424 publications from this institution
The paper aims to highlight recent developments at the Belgian Nuclear Research Centre SCK•CEN in experimental and numerical study of the coupled physical-chemical behaviour of concrete subject to chemical degradation. The discussion mainly focusses on three interlinked research projects covering novel experimental methods to study the alteration of hydraulic and transport properties during carbonation and calcium leaching, a pore scale numerical model to capture microstructural changes due to the above degradation processes and a generic multiscale model aimed at determining evolution of the properties of a macrostructure over the long term. The paper also describes supplementary continuum scale numerical studies concerning concrete-clay interactions and geochemical impact on the physical structure of concrete. Preliminary findings from these studies show encouraging results such as the development of novel leaching, water permeability and diffusion apparatus, a robust pore scale model based on Lattice-Boltzmann method and a mesoscale study focused on the importance of interfacial transition zones on the effective diffusivity for linear and nonlinear diffusion problems.
The reaction rate of Blast-furnace slag (BFS) in slag blened cement is influenced by many factors, such as fineness, chemical composition, temperature, water/solid ratio, vitreous fraction, and replacement level of BFS. These factors shoud be quantitatively taken into account in order to model the reaction process of BFS in blended system ; therefore it is of paramount importance to study the kinetics of slag reaction. In this article, a kinetics model which is similar to the basic hydration rate equation of Portland cement in the HYMOSTRUC model is proposed. The model considers the above mentioned influence factors based on experimental data level. It is concluded from the proposed model that chemical composition and fineness of BFS are the two governing factors determining the basic reaction rate. Nevertheless, other factors also influence the reactivity of BFS in blended systems.
Zeta potential of freshly mixed cement pastes can help to reveal the hydration properties of cement-based materials. In this paper, the zeta potential of cement pastes with different ultra-fine limestone powders addition and chloride concentration in mixing water was measured. The influences of ultra-fine limestone powder and chloride ion concentrations on early hydration properties of cement pastes were studied. The results showed that zeta potential of freshly mixed cement pastes gradually increased with the chloride concentration in mixing water and ultra-fine limestone powders content. Zeta potential of freshly mixed cement pastes can contribute to reveal information on hydration rate and other early hydration properties.
A numerical method to predict the migration coefficient of concrete is presented. According to NT BUILD 492, the rapid chloride migration (RCM) test can be described as a process of charge passed through a concrete conductor. The chloride migration coefficient is simulated on the basis of the phenomenological relationships between conductivity and current, current and flux, flux and the migration coefficient. The conductivity of cement paste is defined as the ratio of pore solution conductivity to formation factor. A prediction of the formation factor is outlined from the cement hydration and HYMOSTRUC3D model. Finally, validation and a discussion on the simulated results are presented.
Concrete is a brittle composite cementitious material that easily fractures under tensile loading. Microcracks can appear throughout the concrete prior to application of any load because of temperature-induced strain and autogenous and drying shrinkage. There is no doubt that these cracks provide preferential access for aggressive agents to penetrate into the concrete, probably causing corrosion of reinforcement steel and degradation of concrete. As a result, the service life of reinforced concrete structures is shortened. Fortunately, concrete has the ability to heal the crack itself without manual efforts when water is present in cracks. This ability is defined as autogenous self-healing. However, the effect of migration of water from cracks into the bulk paste on autogenous self-healing is not clear yet. The aim of this study is to investigate the effect of water migration from cracks into the bulk paste on autogenous self-healing. Nuclear magnetic resonance (NMR) technique was utilized to monitor water migration from cracks into the bulk paste during the process of autogenous self-healing. NMR results show that in the beginning of autogenous self-healing the water in the crack migrates into the bulk paste and the water content of the bulk paste increases significantly. However, after 5-hour autogenous self-healing period the amount of non-chemically bound water in the bulk paste (adjacent to the crack surfaces) determined by NMR decreases instead. It indicates that some of the water coming from the crack was used by additional hydration of unhydrated cement particles in the bulk paste during the process of autogenous self-healing. NMR results reflect that most of the reaction products of additional hydration are formed in the bulk paste adjacent to the crack surfaces, rather than in the crack. Because of the additional hydration caused by the water from the crack, the capillary porosity of the bulk paste adjacent to the crack surfaces decreases significantly. Before this work the filling of cracks is the main concern in term of autogenous self-healing. The densification of the cement paste adjacent to the crack surfaces, which was observed for the first time in this study, will also decrease the ingress of aggressive agents into the bulk concrete matrix and prolong the service life of concrete structures. This observation provides a new insight into autogenous self-healing in cement paste.
Looking into materials is always a dream for scientists and engineers in the field of materials science. Plenty of research activities have been performed to approach this dream, including numerical simulations and lab experiments. In this paper, a tensile test on cement paste is to be simulated virtually on computer. But it is different from previous attempts because we are working at a lower scale, i.e. the micro scale. Although there is a rich set of modeling techniques available to fulfill the task, it is decided to apply 3D lattice model rather than continuum finite element model to carry out the simulation because lattice is a discretized model and it would be more preferable to simulate fracture process at micro scale. The cement paste used in the numerical experiment is virtually simulated by the cement hydration and microstructure formation model HYMOSTRUC3D. The simulation results reflect the ability of lattice modeling techniques and the addressed principles can be used potentially to produce a complete multiscale simulation.
ASR (alkali-silica reaction) is one of the toughest durability problems in engineering. However, the damage induced by ASR is still fairly unpredictable due to the lack of microstructural information of cement-based materials affected by ASR, while the microstructure determines the global performance. In order to fill this gap, a multiscale simulation model of ASR is under development. The basic theory and assumptions about this multiscale model can be found in [8]. This paper illustrates how the microstructure evolution of cement-based materials induced by ASR is achieved by this model. In the model, the entire chemical process including dissolution of reactive aggregate, nucleation and growth of ASR products (alkali silicate complex, calcium alkali silicate complex), is quantitatively simulated based on the kinetic and thermodynamic parameters. Furthermore, the 3D heterogeneous aggregate is numerically simulated using stereology based on the data from 2D thin-section. As a result, the dissolution degree of aggregate, the amount and location of ASR products and the porosity change can be traced. These micro parameters can be used for the simulation of crack formation in mesoscale. Similarly, the macroscale damage can be predicted based on the simulation results from mesoscale.
