International Concrete Abstracts Portal

Testing of admixtures performance is done routinely in industrial laboratories with spread tests in which a cylindrical mould is filled with cement paste and then lifted. The sample spreads and the measured diameter is used to infer the efficiency of the admixture. Recent results link this measurement quantitatively to yield stress. On the other hand, expressions for the dependence of yield stress on interparticle forces and volume fraction of cement have been proposed. In this paper, it is shown that the combination of both approaches brings new insights into the mechanisms by which admixtures modify the rheology of cementitious materials.

A new polysaccharide derivative (NPD) is a thickener that has ionic functional groups and hydrophobic ones. This molecular structure generates unique properties [1], [2]. Generally current common thickeners like cellulose derivatives are much affected by variety and quantity of inorganic metallic ions dissolved from cement. The solubility of thickeners tends to decrease due to metallic ions. Therefore, under ion-exchanged water they add viscosity to water effectively, but the viscous property would be reduced with increase of metallic ions. On the other hand, NPD shows an opposite behaviour. As the concentration of inorganic salt such as NaCl increases, the viscosity of NPD aqueous solution rises significantly [1]. It is supposed that the formation of network structures generated by associating hydrophobic groups in the presence of metallic ions would cause excellent thickening ability. It is expected that NPD will be utilized as a thickener for self-compacting concrete and for anti-segregation in common concrete. Moreover, it is possible to make a so-called two in one product that owns both dispersability and viscous ability, because NPD is mixed with a superplasticizer easily without increasing solution viscosity.

In Japan, the applications of ultra-high-strength concrete having a design strength of 100 MPa are increasing. Such high-strength concrete offers a number of advantages: reduced column cross section for greater useable floor area; broader elasticity range for suppression of earthquake-induced cracking; and longer service life to meet recent demand in Japan. The potential for the use of this class of concrete will increase in Japan. However, ultra-high-strength concrete of this strength range suffers from critical autogeneous shrinkage strain due to its low water-binder ratio (Silica fume is normally used in such the strength range in Japan). The authors developed a shrinkage-reducing superplasticizer by mixing a glycol-based shrinkage reducing component with a superplasticizer for ultra-high-strength concrete developed by the authors, and studied the properties of concrete containing this new shrinkage-reducing SP in this strength range. Compared with plain concrete (without shrinkage reducing component), the shrinkage-reducing SP reduced the autogeneous shrinkage strain by 15 to 30%, and drying shrinkage strain by 3 to 25%. The rate of reduction increased as the W/C decreased and the amount of shrinkage component increased. The authors determined that the new shrinkage-reducing SP is effective in reducing the autogeneous shrinkage strain in ultra-high-strength concrete.

Natural stones have always represented major aesthetic factors in building construction. One of the more ancient and important applications, is the production of decorative floorings. The most famous example in the flooring field is the "Terrazzo alla Veneziana". In recent years, the extensive exploiting of quarries and consequent environmental problems have reduced the availability of good low cost quality stone. In the present work, artificial superplasticized aggregates have been produced and used as decorative elements in this new "Terrazzo" flooring. The substitution of natural stones with these artificial materials is a first application of new practice. These aggregates are produced by combining inorganic binders, finely divided carbonaceous and/or siliceous minerals, superplasticizers and water. The use of nanostructural polycarboxylate superplasticizers allows to obtain very low water to cement ratios and, consequently, the final products are characterised by physical and mechanical properties, similar to those of natural stones. Additional ingredients, such as pigments and other materials have been added in order to obtain artificial aggregates with the desired properties and outstanding aesthetic characteristics.

Aggregate shape, texture, and grading have been known to have a significant effect on the rheological performance of fresh concrete. Moreover, while the optimization of aggregate selection can provide both technical and economical benefits, the availability of materials and construction operations can often dictate the use and proportioning of certain aggregate sources, such as manufactured sands, which can adversely impact the rheology of cementitious mixtures. The use of certain chemical admixtures has been found to often minimize the need to increase cement and water contents in order to overcome the loss of workability that can accompany aggregate sources which feature flat, elongated, angular, and rough particles. In this study, a wide range of natural and manufactured sands, characterized for gradation, mineralogy, shape, texture, and cleanliness, are evaluated for their effect on mortar rheology, with and without a viscosifying-type chemical admixture. While associations between aggregate characteristics and their impact of mortar rheology may not be readily evident, the ability of this class of admixture can be shown to mitigate the rheological effect of certain sands, and in some cases allow for optimizing the mixture to lower paste contents.