The major cause of deteriorating concrete in the infrastructure is corrosion of the reinforcing steel. This is the major mechanism of deterioration of steel used in other applications as well. At present, no technology is available to detect and quantify the corrosion on the steel-reinforcing bars (rebar) hidden inside reinforced concrete or in some other way. The extent of corrosion in these applications is therefore usually unknown until components have severely degraded, making the damage visible.
Ground penetrating radar is an important existing technology that is widely used for locating underground structures. Using radar reflections at around 1 GHz, it is used to locate buried metallic structures. However, it cannot detect corrosion. If this program is successful, it might be incorporated into ground penetrating radar systems, which would result in a mobile sensor that could be used to detect rebar corrosion and measure its extent in transportation structures such as bridge and road beds. In addition, we envision that smaller portable systems could be built for the examination of vertical and more complex structures such as concrete bridge columns and supports.
The economic value of corrosion each year in the US is measured in billions of dollars. This work will provide a new measurement technology for detecting corrosion directly, will advance the science of corrosion prevention in the US infrastructure, and will provide NIST visibility in measurement science that addresses US needs.
We are developing a new technology to detect and quantify the extent of corrosion in embedded rebar using Terahertz (THz) and microwave spectroscopies. This new method will exploit the likelihood that a number of iron corrosion products are antiferromagnetic and absorb THz and microwave radiation. In this program, we will analyze the THz and microwave absorption of various corrosion products, identify the phases that provide the most useful response, and determine the most credible approaches for this technology to be implemented. This effort can be classified as a non-destructive evaluation technology, but using spectroscopy rather than just ordinary imaging.
Two of the most common iron oxides that form during corrosion are antiferromagnetic, hematite and goethite. The absorption of THz and microwaves is different in these antiferromagnetic oxide materials than in the ferromagnetic iron precursor. In an antiferromagnetic material, no externally applied magnetic field is required for THz or microwave absorption due to resonating spins and the resonant frequency can be as high as 120 GHz or larger. Small stray magnetic fields, present in any environment, will give the iron a resonant frequency (FMR), but it will be far smaller. The great difference in resonant frequencies offers the possibility that small quantities of hematite or goethite could be detected in the presence of iron.
Challenge 1: Determine the absorption spectrum of corrosion products in the THz and microwave ranges.
Challenge 2: In the case of rebar in concrete, both the iron and its corrosion products will typically lie centimeters below the surface of the concrete. The attenuation of THz and microwaves by the concrete is a concern.