Taekeun Oh

Nondestructive Bridge Deck Testing with Air-Coupled Impact-Echo and Infrared Thermography

Two different nondestructive test (NDT) methods, air-coupled impact-echo (IE) and infrared (IR) thermography are evaluated on a full-scale simulated reinforced concrete bridge deck containing simulated delamination and cracking defects. The IE data are presented as two-dimensional frequency maps and spectral B-scan lines. The IR data are presented as temperature maps on the concrete surface. The lateral boundaries of the detected delaminations are also indicated in the images. The results obtained from each of the individual NDT methods show reasonably good agreement with most of the actual defects. The advantages and limitations of each method to characterize defects are discussed. The consistency and sensitivity of each method are also investigated. Finally, a simple data fusion technique is proposed to improve effectiveness of the individual test data. The findings from this study demonstrate that the combination of air-coupled IE and IR thermography tests is a practical option for consistent and rapid in situ evaluation of reinforced concrete bridge decks.

Comparison of NDT Methods for Assessment of a Concrete Bridge Deck

The field application of three different nondestructive tests (NDTs)—air-coupled impact echo (IE), infrared (IR) thermography, and sounding (chain drag)—are evaluated in this paper, where an actual in-service concrete bridge deck is tested. Two different contactless IE test equipment sets are deployed as part of an effort to develop new rapid measurement methods. The IE data are presented as two-dimensional frequency maps, and the IR data are presented as temperature maps over the tested area. Sounding (chain-drag) result maps are also presented. For verification of the location of near-surface delamination damage, eight drilled core samples were extracted from the test area. The results obtained from each of the individual NDT methods show reasonably good agreement with the drilled cores in terms of locating near-surface delamination. Finally, the NDT methods are compared across general performance criteria, considering accuracy, testing practicality, and costs. The analysis shows that all of the evaluated NDT methods are comparable, and the chain-drag method is not more accurate and reliable for detection of shallow delamination in the deck.

Improved Interpretation of Vibration Responses from Concrete Delamination Defects Using Air-Coupled Impact Resonance Tests

AbstractThe deteriorating national infrastructure demands improved nondestructive evaluation (NDE) and structural health monitoring methods for existing concrete structures. Vibration resonance tests offer an efficient NDE method to identify and characterize shallow (near-surface) delamination defects that afflict RC structures. However, efficient implementation of effective modal analysis methods for this purpose is hindered by practical testing limitations. This paper studies vibration resonance data from square, rectangular, and circular near-surface delamination defects in concrete using two testing configurations: a coupled source-receiver set configuration (driving point type), which is analogous to the impact-echo (IE) test, and a fixed-source moving sensor configuration, which represents a conventional modal analysis test. All data were collected using contactless air-coupled sensors, which enable efficient data collection from large structures from prepared laboratory samples. An approach to self...

Effect of Cylinder Size on the Modulus of Elasticity and Compressive Strength of Concrete from Static and Dynamic Tests

The primary objective of this study is to investigate the effects of cylinder size (150 by 300 mm and 100 by 200 mm) on empirical equations that relate static elastic moduli and compressive strength and static and dynamic elastic moduli of concrete. For the purposes, two sets of one hundred and twenty concrete cylinders, 150 by 300 mm and 100 by 200 mm, were prepared from three different mixtures with target compressive strengths of 30, 35, and 40 MPa. Static and dynamic tests were performed at 4, 7, 14, and 28 days to evaluate compressive strength and static and dynamic moduli of cylinders. The effects of the two different cylinder sizes were investigated through experiments in this study and database collected from the literature. For normal strength concrete (≤40 MPa), the two different cylinder sizes do not result in significant differences in test results including experimental variability, compressive strength, and static and dynamic elastic moduli. However, it was observed that the size effect became substantial in high strength concrete greater than 40 MPa. Therefore, special care is still needed to compare the static and dynamic properties of high strength concrete from the two different cylinder sizes.

Cost Effective Air-Coupled Impact-Echo Sensing for Rapid Detection of Delamination Damage in Concrete Structures:

Recently, air-coupled impact-echo (IE) tests for rapid damage detection in concrete structures have been reported, where an acoustically shielded, high sensitivity, pre-polarized air-pressure sensor was employed. However, this sensor is expensive and inconvenient compared with conventional (contact sensor) IE testing systems. In this study, a low cost dynamic microphone is evaluated with regard to characterization of delamination damage in a concrete slab using the IE method. Artificial delaminations with various sizes and depths are simulated by embedding plastic double sheets into a concrete slab. Results from both point and areal scanning IE tests that were carried out over the test slab surface are reported. Results show that the dynamic microphone successfully captures impact-echo signals in a contactless manner and without acoustic shielding. Near-surface delaminations in the concrete slab were clearly identified, where the obtained results are equivalent to those results obtained with the high sensitivity sensor.

Application of Impact Resonance C-Scan Stack Images to Evaluate Bridge Deck Conditions

AbstractThis study presents a practical approach for imaging concrete pavement and bridge deck slabs for the presence of delamination defects. Air-coupled impact resonance (impact-echo) test data are presented as a fused image by using stacked spectral C-scans of the tested deck surface. The low frequency (<6  kHz) dynamic vibrational response of delamination defects, primarily caused by the behavior of flexural vibration modes, clearly and accurately reveals the presence of internal shallow delamination defects. Images are constructed with data from air-coupled impact resonance tests carried out on laboratory and field concrete deck samples and an in-service concrete bridge deck. The test data are arranged along surface x- and y-coordinates, with spectral amplitude represented by a coupled gray scale–opacity index. The image parameters are defined by data-driven rule-based criteria, with the presence of near-surface delamination defects indicated by shaded regions. The data manipulation and image constru...

The Measurement of P-, S-, and R-Wave Velocities to Evaluate the Condition of Reinforced and Prestressed Concrete Slabs

The traditional P-wave ultrasonic measurement has been used for the condition assessment of general reinforced concrete structures for a long time, but the effects of prestressing applied to concrete structures such as long-span buildings and bridges on ultrasonic pulse velocity have not been studied clearly. Therefore, this study analyzed the statistical distribution of P-wave ultrasonic pulse velocities in reinforced and prestressed concrete slabs of 3000 × 3000 mm with a thickness of 250 mm. In addition, we measured S- and R-waves to identify experimental consistency by statistical analysis using the Kolmogorov-Smirnov goodness-of-fit test. The experimental results show that the P-, S-, and R-wave velocities increased slightly (2-3%) when prestressing was applied. As expected, the S- and R-wave measurements show better statistical reliability and potential for in situ evaluation than the P-wave because they are less sensitive to confinement and boundary conditions. The experimental results in this study can be used when assessing the condition of prestressed concrete structures through the velocities of elastic waves.

Practical Visualization of Local Vibration Data Collected over Large Concrete Elements

Reinforced concrete bridges represent a significant part of the transportation infrastructure. Restoring, renewing, and extending service life of the existing urban infrastructure are critical challenges facing engineers and managers. An original multidimensional presentation format for nondestructive evaluation point test spectral data is proposed. Time domain signals from air-coupled impact-echo (IE) tests are transformed into the frequency domain and arranged along x and y coordinates of the tested surface to give a three dimensional (3D) data set volume. This article stacks data in both spatial and spectral domains and includes all spectral data up to a natural frequency threshold (5-6 kHz) to isolate the response generated by near-surface delamination defects. Application of this data manipulation and visualization technique to IE data collected from two different reinforced concrete samples demonstrates improved near-surface delamination detection as compared with conventional data presentation formats. Spectral amplitude is indicated by gray scale, giving rise to a “fourth” dimension of information within the data volume. Visual interpretation of the dense data set of spectral information within the volume is enabled by incorporating transparency/opacity control on the data set, where higher spectral amplitudes are associated with more opacity. Consistent rule-based criteria for optimal image parameter determination for a given test structure are established in this article to optimize application for defect detection.

Comparison of Data-Processing Methods by Air-Coupled Impact Echo Testing for the Assessment of a Concrete Slab

The practical application of three different processing methods for impact-echo test data—the classical impact-echo method, the peak frequency map, and the 4D spectrum technique—is performed and analyzed in this study. For comparative evaluation, a concrete slab with simulated delaminations and voids was cast and employed. Two conventional methods, the impact-echo and peak frequency map, are used to determine the relative performance of the 4D spectrum technique, in which the impact-echo data are presented as 4D maps representing x, y, the frequency, and the frequency amplitude for the tested area. For verification, the 4D spectrum technique and the other two methods were applied to three artificial defects at the depth of rebar in the designated concrete slab. The results obtained from comparing the three processing methods show that although three methods are comparable, the 4D spectrum technique is more effective and accurate for the detection of shallow defects in the slab.

Evaluating the Dynamic Elastic Modulus of Concrete Using Shear-Wave Velocity Measurements

The objectives of this study are to investigate the relationship between static and dynamic elastic moduli determined using shear-wave velocity measurements and to demonstrate the practical potential of the shear-wave velocity method for in situ dynamic modulus evaluation. Three hundred 150 by 300 mm concrete cylinders were prepared from three different mixtures with target compressive strengths of 30, 35, and 40 MPa. Static and dynamic tests were performed at 4, 7, 14, and 28 days to evaluate the compressive strength and the static and dynamic moduli of the cylinders. The results obtained from the shear-wave velocity measurements were compared with dynamic moduli obtained from standard test methods (P-wave velocity measurements according to ASTM C597/C597M-16 and fundamental longitudinal and transverse resonance tests according to ASTM C215-14). The shear-wave velocity measured from cylinders showed excellent repeatability with a coefficient of variation (COV) less than 1%, which is as good as that of the standard test methods. The relationship between the dynamic elastic modulus based on shear-wave velocity and the chord elastic modulus according to ASTM C469/C469M was established. Furthermore, the best-fit line for the shear-wave velocity was also demonstrated to be effective for estimating compressive strength using an empirical relationship between compressive strength and static elastic modulus.