How is the real wave velocity found by a Pile Integrity Test (PIT)?
The wave speed in concrete is a function of the concrete mix design strength and age. Consequently, the wave speed for poor, low strength concrete may be as low as 3000 m/sec. Meanwhile, the wave speed for high quality concrete could be as high as 4400 m/sec.
PIT measures the signal versus TIME. The TIME data is converted to LENGTH by multiplying the TIME scale by the input WAVE SPEED, divided by 2. The toe reflection comes in at TIME = 2 x / c, where x is length and c is the wave speed. As a result, the LENGTH on the plot is proportional to the (input) WAVE SPEED. First, an engineer should ensure that an accurate length is known and that a clear toe reflection is observed. Following, the input wave speed can be adjusted until the toe reflection is correctly shown in the plot equal to the known length.
The wave speed should be fairly uniform for concrete with similar mixes and age. Accordingly, testing many piles could result in the determination of a site's usual wave speed. This method may help identify piles that are different from the design length. Pile length uncertainty results in wave speed uncertainty. Henceforth, the best that can be assumed is a general estimate for wave speed based on the concrete strength.