Advanced microwave sensors for the detection of gap in concrete-filled steel tubes

Concrete-metal composite structures have been widely used in infrastructure engineering. Concrete-filled steel tubes (CFSTs) are examples of these structures. For CFSTs, there is a requirement on the shear force transfer between the concrete and steel tubes. However, gap between concrete and steel surfaces may occur due to natural shrinking of concrete and imperfections of concrete and steel caused by not proper manufacturing process. A few methods have been applied for detecting the gaps. Microwave sensor techniques have shown a great potential for this purpose demonstrating the highest sensitivity to the gap. In this research work advanced microwave embedded sensors have been proposed and applied for two cases: 1) early-age concrete when its natural shrinking may lead to gap between steel and concrete surfaces and 2) hardened dry concrete when gap may occurs during service of a CFST. For the first time the feasibility of microwave sensors for the detection and monitoring of gap between the early-age concrete and steel surfaces has been considered taking into account the changes of dielectric constant of concrete during its hydration. It is shown that the sensitivity of the magnitude of reflection coefficient to the changes of gap value is higher with the K-band sensor than with the X-band sensor while these sensors demonstrate the same sensitivity to the changes of dielectric constant and it is significantly lower than the sensitivity of the magnitude to the changes of gap. The results also demonstrate that microwave waveguide sensors with dielectric inserts can be optimized by changing geometrical and electrical properties of the inserts, and they are the best candidates for the application in practice to detect and monitor gaps in CFST at different stages of its life.