What is Impact-Echo Testing?

Impact-echo is a nondestructive test that uses the stress waves of a mechanical impact to find delaminations and measure thickness in concrete.

How it works

An operator taps the concrete surface with a small steel ball or similar impactor. The impact sends a P-wave into the concrete. The wave reflects from the opposite face of the slab and from any flaw that lies in its path. A transducer next to the impact point records the surface motion. A Fourier transform converts the record into a frequency spectrum.

In a plate, the reflections between the two faces give a thickness frequency f. The thickness T follows from T = 0.96 × Cp / (2f), where Cp is the P-wave speed (Carino, equations 6 and 7). A flaw such as a delamination reflects the wave earlier. It gives a peak at a higher frequency and a lower thickness frequency than a sound slab.

What it finds

A flat concrete-air interface that lies parallel to the surface is the easiest flaw to find. Delaminations and voids are examples. The method also finds honeycombed concrete and voids in tendon ducts.

Standard

ASTM C1383 gives two procedures for plates. Procedure A measures the P-wave speed from the travel time between two transducers. Procedure B finds the thickness frequency. A plate in this standard has lateral dimensions of at least six times its thickness.

Limits

• Impact-echo is a contact test at single points, so an operator must be at each point.
• Wave speed varies inside one pavement, so each point needs its own speed measurement.
• Thickness uncertainty is typically 5 to 10 percent.
• The result needs trained staff to interpret.

Airfield use

The sources that this page uses describe highway pavements and bridge decks. No source that was opened names impact-echo for airfield pavement. AC 150/5380-6C section 2.6 says that airports should consider nondestructive testing data but does not name this method.