Acoustic Methods

IMPACT ECHO

Applications
  • Assess dimensions and integrity of concrete and masonry.
  • Map voids, honeycombing and discontinuities (cracks / delaminations).
  • Assess bond quality between slab overlays or the degree of slab support for ground slabs based on signal amplitude and decay.
  • Overlay thickness and slab thickness
  • Crack depth and extent
  • Map delamination
  • Determine if tendon ducts are grouted
  • Estimate compressive strength
  • Determine thickness of tunnel lining, beams where access only from one side is possible
  • Determine pavement thickness
Method

In the impact echo method the surface of a structural element is struck with a steel ball of specific size to induce a low-strain compression wave (P-wave) into the material. The wavelengths of these stress waves are typically longer than the scale of natural in-homogeneities in concrete such as aggregate, air bubbles and micro-cracks. As a result they are only weakly attenuated and multiple reflections of these waves excite natural resonances within the structure. The response is recorded by a sensitive acoustic transducer, placed on the surface near the impact point. The signal waveform is digitised and analysed in both the frequency and time domain to provide information on the element thickness and internal defects such as voids, honeycombing and discontinuities (cracks / delaminations). This technique can also be used to assess bond quality between slab overlays or the degree of slab support for ground slabs based on signal amplitude and decay.

The velocity of an acoustic wave within the material is proportional to its density and elastic modulus. This means that an estimate of the compressive strength can be made. The presence of the reinforcement does not greatly affect the transmission or the signal thus allowing accurate assessments of defects even where steel is densely spaced. Used in conjunction with GPR, this technique can verify the continuity, integrity and mechanical properties of the structure at critical points, providing greater confidence in any conclusion resulting from the investigation.

Limitations:

  • Data is collected at point samples so a large number of points are required to cover an area in detail
  • Surface needs to be smooth
  • The acoustic velocity of the concrete can vary between different batches of concrete and therefore must be recalculated against a known wall thickness for best accuracy
  • The depth of defects within approximately 30 mm of the surface cannot be determined depending on hammer size used.
  • Minimum size of defect that can be detected is approximately 60 mm. This minimum size can increase significantly with depth. Whilst honeycombing comprises of many small defects too small to detect individually, as a cluster they produce identifiable features in the acoustic response.
  • Very fine cracks greater than approximately 0.1 mm wide will block the transition of an acoustic pulse. Hence information below a cold joint or delamination cannot be determined.
  • Plastic has a much lower acoustic impedance than concrete. Hence, little energy is able to be transmitted through plastic layers into the concrete. This may require using larger hammers which produce much lower resolution. It may not be possible to effectively test the structure containing plastic laminations.
  • Depth of penetration is typically limited to 600 – 900 mm in concrete depending on hammer size used.
  • Slow to collect (approximately 25 points/hr)
Data analysis and presentation

Data is analysed in the freqency spectrum where the peak corresponds to the resonant frequency within the structure. The thickness of the slab or depth to the flaw can then be calculated based on the acoustic wave speed of the material. Thicknesses / condition assessment is usually presented in tables and / or on CAD drawings.

SONIC ECHO / IMPULSE RESPONSE

Applications
  • Determine length, continuity and structural integrity of a pile
  • Characterise low strain pile-soil system response
Method

Sonic echo is an acoustic method where a velocity transducer or accelerometer is mounted to the surface of the structure near the centre of the pile or along the edge of the pile. The top of the pile foundation must be accessible. An impact is made near the centre of the pile and the echoes from the bottom of the pile and from any defects are recorded as a velocity-time trace. The depth (d) is calculated based on the time (t) taken for the reflected acoustic wave to return to the surface using the formula d=V*t/2, where V is the wave velocity. In the frequency domain depth is calculated using the formula d=V/2/Δf, where Δf is the difference in resonant frequencies modes. Close agreement between the two methods can give a good indication of the degree of confidence in the result. For a good quality response a depth estimate should be within +/-10% of the actual depth.

In Impulse Response the force-time curve is also recorded. This allows the structures transfer function to be determined which provides further information about the structure. The coherence of the data can also be analysed to indicate data quality.

Limitations:

This method uses a low-frequency impulse, thus this method works best on piles longer than 3 m. These methods work best on piles with length to diameter ratios from 7:1 up to 40:1 although ratios of 4:1 are sometimes possible with careful selection of hammers and displacement transducer type.

All piles produce resonant frequencies across the width of the pile. These waves will decay exponentially, but in short piles these waves swamp the return signal from the bottom of the pile. Post-processing can sometimes be used to filter out this type of response to reveal the bottom echoes. Defects in the pile itself such as: necking, bulging, cracking and voiding; can result in additional reflected waves. Such defects are usually picked up by careful comparison of the signals between sound and defective piles. In the case of a serious defect or discontinuity, it will fully reflect the wave allowing the defective pile to be identified but may prevent detection of the total depth of the pile.

Additional complications in signal generation can occur when the pile penetrates through a soft stratum into a hard stratum. This transition will produce its own return signal. Also piles through dense material will produce results slightly longer than the true length because of the damping effect of the pressure wave along the outer surface of the pile. In soils with high strength, there may be situation where no bottom reflection is visible. This usually happens when the length exceeds 20 to 40 diameters depending on the amount of friction. Typically, maximum length of piles that can be tested is limited to approximately 15 m. In the case where the end of the pile is socketed into rock of similar acoustic properties as the concrete, no return reflection is generated or may be too weak to detect even at lengths shorter than 10 diameters.

To be effective this method requires access to a clean top surface of the pile, the concrete must be more than seven days old and the pile needs to have a constant cross section. In the case where a pile cap is fitted this method cannot be used effectively. This is because the discontinuity in the geometry and the discontinuity of the pour joint results in a loss of energy travelling down the pile. Further, the additional reflections from within the pile cap will clutter the signal return from the bottom of the pile. In the case of a superstructure mounted onto the pile, this can also have adverse effects as vibrations induced in the superstructure can also mask reflections from the bottom of the pile.

Data analysis and presentation

Data is presented in tables.

SLAB IMPULSE RESPONSE

Applications
  • Identify areas of voiding or loss of support below pavements
  • Location of weak concrete
  • Locate areas of delamination within the pavement
Method

Slab Impulse Response is an acoustic method where a velocity transducer (or geophone) is mounted to the surface of the structure and an impact load is applied a few centimetres away. Both the force-time curve and the surface velocity-time curve are recorded. The system is designed to identify areas of voiding below pavements less than 400 mm thick or to quickly locate areas of delamination or weakness within the pavement. The average mobility (velocity/unit force) and flexibility (displacement/unit force) is calculated at each test point. Typically a grid of test points is recorded at a set increment to locate areas with relatively higher mobility or higher flexibility readings. Relatively low mobility and flexibility readings indicate that such areas are more solidly supported or bonded than those areas with relatively high mobility and flexibility.

Data analysis and presentation

Data is presented in tables or contour maps.

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