Slab Impulse Response

Slab IR uses a low-strain impact with an instrumented, rubber tipped hammer to send stress waves through the tested element. The element acts in a bending mode and a velocity transducer, placed adjacent to the impact point, receives this response. Both the hammer and the velocity sensor are linked to a data acquisition for processing and storage.

Both the time trace of the hammer force and the velocity transducer are processed into frequency using the Fast Fourier Transform (FFT) algorithm. Dividing the resultant velocity spectrum by the force spectrum then derives the "mobility". An example of such a mobility plot is given below for a solid concrete member.

The parameters used for the integrity evaluation are:

• The dynamic stiffness (the inverse of the slope below 80 Hz of the mobility plot, the red line)
• The average mobility and the mobility slope (the dotted green line)
• Voids ratio (the ratio between the initial mobility peak to the average mobility)