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

Application of digital image correlation to cement paste

In this paper digital image correlation (DIC) has been applied to study the deformation process of cementitious material at very early age. After mixing of cement-based materials, the cement hydration process begins. Consequently, the ongoing Chemical reactions result in a 3D deformation process (shrinkage). The mechanism affecting the very early age hydration as well as specifically the deformation behavior of cementitious materials is a challenging topic. In view of that, it is essential to determine the significant effect of concrete hardening process on the deformation progression at different stages. The technique of DIC is highly sensitive and allows for the first time in literature an accurate and non-contact optical monitoring of the shrinkage of fresh cementitious material. The displacement of the surface is measured by correlating the different digital images taken at different ages after mixing of the material. The system enables a 3D observation that allows a deeper understanding of the deformation progression. The surface displacement determined by DIC-software (Vic-Snap 2010) is compared to the displacement measured by Linear variable differential transformer (LVDT) sensors for calibration purposes. DIC system realizes a more precise method avoiding the effect of self-weight of the traditional sensor. The purpose of this work is to check the sensitivity as well as the effectiveness of DIC technique, to characterize and better understand the 3D deformation process of fresh cementitious materials

Monitoring Acoustic Emission of Fresh Cement Paste

High strength and durability are the key factors determining the quality of concrete. To achieve the required properties, concrete mix design, and early age hydration, are the main factors affecting the performance of concrete. It is essential to monitor fresh concrete continuously through non-destructive techniques at the moment of mixing. In this study, acoustic emission (AE) has been applied to allow a continuous monitoring of the fresh cement paste. This non-destructive inspection allows the estimation of concrete properties by capturing elastic waves that are nucleated and propagate in the cement past. Moreover, ultrasonic pulse velocity (UPV), capillary pressure and heat evolution monitoring has been applied on cement paste to study the process of hydration mechanism. This study aims to check the sensitivity and effectiveness of AE technique to characterize the ongoing processes in fresh cementitious material and the possibility to contribute to a better monitoring of the process as an additional tool

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

Concrete fracture energy increase by embedding capsules with healing ability : the effect of capsules nature

Concrete is the basic material of infrastructures since it is cost-effective, efficiently produced and strong. Despite its popularity, under service-loads the concrete matrix suffers from flaws that can be crucial for its durability. To overcome the shortcoming in durability, concrete is traditionally reinforced by steel or its mixture is modified by introducing additives that enrich the autogenous crack closure. Nowadays, an alternative solution is proposed namely autonomous healing. Repair polymer agent is encapsulated into tubes and embedded into concrete during mixture. The tubes break as soon as a crack wider than 100 μm is propagated across them. Only at this moment, the agent is released and polymerized. The crack void is sealed and repaired (mechanical features restored as well). The previous years, researchers at the Dept. Mechanics of Materials and Constructions, VUB have studied the mechanical performance of newly developed healing systems and evaluated their repair efficiency. In this study, an additional benefit of autonomous healing is assessed: the short or long tubes contribute as local reinforcement of concrete under tensile load and enhance the fracture toughness. The energy release rate and other fracture mechanics parameters are measured for plain concrete beams tested under three-point bending. The reference case (concrete carrying no healing system) is compared to cases at which different encapsulation systems are applied. Additionally, the study of fracture is correlated to the findings of inspection with different non-destructive techniques. The effect of tubes design (geometry, shape, material) on the fracture toughness is studied leading to the most promising healing system