What is Surface wave testing?
Surface wave testing analyses the propagation of surface waves to determine the mechanical and geometric properties of a concrete element as a function of depth determining layer thickness, concrete stiffness and the locations of defects or areas of weakness.
How does Surface wave testing work?
The Surface wave test works by generating surface waves using a hammer strike on the pavement or ground. Two receivers, placed at a known distance apart, capture these waves. and analyse their dispersion to determine the stiffness profile of layered systems with depth.
By measuring wave velocities, it assesses the condition and thickness of layers, identifying defects like cracks, voids or honeycombing, non-destructively.
The depth penetration of the surface wave test largely depends on the spacing between the receivers and the frequency of the surface waves generated.
Generally, the depth of investigation can range from a few centimetres to several meters, however longer spacings and lower frequencies can be used to probe up to 30m inside concrete elements in optimal conditions.
What is the applicability of Surface wave testing?
Deterioration process | Defects | Control of repairs |
Applicable to processes leading loss | Concrete cover |
How do I carry out Surface wave testing?
While no BSI, ASTM or European standards exist to codify the best practice procedure for Surface wave testing, several industry guides such as ACI 228.2R-13 have been developed which give detailed guidance on the testing method..
A generic procedure based on these documents and academic research papers is as follows:
Ensure the concrete surface is accessible and free from debris.
Determine the test area and mark receiver locations to achieve depth required.
Place two receivers on the surface at the predetermined distance.
Position the impact source midway between the two receivers on a flat surface.
Use an impactor to generate surface waves with a broad range of frequencies.
Monitor the R wave propagation from impact point to both receivers.
Record the received signals and process to identify relationship between wavelength and velocity with varied vibration frequency.
Compute cross-power spectrum to obtain phase differences as a function of frequency.
Calculate phase velocities for each frequency using phase differences and distance between receivers and plot a phase velocity/wavelength dispersion curve.
Perform inversion using forward modelling and trial and error to match the measured dispersion curve with a theoretical curve based on an assumed layered system.
Adjust the layered system model (thickness, density, and elastic constants) and recalculate the theoretical dispersion curve until it matches the experimental curve.
Once a match is achieved, the assumed stiffness profile is considered accurate, providing insight into the condition and properties of the underlying concrete layers.
What equipment and expertise are required for Surface wave testing?
Several systems for conducting SASW tests on concrete are commercially available. Olsen Instruments’ SASW-S system evaluates concrete and pavements for defects and material properties through surface wave dispersion. The setup includes receivers, mounting bars, and impact hammers, complemented by optional WinSASW software for detailed velocity profiling.
Expertise in data interpretation, understanding of geophysical principles, particularly surface wave dispersion and its impact on assessing concrete's material properties is crucial for precise assessments. Operators must be skilled in setting up and calibrating equipment, generating and recording waves, and analysing dispersion curves to infer the stiffness and condition of concrete structures. Knowledge in digital signal processing and interpretation of SASW data, including forward modelling and inversion processes, is crucial for accurate assessment. Training and experience are essential for effectively conducting SASW tests and interpreting their results accurately.
What are the advantages of Surface wave testing?
Assesses stiffness, thickness, and cracks effectively.
Suitable for complex concrete structures like tunnels.
Can evaluate fire damage throughout the structure.
What are the disadvantages Surface wave testing?
Expensive.
Requires high level of expertise.
Requires specific equipment and skilled interpretation.
Effectiveness varies with concrete's heterogeneity.
Depth detection may be limited for deeper defects.
Accuracy influenced by environmental factors.
How accurate is Surface wave testing?
The accuracy of Surface wave testing for concrete defect detection, as outlined by PCTE, is noted to be within 5% for assessing the thickness and stiffness of top layers in concrete structures.
This method's effectiveness in various applications shows that it can reliably measure and evaluate the condition of concrete, asphalt, and other materials.
What are the limits of Surface wave testing?
This test is only suitable for surfaces that are significantly smooth and may require surface preparation to allow accurate testing.
Measurements can be slow and easily influenced by environmental factors so would not be suited for applications that require rapid results or with high environmental vibration such as along high-speed train lines.
Ancillary information
Maturity of test: < 5 years
Qualification & interpretation : Specialised lab
Service disruption: No
Preliminary works: Yes
Time consumption Low (< one hour)
Cost High
Access to element 1 face