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Clays are components in the fine portion of aggregates, less than 75 microns in size (micro-fines), which are usually washed away when producing coarse or fine (manufactured sand) aggregates in quarries. When marginal sources of aggregates are being used, the content of this washed portion can be quite high, and there is an incentive to keep as much of it in the aggregate, including the clays. The present paper presents a comprehensive treatment of the role of clays in terms of the characterization of their composition and quantification of their effects on the rheological and mechanical properties of cementitious systems, as well as the means to mitigate deleterious influences. It is shown that the strategy for neutralizing the effect of micro-fines containing clays on increased water demand in concrete can be quantified in terms of the combination of their content in concrete and their nature as characterized by the methylene blue value (MBV); this is a more rational approach to considering their influence than their content in specific aggregates as specified in standards. The effect of low and medium MBV aggregates on the water requirement can be neutralized by lignosulfonates when their content in the concrete is below a threshold value of about 150 kg/m3; polycarboxylates (PC) are required at higher contents; for high MBV aggregates, a combination of PC and clay mitigating admixture (CMA) is required. It is also demonstrated that with proper treatment, such micro-fines can be turned into useful fillers, enhancing the strength of concrete and thus also serving as a means for reducing cement content. [ABSTRACT FROM AUTHOR]

The object of the present paper was to quantify the effect of crimping of a low modulus polymer yarn on the bonding with a cement paste matrix. Crimping of this kind has been evaluated within the context of optimizing the structure of woven fabric to obtain a strain hardening composite. The concept applied here was based on existing models developed for straight yarns. These models resolve between two stages: the elastic stage (adhesion) and the slip stage (friction). A similarity in the shape of the pull-out load versus slip curve of straight yarns and crimped yarns was found, suggesting that a modeling approach for straight fibers, which consider adhesional and frictional resistance, can also be applied for the crimped yarns. The effect of the yarn crimping on calculated adhesional and frictional components, [[tau].sub.au], and [[tau].sub.fu], was estimated, showing a marked increase with yarn density for both types of bonding mechanisms. These values are considered as equivalent values since their physical significance is limited. Empirical linear correlations was found between the equivalent adhesional and frictional bond strengths and the crimp density. These empirical relations were combined with equation describing the shape of pull-out curves, to calculate the pull-out curves of the crimped yarns. The agreement with the experimental curve was reasonably good. CE Database subject headings: Pull-out resistance; Cements; Bonding; Polymers; Fabrics; Fibers; Textiles.

Cementitious materials have been used in construction for several millennia and through several civilizations. Their use has been revitalized in the recent century, and they have become the major construction material for housing and infrastructure. The amalgamation of the science and engineering of these materials in recent decades has advanced the state of the art in their understanding and development. In a new century where sustainable construction is becoming an overriding consideration, the new scientific concepts can have direct impact on the way we use and formulate cementitious materials. On top of this, the scientific advances have provided the means for the potential use of the current portland cement as a platform to develop composite formulations of a high level of performance equivalent to and in some respects exceeding those of steel and ceramics. This may provide the basis for advanced and efficient construction techniques. The present article reviews the past, present, and future of cementitious materials and highlights the need for a comprehensive approach to maximize the advantages of the newly emerging cementitious materials and concretes. CE Database keywords: Cements; History; Concrete.

Microstructural changes that occur over time around fibers in the cement matrix, leading to an enhanced bond and stiffening of the interfacial transition zone, may result in a reduction in the mechanical properties of the composite. Micromechanical modeling of processes of this kind suggest that such aging trends may occur in microfiber reinforcement but not in macrofiber reinforcement. Degradation in mechanical properties of the composite will show up only when a 'pessimum' combination of parameters exist, particularly brittle microfibers in a very dense matrix. Therefore, in the development of new cementitious composites, interracial effects of this kind should be considered, because aging due to such mechanisms can be demonstrated to occur in practice. Thus, it is not sufficient to assess the durability of cementitious composites by considering only the chemical stability of the fiber in the alkaline cementitious matrix.