Peter Ulriksen

High Frequency Masw For Non-Destructive Testing Of Pavements—Accelerometer Approach

The dispersive nature of surface waves in pavement systems is imaged through a multichannel approach using one accelerometer as receiver and multiple shot points. The image obtained from a wavefield transformation method shows multimodal dispersion curves up to 10 kHz. We present results from a simplified MASW data acquisition method applied to a pavement surface. The method can simulate an arbitrary number of channels. The sensor separation can be chosen arbitrarily small. In these experiments, the upper frequency limit is 10 kHz, which can be increased by the exchange of one sensor. The method is tested by one manual and one automated procedure. Both rely on source-receiver reciprocity. The automated procedure is regarded as necessary when a large number of channels is combined with a small sensor separation. The manual method will not provide the necessary accuracy and endurance for that kind of measurement, but is promising for less complicated setups. We present recommendations for high frequency measurements on pavements. In the subsequent data processing, we follow the procedure of multichannel analysis of surface waves MASW. It has recently been developed as a geophysical method for near-surface investigation. We demonstrate that the MASW technique can identify detailed aspects of the high frequency total wave-field of both surface and body-wave events. Results of dispersion curve extraction indicate that higher modes of surface waves are dominating at depths associated with the transition between the asphalt and the base layer. However, the deviation from the fundamental mode is not large because all modes are converging in an asymptotic manner with increasing frequency. The study indicates that the MASW method is a fast and cost efficient method for measuring pavement stiffness parameters.

Time Break Correction in Multichannel Simulation with One Receiver (MSOR)

Recent investigations in the seismic evaluation of pavement systems indicates that the multichannel approach is indispensable. This is because of the complicated seismic phenomenon that originates from the unique seismic setting of a pavement system. A true multichannel survey would be a formidable task that would require an expensive multichannel (e.g., 48 channel) recording device and so-many receivers with complicated wiring deployed in a small area on the pavement. Instead, the multichannel simulation with one receiver (MSOR) approach can produce a simulated multichannel record by using only one receiver and a single (or two) channel recording device readily available for various types of engineering measurements. For this approach to be an effective alternative, a consistent timing of wave generation at each impact is the most critical condition to be met. Considering the necessary accuracy of tens of microseconds to deal with seismic waves in the range of kilohertz, it seems that a certain degree of inconsistency in time break can always occur in spite of a carefully designed timing mechanism. For a given inconsistency, extraction of the dispersion curve for surface waves is adversely affected most in the high frequencies and least in the low frequencies, making the lower frequencies still useful. This low-frequency dispersion information is then used to construct an impulsive surface wave event that should align perfectly at zero time if there were no time break inconsistency. The appropriate amount of time break correction can therefore be assessed from the amount of misalignment. The correction procedure may continue in an iterative manner because the extractable bandwidth of the dispersion curve would extend after each correction.

Branching of Dispersion Curves in Surface Wave Testing of Pavements

When a set of simulated multichannel seismic records acquired over pavement is processed by a 2-D wavefield transformation technique normally used in the Multichannel Analysis of Surface Waves (MASW) method, several branches are observed in the dispersion-curve image constructed from the transformation. Some investigators theoretically anticipated this branching phenomenon a few decades ago in connection with discontinuities in a layer model that has decreasing stiffness with depth. Although this phenomenon has long been speculated about during measurements with a conventional two-receiver approach, sometimes by attributing the results to several other possible causes like higher modes, an objective observation confirming its link to the predicted theory was never made. The dispersion curve image shows several frequency-phase velocity branches that match fairly well with the discontinuities in the dispersion curve predicted by theory. With a case study and numerical modeling we discuss a new approach that can yield thickness and stiffness of layers in a pavement system simply from the characteristics of this branching phenomenon. These determinations can be made without going through the normal procedure of dispersion curve analysis followed by inversion for the shear wave velocity (Vs) profile.

Frequency selection for coda wave interferometry in concrete structures

HIGHLIGHTSRecommendations as to choice of frequency for CWI in concrete are provided.50–150 kHz signals can be used to find sub‐aggregate sized holes in concrete.50–150 kHz signals can track development of microcracks in concrete.Higher frequency signals limit possible transmission range significantly.Lower frequency signals cannot be used to find such small damage to concrete. ABSTRACT This study contributes to the establishment of frequency recommendations for use in coda wave interferometry structural health monitoring (SHM) systems for concrete structures. To this end, codas with widely different central frequencies were used to detect boreholes with different diameters in a large concrete floor slab, and to track increasing damage in a small concrete beam subjected to bending loads. SHM results were obtained for damage that can be simulated by drilled holes on the scale of a few mm or microcracks due to bending. These results suggest that signals in the range of 50–150 kHz are suitable in large concrete structures where it is necessary to account for the high attenuation of high‐frequency signals.

Continuous wave measurements in a network of transducers for structural health monitoring of a large concrete floor slab

Local, superficial damage was detected and localized on an 8 × 2-m concrete floor slab using a structural health monitoring system. A total of 30 piezoelectric transducers, placed in a grid, transmitted and received continuous ultrasonic waves that were measured using a lock-in amplifier. Tomography was used to create images from the measured amplitude and phase of the continuous waves between all possible transducer pairs. The location of damage induced by impact hits was visible in the resulting images. The signals could easily be detected even between the most distant transducer pairs, indicating the possibility of monitoring even very large concrete structures.

Efficiency of some voice coil transducers in low frequency reciprocal operations.

The feasibility of using different voice coil transducers in applications with reciprocal transducers of mechanical waves is investigated. It was speculated that voice coil transducers could be a more efficient alternative to piezoelectric transducers in low frequency ranges. Five different voice coil transducers, originally constructed for either transmission or reception, were characterized in both modes of operation. A piezoelectric ceramic disk was used for comparison between the transducer types. The results show that voice coils indeed can function as reciprocal transducers and that the most sensitive of the evaluated transducers is more efficient than the piezoelectric disk for low frequencies.

Detecting damage events in concrete using diffuse ultrasound structural health monitoring during strong environmental variations

Diffuse ultrasonic wave measurements used in structural health monitoring applications can detect damage in concrete. However, the accuracy is very susceptible to environmental variations. In this study, a large concrete floor slab was monitored using diffuse wave fields that were generated by continuous-wave transmissions between ultrasonic transducers. The slab was monitored for several weeks while being subjected to changes in environmental conditions. Subsequently, it was damaged using impact hits, resulting in centimeter-scale cracking. The variations caused by the environment masked the effects of the damage in the measurements. To address this issue, the Mahalanobis distance was used to distinguish between the influence of the damage and the influence of the environmental variations. The Mahalanobis model uses amplitude and phase measurements of continuous waves at a set of different frequencies as inputs. A moving window approach was applied to the baseline data set to account for slow trends. This study shows that this technique greatly suppresses most of the variations caused by environmental conditions. All damage events in our data set have been detected.

Portable Seismic Acquistion System (PSAS) for Pavement MASW

A seismic method (e.g., surfaceor body-wave method) has been often used in pavement engineering to evaluate such critical constructional parameters as the E-modulus and Poisson’s ratio. Conventional method usually uses one or two-channel recording device (e.g., dynamic signal analyzer) for data acquisition whose cost is by no means trivial. In addition, recent applications with the multichannel approach have drastically improved the effectiveness of the seismic method in general and proven a greater potential of the method than ever. A true multichannel approach, however, would require a multichannel recording device (e.g., a 48-channel seismograph) and so-many accelerometers deployed simultaneously. The high-cost aspect of seismic method would make this otherwise-effective method excluded from consideration during the early stage of project planning. Instead, we propose a cheap, compact, and convenient seismic system that can be used with either conventional or multichannel approach. This Portable Seismic Acquisition System (PSAS) consists of a laptop computer, one or two accelerometers, and a hammer. A 16-bit PC-card (PCMCI bus) readily available nowadays is equipped into the computer as a data acquisition board. With this system, the multichannel measurement is simulated through repetitive generation of seismic waves along a linear survey line at different distances from the receiver fixed at a surface point. Data can be processed directly in the field on the same portable computer, only seconds after data acquisition. In combination with the robust dispersion curve analysis by the multichannel approach, this system creates new possibilities for seismic non-destructive testing (NDT) of pavements with on site evaluation. Data acquisition flow chart, signal conditioning, triggering, and other key features of the system are explained. We also present a case of evaluating pavement concrete thickness, E-modulus, and Poisson’s ratio directly in the field using the proposed system.