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Recycled-aggregate concrete is known for its higher tendency to shrink with respect to concrete prepared with ordinary aggregate, at least when both coarse and fine recycled aggregate are used. In this work, an attempt was made to manufacture recycled-aggregate concrete that is less sensitive to shrinkage. Seven different concrete mixtures were prepared with the same water-to-cement ratio of 0.45 by various kinds of coarse aggregate: ordinary natural gravel, recycled-concrete aggregate from a suitable treatment of precast-concrete scraps, or a recycled-rubble aggregate from a crushing plant in which rubble from building demolition is treated. Different kinds of water-reducing admixtures were also tested: the first one was based on polycarboxylate polymer, the other two were also based on polycarboxylate polymer incorporating a shrinkage reducing group. The latter two are characterized by a different formulation to assure either a set-accelerating or a retarding effect. The pure superplasticizing admixture was added at a dosage of 0.8%, by mass of cement, while the multifunction admixtures were added at a dosage of 1.6% and 2.0%, by mass of cement, for the accelerating and the retarding types, respectively. Compressive strength tests were carried out at different curing times, and free-drying shrinkage was measured up to 70 days of age. The results were positive, particularly in terms of very low shrinkage of recycled-aggregate concrete containing the shrinkage reducing admixture.

Dynamic and static stability of self-consolidating concrete (SCC) affect the production, transport, and overall performance of the concrete. Viscosity- modifying admixtures (VMAs) are often incorporated to enhance the stability of SCC, especially in the case of cast-in place concrete. The influence of a new type of polysaccharide, diutan gum, on key characteristics of SCC targeted for the construction and repair of concrete infrastructure is examined. The investigation compares the performance of SCC made with diutan gum and different types of high-range water reducing admixtures (HRWRAs), including polynaphtalene sulphonate (PNS)-based and two polycarboxylate polymer (PCP)-based HRWRAs. The effect of admixture combination on workability, rheology, stability, and setting time was investigated. The robustness of optimized VMAHRWRA system and key durability parameters of the concrete were also determined. Test results indicate that the use of diutan gum increases the plastic viscosity and yield stress of SCC. Regardless of the VMA and HRWRA combination, the use of diutan gum can significantly decrease segregation and bleeding, and lead to greater homogeneity of the concrete during the dormant period of cement hydration. The performance of SCC made with diutan gum depends on the type of PCP in use. Out of two PCP-based HRWRA, one system resulted in lower HRWRA demand, increase in viscosity with the increase in VMA content, and greater stability. The use of PCP and diutan gum is shown to lead to adequate robustness where the SCC can tolerate small changes in sand moisture content without significant effect on SCC properties. SCC made with medium dosage of diutan gum and PCP is shown to develop adequate air-void system and excellent frost durability and resistance to de-icing salt scaling.

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.