What is the usual range of concrete wave speeds that should be used with the ACT?

The usual values of good concrete wave speeds are between 3500 m/s (11,500 ft/s) and 4500 m/s (14,500 ft/s).

Posted: 5/13/2009

I measured the wave speed on a slab with known thickness and got a value of about 2500 m/s (8200 ft/s). Using this wave speed my thickness measurement was incorrect, but by increasing the wave speed to about 3500 m/s (11,500 ft/s) I got the correct result. What am I doing wrong?

In order to determine the wave speed it is necessary to detect the surface P-wave. This wave is much smaller in amplitude than a reflected or Raleigh wave. If you are getting too low a wave speed it is likely that the surface P-wave is not distinguishable and the first reflected wave is being used for wave speed determination. The wave speeds determined with the ACT should be within the range specified on Question Q1018, unless the concrete quality is unusually low (in which case the wave speed can be below 3500 m/s or 11,500 ft/s) or unusually high (in which case the wave speed can be above 4500 m/s or 14,500 ft/s). To increase the amplitude of the P-wave:

1. Remove old coupling putty and apply a thin layer (1mm) of fresh coupling putty
2. Push down onto the probes while triggering
3. If the concrete has brush strokes, ensure the probes are parallel to the grooves
4. Move the probes closer together (450mm – 550mm)
5. Smooth the surface between the probes if the above fails to sufficiently improve results

Posted: 5/13/2009

I tried to measure the thickness of a slab which I know is less than 200 mm (8 inches) thick. However, I get a much higher reading than that, unless I touch the “Next Peak” button. What can I do to avoid that?

Sometimes with slabs less than 200 mm or 8" thick there is an additional low frequency resonance peak, which will result in a high thickness reading. To prevent the system from detecting this peak, it is necessary to adjust the lower frequency limit (using the LOW button) to 5500 Hz (the default value of LOW is 2000 Hz). This spurious low frequency resonance can be minimized by applying a thicker layer (2 mm or 1/16”) of fresh coupling putty and maintaining low CS values. In this case it is also recommended to place the probes on the surface lightly, without pushing them down.

Posted: 5/14/2009