Megan E. McGovern

Detection and assessment of wood decay in glulam beams using a modified impulse-echo approach

A glulam beam retired from the field and without visible indications of wood decay was used. Towards detection and assessing wood decay, X-ray computer tomography and ultrasonic measurements were carried out. It was observed that decrease in mass density with increasing levels of wood decay affects x-rays attenuation and allows radioscopy to detect and assess wood decay. To detect and assess decay when only one lateral side of the beam is available, a modified impulse-echo was developed. The modified impulse-echo approach is based on observing the dynamic response of the glulam beams to the drop of a steel sphere onto a steel plate coupled to the glulam beam lamina. It was observed that monitoring certain frequency bands allows detection and assessment of wood decay. The selection of these frequency bands requires knowledge of the nominal beam transverse dimensions. Because of the high ultrasonic material attenuation values of decayed wood as compared with those of sound wood, the area under the power spectral density in these frequency bands is smaller in regions of decayed wood. Results show that results from both X-ray computer tomography and impulse-echo measurements are consistent with each other and can be used to detect and assess wood decay in structural lumber.

Use of Nonlinear Acoustic Measurements for Estimation of Fracture Performance of Aged Asphalt Mixtures

This study evaluated the oxidative aging of asphalt mixtures by both nondestructive and fracture performance tests and investigated the possibility of estimating the low-temperature fracture properties of aged mixtures by using a nonlinear ultrasonic approach. Asphalt mixture samples, oven aged for 12, 24, 28, 32, and 36 h at 135°C, were compacted, made into test specimens, and evaluated by using a noncollinear wave-mixing approach. In addition, the embrittlement temperature and fracture energy of the replicate samples were assessed by using the acoustic emission test and the disk-shaped compact tension test. Comparison of results from the three testing methods clearly showed similar trends. This finding suggests that the fracture properties of mixtures can be estimated by using the noncollinear ultrasonic wave-mixing approach, which is a nondestructive method which does not require core extraction in the field.

Damage characterization in dimension limestone cladding using noncollinear ultrasonic wave mixing

Abstract. A method capable of characterizing artificial weathering damage in dimension stone cladding using access to one side only is presented. Dolomitic limestone test samples with increasing levels of damage were created artificially by exposing undamaged samples to increasing temperature levels of 100°C, 200°C, 300°C, 400°C, 500°C, 600°C, and 700°C for a 90 min period of time. Using access to one side only, these test samples were nondestructively evaluated using a nonlinear approach based upon noncollinear wave mixing, which involves mixing two critically refracted dilatational ultrasonic waves. Criteria were used to assure that the detected scattered wave originated via wave interaction in the limestone and not from nonlinearities in the testing equipment. Bending tests were used to evaluate the flexure strength of beam samples extracted from the artificially weathered samples. It was observed that the percentage of strength reduction is linearly correlated (R2=98) with the temperature to which the specimens were exposed; it was noted that samples exposed to 400°C and 600°C had a strength reduction of 60% and 90%, respectively. It was also observed that results from the noncollinear wave mixing approach correlated well (R2=0.98) with the destructively obtained percentage of strength reduction.

Linear and nonlinear characterization of limestone rock using a non-collinear ultrasonic wave mixing

Characterization of dolomitic limestone rock is presented using linear and nonlinear ultrasonic approaches. The linear approach is based upon the concept of complex moduli, which is estimated using ultrasonic dilatational and shear phase velocity measurements and the corresponding attenuations. The nonlinear approach is based upon noncollinear wave mixing, involving mixing of two dilatational waves. Criteria were used to assure that the detected scattered wave originated via wave interaction in the limestone and not from nonlinearities in the testing equipment. These criteria included frequency and propagating direction of the resultant scattered wave, and the time-of-flight separation between the two primary waves and the resulting scattered wave. Three cases of non-collinear interaction of two longitudinal waves are presented and discussed including one that requires only access to a plane surface of the stone test sample.

Nonlinear Ultrasonic Damage Characterization of Limestone

ABSTRACT Characterization of dolomitic limestone rock samples with increasing levels of damage is presented using a nonlinear ultrasonic approach. Limestone test samples with increasing levels of damage were created artificially by exposing virgin samples to increasing temperature levels of 100, 200, 300, 400, 500, 600, and 700°C for a ninety minute period of time. These samples were first characterized using ultrasonic dilatational and shear phase velocity measurements and corresponding attenuations. Then, a nonlinear approach based upon noncollinear wave mixing of two dilatational waves was used. Criteria were used to assure that the detected scattered wave originated via wave interaction in the limestone and not from nonlinearities in the testing equipment. It was observed that both the dilatational velocity and the noncollinear wave mixing approach are able to characterize the level of damage in limestone rock. However, the nonlinear approach is more sensitive to damage accumulation by about two orders of magnitude.

Nondestructive Field Evaluation of Aging Levels of Rejuvenated Asphalt Concrete Pavements

This study used a nonlinear ultrasonic approach with noncollinear wave mixing of subsurface longitudinal waves that was capable of evaluating the efficacy of rejuvenators in restoring the original properties of aged asphalt concrete (AC) pavements. AC specimens that were oven aged for 36 h at 135°C were treated on the top surface with a rejuvenator (10% of binder by weight) and left to dwell for prescribed periods of time. After the dwell time, each specimen was nondestructively evaluated. Because in the field, AC pavement acoustic properties (ultrasonic velocities and corresponding attenuations) are unknown, a predetermined fixed-incident angle (based on the AC mixture type) was used; this use allowed for practical implementation in the field. It was observed that the rejuvenator had the effect of restoring the material responses of oven-aged samples to those corresponding to virgin samples. It was also observed that the rejuvenating action was not uniform and depended on the samples’ porosity because of the stochastic nature of AC.

Assessment of the aging level of rejuvenated hot mixed asphalt concrete pavements

The efficacy of asphalt rejuvenator on restoring the properties of oxidatively aged asphalt was tested via a non-collinear ultrasonic subsurface wave mixing technique modified for field use. Longitudinal transducers were mounted on angle wedges to generate subsurface dilatational waves to allow for pavement evaluation when there is only access to one side. Because in the field the asphalt concrete (AC) pavement properties (i.e., ultrasonic velocities and attenuations) are unknown, a pre-determined fixed incident angle (based on the AC mixture type) was used, which allows for practical implementation in the field. Oxidative aged AC specimens were coated with rejuvenator (10% by weight of the binder) and left to dwell for varying amounts of time. Once the dwell time reached the desired amount, the specimen was immediately ultrasonically tested. The frequency ratio, f2/f1, at which the interaction took place and the normalized nonlinear wave generation parameter, β/β0, were recorded and compared against a reference plot. It was observed that the rejuvenator had the effect of restoring the nonlinear properties to those corresponding to a virgin sample after a sufficient amount of dwell time. The ability of the rejuvenator to fully penetrate and act on the binder was observed to be dependent on the porosity and aggregate structure, and thus varied for each specimen. As a result, some portions of the binder were restored to a greater extent than others. This non-uniform nature was captured via the nonlinear ultrasonic technique.

Linear and nonlinear ultrasonic characterization of in limestone

Characterization of dolomitic limestone rock samples with increasing levels of damage is presented using linear and nonlinear ultrasonic approaches. Limestone test samples with increasing levels of damage were created artificially by exposing virgin samples to increasing temperature levels of 100, 200, 300, 400, 500, 600, and 700oC for a ninety minute period of time. The linear characterization is based upon the concept of complex moduli, which is estimated using ultrasonic dilatational and shear phase velocity measurements and corresponding attenuations. The nonlinear approach is based upon non-collinear wave mixing, involving mixing of two dilatational waves. Criteria were used to assure that the detected scattered wave originated via wave interaction in the limestone and not from nonlinearities in the testing equipment. These criteria included frequency and propagating direction of the resultant scattered wave, and the time-of-flight separation between the two primary waves and the resulting scattered wave. It was observed that both the linear and nonlinear approaches are able to characterize the level of damage in limestone rock.

Characterization of oxidative aging in asphalt concrete pavements using its complex moduli

Asphalt concrete mixtures with different levels of oxidative aging, prepared by oven-aging the mixture at 135 °C for different amounts of time, were used to study the effects of oxidative aging upon the ultrasonic phase velocities and attenuation measurements. It was a observed that both the dilatational and shear velocities increase up to approximately 24 hours of aging after which they significantly decease with aging. Also, both the dilatational and shear attenuation decrease up to around 24 hours of aging, after which both attenuations strongly increase. These results are consistent with results obtained using the mechanical Disk-shaped Compact Tension (DC[T]) fracture tests. Based upon these velocity and attenuation measurements, the dynamic moduli were calculated. It was observed that the dynamic moduli increase from 0 hours to 24 hours and decrease from 24 to 36 hours of oven-aging. The modulus obtained using ultrasonic measurements is also compared with the modulus obtained using the AASHTO recommended mechanical testing. The differences are due to scattering effects, which are present in ultrasonic testing. It was also observed that to avoid the uncertainty associated with assuming a suitable value for the Poisson’s ratio, both the dilatational and shear velocities and corresponding attenuation measurements must be carried out. Furthermore, to eliminate the need for traditional mechanical testing during estimation of complex moduli, frequency-dependent ultrasonic measurements must also be carried out.