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.
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
Theory – Stress (Sound or pressure) wavesFigure 1: A mechanical impact creates stress waves through a material. These(reflected)waves can be measured to gage depth of a discontinuity. Certainwaves will dominate based on the location of the discontinuity. Wave velocitiesmust be known to determine depth of flaw.
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
Purpose• Plain, reinforced and post-tensioned concrete• Can be used to sense: – Cracks – Delamination – Voids – Honeycombing – Debonding
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
InterpretationPeaks are associated with a certain “characteristic frequency”, specifyinga distance to the discontinuity.
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
EquipmentBasic Test Set Up Portable Impact Echo (PIES) Test Set Up
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
ASTM C 1383 ProceduresFigure 5: Two-step procedure for measuring plate thickness:Procedure A is used to determine the P-wave speed and ProcedureB is used to determine the thickness frequency.
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*
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