International Concrete Abstracts Portal

Self-healing cementitious composites are a broad category of smart construction materials to which strong and highly qualified research efforts are currently being devoted worldwide, with the aim of providing a sound scientific background to their consistent, and – design-wise – “consciously safe”, use in the engineering practice. Tailored additions can be employed to enhance the self-healing capacity, among which the so-called crystalline admixtures, play a prominent role. Crystalline admixtures consist of proprietary active chemicals, which, because of their hydrophilic nature, react with water and cement particles in the concrete to form calcium silicate hydrates, increasing the density of the CSH phase, and/or pore-blocking precipitates in the existing micro-cracks. The mechanism is analogous to the formation of CSH and the resulting crystalline deposits become integrally bound with the hydrated cement paste, thus contributing not only to a significantly increased resistance to water penetration but also to the healing of the existing damages and cracks. This paper summarizes the results of a wide experimental investigation jointly performed by Politecnico di Milano (Italy), Indian Institute of Technology Madras, Chennai (India) and Universitat Politecnica de Valencia (Spain) to assess the effectiveness of different commercially available crystalline admixtures on the self-healing capacity of cement based materials.

The concrete industry is among the most resource and energy-consuming human activities. Therefore, several solutions are currently under investigation with the aim of reducing the environmental impacts of concrete production. These solutions often consist of partially replacing ordinary “natural” constituents (i.e. aggregates, cement, water, fibers) with recycled ones, in view of the twofold objective of reducing both the demand of raw materials and the amount of waste to be disposed in landfills. Therefore, this paper deals with sustainable concrete made with recycled concrete aggregates and coal fly-ash in partial substitution of natural aggregates and Portland cement, respectively. Particularly, it reports the results of a wide experimental activity intended at investigating both the mechanical properties and the durability performance of this material by measuring the time evolution of its compressive strength, as well as some relevant durability-related properties, such as water permeability, carbonation resistance and chloride penetration. The results reported herein unveil the synergies arising by employing the aforementioned recycled materials and industrial by-products in partial replacement of the ordinary concrete constituents.

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.