This paper studies an air-coupled ultrasonic scanning approach for damage assessment in steel-clad concrete structures. An air-coupled ultrasonic sender generates guided plate waves in the steel cladding and a small contact-type receiver measures the corresponding wave responses. A frequency-wavenumber (f-k) domain signal filtering technique is used to isolate the behavior of the fundamental symmetric (S0) mode of the guided plate waves. The behavior of the S0 mode is sensitive to interface bonding conditions. The proposed inspection approach is verified by a series of experiments performed on laboratory-scale specimens. The experimental results demonstrate that hidden disbond between steel cladding and underlying concrete substrate can be successfully detected with the ultrasonic test setup and the f-k domain signal filtering technique.
Carbonation causes a physicochemical alteration of cement-based materials, leading to a decrease of porosity and an increase of material hardness and strength. However, carbonation will decrease the pH of the internal pore water solution, which may depassivate the internal reinforcing steel, giving rise to structural durability concerns. Therefore, the proper selection of materials informed by parameters sensitive to the carbonation process is crucial to ensure the durability of concrete structures. The authors investigate the feasibility of using linear and nonlinear dynamic vibration response data to monitor the progression of the carbonation process in cement-based materials. Mortar samples with dimensions of 40×40×160 mm were subjected to an accelerated carbonation process through a carbonation chamber with 55% relative humidity and >95% of CO2 atmosphere. The progress of carbonation in the material was monitored using data obtained with the test setup of the standard resonant frequency test (ASTM C215-14), from a pristine state until an almost fully carbonated state. Linear dynamic modulus, quality factor, and a material nonlinear response, evaluated through the upward resonant frequency shift during the signal ring-down, were investigated. The compressive strength and the depth of carbonation were also measured. Carbonation resulted in a modest increase in the dynamic modulus, but a substantive increase in the quality factor (inverse attenuation) and a decrease in the material nonlinearity parameter. The combined measurement of the vibration quality factor and nonlinear parameter shows potential as a sensitive measure of material changes brought about by carbonation.
Carbonation is an important deleterious process for concrete structures. Carbonation begins when carbon dioxide (CO2) present in the atmosphere reacts with portlandite producing calcium carbonate (CaCO3). In severe carbonation conditions, C-S-H gel is decomposed into silica gel (SiO2.nH2O) and CaCO3. As a result, concrete pore water pH decreases (usually below 10) and eventually steel reinforcing bars become unprotected from corrosion agents. Usually, the carbonation of the cementing matrix reduces the porosity, because CaCO3 crystals (calcite and vaterite) occupy more volume than portlandite. In this study, an accelerated carbonation-ageing process is conducted on Portland cement mortar samples with water to cement ratio of 0.5. The evolution of the carbonation process on mortar is monitored at different levels of ageing until the mortar is almost fully carbonated. A nondestructive technique based on nonlinear acoustic resonance is used to monitor the variation of the constitutive properties upon carbonation. At selected levels of ageing, the compressive strength is obtained. From fractured surfaces the depth of carbonation is determined with phenolphthalein solution. An image analysis of the fractured surfaces is used to quantify the depth of carbonation. The results from resonant acoustic tests revealed a progressive increase of stiffness and a decrease of material nonlinearity.
Frost resistance of concrete is a major concern in cold regions. RILEM (International union of laboratories and experts in construction materials, systems and structures) recommendations provide two alternatives for evaluating frost damage by nondestructive evaluation methods for concrete like materials. The first method is based on the ultrasonic pulse velocity measurement, while the second alternative technique is based on the resonant vibration test. In this study, we monitor the frost damage in Portland cement mortar samples with water to cement ratio of 0.5 and aggregate to cement ratio of 3. The samples are completely saturated by water and are frozen for 24 hours at -25°C. The frost damage is monitored after 0, 5, 10, 15 and 20 freezing-thawing cycles by nonlinear impact resonance acoustic spectroscopy (NIRAS). The results obtained are compared with those obtained by resonant vibration tests, the second alternative technique recommended by RILEM. The obtained results show that NIRAS is more sensitive to early stages of damage than the standard resonant vibration tests.
Metal corrosion is a significant problem for the US concrete infrastructure. Accurate and continuous corrosion sensing methods would help reduce this cost and enable effective health monitoring and service life prediction. In this paper, recent efforts to apply giant magneto-resistive response (GMR) and eddy current sensors for corrosion sensing are described. The sensors are applied in passive and active sensing configurations, neither of which require excavation of the concrete, so remote sensing at a surface and internal sensing with an embedded unit are possible. The passive and active testing configurations are described. Then experimental results for aluminum corrosion are presented, with the aim of identifying existing corrosion state to date and rate of active corrosion at time of sensing.
The corrosion of steel reinforcing bars in concrete is a significant problem for the US infrastructure, and the need for
effective corrosion sensing in structures clearly exists. This paper reviews recent developments in corrosion sensing.
First the bases and applications of several different types of embedded corrosion sensors are discussed. We then present
a new basis for corrosion sensing based on magnetic field measurement. Magnetic field measurement allows both the
extent and rate of corrosion to be measured using active and passive sensing configurations respectively. These two
sensing configurations are briefly described. Both active and passive approaches can be applied without excavation of
the concrete, so either remote sensing at a surface or internal sensing with an embedded unit are possible. The feasibility
of using recently-developed high sensitivity miniature GMR (Giant Magneto-Resistive) sensors in the magnetic sensing
configuration is investigated. Initial tests with GMR sensors passive show promise for the passive sensing configuration.
Determination of crack depth in field using the self-calibrating surface wave transmission measurement and the cutting
frequency in the transmission function (TRF) is very difficult due to variations of the measurement conditions. In this
study, it is proposed to use the measured full TRF as a feature for crack depth assessment. A principal component
analysis (PCA) is employed to generate a basis of the measured TRFs for various crack cases. The measured TRFs are
represented by their projections onto the most significant principal components. Then artificial neural network (ANN)
using the PCA-compressed TRFs is applied to assess the crack in concrete. Experimental study is carried out for five
different crack cases to investigate the effectiveness of the proposed method. Results reveal that the proposed method
can be effectively used for the crack depth assessment of concrete structures.
Proc. SPIE. 4337, Health Monitoring and Management of Civil Infrastructure Systems
KEYWORDS: Statistical analysis, Data modeling, Databases, Inspection, Nondestructive evaluation, Optical inspection, Geographic information systems, Finite element methods, Bridges, Information technology
The importance of rational decision-making for optimum resource distribution of civil infrastructure systems management is well recognized. Bridges, serving as node points of the highway transportation system, are critical components of the nation's infrastructure. As the nation's bridge population is aging, management decisions must be based on an objective, complete, accurate and compatible information for maximum reliable bridge lifecycle. For bridges sharing common materials, similar geometric design attributes and behavior mechanisms, fleet-strategies for health monitoring would offer significant advantages. Improvements from fleet health monitoring would lead to objective engineering knowledge for optimal decision making. This paper provides an overview of fleet health monitoring concept, then summarizes an on-going research on re- qualification of reinforced concrete T-beam bridge population in Pennsylvania.
A nondestructive evaluation technique for monitoring damage due to crack growth in concrete beams is presented in this paper. The technique is based on monitoring the resonant frequencies of vibration. An introduction to the concepts of vibrational modes in beams is detailed first followed by a description of the experimental procedure for resonance frequency measurement. The result of a finite element (FE) analysis that is performed to identify the different modes of vibration of the beam specimen, are then presented. The resonance frequencies determined by the FE analysis are shown to match closely with the experimental values. A notch is introduced in the specimen and the effect of notch length on the resonant frequencies is studied by varying the notch depth. Experimental results are compared with the result of FE simulation of the beam with different notch lengths. The influence of a real crack on the frequencies of the vibrational modes is also studied by loading a specimen in a three-point bending configuration and propagating a crack in a controlled manner using a closed-loop testing machine. Analysis of the obtained data is performed to evaluate the response of the different vibrational modes of the concrete beam specimen to varying crack and notch lengths. Frequencies of the vibrational modes decrease consistently with increasing crack and notch lengths. There is a larger decrease associated with increasing notch length. Resonance frequencies are shown to be sensitive to crack and notch growth in concrete beams and can be used to effectively monitor the decrease in structural stiffness due to crack progress progression.
There is a need for non-destructive evaluation (NDE) techniques which can effectively determine the extent of damage (cracking) in concrete structures. Non-destructive, one-sided surface wave attenuation measurement is a very sensitive and practical tool for such characterization. A technique for practical determination of frequency-dependent surface wave attenuation is introduced and demonstrated to be sensitive to damage in free concrete slabs. A theoretical model for the attenuation response in undamaged free slabs is introduced and shown to accurately predict experimentally obtained responses in concrete within certain frequency limits. The theoretical model is then used to investigate the practical application of the attenuation technique to concrete pavement NDE in terms of slab depth and subbase conditions. Theoretically obtained data are presented for a variety of pavement types. Based on the presented results of the theoretical model, conclusions concerning practical application of the technique to pavement inspection are given.
The Center for Quality Engineering and Failure Prevention (CQEFP) at Northwestern University is actively involved in the development of new stress-wave based non-destructive evaluation techniques for airport pavements. This paper summarizes recent accomplishments and outlines current research directions. The development of a new stress-wave source is detailed first. The stress-wave generating technique allows for a high degree of control of the input stress wave while at the same time enabling the generation of significant wave amplitudes. Experimental results on concrete specimens demonstrate the controllability and penetrating ability of the developed stress wave generation technique. Its performance is compared to that of an impact source. Experimental results from an existing stress-wave based NDE technique, the impact-echo method, are presented and limitations of that approach are demonstrated. Finally, directions of future airport pavement NDE research at CQEFP, which focus upon application of the developed stress wave generation technique to pavement NDE problems, are outlined.