SASW and USW are used to determine pavement thickness as well as to evaluate determine linear elastic modulus of pavement layers. The USW technique, and presumably the SASW and MASW techniques, can be used to measure modulus of base materials by adjusting the sensor spacing and frequency range of the instrument. Since the materials are not consolidated the velocities and moduli will be lower. This requires lower frequencies for the investigation.
Basic Concept: The surface wave methods including Spectral Analysis of Surface Waves (SASW), Multi-Channel Analysis of Surface Waves (MASW), or Ultra Sonic Surface Wave (USW) methods are used to determine linear elastic modulus of pavement layers. These methods operate by measuring surface seismic waves generated by an impulsive source (i.e. a hammer or electrically activated solenoid). As long as the wavelengths studied are smaller than the thickness of the exposed layer in question, the modulus computed will be indicative of that layer. The Ultra Sonic and SASW methods use this approach with two receivers to measure just the properties of the exposed layer. These methods measure in a few seconds and the instruments are easily portable. The MASW method uses more receivers (channels) to get information on all of the layers in a pavement system, as described in Multchannle Analysis of Surface Waves Methods.
Figure 76. Seismic waves generated.
Data Acquisition: All of the systems to measure velocity and determine modulus use some method of deploying a source with two or more accelerometers located at known distances, usually inline, from the source. When the source is fired the short-duration pulse creates a packet of seismic waves that travel through the body and along the surface of the pavement. See figure 76.
Data Processing: In most systems, compressional waves arrive first at the receivers followed by shear waves and then surface waves. However, more than fifty percent of the energy in the wave train is in the surface waves, making them more easily studied. Figure 77 illustrates the relative arrival of the wave train at a near and far receiver. Figure 78 shows how two or more receivers can be used to measure the travel time between them using the recorded waves.
Figure 77. Recorded waveforms.
Several companies manufacture instruments for SASW; two sets of such equipment are illustrated below in figure 79.
Data Interpretation: It is easier to measure surface wave velocity, but shear wave velocity is required to compute Young's modulus. The conversion equation below is used.
Calculate Shear Wave Velocity (VS) from Raleigh (or Surface) Wave Velocity (VR)
VS = VR (1.13 - 0.16 ν)
Following the conversion to shear wave velocity, the computation of Young's modulus is made with the formula below.
Calculate Young's Modulus (E)
E = 2 (ρ) VS² (1 + ν)), where ν) = Poisson's Ratio, ρ = Mass Density
The above computation is usually made for a number of frequencies in the surface wave data packet, using Fourier analysis. These values can be averaged over an appropriate number of frequencies (or wavelengths) to obtain an average modulus for a pavement of a certain thickness.
Figure 78. Velocity calculation.
Calculate Shear Wave Velocity (VS) from Raleigh (or Surface) Wave Velocity (VR)
VS = VR (1.13 - 0.16 ν)
Following the conversion to shear wave velocity, the computation of Young's modulus is made with the formula below.
Calculate Young's Modulus (E)
E = 2 (ρ) VS² (1 + ν),
where
ν = Poisson's Ratio,
ρ = Mass Density
The above computation is usually made for a number of frequencies in the surface wave data packet, using Fourier analysis. These values can be averaged over an appropriate number of frequencies (or wavelengths) to obtain an average modulus for a pavement of a certain thickness.
The modulus of pavement can be specified before construction using techniques developed for Texas Department of Transportation (see Bibliography). Modulus standards are beginning to be set by AASHTO and other organizations.
Advantages: This is a test method that can take the place of lab testing of cores or cylinders. Test can be made within hours of construction.
Limitations: These measurement systems must occupy a particular location to be tested and remain stationary during the measurement process, with set and measurement time proportional to the number of receivers used. Therefore, road and bridge closures would be necessary for surveying existing pavements.
Figure 79. Spectral Analysis of Surface Waves Instruments (a) Portable Seismic Property Analyzer (PSPA) by Geomedia Research and Development, (b) SASW 1-S by Olson Instruments, Inc.
Spectral Analysis of Surface Waves and Impact-Echo Combined
Ultrasonic Seismic (SASW) and Impact Echo, particularly when applied together in an integrated instrument, such as the Portable Seismic Property Analyzer (PSPA), are high-frequency, acoustic (seismic) geophysical methods. Concrete condition assessment by integrated ultrasonic methods has recently been applied to a number of engineering problems associated with both new and aging concrete structures. Recent work on evaluating concrete integrity, particularly looking for the formation of corrosion-induced delamination in early, moderate, and late stages of development, has demonstrated that these integrated methods show considerable promise in terms of current and potential capability.
In the transportation sector, these integrated surveys can be used for quality assurance (QA) verification of new construction (thickness determination and homogeneity of concrete pour, even segregation of aggregates or suspected voids) and calculation of mechanical properties on bridge decks and pavements.
Ultrasonic methods implemented in various forms of integrated ultrasonic seismic devices, such as the Portable Seismic Property Analyzer (PSPA), can be successfully used to assist in evaluating concrete pavement structure condition, also alone (like GPR) or in tandem with other geophysical and/or ground truth sampling. However, it is always recommended to use a multiple-method approach.
Of special interest are three ultrasonic techniques: ultrasonic body-wave (UBW), ultrasonic surface-wave (USW), and impact echo (IE) geophysical methods that can be used separately or in combination. The first two are used in concrete Characterization, and the IE method is used primarily in delamination detection once a propagation velocity of the seismic wave in the concrete is established. The primary advantage that the IE method has over the current practice of chain dragging is that it allows detection of delamination zones in various stages of deterioration.
Finite-element simulations of two probable scenarios of delamination progression, demonstrated that periodic monitoring of concrete pavement structures by IE enables improved prediction of the deterioration processes. Three-dimensional data visualization techniques constitute important components in condition assessment and delamination detection using the IE method. Results include three-dimensional translucent visualizations of a concrete pavement structure section, horizontal cross sections through all distinctive zones (including a zone of delamination), and vertical cross sections along chosen test lines. Visualization techniques enable the PSPA to be used as a kind of concrete pavement structure sonar device.
Combined ultrasonic seismic techniques, however, do require more sophisticated analysis than the GPR and half-cell corrosion potential methods or the GPR or pachometer. The pachometer is not a recommended tool for QA verification of rebar PPD if accuracy within 0.64 cm is consistently expected, and if high repeatability is desired. On some of the QA applications, however, such as determination of concrete integrity on new and fairly uniform structures, use of the acoustic techniques can be straightforward and very simple. A simple impact-echo device used to measure concrete thickness, for example, is a concrete thickness meter that can accurately determine thickness for QA verification purposes.
Various methodologies are more fully explained in Bridge Superstructure.