Self-healing of cracks in an ultra high performance concrete, considered as a model material, is investigated in this paper. An experimental program is carried out in order to quantify the phenomenon, which has been mainly highlighted by means of water permeability tests until now. Mechanical behaviour of self-healed concrete under three points bending, and acoustic emission analysis of the cracking mechanisms are reported. The mechanical tests demonstrate a recovery of the global stiffness, depending on the time of healing, for specimens initially cracked and then self-healed, and a slow improvement of structural strength. The acoustic emission (AE) analysis is performed in order to show that the mechanical response is due to new crystals precipitating in the crack. The microcracking of these products during three points bending tests is highlighted and an energy analysis provides insights about the cracking process of healed concrete, including damage of the newly formed crystals and continuation of the crack propagation.

Up to now, glass capsules, which cannot resist the mixing process of concrete, have been mostly used in lab-scale proof-of-concept to encapsulate polymeric agents in selfhealing concrete. This study presents the design of polymeric capsules which are able to resist the concrete mixing process and which can break when cracks appear. Three different polymers with a low glass transition temperature T g have been extruded: Poly(lactic acid) (PLA) (T g = 59 °C), Polystyrene (PS) (Tg = 102 °C) and Poly(methyl methacrylate/n-butyl methacrylate) (P(MMA/n-BMA)) (Tg = 59 °C). After heating the capsules prior to mixing with other components of the mix, to shift from a brittle state to a rubbery state, their survival ratio considerably increased. Moreover, a part of the capsules, which previously survived the concrete mixing process, broke with crack appearance. Although some optimization is still necessary concerning functional life of encapsulated adhesives, this seems to be a promising route.

International audienceLeaching and external sulphate attack on concrete lead to dissolution of hydration products, mainly portlandite, and in case of ingress of sulphate ions to formation of expansive products such as gypsum and ettringite. Durability of concrete exposed to these environments is still based on prescriptive specifications. The aim of the study is to provide experimental data to design performance based tests and specifications for a comparative approach. Tests have been developed to study these degradations in controlled conditions at constant pH. A leaching test has been performed on concrete. Indicators can be deduced from the leaching test and they are sensitive to the variations of potential durability. An accelerated test using a high sulphate concentration has been performed on mortars. Expansion results are sensitive to mortar mix proportions and the test could be used to qualify cements or binders in a comparative approach. Concrete specimens are exposed to external sulphate attack at two levels of sulphate concentration to provide long-term data that can be used to study the relevance of the accelerated tests and models which are being developed. Results from both series of tests show a correlation between the resistance to leaching and the resistance to external sulphate attack

International audienceA semiempirical model is proposed to predict the evolution of chemical shrinkage and Ca(OH)(2) content of cement paste at early age of hydration. The model is based on chemical equations and cement compound hydration rates. Chemical shrinkage and Ca(OH)(2) amount are computed using the stoichiometric results of the hydration reactions considered in the model and the density of hydration products and reactants. The model validation is conducted by comparison between computed and experimental results achieved on ordinary cement pastes with different water-to-cement (w/c) ratios (0.25, 0.30, 0.35 and 0.40) cured at 10, 20, 30, 40 and 50 degreesC, respectively. Hydration degree and Ca(OH)(2) content are determined using the thermogravimetric analysis (TGA) and chemical shrinkage evolution using a gravimetric method. The comparison reveals a good consistency between modelled and experimental data at early age of hydration

International audienceSelf-compacting concrete (SCC) mixtures are usually designed with higher volumes of paste than vibrated concrete mixtures. The results reported in this paper come from a study of nine SCC concrete mixtures. Volume of paste was varied between 291 and 457 l/m3. One of the mixtures had already been used in a large scale test, and the others were designed by varying several parameters of the reference concrete mixture. Mechanical properties, shrinkage, fracture parameters and fracture process zone (FPZ) size were measured. Fracture behavior was characterized by means of three-point bending tests and acoustic emission analysis. From the experimental results, increasing the volume of paste has a restricted effect on strength, unless water content varies. Strength, elastic modulus and fracture resistance slightly decrease with an increase in paste content. Volume of paste causes an increase in shrinkage and cracking due to shrinkage. Fracture and acoustic emission analysis show that increasing the volume of paste tends to make SCC more brittle

In order to build sustainable structures, the study of mechanical behavior must integrate with local phenomena, e.g. fracture propagation and localization zone. Fracture in concrete usually develops in the form of localized zone of microcracks which then coalesce into macrocrack of significant crack openings. In this paper, fracture process in geometrically scaled concrete beams under bending test is analyzed. Acoustic emission (AE) and digital image correlation (DIC) techniques are simultaneously applied to identify fracture parameters such as crack openings and size of fracture zone. The AE technique is useful to identify the location of fracture growth due to microcracks and macrocrack, however, DIC is useful to measure crack openings at various locations of crack. The location of crack tip is also estimated from both techniques. It is observed that the two techniques in coupled position proved effective in identifying the fracture process zone and cracking mechanisms of concrete.

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