Lamb Wave Basis for Impact-Echo Method Analysis

The impact-echo method has been developed over the past 20?years and is now widely used in the nondestructive evaluation of concrete. However, some practical issues remain unresolved, such as the physical basis for the empirical correction factor (β) used to obtain thickness mode frequency. A new approach based on guided wave theory is proposed in this paper: that the impact-echo resonance in plates corresponds to the zero-group-velocity frequency of the S1 Lamb mode. A numerical model is developed, verified by experiment, and then shown to adequately simulate the dynamic response of a concrete plate. Using this model the thickness resonance mode is identified and found to accurately match that particular Lamb mode in terms of shape and frequency. New values for β based on the Lamb mode model are computed and dependence on material Poisson’s ratio is demonstrated.     

Nondestructive Assessment of Damage in Concrete Bridge Decks

Impact-echo tests were performed on a precast, reinforced concrete bridge slab that was removed from a maintenance bridge built in 1953 in South Carolina. Impact-echo tests were first performed to nondestructively assess the initial condition and the distribution of damage throughout the slab by analyzing the variation in propagation wave velocity. It was found that the velocity varied by as much as 900?m/s throughout the slab. After the in-service condition was assessed, the slab was subjected to a full-scale static load test in the laboratory and impact-echo tests were again performed, this time to evaluate the initiation and progression of damage (stiffness loss and crack development) within the slab. After structural failure of the slab, a reduction in propagation wave velocity up to 6% was observed correlating to a reduction in slab stiffness. Cracks were detected within the concrete slab that were not visible from the surface. Areas with preexisting damage experienced more crack growth when subjected to the load test than those that were initially intact. Locations exhibiting stiffness loss, crack propagation, and localized damage can be differentiated such that the method can be used to make decisions between rehabilitating and replacing concrete bridge decks depending upon the severity of damage.     

Nondestructive Sample Quality Assessment of a Soft Clay Using Shear Wave Velocity

While improvements in equipment and sampling methods have enabled collection of better quality samples of soft clays for more reliable engineering design and performance prediction, current sample quality assessment methods typically require destructive laboratory testing performed long after samples are taken. This paper describes a nondestructive technique for sample quality assessment of soft clays using shear wave velocity. A portable bender element device was used to measure shear wave velocity (Vvh) in the field immediately following collection of Sherbrooke block, tube, and split spoon samples of Boston blue clay. Vvh values were compared to in situ values from seismic piezocone (VSCPTU) tests. The ratio Vvh/VSCPTU was compared with results from a conventional, laboratory-based assessment method. Results indicate a consistent correlation between laboratory-based methods and the Vvh/VSCPTU ratio, which ranges from Vvh/VSCPTU = 0.77 for the block samples to 0.28 for split spoon samples. The portable bender element device and nondestructive assessment technique offer the potential for field quality assessment and allow for real time adjustments to sampling techniques and/or more effective selection of samples for laboratory testing.     

Nondestructive and Laboratory Evaluation of Damage Gradients in Concrete Structure Exposed to Cryogenic Temperatures

This paper discusses the use of pulse velocity, dynamic Young’s modulus of elasticity, and air permeability of concrete to evaluate the extent of damage and damage gradients to a concrete structure exposed to thermal shock and subsequent cryogenic temperatures. Liquefied natural gas (LNG) is maintained in liquid form at cryogenic temperatures typically below ?160°C (?260°F). The elevated concrete pedestal and precast concrete piles supporting a LNG storage tank were exposed to cryogenic temperatures following a leak of the LNG. The engineering assessment of the concrete structure consisted of a nondestructive evaluation phase using ultrasonic pulse velocity and a subsequent laboratory phase based on concrete cores. Dynamic Young’s modulus of elasticity and air permeability index of 25?mm (1?in.) thick disks sawed from the cores were determined. Analyzing concrete disks at 25?mm (1?in.) increments permitted assessment of changes in these properties with depth and enabled evaluation of depth of damage and damage gradients. The laboratory study confirmed that the distressed zone was limited to a near-surface area of concrete as suggested by the results of pulse velocity testing.     

Evaluation of Fire Damage to a Precast Concrete Structure Nondestructive, Laboratory, and Load Testing

This article discusses the use of nondestructive and laboratory testing techniques and load testing in evaluation of fire damage to precast prestressed concrete members in a parking structure. The in situ evaluation phase consisted of nondestructive testing of concrete using ultrasonic pulse velocity and radiographic exposures to locate tendons prior to the removal of cores. Flexural strength of concrete and dynamic Young’s modulus of elasticity and air permeability index of 25?mm (1?in.) thick disks sawed from the cores were determined in the subsequent laboratory testing phase. Analysis of concrete properties at small depth increments permitted assessment of whether a damage gradient was present and the nature of any gradient found, as expressed by changes in these properties. Based on the compromise in material properties indicated by nondestructive and laboratory testing, two affected double-tees were load tested. The deflection pattern observed during load testing confirmed the compromise indicated by the findings of the testing program.     

Detection of Common Defects in Concrete Bridge Decks Using Nondestructive Evaluation Techniques

The transportation infrastructure in the United States is deteriorating and will require significant improvements. Consequently, innovations in the area of transportation infrastructure maintenance and rehabilitation are keys to the health and wellness of this valuable national asset. A major component of maintenance and rehabilitation is the ability to accurately assess the condition of the transportation infrastructure. This can be accomplished in part by using nondestructive evaluation techniques. Several nondestructive techniques have been used on concrete bridge decks and have proven to be efficient and effective. This paper aims at studying the different nondestructive evaluation techniques used in the assessment of concrete bridge deck conditions. An experimental investigation to evaluate the ability of infrared thermography, impact echo, and ground penetrating radar to detect common flaws in concrete bridge decks is developed and discussed. Results from this study showed the ability of these methods to detect defects with varying precision. Capabilities of the methods were verified and comparisons among the methods were made.     

Ultrasonic Pulse Velocity in Nondestructive Evaluation of Low Quality and Damaged Concrete and Masonry Construction

This paper discusses applications of ASTM C 597 “Standard Test Method for Pulse Velocity Through Concrete” technique to field detection of damage to concrete in service and field quality assessment of cast-in-place concrete and masonry under construction. Four unique field investigation and validation studies are discussed in this paper. The first part includes field assessments of concrete members under construction with questionable quality. Case studies include detection of zones of high air content and low strength concrete in a cast-in-place, posttensioned structure and detection of voids and honeycombs in poorly consolidated cast-in-place beams. The third case study pertinent to construction involves detection of poorly consolidated collar joints in a masonry rehabilitation project. In addition to assessments during construction and rehabilitation, this paper also discusses assessment of damage to concrete structures in service. The case study included in the paper involves exposure to elevated temperatures during a fire at a precast, double tee concrete parking structure. Nondestructive evaluation (NDE) testing findings were validated by subsequent laboratory testing or selective demolition to confirm NDE findings. This paper should be of value to practicing engineers interested in application of pulse velocity testing technique in field assessments similar to ones discussed. This paper should also be of value to researchers interested in applicability of pulse velocity to research concerning properties of concrete subjected to the damage mechanisms associated with elevated temperatures.     

Detection of Ungrouted Cells in Concrete Masonry Constructions Using a Dielectric Variation Approach

In this study, a new technique for detecting ungrouted cells in concrete block masonry constructions was developed. The concept, based on detecting the local dielectric permittivity variations, was employed to design coplanar capacitance sensors with high sensitivities to detect such construction defects. An analytical model and finite element simulations were used to assess the influence of the sensor geometrical parameters on the sensor signals and to optimize the sensor design. To experimentally verify the model, the dielectric properties of various materials involved in concrete masonry walls were measured. In addition, a masonry wall containing predetermined grouted and ungrouted cells was constructed and inspected using the developed sensors in a laboratory setting. Moreover, different capacitance sensors were designed and compared with respect to their sensitivity, signal-to-noise ratio, and coefficient of variation of the inspected measurements. Excellent agreements were found between the experimental capacitance signal response parameters and those predicted by the analytical and finite element models. The proposed sensor design, coupled with a commercially available portable capacitance meter, would facilitate employing this technique in the field for rapid inspection of masonry structures without the need for sophisticated data analyses usually required by other more expensive and time consuming methods.     

Damage Detection of FRP-Strengthened Concrete Structures Using Capacitance Measurements

In this study, a new concept for detecting air voids, water intrusion, and glue infiltration damages in fiber-reinforced polymers (FRPs)-strengthened concrete structures was developed. The concept, based on detecting the local dielectric permittivity variations, was employed to design coplanar capacitance sensors (CCSs) to detect such defects. An analytical model was used to introduce the sensor operation theory and analyze the influence of different sensor parameters on the output signals and to optimize sensor design. Two dimensional finite element (FE) simulations were performed to assess the validity of the analytical results and to evaluate other sensor design-related parameters. To experimentally verify the FE model, dielectric properties of various materials involved in FRP-strengthened concrete systems were measured. In addition, two concrete specimens strengthened with FRP laminates and containing preinduced defects were constructed and inspected in a laboratory setting. Good agreement was found between experimental capacitance measurements and those predicated by the FE simulations. The proposed CCS design, coupled with commercially available portable capacitance meters, would facilitate field implementation of the proposed technique for rapid inspection of FRP-strengthened concrete structures without the need for sophisticated data analyses usually required by other more expensive and time consuming methods.     

Fundamental Frequency Testing of Reinforced Concrete Beams

Tests were conducted to measure the fundamental frequencies of reinforced concrete beams. Beams were tested prior to load application and after they had been loaded to various fractions of their ultimate moment capacity. Dynamic testing was performed in an unloaded state in both the direction of loading and in the direction perpendicular to loading. Resulting fundamental frequencies were used to determine the dynamic flexural stiffness (EdI) relative to the undamaged flexural stiffness. Results show that fundamental frequency tests can effectively measure decreases in dynamic flexural stiffness caused by flexural cracking. However, the effective moment of inertia in the relaxed state is not accurately predicted by American Concrete Institute recommendations for computing static beam deflections. Equations were developed to describe the effective flexural stiffness of unloaded, cracked beams. A relative dynamic flexural stiffness value of 70 provides a conservative prediction that a beam has failed by being loaded to its ultimate moment capacity.     

Diagnosis of Reinforced Concrete Structural Damage Base on Displacement Time History using the Back-Propagation Neural Network Technique

A state-of-the-art methodology is proposed for damage diagnosis of structures, such methodology being presented in the example of a simply supported reinforced concrete (RC) beam. The severity and location of defects within the RC structures can be assessed much more conveniently by using the back-propagation neural network technique. A simply supported RC beam with specified size (i.e., rectangular cross section and 4 m span) and assumed defects is theoretically analyzed by a finite-element program to generate training and the testing of numerical examples necessary to assess the damaged RC structure by using the neural network (NN). Numerical examples are then generated according to the displacement time history of the defected beams loaded by an impact force at the beam center. In addition, 10 sets of test beam with the assumed damage and same specified size of the numerical examples are constructed in full scale. The damage scenario of each test beam is also diagnosed by using the well-trained NN according to the displacement time history, which is the history of the responses caused by the impact loading acting at the beam centers. Based on the study and test results, the damage scenarios of the 10 sets of test beams are successfully classified.     

Global Monitoring of Large Concrete Structures Using Acoustic Emission and Ultrasonic Techniques: Case Study

Global monitoring of civil structures is a demanding challenge for engineers. Acoustic emission (AE) is one of the techniques that have the potential to inspect large volumes with transducers placed in strategic locations of the structure. In this paper, the AE technique is used to characterize the structural condition of a concrete bridge. The evaluation of AE activity leads to information about any specific part of the structure that requires attention. Consequently, more detailed examinations can be conducted once the target area is selected. In this case, wave propagation velocity was used as a means to evaluate, in more detail, the condition of the region indicated by the AE analysis.     

Using Scanning Lasers to Determine the Thickness of Concrete Pavement

Due to limited budgets and reduced inspection staff, state departments of transportation are in need of innovative approaches for providing more efficient quality assurance on concrete paving projects. In Iowa, the current technique is to take core samples of the pavement, which is a labor intensive, destructive process. Due to these limitations, a limited number of cores are used to estimate the pavement thickness. Any method that can reduce or eliminate cores and increase the statistical accuracy of the thickness estimate will be beneficial. One method, which uses a laser to scan the surface of the base prior to paving and then to scan the surface after paving can determine the thickness at any point. Also, scanning lasers provide thorough data coverage that can be used to calculate thickness variance accurately and identify any areas where the thickness is below tolerance. The laser scanning methodology for this study involved the following: (1) investigating characteristics of the paving process; (2) using a laser scanner on three different sites; (3) processing the data to create clean surface models; (4) performing statistical analyses to determine thickness variability; and (5) summarizing the results.     

Assessment of Fire Damage to a Reinforced Concrete Structure during Construction

This article summarizes an engineering evaluation of the extent of fire damage to a concrete structure under construction. The fire occurred in a portion of the reinforced concrete structure and visibly damaged a load bearing exterior foundation wall. The purpose of the assessment was to promptly evaluate the in situ condition of the wall and recommend necessary repair or replacement options prior to commencement of backfilling and the concrete construction to be supported by the subject wall. The engineering assessment of the damaged wall included a nondestructive evaluation phase consisting of ultrasonic pulse velocity testing and a laboratory testing phase on the concrete cores removed from the damaged wall. Dynamic Young’s modulus of elasticity and an air permeability index of 25?mm (1?in.) thick disks sawed from the cores were determined. Analysis of properties of 25?mm (1?in.) concrete specimens permitted assessment of the presence and degree of any damage in smaller depth increments compared to the size of a compressive strength core. Significant differences were not indicated by compressive strength of cores, however, the in situ nondestructive testing and laboratory testing of the disks were effective in determining the depth of damage, as a result of the fire. The results of the nondestructive and laboratory evaluation indicated that the distressed zone of the concrete was limited to a near-surface layer. Repair recommendations were based on removal and replacement of the affected concrete sections identified by the testing program.     

New Method for the Evaluation of Material Damping Using the Wavelet Transform

Material damping is a fundamental parameter required for dynamic analysis of geotechnical and civil infrastructure. The material damping ratio is very difficult to measure in situ. A new methodology for in situ measuring of material damping using surface waves is presented in this work. This methodology is successfully evaluated on laboratory scale models and numerical simulations. Ultrasonic waves are used in this work because of the size of the laboratory models. The output force of an ultrasonic piezoelectric transmitter is modeled by using a Morlet function. The wave attenuation and phase variation of propagating surface waves with distance are analyzed using the wavelet transform. Numerical results show that the material damping ratio calculated using the wavelet transform gives a global value that represents an average damping ratio for the frequency bandwidth imposed by the seismic or ultrasonic source. Experimental results, from tests on a cemented sand and a concrete plate, show good agreement with published damping values.     

Comparison of Pulse Velocity and Impact-Echo Findings to Properties of Thin Disks from a Fire Damaged Slab

The engineering assessment of fire damage to a concrete slab provided the opportunity to compare the results of in situ, nondestructive evaluation (NDE) techniques and laboratory testing of specimens taken from cores extracted from the fire damaged slab. This paper discusses and compares results of in situ pulse velocity and impact-echo testing with dynamic elastic modulus and air permeability index test results of 25?mm (1?in.) thick disks sawed from concrete cores removed from selected areas of the damaged slab. Both the NDE techniques and the laboratory testing of thin disks identified the presence of damage as a result of the fire. Analysis of the relatively thin concrete specimens permitted assessment of the presence and degree of damage in thin layers, and provided important and useful data on concrete properties for engineering assessment which was not available from NDE alone. Compressive strength results were consistent with the results of other tests but largely inconclusive by themselves. Impact-echo testing was able to identify the presence of a severely deteriorated concrete layer but could not identify the extent or depth of damage or clearly identify less damaged areas. A distressed layer of concrete was found by subsequent laboratory testing to be limited to a near-surface zone in some areas as suggested by the pulse velocity evaluation, but pulse velocity based analysis resulted in an overestimate of the depth of the damage. The findings highlighted a shortcoming of using conventional strength testing alone on investigations involving relatively thin layers of damage and pointed out several key limitations in the use and interpretation of nondestructive evaluation and associated analysis in a field assessment project.     

Detection of Stiffness Reductions in Concrete Decks with Arbitrary Damage Shapes Using Incomplete Dynamic Measurements

A new refined nondestructive evaluation technique for concrete decks with arbitrary damage shapes is presented, and its utility in detecting the location and extent of the damage using only a single dynamic measurement signal is demonstrated. Six unknown parameters are considered to determine the damage distribution, which is a modified form of the bivariate Gaussian distribution function. Using a combination of the combined finite-element method (FEM) and the advanced uniform microgenetic algorithm, the various influences of different measurement locations on the damage detection are studied. In addition, the effect of noise is simulated in order to study the influence of the measurement errors and the uncertainty of the method. The sample studies demonstrate the excellence of the proposed method from the standpoints of its computation efficiency as well as its ability to investigate the complex distribution of an arbitrary stiffness reduction.     

In Situ Estimate of Shear Wave Velocity Using Borehole Spectral Analysis of Surface Waves Tool

In situ field testing has been performed over the past several years at a silty sand site in Austin, Tex. using the borehole spectral analysis of surface waves (SASW) tool to develop the technique and assess the validity of the method. The borehole SASW tool is an inflatable pressuremeterlike device that allows surface wave measurements to be performed along the wall of an uncased borehole while varying the in situ states of stress. Field results demonstrate the applicability of borehole SASW testing as a method to characterize soil sites and provide information about in situ shear wave velocity and the relationship between shear wave velocity and state of stress. Results from a borehole SASW test conducted at the Austin site are presented herein to demonstrate the applicability and validity of the method.     

Identification and Validation of a Discrete Element Model for Concrete

The use of a three-dimensional discrete element method (DEM) is proposed to study concrete structures submitted to dynamic loading. The aim of this paper is to validate the model first in the quasistatic domain, and second in dynamic compression, at the sample scale. A particular growing technique is used to set a densely packed assembly of arbitrarily sized spherical particles interacting together, representing concrete. An important difference from classical DEMs where only contact interactions are considered, is the use of an interaction range. First, the correct identification of parameters of the DEM model to simulate elastic and nonlinear deformation including damage and rupture is made through quasistatic uniaxial compression and tension tests. The influence of the packing is shown. The model produces a quantitative match of strength and deformation characteristics of concrete in terms of Young’s modulus, Poisson’s coefficient, and compressive and tensile strengths. Then, its validity is extended through dynamic tests. The simulations exhibit complex macroscopic behaviors of concrete, such as strain softening, fractures that arise from extensive microcracking throughout the assembly, and strain rate dependency.