Concrete is the most widely used for construction materials in the world. Water content of concrete is an important parameter in terms durability of concrete structures. Terahertz (THz) waves, for which concrete is a porous and absorbable material, have been studied in order to establish a new non-contact inspection technology for maintenance of concrete structures. In this study, THz transmittance and reflectance of concrete in drying process were measured with a 60 GHz GUNN diode and absorption coefficient is analyzed for concrete with various water contents. It is shown that quantitative detection below 10% is possible for the water content at surface area of concrete.
Terahertz (THz) waves are electromagnetic waves in the frequency range between approximately 0.1 and 10 THz, which is just the middle of the radio wave and the light wave. Therefore, THz wave has both of characteristics as represented by transparency of radio wave to non-polarized substance and good reflectivity to metal of light wave.
1 THz corresponds to quantum energy of about 4.1 meV, which is relatively as low as one hundred thousandth of X-ray. Hence, it is safe for human body. THz waves are expected as non-destructive evaluation. Our group has created a database of THz permeability characteristics for industrial materials, and successfully constructed non-destructive THz diagnosis of building blocks [
In this study, we report the non-contact inspection of water content inside the concrete by sub-THz waves which are relatively low frequency in the THz band. Water has a large absorption in the THz frequency region. THz wave has a high sensitivity in detecting water in concrete. Based on quantitative relation between THz absorption coefficient and water content, which is measured in this study, non-destructive THz inspection is available for water content in concrete of infrastructure buildings itself. THz wave generator and detector are recently developed with small size.
From the Roman period, concrete is widely used in our lives such as buildings, bridges, dams due to its good mechanical properties, durability, workablity, and accessibility with low economical cost. To establish a sustainable society, the durability of concrete structures with proper maintenance work becomes more and more important. When constructing a new concrete, it is not obtaining strength from the beginning. It obtains strength by hydration reaction by mixing cement and water in casting. In case that enough water is not provided at the early hydration stage, concrete has risks of insufficient strength and durability due to rough microstructure. Moreover, corrosion of reinforcing steel bars of concrete structure is strongly affected by the presence of water. Therefore, it is important to measure the water content at concrete structures.
As a traditional inspection method for the water content in concrete, a high frequency capacitance measurement method is available now [
u = m − m 0 m 0 × 100 (1)
which u is water content, m is mass at measurement and m0 is mass at dry condition. In this study, transmittance and reflection intensity measurements were carried out for the concrete as a function of water content.
A schematic diagram of the transmittance and reflection measurement system is shown in
Using the transmittance and the reflectance obtained by this measurement, the absorption coefficient was calculated by
α = − ln ( T 100 − R ) x (2)
which T is transmittance, R is reflectance and x is width of sample [
Since it is difficult to measure the transmission wave from the sample in the field measurement, we consider the quantitative detection possibility using only
the reflected wave. The measurement result of the reflection intensity from the concrete when the thickness is 7 mm is shown in
water evaporates from the surface, the THz wave penetrates deeper and the absorption of the THz wave by the concrete becomes larger. From
For concrete with 0 - 10 water contents, absorption coefficient was measured using 60 GHz emitter at room temperature. The absorption coefficient increases slowly with increasing water content up to 10%. This feature can be taken as evidence for hydrated water in concrete. Above 10%, the absorption coefficient increases steeply, which is attributed to free water. The water content can be measured below 10% at surface area of concrete.
This study is partially supported by “Fundamental Research and Human Resources Development Program for Nuclear Decommissioning related to Integrity Management of Critical Structures including Primary Containment Vessel and Reactor Building, and Fuel Debris Processing and Radioactive Waste Disposal” carried out under the Center of World Intelligence Project for Nuclear S & T and Human Resource Development by the Ministry of Education, Culture, Sports, Science and Technology of Japan.
Tanabe, T., Kanai, T., Kuroo, K., Nishiwaki, T. and Oyama, Y. (2018) Non-Contact Terahertz Inspection of Water Content in Concrete of Infrastructure Buildings. World Journal of Engineering and Technology, 6, 275-281. https://doi.org/10.4236/wjet.2018.62016