The final properties of cementitious materials (strength and durability) strongly depend on the mix proportions and the fresh state of the latter. It is therefore imperative to investigate the early stages, assess the quality of the mixes as well as monitor their time evolution. In this direction, ultrasonic measurements, since many decades, have been proposed as the most efficient tool for quality control and condition characterization due to their ability to inspect, detect, locate and continuously monitor the material’s performance throughout the entire lifetime. However, wave propagation can be quite complicated, especially if the material heterogeneity and wave-microstructure interactions are taken into account. For this reason, in the current study, the ultrasonic experiments are complemented by numerical analyses of wave propagation offering the advantage of easier, faster, repeatable and parametric implementation. The strong dispersion and attenuation trends observed in both the experiments and the numerical tests make, herein, the additional implementation of scattering theories necessary as the third pillar. The results show good match between the experimental and the numerical methods as well as between the numerical simulations and scattering theories, thus providing a more holistic insight of wave propagation in microstructured cementitious materials. In the framework of this study, cement pastes and mortars (containing sand or glass beads as aggregates) are investigated, while the results are demonstrated in terms of pulse velocity and attenuation as a function of frequency revealing interesting information on the influence of the aggregate content on the quality of the mixes.
Nowadays, more and more, the monitoring of concrete’s setting and hardening as well as concrete’s condition assessment and mechanical characterization is realized with the Ultrasonic Pulse Velocity technique. However, despite its increasing use, the high potential and the vast applicability over a wide range of materials and structures, the aforementioned nondestructive testing technique is only partially exploited since a) a default pulse usually not selected by the user is transmitted, b) a single frequency band dependent on the testing equipment (pulse generator and sensors) is excited and c) usually the first part of the signal is only considered. Moreover, the technique, as defined by its name, is based on pulse velocity measurements which strongly rely on a predefined threshold value for the calculation of the travel time between the transmitting and receiving sensor. To overcome all these issues, in the current experimental campaign, user-defined signals are generated, a broad range of ultrasonic frequencies is excited, while the full length of the signal is also taken into account. In addition, the pulse velocity measurements are replaced by the more advanced phase velocity calculations determined by reference phase points of the time domain signals or by phase differences of the signals transformed in the frequency domain. The experiments are mainly conducted in hardened concrete specimens but the aggregates are substituted by spherical glass beads of well-defined sizes and contents in order to better control the microstructure. Reference liquid media are also examined for comparison purposes. The results in both cases show strong dispersive trends indicated by significant changes in the phase velocity.
In construction sector marble and granite are widespread because of their unique properties through the centuries. The issue of repair in these materials is crucial in structural integrity and maintenance of the monuments through the world, as well as in modern buildings. In this study fracture experiments on granite specimens are conducted. The goal is to compare the typical acoustic emission (AE) signals from different modes (namely bending and shear) in plain granite and marble specimens as well as repaired in the crack surface with polyester adhesive. The distinct signature of the cracking modes is reflected on acoustic waveform parameters like the amplitude, rise time and frequency. Conclusions about how the repair affects the mechanical properties as well as the acoustic waveform parameters are drawn. Results show that AE helps to characterize the shift between dominant fracture modes using a simple analysis of AE descriptors as well as the integrity of the specimen (plain or repaired). This offers the potential for in-situ application mainly in the maintenance of the monuments where the need for continuous and nondestructive monitoring is imperative, but always care should be taken for the distortion of the signal, which increases with the propagation distance and can seriously mask the results in an actual case.
Protecting the environment and future generations against the potential hazards arising from high-level and heat emitting radioactive waste is a worldwide concern. Following this direction, the Belgian Agency for Radioactive Waste and Enriched Fissile Materials has come up with the reference design which considers the geological disposal of the waste in purely indurated clay. In this design the wastes are first post-conditioned in massive concrete structures called Supercontainers before being transported to the underground repositories. The Supercontainers are cylindrical structures which consist of four engineering barriers that from the inner to the outer surface are namely: the overpack, the filler, the concrete buffer and possibly the envelope. The overpack, which is made of carbon steel, is the place where the vitrified wastes and spent fuel are stored. The buffer, which is made of concrete, creates a highly alkaline environment ensuring slow and uniform overpack corrosion as well as radiological shielding. In order to evaluate the feasibility to construct such Supercontainers two scaled models have so far been designed and tested. The first scaled model indicated crack formation on the surface of the concrete buffer but the absence of a crack detection and monitoring system precluded defining the exact time of crack initiation, as well as the origin, the penetration depth, the crack path and the propagation history. For this reason, the second scaled model test was performed to obtain further insight by answering to the aforementioned questions using the Digital Image Correlation, Acoustic Emission and Ultrasonic Pulse Velocity nondestructive testing techniques.
The propagation of longitudinal waves through concrete materials is strongly affected by dispersion. This is clearly indicated experimentally from the increase of phase velocity at low frequencies whereas many attempts have been made to explain this behavior analytically. Since the classical elastic theory for bulk media is by default non-dispersive, enhanced theories have been developed. The most commonly used higher order theory is the dipolar gradient elastic theory which takes into account the microstructural effects in heterogeneous media like concrete. The microstructural effects are described by two internal length scale parameters (g and h) which correspond to the micro-stiffness and micro-inertia respectively. In the current paper, this simplest possible version of the general gradient elastic theory proposed by Mindlin is reproduced through non-local lattice models consisting of discrete springs and masses. The masses simulate the aggregates of the concrete specimen whereas the springs are the mechanical similitude of the concrete matrix. The springs in these models are connecting the closest masses between them as well as the second or third closest to each other masses creating a non-local system of links. These non-neighboring interactions are represented by massless springs of constant stiffness while on the other hand one cannot neglect the significant mass of the springs connecting neighboring masses as this is responsible for the micro-inertia term. The major advantage of the presented lattice models is the fact that the considered microstructural effects can be accurately expressed as a function of the size and the mechanical properties of the microstructure.
The Impulse Excitation Technique (IET) is a useful tool for characterizing the structural condition of concrete.
Processing the obtained dynamic parameters (damping ratio, response frequency) as a function of response amplitude,
clear and systematic differences appear between intact and cracked specimens, while factors like age and sustained load
are also influential. Simultaneously, Acoustic Emission (AE) and Ultrasonic Pulse Velocity (UPV) techniques are used
during the three point bending test of the beams in order to supply additional information on the level of damage
accumulation which resulted in the specific dynamic behavior revealed by the IET test.
Fatigue assessment includes estimation of the expected damage accumulation and the remaining life-time of the
structure. This work is based on output-only vibration measurements at a limited number of locations provided by a
sensor network installed on the structure. For the fatigue damage assessment, the stress time responses are obtained by
using the vibration sensor data and a modal expansion approach enabling predictions of stresses at positions where
sensor installation is not possible. A methodology for the prediction of stresses based on the combination of a finite
element numerical model and the accelerations recorded at measurement locations is presented in this paper.
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