermography with induction heating combined with 3-D microwave imaging—a research cosponsored by the US National Research Council NRC.
A corroded steel bar that has been heated above ambient temperature will cool down more slowly than an uncorroded one, and the difference in rate of cooling can be used to detect the corrosion. The rebar can be heated in the concrete by using an AC (alternating current) electrical induction heater, external to the concrete structure. The temperature at the surface of the rebar can be detected nondestructively by the reflectance of a microwave beam, since the reflectivity is temperature dependent. The induction heater can be cycled on and off to produce temperature pulses in the rebar, and thus the heating and cooling can be cycled repeatedly. By scanning the microwave beam over the rebar it is possible to image the corrosion layer. Apart from infrared thermography with induction heating and a new microwave camera, the used methods will consist of an ultrasonic linear array for early crack detection, commercial ground penetrating radar, and electrochemical reference methods.
To realize this ambitious plan an expert's committee is founded, involving corrosion specialists and NDE experts from the US and Europe. The focus of this collaborative effort is to design and build concrete test specimens, artificially induce and monitor corrosion, periodically perform multisensor NDE inspections, followed by 3-D imaging and destructive validations.