Nondestructive evaluation techniques using impact-generated stress waves in concrete

In this dissertation, non-destructive evaluation techniques using impact-generated stress waves for the measurement of longitudinal and the surface wave velocities and the surface wave transmission coefficient across a surface-breaking crack or a notch in concrete are developed.; In the first part, a technique for one-sided measurement of longitudinal and surface wave velocities in concrete is presented. This technique is applicable to concrete structures such as pavements where only one surface is accessible. A solenoid-based impactor is employed as a reliable stress wave source to generate consistent and repeatable stress waves in concrete without damaging the surfaces of the test specimens. Systematic methods are used to compensate for highly incoherent signal noise levels (using signal summing) and signal dispersion (using curve fitting) for accurate determination of the stress wave arrival time in the time domain signal. Accurate and reliable measurements of the longitudinal and surface wave velocities are obtained on the surface of the test specimens. One-sided stress wave velocity measurements are also performed to observe the effect of the moisture content gradient in concrete specimens within concrete on the stress wave velocities and to monitor the strength gain at early curing stages. The feasibility of monitoring the strength gain in very young concrete using one-sided velocity measurement is shown.; In the second part, a self-calibrating surface wave transmission measurement technique is presented and used to measure the surface wave transmission coefficient for notches and surface-breaking cracks in concrete structures. The measured signal transmissions are not affected by effects from the experimental set-up such as unknown characteristics of the receiver and the stress wave source, and variation of impact event or receiver coupling. The signal transmission for varying surface-breaking crack/notch depths, the types of concrete, and the crack opening condition are displayed. Using the developed self-calibrating transmission measurement scheme, a repeatable and reliable coefficient for surface wave transmission across normalized surface-breaking crack/notch depths in concrete is obtained. A boundary element method study is also included in this dissertation for comparison with and verification of measured surface wave transmission results.