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As suggested in the comments PIR detector, ultrasonic sensor or micro phones are alternate to detect the motion or humans in floor. Accelerometer or piezo transducer might be the better option to detect human presence through vibration.

Additional I will also take look at stain gauge or pressure sensors. The once that I am thinking might need to be connected at a pivot point. I am not sure how this will work on a concert floor.

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Based on working on similar problems having a good mechanical coupling is essential to get accurate and strong signal from the sensors.

It appears that you are using hot glue to attach the ADXL 345 sensor module to the floor. My experience working with hot glue, is that hot glue tends to absorb some to the vibration. It is important to get a good mechanical coupling between the sensing element and the concrete floor. I would try to use Cyanoacrylates (super glue) to attach the ADXL 345 sensor module to the concrete floor to get a good mechanical coupling.

Note: If you use Cyanoacrylates to obtain a good mechanical coupling you will not be able to remove the ADXL 345 sensor module from the floor without destroying the ADXL 345 sensor module.

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