Theory
The James Vu-Con™ System uses the impact echo method to evaluate concrete and masonry structures. The impact echo method is based on the use of impact generated stress waves that propagate through the material and are reflected by both the material’s external surfaces and internal flaws.

This method can be used to make accurate non-destructive measurements of thickness in concrete slabs and plates and to locate internal flaws such as honey combing debonding, and delaminations. It can measure thickness, locate cracks, voids and other defects in masonry structures where mortar bonds the masonry together. Finally, the impact echo method is not affected by the presence of steel reinforcing bars.

The method works by creating a short duration mechanical impact on the surface of the material under examination. This is typically performed by small steel balls that produce low frequency stress waves that propagate through the material and reflect off of other surfaces and internal flaws back to the surface used for testing (See diagram, top of opposite page.) By recording and analyzing the vibration from the mechanical impact at the surface, thickness and other physical features (referred to in the preceding paragraph) can be determined. The advantage of using an impact rather than other more classic ultrasonic techniques is the low frequency and the long wave length of the stress waves that are produced. Low frequency stress waves treat concrete and masonry as a single elastic homogeneous material as they propagate through.

Results
The resulting amplitude waveform from the transducer can be displayed for analysis both with respect to time and frequency. This is illustrated by graphs. The time domain waveform can be described as a sum of a series of different frequency waveforms.

This allows the user to:

• Analyze the dominant frequencies of vibration found from the impact and determine thickness as well as distance to features within the material.
• Compare the frequency spectra to obtain indications of homogeneity of the concrete.
• Calculate the rate of decay of the vibrations to glean information regarding the sub-grade.

The Vu-Con™ System represents the latest technology in concrete ultrasonic analysis, allowing the engineer to “see” into the concrete surface.

Diagram of Vu-Con™ System Test.

Display of Frequency Spectra for a single test.

Display of time domain waveform.