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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.

Recently, the concrete workability has become regarded as much more important due to large demands for high durable concrete with low water content and unstable quality of concrete ingredients. The new superplasticizer was found to provide better workability controlling the rheological properties of the fresh concrete than the previous polycarboxylate type superplasticizers regardless of the quality of concrete ingredients. The framework structure of the new superplasticizer was a hyper-branched polymer in which specific function monomers were co-polymerized. The new polymer of the hyper-branched structure was more condensed than that of polycarboxylate type polymers and was highly adsorptive to cement particles, which made a higher density of the polymer adsorption onto cement surfaces possible. This dense and strong adsorption of the superplasticizer polymer molecules provided the unique rheological properties of the concrete or the mortar such as the lower viscosity of the concrete giving much easier handling. This new superplasticizer also provided the robustness against varieties of the concrete ingredients qualities. This superplasticizer for new generations has already been made practicable successfully for several constructions, and this technology will bring various possibilities and merits in concrete construction fields such as shortening the construction period thanks to better workability.

Superplasticizers are considered to be the most important chemical admixtures in cement mixtures due to their influence on the hardened concrete properties (related to the water-cement ratio) as well as on those of the concrete in the fresh state (workability and workabilty loss). During the last decade a new family of polymers based on polycarboxylate as the main polymer chain and polyether as side graft chains have been developed. This new family of products appear to be in general more effective in terms of higher water reduction, lower slump loss and lower retarding effect at very early ages. More recently these performances have been enhanced in view of some specific applications: a) in precast concrete structures, the very early strength (such as at 12-16 hours) can be increased even in cold climates and in the absence of steam curing; this effect depends on the number and length of the polyether graft chain which are responsible for the dispersion effect related to the steric hindrance; b) in ready-mixed concrete mixtures, the slump maintenance behavior can be designed as a function of the transport time and placing temperatures, so that fresh superplasticized concrete mixtures can be transported from the batching plant to the job site without any slump loss even in summer times; this effect depends on the gradual liberation of special molecular groups which are responsible for the adsoption of these polymers on the surface cement grains; c) a new series of polycarboxylate polymers has been synthetized in which functional chemical groups, acting as shrinkage-reducing admixtures, again are gradually liberated by the superplasticizer polymer as a function of the pH of the aqueous phase related to the cement hydration.

SP-239 This Symposium Publication includes 36 papers selected from a conference that took place in Sorrento, Italy, in October 2006. Topics include cementitious systems, ultra-high-strength concrete, artificial superplasticized aggregate, mortars, and self-consolidating concrete.

Polycarboxylate polymers as superplasticizers have revolutionized concrete technology in the past years. Today most of SCC and high performance or ultra high strength concrete is produced with this new kind of polymers. Their comb-like structure consisting of an ionic backbone and non-ionic polyalkylene glycol side chains offers a huge "playground" for polymer design. Polymers with different side chain chemistry, length, grafting density as well as with different backbone ionic content, structure and length are on the market, which offers the customer an almost confusing range of superplasticizers. Different concrete applications often require different and chemically optimized superplasticizers. The universal admixture is still a challenge for the researchers. In order to better understand the structure-performance relationship of polycarboxylates, systematic variations of comb type superplasticizers were produced in the laboratory and the influence of the changes in polymer structure on the performance in cement paste, mortar and concrete was studied. Clear effects of the structural changes on the performance regarding water reduction, flow improvement and slump retention can be shown. They stress the importance of a good adsorption and adequate surface coverage to achieve good dispersion. All can be directly influenced by the polymer design depending on the desired properties of the superplasticizer.