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

Surface wave dispersion in cement-based media: Inclusion size effect

Abstract This paper presents the experimental study of surface wave propagation in cementitious material with different shape and size but same volume content of thin inclusions that simulate distributed damage. The Rayleigh wave velocity changes almost up to 20% depending on the inclusion shape, while the longitudinal velocity, the experimental variance, as well as the coherence of the signals is also affected. It is demonstrated that the material is strongly dispersive and caution should be taken for the interpretation of the wave measurements since the velocity is sensitive not only to the damage content but also to the ''crack'' size.

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