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Requirements on material properties for extrusion-based additive manufacturing mostly focus on the rheological behavior of the cementitious material being printed. The layer interface strength is therefore often considered to result from a proper mixing or remixing of two consecutive layers induced by the deposition process itself and therefore from the material thixotropic behavior. We show however here that, in the case of smooth interfaces, the drop in interface strength finds its origin in the water evaporation from the free surface occurring during the short time interval between two successive layers. Our results and their analysis within the framework of drying physics suggest that the water loss is localized in a dry region at the free surface leading to an incomplete cement hydration and high local porosity. We moreover compare here various experimental protocols allowing for the assessment of a drop in bond strength.

The aim of this paper is to present a real-time relation between the evolution of the electrical resistance and the crack width opening in a saturated concrete sample. The splitting (Brazilian) test has been enhanced and adapted so that the monitoring of a single crack opening and the electrical resistance of the specimen are possible. The experimental results showed that the electrical resistance is constant before the peak (when no crack is observed) and decreases only when the peak of load is reached and when a crack is initiated. No threshold value and no delay between the crack opening and the resistance evolution have been observed, even in the case of a breathing crack. The evolution of the electrical conductance has then been modeled by taking into account the (simplified) morphology of the crack and assuming a constant value for the electrolyte conductivity. The model and the experimental conductance through a traversing crack have proved to be in good agreement.

A model is proposed that accounts for the isothermal drying process of hardened cement-based materials. It constitutes a further development of previous works. The equations of isothermal drying are derived (i) from mass balance equations written for the liquid water phase, water vapour and dry air, (ii) from the Fick's law governing the relative diffusion process of water vapour and dry air to the gaseous mixture, and (iii) from the Darcy's law describing the transport of wet air and liquid water. Intrinsic liquid water (Kl) and gas (Kg) permeabilities are distinguished, since the concept of intrinsic permeability, which is independent of the fluid nature, is not relevant for a cementitious material. New laws for gas transfers are introduced according to measurements on concrete specimens. Thus, a semi-empirical law gives the effective diffusion coefficient of water vapour vs. porosity and degree of liquid water saturation. In the same way, a new function, expressing the relative permeability to gas with respect to this degree of saturation, is proposed on the basis of experimental results. In order to describe the global movement of gas, viscous and slip flows are taken into account according to the Klinkenberg's concept. A numerical study shows, on the one hand, that a gas depression (below the atmospheric gas pressure) can be observed and, on the other hand, that transfers of water in the gas phase may significantly contribute to the drying of cementitious materials in addition to liquid water transport by capillarity movements. Simplified approaches and their range of application are presented.

Within the framework of the evaluation and the prediction of chloride-induced corrosion risks, simple and rapid AgNO3 spray tests can be proposed for various issues. This paper forms the first part of a series. In this first part, the Maultzsch procedure (spraying of 0.1-N AgNO3 solution + K2CrO4) as well as the procedure based on the use of a sole AgNO3 solution have been investigated on a broad range of concretes. These procedures have also been compared to the Collepardi procedure (spraying of 0.1-N AgNO3 + fluoresceine). The feasibility of these colorimetric techniques on saturated specimens cast in laboratory, after non-steady-state diffusion tests carried out in various conditions, is discussed. In addition, the results obtained from applying such spray tests in field conditions on cores drilled out from various RC test specimens exposed to a marine environment (tidal zone) and to a road and cold environment (freezing-thawing cycles and spraying of deicing salts) are presented. Colorimetric methods have in particular been applied here to the assessment of the average chloride penetration depth and of its evolution versus time (kinetics). Moreover, the detection threshold of these techniques has been investigated in various environments. The possible sources of discrepancy on the results have been analysed.