It is well known that the behavior and properties of construction materials largely rely on the characteristics of their internal microstructure. It is important the curing process in freshly poured cementitious materials to be understood to successfully carry out every stage of construction development. Shortly after the mixing procedure, at the state when the suspension transmutes from the liquid to the solid-state phase, the ultrasonic wave propagation and the low pulse velocity of cement-based materials exhibit simultaneously a significant decrease. This is followed by an increase in both the ultrasonic pulse velocity and the signal amplitude. The point of solidification is responsible for the load-bearing capacity of the cement composite and its long-term behavior. At the point of phase change which occurs during curing, the nonlinear behavior of the material exhibits a notable sensitivity. This work aims at the comparison between nano-enhanced and plain cement-based composites regarding their hydration process. Multi-walled carbon nanotubes (MWCNTs) have been used as nano-enhancement in the cement paste specimens. The MWCNTs were synthesized via catalytic chemical vapor deposition, while a water-based superplasticizer was selected as the dispersion agent. The early stages of freshly poured materials were monitored using nonlinear elastic waves. A contact ultrasonic transducer and a noncontact optical detection device (Laser Doppler Vibrometer) were used for the experimental measurements. This method assesses the amplitudes of harmonic vibrations of an elastic wave with a specific fundamental frequency, propagating through the material, leading to the evaluation of its internal structure.
An essential issue in materials research, quality control, and in practical planning and implementation of construction projects, is the understanding of the curing process of fresh cement-based materials. Immediately after mixing, cementitious materials exhibit a significant damping effect on ultrasonic wave propagation together with low pulse velocity. During the curing process, ultrasonic waves, especially the nonlinear acoustic behavior of the material, are sensitive to the point at which the solid phase appears. After this point, the ultrasonic pulse velocities and signal amplitudes increase continuously. The point of solidification is of practical significance since the connectivity of the solid phase is responsible for the load-bearing capacity of the cement composite and its long-term behavior. The aim of this study is to monitor the early stages of fresh cement-paste composites during the hydration process using nonlinear elastic waves. The measurements in this work were performed using a combination of contact ultrasonic transducer and noncontact optical detection measurement device. The principle of operation of the detection device is based on the doppler effect. Using this technique, the amplitudes of harmonic vibrations of an elastic wave with a fundamental frequency propagating through the material can be assessed. This leads to the evaluation of important materials characteristics, such as the changes in internal microstructure of fresh concrete during curing, the evolution of higher order elastic contants of the material expressed in the form of nonlinear parameters, as well as the longitudinal wave velocity.
The demolition waste that is produced by the construction industry is one of the highest generators of solid waste worldwide. Moreover, the construction industry consumes massive amounts of all extracted natural resources. It is thus crucial to encourage more sustainable, environmental and economical construction practices. Nowadays, the proper modification of the recycled aggregates is of high demand because they mitigate the main disadvantages of recycled aggregates, like the increased porosity and water absorption. Although a lot of research has been performed in the modification of coarse recycled aggregates, the modification of fine recycled aggregates has not been adequately investigated. In this study, the outer surface of fine recycled concrete aggregates has been modified by coating them with three different types of modification cement paste, spreadable, elastomeric, water-soluble sealant, and a mixed variety of both modifications. The coated samples were nondestructively examined through the thickness (longitudinal mode), and the ultrasound velocity measurements were processed by a new developing method based on MATLAB that can provide automatically enhanced and high-quality data. The goal is to compare the influence of the cement paste coating of the fine recycled concrete aggregates utilizing ultrasound velocity measurements during the hardening of mortar. Results showed that the coating modification of fine recycled concrete aggregates affects the water absorption, as well as the elastic properties of the mortar. This led to a better understanding of the mechanism of hydration in recycled aggregates mortars, as well as in recycled aggregates concrete.
Different measurement methodologies have been used in bridge monitoring due to seismic, environmental and operating loading. Moreover, bridge monitoring systems have influence in the smooth operation of the traffic load in big cities and thus it’s crucial to encourage monitoring techniques that are flexibly adaptable to various construction building models. Even though a lot of research has been performed in bridge health monitoring in order to identify damages or deterioration of the structural elements there is still a need for a method that could combine multisensor techniques in big data processing for multisource loading. The behavior of the cable-stay bridge model is being monitored via acoustic emission and 2D laser Doppler vibrometry systems. In each case, both static and dynamic loading conditions have been applied. The goal is to correlate the results of these nondestructive evaluation techniques during static and dynamic response of different support situations. The purpose of this work is to improve bridge design and enable the detection of distributed failures during a multifactor loading system.
Infrared thermography (IRT) is a well-established and well-documented nondestructive evaluation (NDE) technique which has been proved as one of the critical assessment tools providing not only qualitative but also quantitative results useful for various applications. Even though many post processing methodologies have been used for thermal imaging analysis, there is still a need for a methodology that could possibly reduce the noise, improve the Signal to Noise Ratio (SNR) and focus on a specific area of interest reconstructing automatically the thermal image. This work deals with fine-tuning the IRT method in order to assess the detectability of damage in composite materials.
In recent years, the damage assessment by means of Laser Doppler Vibrometry (LDV) has become very attractive as it provides non-contact, non-destructive, accurate and improved evaluation of advanced materials. This study deals with the development of advanced software based on LabVIEW in order accurate and automated measurements of acoustic activity to be achieved. Furthermore, this automated method was applied for damage detection in aluminum 1050 Η16 undergone cyclic mechanical loading. LDV was used to measure the amplitude of a Rayleigh surface wave propagating in aluminium specimens. Rayleigh waves are experimentally generated with a piezoelectric transducer and detected by LDV. The proposed measurement technique is used to assess the damage and its evolution, in terms of the increasing amplitude of Rayleigh wave, in 1050 H16 specimens under cyclic mechanical loading. In addition, the reduction in the Rayleigh wave velocity it depends on ultimate fatigue strength of material. The development of this process allows the automated, improved and detailed damage assessment of composite materials.
Scanning acoustic microscopy uses a focused acoustic beam to investigate local elastic properties on the surface of a material. The measurement is based on the difference in propagation time between the direct reflection and the Rayleigh wave. This work deals with the development of a fully automated acoustic microscopy method in order to determine the near-surface elastic property and map sub-surface features in metallic and composite materials. This method allows for the detection and analysis of Rayleigh waves, which are sensitive to subtle changes in the material’s local elasticity. Via this process, the periodicity of the V(Z) curve can be initially assessed and the local Rayleigh velocity of the material is determined. In this work, the automated acoustic microscopy method was applied for the assessment of aluminum and Al-SiC metal matrix composites.