97 research outputs found
Monitoring early-age acoustic emission of cement paste and fly ash paste
In this study, a combined approach of several monitoring techniques was applied to allow correlations between the AE activity and related processes such as shrinkage and settlement evolution, capillary pressure and temperature development in fresh cementitious media. AE parameters related to frequency, energy, and cumulative activity which exhibit sensitivity to the particle size distribution of cement paste are compared with inert fly ash (FA) leading to isolation of the mechanical sources from the chemical ones. Characterization of the origin of different processes occurring in cement paste during hydration is complex. Although acoustic emission (AE) monitoring has been used before, a qualitative relation between the microstructural formation or other early-age processes and the number or parameters of AE signals has not been established. The high sensitivity of AE enables the recording of elastic waves within the cementitious material, allowing the detection of even low-intensity activities
Monitoring of fresh concrete curing by combined NDT techniques
Ensuring the quality of fresh concrete and suitable curing conditions substantially reduces
the possibility of future failure to perform as designed. However, the most reliable
examination for concrete is mechanical testing after hardening. In order to obtain better
control on the process from very early age, this study describes a combined approach of
several monitoring techniques. Acoustic emission is used to record the numerous events
occurring during the first hours when concrete is in liquid form as well as later when
hardening takes place and drying shrinkage cracking is exhibited. In addition, pressure
sensors follow the development of capillary pressure in the matrix and indicate the moment of
air entry into the system. Settlement and shrinkage, measured both non-contact by digital
image correlation and conventionally, as well as temperature shed light into the complex
processes occurring into fresh concrete and help to verify the sources of AE. The final aim is
to develop a methodology to assess the quality of the fresh concrete from an early age, to
possibly project to the final mechanical properties and to ensure a proper service life
Acoustic and Elastic Waves: Recent Trends in Science and Engineering
The present Special Issue intends to explore new directions in the field of acoustics and ultrasonics. The interest includes, but is not limited to, the use of acoustic technology for condition monitoring of materials and structures. Topics of interest (among others): • Acoustic emission in materials and structures (without material limitation) • Innovative cases of ultrasonic inspection • Wave dispersion and waveguides • Monitoring of innovative materials • Seismic waves • Vibrations, damping and noise control • Combination of mechanical wave techniques with other types for structural health monitoring purposes. Experimental and numerical studies are welcome
Monitoring the reduction in shrinkage cracking of mortars containing superabsorbent polymers
Ultra-high performance concrete (UHPC) is characterized by a low water-to-cement ratio, leading to improved durability and mechanical properties. However, the risk for autogenous shrinkage and cracking due to restrained shrinkage increases, which may affect the durability of UHPC as cracks form pathways for ingress of aggressive liquids and gases. These negative features can be prevented by the use of superabsorbent polymers (SAPs) in the mixture. SAPs reduce autogenous shrinkage by means of internal curing: they will absorb water during the hydration process and release it again to the cementitious matrix when water shortage arises. In this way, hydration can continue and shrinkage is diminished
Assessment of the effect of nanosilica on the mechanical performance and durability of cementitious materials
Over the last years, nanotechnology is getting more attractive and nanomaterials are being used more commonly in construction industry. One of these materials is nanosilica: the nano-sized, engineered form of silica fume. The replacement of cement by these nanoparticles is said to enhance both the mechanical performance and the durability of the concrete material. In this paper colloidal silica will be used, which is nanosilica in solution. A characterization of mortar mixtures containing different amounts of silica is done and a comparison is made with respect to a reference mixture
Experimental techniques synergy towards the design of a sensing tool for autonomously healed concrete
The first-generation of autonomously healed concrete elements is under construction:
beams (SIM-SECEMIN project, Flanders Belgium), one-way flat slabs (MeMC, VUB, Belgium) and
wall panels (Materials4Life project, UK) are designed with the embedment of encapsulated repair
agent. In the presence of cracks, capsules rupture releasing the agent that fills the crack void. The
released agent seals and mechanically restores the crack discontinuity. This automatic process can
be repeatable using vascular networks that carry the agent and release it at different locations into
concrete. The innovative design is built up following several series of laboratory-scale beam tests
configured over the last decade. This paper discusses the application of numerous experimental
techniques that assess the mechanical performance of autonomously healed concrete: Acoustic
Emission, Ultrasound Pulse Velocity, Optical Microscopy, Digital Image Correlation, Capillary
Water Absorption, Computed Tomography. The study focuses on the performance and efficiency
of each method on laboratory and real-scale tests. The techniques with the most promising output
are selected and combined in order to design a sensing tool that evaluates healing on real
applications
Elastic wave monitoring of cementitious mixtures including internal curing mechanisms
The mitigation of autogenous shrinkage in cementitious materials by internal curing has been widely studied. By the inclusion of water reservoirs, in form of saturated lightweight aggregates or superabsorbent polymers, additional water is provided to the hydrating matrix. The onset of water release is of high importance and determines the efficiency of the internal curing mechanism. However, the monitoring of it poses problems as it is a process that takes place in the microstructure. Using acoustic emission (AE) sensors, the internal curing process is monitored, revealing its initiation and intensity, as well as the duration. In addition, AE is able to capture the water evaporation from saturated specimens. By ultrasonic testing, differences in the hydration kinetics are observed imposed by the different methods of internal curing. The results presented in this paper show the sensitivity of combined AE and ultrasound experiments to various fundamental mechanisms taking place inside cementitious materials and demonstrate the ability of acoustic emission to evaluate internal curing in a non-destructive and easily implementable way
Conclusions
The state of the art of structural health monitoring damage detection systems reviewed in this book shows that it is a promising area of technologies. SHM damage detection systems in civil aviation are still mostly limited to lab applications because there are still issues, which need to be solved for such systems to be integrated in an aircraft structure. Therefore, further research is needed to solve the current drawbacks/limitations of the existing SHM approaches such that this technology can be used in aircrafts. Despite the current limitations, SHM application for damage detection in aircrafts would make the flying safer and the structure lifetime longer and reduce the maintenance time and costs considering that the maintenance could be performed not at the predetermined intervals, but upon the need based on the condition that would be determined by the SHM systems used. We conclude some of the important differences and the common challenges to the methods reviewed in this book and provide an outlook on the next steps to a successful implementation
Damage detection and healing performance monitoring using embedded piezoelectric transducers in large-scale concrete structures
Concrete keeps being the leading structural material due to its low
production cost and its great structural design flexibility. However, concrete is prone
to various ambient and operational loads which are responsible for crack initiation
and extension, leading to decrease of its anticipated operational service life. The
current study is focusing on the use of ultrasonic wave propagation techniques based
on low-cost and aggregate-size embedded piezoelectric transducers for the online
monitoring of the damage state and the healing performance in concrete structures
with an autonomous healing system in the form of encapsulated polyurethane-based
healing agent embedded in the matrix of concrete. The crack formation triggers the
autonomous healing mechanism which promises material recovery and extension of
the operational service life. The proposed technique is applied on large-scale, steel
reinforced, concrete beams (150mm × 250 mm × 3000 mm), subjected to four-point
bending. After the capsules are broken and the healing agent is released, which
results in filling of the crack void, and polymerized, the concrete beams are
reloaded. The results demonstrate the ability of the monitoring system to detect the
initiation and propagation of the cracking as well as to assess the performance of the
self-healing system
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