A diffusion -reaction numerical model is proposed to simulate the response of concrete exposed to external sulfate attack. Diffusion properties are modified based on the strain reached and the ratio of porosity filled by ettringite. A direct and intuitive approach is proposed for the consideration of the diffusion in a cracked porous media based on the constitutive law of the material. A methodology to compute expansions based on a more realistic consideration of the concrete porosimetry is presented, by which it is possible to distinguish different strain contributions from different pore sizes. The described approach also allows the consideration of different capacities to accommodate expansive product for each pore size considered and the faster filling rate existent in small pores. Critical parameters of the numerical model developed are recognized and

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Design codes promote a limitation of permeability (indirectly of porosity) to reduce the sulphate ingress and improve the resistance of concrete and mortar to external sulphate attack (ESA). However, porosity could also have a positive effect on durability by generating additional space to accommodate the expansive phases. The aim of this study is to evaluate the role of porosity in ESA. For that, changes at the macro-scale, phase composition and pore network are monitored for mortar compositions with different pore-size distribution. Results indicate the existence of two mechanisms: the capacity to accommodate expansive phases controls the durability during the initial stages of the attack, while at later stages durability is defined by the permeability. Results from specimens with air-entrainer suggest that the intentional increase of porosity towards maximising the capacity to accommodate expansive products might be a valid approach in order to reduce the expansive forces generated during ESA.

14The external sulfate attack is a degradation process that causes expansion and cracking 15 in concrete structures. Due to the absence of simplified methodologies to predict the 16 potential damage, codes specify that sulfate resistant cement should be used whenever

The objective of this paper is to evaluate the influence of sulfate exposure on the pore network development of several Portland cement matrices. MIP, XRD and SEM analysis were performed at different ages in samples exposed to sulfates after 2 days of casting. Results suggest that patterns of precipitation of the expansive products are linked to the degree of refinement of the pore network. During early stages of exposure, large pores concentrate a higher proportion of the expansive product formed. At later stages, precipitation evolves towards finer pore sizes.

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Research on external sulfate attack (ESA) is usually performed on small scale specimens under free expanding conditions. However, most field structures cannot expand freely due to confinement induced by the ground or other elements from the structure. As a result, ESA usually develops in confined conditions. This work aims to assess the interaction of gradual biaxial compression stresses generated by confinement with the ESA. Visual appearance, mass and ultrasonic velocity were monitored to characterize the macro-scale behavior of free and restrained mortar samples. Changes on phase composition and crystal morphology induced by confinement were evaluated by X-Ray diffraction and scanning electron microscopy. The confining stresses generated were estimated during the attack. Results indicate that ESA is not developed equally in free and confined specimens. The confinement limits sulfate availability, reduce the amount of ettringite precipitated and might induce changes on crystal morphology that reduce the degradation caused by the ESA. Keywords: Concrete (E); Durability (C); Sulfate attack (C); Ettringite (D); Confinement; 1. INTRODUCTION External sulfate attack (ESA) is a degradation process that affects concrete structures. It is caused by the interaction between the reactive phases of the cement paste and sulfate ions from an external source. The continuous sulfate exposure may lead to cracking, spalling, softening and disintegration. Fortunately, in typical service conditions the attack usually develops during decades before causing high degrees of damage. Previous research on the interaction between constant compressive loads and ESA [1-3,9] suggests that the effects of confinement will probably depend on the stress level. Therefore, for a precise assessment of this phenomenon, it is necessary to link the different behavior of confined and free specimens with the stress level applied. The monitoring of the confining stresses generated during the attack requires complex experimental set ups. Only one study was found in the literature able to capture this phenomenon [10]. Mullauer et. al [10] applied different degrees of confinement to thin-walled mortar cylinders exposed to sulfates by a specially constructed stress cell. Even though the main objective was to estimate the stresses generated during the attack, it indirectly pointed some possible positive effects of confinement on the durability. Evolution of the phase composition with and without confinement showed that both ettringite and gypsum were partially suppressed at late stages of the attack by increasing the degree of confinement. Unfortunately, other potential effects of stresses generated by confinement remain practically unexplored and are ignored during the assessment of the ESA. The present work aims to shed light on this phenomenon and identify which processes (ionic transport, phase composition or crystal morphology) might be altered by the confining stresses. One of the key aspects to study this phenomenon is the experimental set up adopted t...

This paper studies the microstructural and tribological differences of layers formed during the ferritic nitrocarburizing of AISI 4340 alloy steel and AISI 347 stabilized stainless steel. The samples were exposed to different times of immersion in a nitrocarburizing bath (60, 75, 90, 105 and 120 min) at 580 °C. Subsequently, they were subjected to an oxidation process at 480 °C in order to form a Fe3O4 layer. Surface microstructural studies were carried out by SEM-EDS and x-ray diffraction (XRD). Wear and friction coefficient of nitrocarburized samples and non-treated samples were studied by pin-on-disk test. The results show two well-differentiated zones in AISI 4340 steel: an outer oxides layer, a white layer or compound layer and a diffusion zone. However, the compound layer was not found in AISI 347 steel. In both steels, the specific wear coefficient (k) of nitrocarburated samples is about thirty times lower than the reference samples.