Detection and Thickness Estimation of Water Layer in Layered Medium Based on Multi-Reflection of Oblique Incident Ultrasonic Wave

Abstract

A reinforced concrete (RC) slab of a bridge is sometimes strengthened by attaching a steel plate on the bottom of the slab to enhance the flexural capacity. In this case, epoxy is injected between steel plate and concrete as the adhesive, together with anchor bolts. However, the reinforcement approach has a problem of water invasion into the top surface of a steel plate through an additionally damaged concrete slab, which will lead to a serious corrosion problem of a steel plate. Hence it is of great importance to detect the existence of water layer in the steel-epoxy-concrete layered medium and acquire information on thickness and distribution area of water. In this study, an ultrasonic multi-reflection approach through oblique incidence is developed for thin liquid layer detection. Firstly, reflection and transmission coefficients of multi-layered media including a liquid layer are calculated theoretically. Experiments on multi-layered configurations are conducted with different water layer thicknesses and bottom layer materials. By analyzing the experimental results, the existence of water layer can be clearly distinguished. Through comparison with the theoretical wave travelling time in the water layer, the thickness of the water layer can be estimated from the time interval of reflected wave groups. Different materials as a bottom layer can also affect the reflection notably, which shows good agreement with the calculated water-solid reflection coefficients.