Circuit that measures the speed of sound in concrete
You don't need a circuit, I would use:


hammer (please see @hoosierEE comment, hammer may be overkill)
two small piezoelectric microphones (even a piezo speaker might work)
2-channel digital oscilloscope


Glue / tape a piezo microphone to each side of the concrete. Connect piezo 1 to probe 1, the other to probe 2. Turn on both channels. Set the scope to trigger and hold off probe 1. Tap the concrete with the hammer next to piezo 1. The scope should trigger and then you can work out the difference between the initial and final pulses. Do multiple measurements to increase accuracy.

This will be much cheaper and less time consuming than other projects. As a bonus you will have a digital oscilloscope for other pracs such as motors, microphones, etc.