Bridge – FHWA InfoTechnology

Bridge - Ultrasonic Surface Waves (USW)

Target of Investigation

The ultrasonic surface waves (USW) technique has the following applications:⁽¹⁾

Concrete quality control and evaluation (modulus and strength, using the correlation with modulus).
Indirect assessment of delamination in bridge decks.
Estimates of the depth of vertical cracks within bridge decks or other elements.
Condition assessment through evaluating probable material damage from various causes: alkali-silica reaction (ASR), fire, freeze-thaw, and other cracking processes.

Description

The USW technique is an offshoot of the spectral analysis of surface waves (SASW) method used in evaluating material properties (elastic moduli) in the very near surface zone. SASW utilizes the phenomenon of the surface wave dispersion, i.e., velocity of propagation as a function of frequency and wavelength, in layered systems to obtain information about layer thickness and elastic moduli.⁽²⁾ The USW test is identical to the SASW test except that the frequency range of interest is limited to a narrow high frequency range where the velocity of the surface wave (phase velocity) does not vary significantly with frequency. The surface wave velocity can be converted to the material modulus, concrete modulus in the case of bridge decks, using either the measured or assumed mass density or Poisson’s ratio of the material. A USW test consists of recording the response of the deck at two receiver locations due to an impact on the surface of the deck.

Ultrasonic Surface Waves (USW) Equipment. The pictured USW equipment has three vertical, cylindrical objects and one metal box, all attached to two large, rounded, horizontal connecting bars. The two cylindrical objects closest to the box are sensors. The third cylindrical object on the right end of the connecting bar is an impact source. Yellow cables run from the metal box to each of the cylindrical objects. — Figure 1. Photo. Ultrasonic Surface Waves (USW) Equipment.⁽²⁾

Physical Principle

Surface waves are elastic waves that travel along the free surface of a medium and, in most cases, is the predominant part of energy in comparison to body waves—compressive (P- waves) and shear waves (Swaves).⁽²⁾ The arrival of the surface (Rayleigh) wave follows the arrival of the two body wave components because it is the slowest one (figure 2).

Typical Time Record Used in Surface-Wave Method. The x-axis of the graph is Time, and the y-axis is Amplitude. A plotted Time-Amplitude record has three labeled points. Closest to the y-axis, with the shortest time measurement, is point P, for a compression wave arrival. The next labeled point is S, for a shear wave arrival. The amplitudes of the shear wave are higher than of the compression wave. The furthest labeled point, and thus with the longest time measurement, is R, for a surface or Rayleigh wave. The surface wave has the highest amplitude of the three waves. — Figure 2. Graph. Typical Time Record Used in Surface-Wave Method.⁽²⁾

The surface waves propagate radially from the impact source, forming a cylindrical front with a velocity dependent on the elastic properties of the medium. The waves propagating in a heterogeneous medium are dispersive; that is, waves of different wavelengths or frequencies travel with different velocities. Thus, the information about the subsurface can be obtained through measuring the phase velocity versus frequency relationship (dispersion curve) and back calculation of the dispersion curve to obtain the profile of the tested system. 

Surface wave energy extends to a depth of approximately one wavelength. At wavelengths less than or equal to the thickness of the layer, the velocity of surface-wave propagation is more or less independent of wavelength (figure 3). Therefore, if the measurement is limited to wavelengths not exceeding the thickness of the deck, the velocity of surface waves will be dependent only on the concrete modulus. An average velocity is used to correlate it to the concrete modulus. Significant variation in the phase velocity indicates the presence of delamination or other anomaly. Elastic waves are generated by means of impacts, detected by a pair or an array of receivers and recorded by a transient recorder.

Surface-wave Velocity vs. Wavelength (top) and Evaluation of a Layer Modulus by SASW (USW) Method (bottom). The top figure has two, two-dimensional components. On the left is a two-layer half-space, a rectangle with the top part labeled V subscript R1 and the bottom part labeled V subscript R2. On the right is a plot of a dispersion curve. The x-axis is Phase velocity. The y-axis is Wavelength. A plotted line has two sections. The top part of the curve corresponds to V subscript R1 and descends vertically. The bottom part of the curve corresponds to V subscript R2 and curves downward to the left, indicating decreasing Phase velocity and decreasing wavelength. The bottom portion of the figure is a schematic of the use of ultrasonic surface waves equipment. The bottom figure is a surface with an impact source on the left and two receivers to the right. Signals from the receivers go up to a display of Coherence and Phase, which outputs a dispersion curve with a Wavelength versus Phase velocity plot, which in turn produces a Young’s Modulus Profile plot of Depth versus Young's modulus. The Young’s Modulus Profile is approximately a vertical line and is described as “Wavelength considered less than layer thickness.” — Figure 3. Illustration. Surface-wave Velocity vs. Wavelength (top) and Evaluation of a Layer Modulus by SASW (USW) Method (bottom).⁽²⁾

Data Acquisition

The basic components of USW equipment include a source, at least one pair of receivers, and a portable computer with controls and data acquisition software.⁽¹⁾ The USW equipment enables both automatic and semi-automatic data collection and data processing. Figure 4 shows an example of a simple USW device, called portable seismic property analyzer (PSPA), with a solenoid type impact source and two receivers (accelerometers). Once the device is placed on the ground, a series of impacts from the source is detected by the receiver pair and recorded on a portable computer.

USW Testing Using PSPA in a Mobile Cart. A worker is on a bridge deck with a portable seismic property analyzer in a mobile cart. A portable computer is on a shelf mounted near the top of the cart handles. — Figure 4. Photo. USW Testing Using PSPA in a Mobile Cart.⁽¹⁾

Data Processing

In most systems, compressional waves arrive at the receivers first, followed by shear waves and then surface waves.⁽³⁾ However, more than two-thirds of the energy in the wave train is in the surface waves, making them more easily studied. Figure 5 illustrates the relative arrival of the wave train at a near and far receiver. Figure 6 shows how a phase lag in the arrival of a wave component to two or more receiver locations can be used to measure the travel time between them. The received signals are processed, and a subsequent calculation scheme is used to calculate the concrete modulus directly in the field (figure 7).⁽⁴

Recorded Waveforms. The x-axis of the graphs is labeled Time in milliseconds. The graphs contain two plots, one for a recording by a near receiver, and one for a recording by a far receiver. The near receiver waveform is large initially and then gradually dissipates. The far receiver waveform takes longer to reach its peak and is not as large as the near receiver waveform. — Figure 5. Graph. Recorded Waveforms.⁽³⁾

Velocity Calculation. The schematic is of a surface being impacted by a source on the left. The impacts are being recorded by two receivers on the right. t subscript 1 is the arrival time at the receiver closest to the impact, and t subscript 2 is the arrival time at the receiver furthest from the impact. Associated with time t subscript 1 is phase angle phi subscript 1. Associated with time t subscript 2 is phase angle phi subscript 2. Two equations are given: t equals t subscript 2 minus t subscript 1, and phi equals phi subscript 2 minus phi subscript 1. — Figure 6. Illustration. Velocity Calculation.⁽³⁾

Testing Device (left) and Screen (right) During Data Collection. The left photo is an ultrasonic surface waves testing device. It has three vertical, cylindrical objects and one metal box, all attached to two large, rounded, horizontal connecting bars. Two of the cylindrical objects are on the right end of the connecting bars. The third cylindrical object and the metal box are on the left end of the connecting bar. Yellow cables run from the metal box to each of the cylindrical objects. The right photo is a display from the testing device’s computer screen. Two portions are highlighted with labels: Average Modulus and Modulus versus Depth. — Figure 7. Photo. Testing Device (left) and Screen (right) During Data Collection.⁽²⁾

Data Interpretation

Data from USW testing are typically presented in terms of the concrete modulus distribution (figure 8). Very low modulus is often an indication of presence of delamination or cracking and does not necessarily represent the actual concrete modulus at the test location.

Concrete Modulus Distribution from USW Testing. The x-axis is Longitudinal Axis in feet and ranges from 0.5 to approximately 18.5. The y-axis is Transverse Axis in feet and ranges from 1 to 7. Beyond the right edge of the graph is a color scale of modulus in kilopounds per square inch. The scale ranges from 2500 to 5000, with corresponding colors ranging from red (low modulus), to yellow, to green, to blue (high modulus). The face of the map is completely covered with the colors. Blues and greens are the most prevalent colors. Reds and yellows are more prevalent in the center of the map. — Figure 8. Contour Map. Concrete Modulus Distribution from USW Testing.⁽⁵⁾

Advantages

Repeatable.
Provides modulus of the concrete for appropriate design.
Can take the place of lab testing of cores or cylinders.
Can be conducted within hours of construction.
Can characterize the depth of vertical cracks.

Limitations

Slower data collection.
Road and bridge closures necessary for surveying existing pavements.
Expertise and training required for equipment setup, data collection, data processing, and data interpretation.
Higher level expertise and experience required to interpret test results for concrete deck deteriorations such as debonding/delamination.
More complicated SASW modulus evaluation for layered systems like decks with asphalt concrete overlays.

References

Gucunski, N., et. al., Nondestructive Testing to Identify Concrete Bridge Deck Deterioration, Report S2-Ro6A-RR-1, SHRP2 Renewal Research, Transportation Research Board, 2013.
Gucunski, N., et.al., Comprehensive Bridge Deck Deterioration Mapping of Nine Bridges by Nondestructive Evaluation Technologies, Project SPR-NDEB(90)-8H-00, Center for Advanced Infrastructure and Transportation, Rutgers University, January, 2011.
Central Federal Lands Highway, “Spectral Analysis of Surface Waves (SASW) and Ultra Sonic Surface Wave (USW) Methods,” (Website) Lakewood, CO. Accessed online: February 13, 2015. http://www.cflhd.gov/resources/agm/engApplications/Pavements/413SpecAnalySurfWaveandUltrSonicSurfWaveMethods.cfm
ACI, Nondestructive Test Methods for Evaluation of Concrete in Structures, ACI 228.2R98, American Concrete Institute, June, 2013.
Azari, H., Yuan, D., Nazarian, S., and Gucunski, N., “Sonic Methods to Detect Delamination in Concrete Bridge Decks: Impact of Testing Configuration and Data Analysis Approach,” Transportation Research Record (TRR): Journal of Transportation Research Board, No. 2292, Transportation Research Board of the National Academics, Washington, DC, pp. 113-124, 2012.