International Concrete Abstracts Portal

In relation to reinforced concrete high-rise buildings built in the Fifties and Sixties of the 20th Century, it has acquired importance, in the last few years, the analysis of the capabilities to withstand various kinds of environmental risks, defined according to actual parameters. The provisions prescribed by new structural design codes practiced today, indeed, have substantially changed both design actions and verification procedures as well, if compared to the building criteria in use in the past. This kind of analysis gives evidence to specific design performances which are seen as prevalent nowadays but were not considered in older versions of the codes, as the earthquake loads. In the present work this problem is discussed with reference to the case study offered by the Milan Municipality 25 story r.c. building erected in Milano in the ‘60s. Typically, this kind of buildings were designed for the effect of vertical loads and wind lateral loads only. At present, after being recognized of strategic importance for the society, they have to be verified also for the seismic resistance. Although the seismic hazard is classified as low in the area of Milano, design seismic forces are a little more severe than wind actions for this building, due to the limited ductility resources available in the structural elements, mainly in the shear walls. Consequently, the value which can be assigned to the load reduction factor is extremely low.

Natural fibres are a waste product of food and agriculture industry to which a great potential of use as dispersed reinforcement in cementitious composites has been recognized, making them a valuable source of income for developping communities and countries, where they are abundant and can be harvested with minor investments. A further value to the use of natural fibres in cementitious composite as promoters and facilitators of self healing has been recently confirmed by preliminary investigations. Thanks to their microstructure, natural fibres are able to create a porous network through which the moisture can be distributed throughout the cementitious matrix and activate the delayed hydration reactions which, together with carbonation ones, can be responsible of the autogeneous healing of cracks. The authors have undertaken a comprehensive experimental programme to investigate the efficacy of different types of natural fibres, when used in combination with industrial fibres (steel), to promote and enhance the self healing reactions in HPFRCCs. Influence of environmental conditions has also been studied. The effects of self healing on the recovery of flexural performance has been quantified; healed cracks and effects of healing on fiber matrix bond have been visualized through optical digital microscopy.

A key concept of sustainability is the preservation of resources, thus adding life to existing concrete structures by means of durable strengthening and rehabilitation methods is a key objective. Composite materials, such as FRCM (Fabric-reinforced Cementitious Matrix), have proven to be a viable option for increasing durability of existing building stock. Experimental works show that the main failure mode of FRCM, applied to masonry or concrete substrates, is by debonding at the fabric/matrix interface. Here, the idea is to use an epoxy coating and a layer of quartz sand in order to increase the adhesion of the fabric with the matrix. The effectiveness of coating treatments was studied by means of tensile tests, as indicated in AC434 Annex A. Tests were carried out on seven different types of fabric, with different levels of pre-impregnation and with or without quartz sand applied to the fabric surface. Experimental evidence shows a promising enhancement of the bond between fabric and matrix and, therefore, of the entire strengthening system even with the use of low percentages of resin, depending on the type of mortar.

The action of high temperature in concrete is a field of much interest and attention due to its strong influence in strength, durability and serviceability conditions. Long-term exposures to high temperature fields strongly affect the most relevant mechanical properties of concrete materials such as cohesion, friction, stiffness and strength. In this work, two alternatives approaches for the analysis of failure behavior of concrete subjected to high temperatures are discussed and their predictions analyzed. Specifically, a thermodynamic gradient poro-plastic model based on the continuous or smeared-crack approach and an interface model based on the discrete crack approach are developed. After describing the main aspects of both models, this work focuses on the analysis of their results in terms of the degradation of concrete durability and strength capacities when subjected to severe thermal fields. The results demonstrate the comparative advantages of the discrete approach to analyze at both the macroscopic and mesoscopic scale the complex degradation processes of concrete constituents at high temperature, thanks to the robustness, stability and overall simplicity of the discrete model approach. Furthermore, the results show the capabilities of the continuous model to analyze the durability degradation of concrete at material level.

The durability of concrete structures is particularly susceptible to aggressive environments, in particular to the penetration and diffusion of chloride ions. Hence, a reliable prediction of the chloride diffusivity is mandatory to schedule efficient maintenance as well as to estimate the service and ultimate life of concrete structures. This is a non-trivial task because the chloride diffusion process is clearly a multiscale problem since it is influenced by different factors acting at different length and time scales, including the ability of some phases of the hardened cement paste (HCP) to interact with chloride ions. In the present work the chloride diffusivity of HCP is numerically simulated using a modified version of Fick’s law accounting for the ability of some HCP phases to bind chloride ions. The 3D HCP microstructures for the analyses are generated artificially, using the software CEMHYD3D, as well as segmented starting from real X-ray images, and in all cases are discretized using a voxel-based mesh. The effective (homogenized) coefficient of diffusivity, to be used for mesoscale analyses, is obtained through upscaling and is validated using data from the literature. Finally, comparisons between real and artificially generated HCP microstructures are performed and discussed.