Guided Waves Propagation (GW) techniques


Main objective

Detection of the damage in structural health monitoring of the reinforced concrete can be studied with guided waves propagation survey as a promising and non-destructive testing. Guided waves technique enables examination within seconds on large areas and with limited number of sensors mounted on the structure. Guided waves can propagate over many tens of meters therefore it is possible to monitor areas that are very costly to inspect using other methods; for instance, insulated pipelines or buried sections of road crossings.

In variety of fields guided waves can be used for inspection of reinforcement and other metal structures, because of sensitivity greater than other conventional non-destructive methods. Guided waves techniques give possibility to inspect the state of the structure for destructive changes in a semi-automated way. Used in rapid testing or screening tools to detect, locate and classify corrosion defects. Permanently installed sensors allow critical sections of infrastructures, such as bridges, to be regularly and efficiently monitored. Regular data collection from fixed positions greatly enhances the usefulness of guided waves.


Functioning mode

Guided waves technique can be performed in two modes of wave generator frequency:* high frequency mode – used for continuous monitoring

  • low frequency mode – used for short-term measurements

From the point of methodology there are two important approaches:* pulse-echo based mode – structure is excited with narrow pulse and the sensors detect the echo of the pulse coming from discontinuities; signals from the defects are filtered out and defect location can be obtained by use of the wave speed; reflected signal associated with the damage detected,

  • pitch-catch based mode – pulse is sent across the sample and the sensor at the other end receives the signal; studying various characteristics of the received signal such as amplitude, frequency content, delay in time of transmit information about damage can be extracted; diffracted signal associated with the damage detected.

Guided waves propagation system includes device for wave generation and acquisition system of spring waves.

There are few transducer technology used:* Systems with piezoelectric transducers

The sensors are embedded or mounted on the surface of the inspected element. Piezoelectric sensors are inexpensive and available in different thicknesses. Materials used in production of piezoelectric are mostly zirconium titanate ceramics and polymer film made from polyvinylidene fluoride. * Systems with piezocomposite transducers

Piezocomposite transducers were introduced mostly to overcome the issue with brittleness of standard ceramic piezoelectric transducers. * Systems with non-piezoelectric transducers:

  • fiber optic sensors,
  • flat magnetostrictive sensors.

Fiber optic sensors are taken into consideration where comes to long term structural monitoring with guided waves, however to acquire the associated support equipment higher cost will be generated. Flat magnetostrictive sensors consist of a thin nickel foil with a coil which can be permanently bonded to the surface of inspected element .


  • Impact-Echo test (IE) – general working principle is shown on the Fig.1. Instrumentation consists of:
    • a mechanical impactor capable of producing short-duration impacts, the duration of which can be varied,
    • a high fidelity receiver to measure the surface response,
    • data acquisition – signal analysis system to capture, process and store the waveforms of surface motion.

The duration of the impact is critical for success of an impact-echo test. The impact gives rise to modes of vibration and the frequency of these modes is related to the geometry of the test object and the presence of flaws.
Fig 1. Scheme of the method.

* Ultrasonic Pulse Velocity Test (UPV) – a pulse of longitudinal vibrations is produced by an electro−acoustical transducer, which is held in contact with one surface of the concrete under test. When the pulse generated is transmitted into the concrete using a liquid coupling material such as grease or cellulose paste, it undergoes multiple reflections at the boundaries of the different material phases within the concrete. A complex system of stress waves develops, which include both longitudinal and shear waves, and propagates through the concrete. The first waves to reach the receiving transducer are the longitudinal waves, which are converted into an electrical signal by a second transducer. Three modes are generally used:

  • Opposite faces (direct transmission) mode
  • Adjacent faces (semi−direct transmission) mode
  • The same face (indirect or surface transmission) mode.

Fig 2. Transducers arrangement

* Spectral Analysis of Surface Waves (SASW) - uses the dispersive characteristics of surface waves to determine the variation of the surface wave velocity (stiffness) of layered systems with depth.

Process/event to be detected or monitored

Guided waves techniques can be generally applied to detect:* the presence of discontinuity from the wave signal diffracted by the crack.

  • detection of delamination and debonding of the concrete structures.
  • surface cracks depth
  • homogeneity of concrete
  • quality variation of concrete
  • detection of voids, imperfections
  • determination of the age of concrete.

Impact Echo investigations are performed to assess the condition of slabs, beams, columns, walls, pavements, runways, tunnels, dams, and other structures. voids, honeycomb, cracks, delaminations and other damage in concrete, wood, stone, and masonry materials can be found utilizing the IE method. IE investigations are also performed to predict the strength of early age concrete if the member thickness is known, and to measure the thickness of structural members.

Spectral Surface Waves Analysis can be applied for:

• determination of pavement system profiles including the surface layer, base and subgrade materials
• determination of surface opening crack depths
• freeze-Thaw damage depth measurement
• fire damage depth measurement
• determination of abutment depths of bridges
• condition assessments of concrete liners in tunnels, and other structural concrete conditions

Physical quantity to be measured (e.g. actions, displacements, deformations, dynamic structural properties, material properties including mechanical, electrical and chemical properties, relative displacements of the two sides of a crack, etc.).

Strength of longitudinal and/or flexural wave transmission along a rebar:* amplitude of the signals [mv],

  • amplitude of the noise [mV],
  • ringing zone duration [µs],
  • speed of the wave [m/s],
  • thickness of elements [mm].

Induced damage to the structure during the measurement

No damage is induced during the measurement.

General characteristics

Measurement type (static or dynamic, local or global, short-term or continuous, etc.)

Guided waves techniques are dynamic and both local and global measurement, which offers both short-term and continuous monitoring.

Measurement range

    • Guided Wave propagation:

Range depends on the thickness of the sample and the test method. For 5 mm specimen the maximum frequency range is around 250 kHz with a 10 mm minimum wavelength. Transducers working frequency usually lies between 100 up to 240 kHz.
Wave generator – work parameters and example of ranges:


range of work frequencies: usually from a few ten to several hundred hertz, 30 – 350 kHz
  • number of transmitting/measuring channels - 1/15 (16) or 1/7 (8)
  • number of periods of lambda waves packs: 1-16
  • modulation of wave packets: Hanning window, triangular, rectangular
  • delay between generated packets: 1-4095 ms;
  • amplifier output voltage: ±100 V
  • regulation of the receiving circuit: .1, ½, ¼, ⅛, 1/10, 1/20, 1/40, 1/80
  • sampling frequency of the measurement path: 2,5 MHz
  • analog-to-digital converter: 24 bit.
  • Ultrasonic Pulse Velocity Test (UPV):
  • transit time measurement Range 0.1 – 9999 µs + auto-ranging.
  • resolution 0.1 µs.
  • Impact-echo method:


Table 1. Data acquisition parameters

Measurement accuracy

  • Impact-echo method - the defect depth determination is possible with accuracy ranging from 60.00 - 99.05%. Stronger concrete gives better accuracy in determining the depth of defect. The size of the smallest flaw that can be detected increases as the flaw depth increases.
  • Ultrasonic Pulse Velocity – the compressive strength evaluation by means of UPV can be performed with up to 94% accuracy.
  • SASW measurements are accurate to within 5% for the determination of the thickness and stiffness of the top layer during pavement layer analysis or of the concrete liner of a tunnel.

Background (evolution through the years)

First studies over guided waves propagating were performed in the seismology domain in early 1920. Guided wave technique was developed in 1990 for the rapid survey of pipes, for the detection of both internal and external corrosion. The main area of application has been to pipes and pipelines .


General points of attention and requirements

Design criteria and requirements for the design of the survey

The wave should propagate in a spiral - a necessary condition that various types of reflected waves do not disturb the measurement and the attenuation along the way should be small enough to be able to record the returning wave (too much attenuation extinguishes the wave energy). For proper analysis of the failures there should be noted some information about the structure in undamaged state or stage when the inspection is planned. Surface should be prepared before mounting the sensors – evened with sandpaper and degreased.

Procedures for defining layout of the survey

No specific procedures.

Design constraints (e.g. related to the measurement principles of the monitoring technologies)

The SASW method can be performed when an accessible surface is available for receiver mounting and impacting.

Sensibility of measurements to environmental conditions

In case of Impact-Echo method special attention should be given to the surrounding environment. If the stiffness of bedrock or underlying slab is very close to those of the main concrete element, the accuracy of the method will be affected.
When testing with SASW, in inclement weather, receivers must be protected against rain drops. Care should be taken when lateral variations might be expected at the site because they may more easily affect long arrays. Passive methods may face difficulties in very quiet sites where the level of ambient vibrations is very low or in case of stiff soil to rock conditions, where the mechanism of generation and propagation of surface waves is less efficient.


Procedures for calibration, initialisation, and post-installation verification

For accurate results it is necessary to have good practice and theoretical knowledge of the effects of damage on the wave characteristics. Base signal line should be obtained in the healthy state for use as reference in comparison with results in time. Optimization of the wave propagation in initial phase can be done by adjusting frequency and number of cycles. Data cleansing should be performed before the actual measurement.

* Ultrasonic Pulse Velocity Test (UPV)

The distance (path length) between the transducers should be measured as accurately as possible. It is very important to ensure adequate acoustic coupling of the transducers to the surface under test. A thin layer of couplant should be applied to the transducer and the test surface. In some cases it may be necessary to prepare the surface by smoothing it. For compound measurements and uniformity testing a test grid should be drawn out on the surface. Rebars affect the ultrasonic measurements as the signal will travel faster through the rebar than through the concrete. The location of rebars should be determined using a rebar locator.

Procedures for estimating the component of measurement uncertainty resulting from calibration of the data acquisition system (calibration uncertainty)

Guided wave behaviour is directly affected by thickness of the specimen and frequency used. Measurement uncertainty can be evaluated with statistical methods.

Requirements for data acquisition depending on measured physical quantity (e.g. based on the variation rate)

Data processing algorithm pass the signals along the same radial line.


Requirements and recommendations for maintenance during operation (in case of continuous maintenance)

Signal processing algorithm should be capable of running in real-time and frequent intervals, possibly during operation of the structure. System should be able to distinguish between signal changes due to damage processes and changes to environmental conditions. These environmental changes should be compensated with efficient signal-processing methods. Elimination of the sensitivity to temperature changes.

Criteria for the successive surveying campaigns for updating the sensors. The campaigns include: (i) Georeferenced frame, i.e. the global location on the bridge; (ii) Alignment of sensor data, relative alignment of the data collected in a surveying; (iii) Multi-temporal registration to previous campaigns; and (iv) Diagnostics.

No data available.


The inspection report should include: * date of the inspection,

  • dimensions of the investigated area or sketch of the structure,
  • ambient temperature,
  • registered signals.

In case of continuous monitoring usually the system is equipped with a software and reports are generated automatically with scans, reflection annotations and operator notes are stored in the cloud.

Lifespan of the technology (if applied for continuous monitoring)

The guided waves technique can be used in short-term inspection and continuous monitoring over large volumes with mounted sensors. Continuous measurement can take from few hours to years.

Interpretation and validation of results

Expected output (Format, e.g. numbers in a .txt file)

  • Guided Wave Propagation method:

Time domain response vs amplitude of piezo sensors in a graphical form on the computer.

An example of base signal with the recorded signal from a rebar in concrete structure is shown below:

Fig. 3. Time domain response from the inspected concrete area

In the pictures below there is visible an output without any damage detected and additional signal with lower amplitude in the middle on the second graph resulted from crack in the damaged rebar.

Fig. 4. Time domain response for healthy and damaged structure

* Impact - Echo Method

Test results can be saved to a file in the computer for later examination and analysis, and for printing copies of test results. The results of a single test are stored as a unique record in a random access file. The file can be on the hard disk or on a floppy disk or another storage device connected the computer.

Fig 5. Test Record

  • Ultrasonic Pulse Velocity Test :

Fig. 6. Ultrasonic test output

  • Spectral Analysis of Surface Waves (SASW)


Fig 7. SASW Output

Interpretation (e.g. each number of the file symbolizes the acceleration of a degree of freedom in the bridge)

Wave propagation characteristics corresponds to different structural conditions. The results of the experiment show measured excitation vibrations and then reflected mechanical waves from the defects. Knowing the material and dimensions of the object, it is possible to calculate whether the given reflection comes from damage to the structure. Higher signal strength at the receiving end indicates debonding and corrosion. * Impact - Echo Method – example response of the concrete slab with detected voids:

Fig 8 Concrete slab with voids


Specific methods used for validation of results depending on the technique

Numerical methods used for studies on relationship between arrival time, amplitude of waves – models used for fitting with experimental data. In order to extract effective information on damages from received signals it is usually necessary to perform a simulation of the issue by numerical method.
selective verification of key conclusions after Impact-Echo test should be undertaken by drilling cores or drilling holes in combination with visual inspection using a borescope. Verification is especially important when evaluating a complex structure.

Quantification of the error

Numerical simulations of wave propagation, Finite Element Method, Boundary Element Method are used and models are compared with experimental data.

Quantitative or qualitative evaluation

The global matrix approach is applied for quantification of the reflection and transmission coefficients of discontinuities from measured data.

Detection accuracy

Detection accuracy depends on the signal-to-noise ratio which can be enhanced depending on the system configuration and environmental conditions. Statistical averaging is used to reduce global noise and improve the accuracy. For Ultrasonic Pulse Velocity (UPV) depending on the detected quantity – the accuracy level can be on the level of 97,0 % .


  • propagation over long distances
  • sensitivity to different type of flaws
  • detection of damages from remote position, with use of surface-mounted sensors
  • combined with ultrasonic tomography give information such as localization of the damages in the cross-section, degree of degradation, determination of which rebars are damaged
  • performance over long range with an accurate sensitivity
  • testing of multi-layered structures
  • fully automated data collection


  • different damage configurations requires numerical analysis
  • difficulties in extracting desired wave modes
  • dependence on the thickness and shape
  • interpretation of the results highly dependent on the operator.

Possibility of automatising the measurements

In case of impact- echo method there are some attempts for automatisation of the whole measurement and an example can be found in - . However data collection in all guided waves techniques is fully automated.


High power requirements for the system used for structural health monitoring in real-time for excitation of guided waves with an accurate scan range.

Existing standards

  • BS 9690-1:2011, Non-destructive testing. Guided wave testing. General guidance and principles,
  • BS 9690-2:2011, Non-destructive testing. Guided wave testing. Basic requirements for guided wave testing of pipes, pipelines and structural tubulars,
  • ASTM E2775 – 16, Standard Practice for Guided Wave Testing of Above Ground Steel Pipework Using Piezoelectric Effect Transduction,
  • E2929 – 13, Standard Practice for Guided Wave Testing of Above Ground Steel Piping with Magnetostrictive Transduction,
  • ISO 18211:2016, Non-destructive testing — Long-range inspection of aboveground pipelines and plant piping using guided wave testing with axial propagation,
  • PCN/GEN GUIDED WAVE TESTING ISSUE, 1 Rev A, 1st January 2013 – General requirements for qualification and PCN Certification of Guided Wave testing personnel.


Relevant knowledge fields Civil engineering:* adhesive bonding

  • diagnostics of screw connections
  • insulated, underground pipelines. Geotechnics:* diagnostics of rock and ground anchors

  • strengthening of slopes and tunnels walls
  • seismic exploration
  • erosion process. Aerospace structures:* aircraft fuselage structures

  • rocket motors
  • fuel tanks.

Performance Indicators

  • cracks
  • holes
  • wire break
  • loss of section
  • frequency
  • vibrations/oscillation
  • obstruction/impeding
  • displacement
  • reinforcement bar failure/bending
  • stirrup rupture
  • debonding
  • delamination
  • spalling
  • tensioning force deficiency.

Type of structure

  • suspension bridges
  • wind turbine blade
  • metallic structures
  • adhesive joints

Spatial scales addressed (whole structure vs specific asset elements)

Guided waves propagation inspection is addressed to whole structures since wave excited at one end of the structure propagate through the volume and the waveforms can be registered at place.


  • steel
  • concrete
  • glass fibres
  • polymers
  • composites.

Available knowledge

Reference projects

WI-HEALTH: Wireless network for total SHM of bridges (EU FP7 2011-2013).


Future considerations on development of systems for structural health monitoring concern use of carbon and boron nanotubes as transducers. High cost and other problems are limiting use of nanocomposites in monitoring for now.

Manufacturers websites:

Guided Ultrasonics LTD.

Guided Ultrasonics LTD. WavePro4tmanalysis software

Guided Ultrasonics LTD. Training Descriptions


Mistras Hellas A.B.E.E. – Impact-echo technology overview, products, software

ACS-Solutions GmbH Science – Ultrasonic test equipment

Olson Instruments – Spectral Analysis of Surface Waves - Products

Olson Instruments – Impact Echo Scanning

General information:

Ultrasonics Pulse Velocity test methods – overview

Ultrasonic Pulse Velocity Test for Concrete - video

Guided Wave Ultrasonic Inspection – video

Impact – Echo method – how to test concrete - video

Impact-Echo – User’s Manual


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