How do I find the real wave velocity from a Pile Integrity Test (PIT)?
The wave speed in concrete is a function of concrete mix design strength and age of concrete. The wave speed for really poor low strength concrete might be as low as 3000 m/sec and for high quality concrete as high as even 4400 m/sec. PIT measures the signal versus TIME. The TIME data is converted to LENGTH by multiplying TIME scale by the input WAVE SPEED divided by 2 (since the toe reflection comes at TIME = 2 x / c, where x is a length and c is the wave speed). So the LENGTH on the plot is proportional to the (input) WAVE SPEED. If the length is accurately known and a clear toe reflection is observed, then adjust the input wave speed until the toe reflection is correctly shown in the plot as equal to the known length. For generally similar concrete mix and generally similar age after casting concrete, the wave speed should be fairly uniform, so by testing many piles and you can find the usual wave speed for a site, and this may help spot piles where the length is different than the design length. If you are not certain of the length of any pile, then there is uncertainty also in the wave speed and the best you can then assume is a general estimate for wave speed based on the concrete strength.