[en] We propose an investigation combining numerical and analytical tools to estimate the ageing viscoelastic properties of cement-based materials within a multi-scale framework.With analytical homogenization the properties at the cement paste and mortar scales are estimated by a combination of Generalized Self-Consistent (GSC) and Mori-Tanaka (MT) schemes. With numerical homogenization the effective properties at the concrete scale are estimated. Numerical homogenization has the advantage of allowing assessing local information and to study more complex geometries. This combined strategy constitutes a promising tool to investigate how different mechanisms leading to ageing at the hydrated products scale, as well as other features of cement-based materials such as the Interfacial Transition Zone (ITZ), affect the viscoelastic behaviour at superior scales. In this context, we study the solidification of non-ageing constituents as the mechanisms leading to the ageing behaviour combined or not with a space-filling process in C-S-H. Relaxation and creep results are presented. (authors)

[en] This second paper presents a series of 4 crack propagation tests with the same experimental protocol as in a companion paper, but with some significant loading modifications. The first difference is that the loading is composed of in-plane rotation in addition to tension and shear translations. The second difference is that the loading is manually changed during the tests, depending on the crack tip location. This leads to tests with several bifurcations, and/or different loading ratios during the same test. One of them leads to mode I + II, and then mode I + III crack propagation. Some tests end with instabilities while others are controlled to be stable up to the complete failure of the specimen. In some cases, crack closure and friction between the crack faces occur. (authors)

[en] A simple, analytical model was proposed to account for the effect of moderate temperature on water retention in cement-based materials. The model takes advantage of the simplicity and flexibility of van Genuchten equation. It was first calibrated using experimental results taken from literature and then successfully validated on other results. The presented model provides a valuable tool for the description of water transport in unsaturated cementitious materials submitted to moderate temperatures (up to 85 C). (authors)

[en] A semi-analytical model of hydration kinetics based on the boundary nucleation and space-filling growth for early hydration and a diffusion-controlled kinetics for late hydration is proposed. The objective is to develop a simple model that can be coupled with more complex approaches required for the estimation of properties at early-age, but without neglecting some key mechanisms driving the kinetics. The particle size distribution (PSD) of cement is explicitly accounted for. The impingements are modeled by means of an exponential boundary condition in which a limited reaction zone is defined. Numerical solutions for the model are obtained and compared to experimental data. The model can reproduce some relevant features regarding hydration kinetics such as calorimetric data and the influence of the w/c ratio on late kinetics. (authors)

[en] This paper presents a modeling strategy to simulate the corrosion of steel reinforcement in atmospheric environment due to carbonation of concrete. Its principal objectives are to analyze the effects of the progressive formation of corrosion products at the steel/concrete interface on concrete cover cracking. The approach is based on modeling studies carried out independently on carbonation, corrosion and creep. These models are coupled and integrated into a numerical 3D simulation procedure for investigating the behavior of concrete meso-structures. A visco-damage model is used to reproduce both creep and damage behaviors of mortar, and the approach is applied to the simulation of a 3D reinforced concrete meso-structure including explicitly the coarse aggregates. The numerical results highlight the influence of aggregates and the effects of creep on crack initiation and propagation. (authors)

[en] Wollastonite-based brushite cements are used for refractory material applications, but they may also offer prospects for the solidification/stabilization of hazardous waste. These binders are formed by the reaction of wollastonite with an orthophosphoric acid solution containing borax and metallic cations (Al³⁺, Zn²⁺). This work provides new insights into their setting and hardening process using a panel of characterization techniques (XRD, NMR, rheology...). A multi-step hydration process is evidenced by the formation of several products: amorphous silica, monocalcium phosphate monohydrate (Ca(H₂PO₄)₂.H₂O or MCPM), that precipitates transiently during the first stage of hydration, and brushite (CaHPO₄.2H₂O) that crystallizes at higher pH (3). In addition, an amorphous phosphate phase containing aluminum, calcium and zinc massively precipitates at the beginning of hydration, and gets richer in calcium as hydration progresses. Setting of the cement paste occurs when brushite starts to form. The successive formation of MCPM and brushite is well predicted by thermodynamic modelling. (authors)

[en] Two sets of activating solutions with increasing sodium hydroxide contents were prepared either with or without silicates. Their buffer capacities, i.e. their ability to resist changes in pH, were determined and compared using the Hammett acidity function, a measure of acidity appropriate for concentrated solutions. This is the first time the Hammett acidity function of sodium silicate solutions has been measured. The effects of the buffer capacity and of the initial Hammett acidity function on the reactivity of metakaolin-based pastes were assessed using isothermal conduction micro-calorimetry. The reactivity of metakaolin in sodium hydroxide solutions is shown to be directly related to the initial Hammett acidity function, whereas for sodium silicate mixtures, the buffer capacity is a more pertinent parameter. The mechanism deduced for the role of hydroxide ions during geopolymerization also highlights the role of silicate species as a hydroxide reservoir that nurtures the dissolution process. (authors)

[en] In this study, the structure of synthetic magnesium silicate hydrated phases (M-S-H) and the distribution of water as a function of the Mg/Si ratio were investigated. Different analytical techniques indicated that M-S-H phases arc nano-crystallite hydrated phyllosilicates with relatively short coherence length (1.2 nm) and small particle size (> 200 m²/g). Zeta potential and cations exchange capacity measurements showed a negative surface charge on M-S-H from the deprotonation of the silanol groups. The negative surface charge is compensated by exchangeable magnesium or others cations. Combined analyses were used to characterize the water distribution in M-S-H revealing the following: i) confined water; its amount depends mainly on the relative humidity ii) very confined water (or possibly water coordinated to magnesium), and iii) hydroxyl groups bound to magnesium and to silicon. (authors)

[en] One of the main objectives of the APPLET project was to quantify the variability of concrete properties to allow for a probabilistic performance-based approach regarding the service lifetime prediction of concrete structures. The characterization of concrete variability was the subject of an experimental program which included a significant number of tests allowing the characterization of durability indicators or performance tests. Two construction sites were selected from which concrete specimens were periodically taken and tested by the different project partners. The obtained results (mechanical behavior, chloride migration, accelerated carbonation, gas permeability, desorption isotherms, porosity) are discussed and a statistical analysis was performed to characterize these results through appropriate probability density functions. (authors)

[en] The ability of carbonated mortar to provide corrosion protection of embedded steel was studied on mortars prepared with low-alkalinity binders. Three mortars were investigated, mortar based on ordinary Portland cement, mortar based on high volume of fly ash and silica fume and mortar based on slag and nano-silica. Analysis of impedance spectra after carbonation revealed an increase in electrical resistivity of mortar based on OPC and significant decrease in electrical resistivity of low-alkalinity mortar. After exposure to cycles of wetting and drying steel embedded in carbonated low-alkalinity mortars has lower open circuit potential, lower charge transfer resistance and significantly higher corrosion current density compared to steel embedded in OPC mortar. To explain differences in corrosion protection properties, an extensive microstructural testing of carbonated mortar was performed. Difference in corrosion properties were explained by the changes in mortar microstructure, mainly changes in pore size distribution. (authors)