| The techniques for the measurement of the changes of acoustic impedance, wave speed, and attenuation in concrete are studied on a virgin sample and a specimen after exposure to a hydrostatic pressure of about 3 kbar. The experimental procedure is to use a modified split Hopkinson pressure bar(SHPB) test method in which a 50 in. long, 3/4 in. diameter steel impact bar at a velocity of up to 140 ips is used to strike the end of a cylindrical concrete specimen which rests on a steel load bar. This load bar is 72 in. long and has two semiconductor strain gage bridges that measure the passage of the waves through the load bar. For an impact bar of 50 in. in length, the duration of the pulse is about 500 {dollar}mu{dollar}sec. This gives sufficient time to measure the reflections of the wave from the concrete steel interfaces. The arrival time of the reflected wave relative to the initial wave is a measure of the propagation velocity in the concrete, and the amplitude of the first reflection is a measure of the acoustic impedance mismatch between the concrete and the steel bar. The records of waves from the impact test of virgin and densified concrete specimens are compared, and the characteristics of each of them are catalogued. Additionally, an impact test with a pre-load condition is performed on a granular media(sand) and on a densified concrete specimen. The tests on sand are conducted to compare it with the densified concrete specimen. |
