Ultrasonic waves are commonly used for non-destructive evaluation of concrete structures. For fresh concrete, ultrasonic waves have also been used to monitor concrete setting and strength development at early ages. However, the conventional ultrasonic test setup typically needs access to the two opposite sides of concrete structures, which is not always possible for in situ field testing. In this paper, embedded piezoceramic bender elements are proposed to measure ultrasonic shear (S) waves in fresh cement paste, mortar and concrete. The shear wave velocities are obtained from B-scan images of a collection of recorded signals over time. Experimental results indicate that the shear wave velocity is closely related to the setting time, and this relationship is independent of air void content and w/c of cement pastes. The low cost bender elements can also be of use to monitor the setting of fresh concrete and the long term evaluation of hardened concrete.

The impact-echo method is a commonly used nondestructive testing technique for elastic plates in civil engineering. The impact-echo mode corresponds to the frequency at zero group velocity of S1 Lamb mode. Recent development of the air-coupled impact-echo (ACIE) method introduces the possibility for rapid scanning of large structures and increases the practicality of in situ measurements. However, sensors used in ACIE are susceptible to ambient noise, which complicates in situ ACIE measurements. This letter presents the results of ACIE measurements taken using a parabolic reflector together with standard measurement microphones to increase the signal to noise ratio for ACIE measurements. The signal gain and effects of sensor location with respect to impact location are discussed.

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A time domain analytical solution is presented to calculate the pressure response along the axis of a paraboloidal reflector for a normally incident plane wave. This work is inspired by Hamilton's axial solution for an ellipsoidal mirror and the same methodology is employed in this paper. Behavior of the reflected waves along reflector axis is studied, and special interest is placed on focusing gain obtained at the focal point. This analytical solution indicates that the focusing gain is affected by reflector geometry and the time derivative of the input signal. In addition, focused pressure response in the focal zone given by various reflector geometries and input frequencies are also investigated. This information is useful for selecting appropriate reflector geometry in a specific working environment to achieve the best signal enhancement. Numerical simulation employing the finite element method is used to validate the analytical solution, and visualize the wave field to provide a better understanding of the propagation of reflected waves. This analytical solution can be modified to apply to non-planar incident waves with axisymmetric wavefront and non-uniform pressure distribution. An example of incident waves with conical-shaped wavefront is presented.