3. Introduction
• Non-destructive test
• Looks for flaws/imperfections in the material
• Can estimate thickness to within 3%
• Primarily used for concrete & masonry
• Approx. 25 years old
o Developed from ultrasonic pulse echo (1940’s)
• Similar idea to chain dragging
4. Theory – Stress (Sound or pressure) waves
Figure 1: A mechanical impact creates stress waves through a material. These
(reflected)waves can be measured to gage depth of a discontinuity. Certain
waves will dominate based on the location of the discontinuity. Wave velocities
must be known to determine depth of flaw.
5. Instrumentation
• Steel ball (4-15mm) impacts Source: http://ciks.cbt.nist.gov/~carino/ie_Fig2.GIF
(2-10 m/s) the concrete,
creating stress waves
(<80kHz & λ=5cm-6m).
o Transmitter Pulse echo
o Can be spring loaded
o Start from large impactor
small
• Transducer measures
surface displacements
o Placed adjacent to impactor
o Measures primarily P-waves
o Piezoelectric
6. Purpose
• Plain, reinforced and
post-tensioned concrete
• Can be used to sense:
– Cracks
– Delamination
– Voids
– Honeycombing
– Debonding
7. Operation
• Impact produces stress
waves in the material;
reflected waves from voids
are detected by the
transducer
• These reflected waves set up
a resonance condition
having a distinctive
frequency
• Waveform is transformed
into spectra (FFT)
• Should have an idea of what
to look for
o Size of flaw
• Operate parallel to regular
occurring grooves
9. Strengths and Weaknesses
• Only need access to one surface • Data can be difficult to interpret;
especially on thick plates or on
layered materials (overlays, soil)
o Layered- needs special attention
• Internal flaws can be detected
• Small voids can be missed
o limited by size of wavelength
• Can determine depth of the
internal flaws • Complicated geometries poses
difficulties
• Easily repeatable • Flaws beneath sensed flaw must be
evaluated from the opposite side
• Can construct a map of • Flaw detection length constraints:
discontinuities o Lmin = d/4, L>d/3
• Requires adequate frequency
resolution
12. Use of Each Transducer
• Cylindrical – For testing in narrow and
confined spaces
• Pistol Grip – is easier to use and well suited to
flat surfaces.
• Dual Head – Used for independent
measurements of wave speed which can be
used to determine depth and thickness
13. ASTM C 1383 Procedures
Figure 5: Two-step procedure for measuring plate thickness:
Procedure A is used to determine the P-wave speed and Procedure
B is used to determine the thickness frequency.
14. Suppliers and Costs
• Impact Echo Instruments www.impact-echo.com
– System “A” - $12,500
– System “B” - $11,500
– System “C” - $9,500
• Olson Instruments www.olsoninstruments.com
– IE 1 - $5,000
– IE 2 - $10,000
– IE T - $13,000
• Qualitest USA www.WorldofTest.com
– PIES System - $16,995
*does not include prize of laptop computer*
15. Test Standards
• ASTM Standard C1383-98a for measuring the
p-wave speed and thickness of concrete plates
using the impact echo method
• ACI 228.2R-98 Nondestructive Test Methods
for Evaluation of Concrete Structures
Editor's Notes
Point out surface. Above z=0 is air/solid interface. Below z=0 is material.
Long wavelengths allow the material (concrete, rock, masonry) to be modeled as a homogenous material.
Reorganize limits weaknesses
Beta is a correction factor for the geometry. Beta = 0.96 for slabs. If a crack is 1~1.5x the depth, the crack will behave as a plate of corresponding depth.
Sampling rate should be 10x faster than the fastest event you want to sample. Surface defects hard to measure (includes voids, delamination) Length of the flaw needs to be larger than ¼ the depth of the crack in order to be detectable
The dual head transducer is used to measure the speed of the P wave generated by an adjacent source.