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One-pack hybrid admixture (HSP) combining high fluidity, viscosity-reducing property, and shrinkage-reducing capability for high strength concrete with a target strength of 60 to 150 MPa was developed and examined. HSP showed high fluidity and viscosity-reducing performance, as well as excellent autogenous shrinkage-reducing effect to achieve a reduction in the autogenous shrinkage of 21 to 44% with respect to that of concrete containing a conventional high-range water-reducing admixture. HSP was therefore found to be a simple and effective solution to shrinkage problems for a wide range of high strength concretes.

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.

The interactions between six commercial cements and three types of plasticizers, namely naphthalene sulfonate formaldehyde condensate (SNF), lignosulfonate (LS) and polyacrylate grafted with polyether (PA) are investigated using cement paste. PA was found to be the most efficient plasticizer of the three tested even though it was found to present lower adsorption onto cement than SNF and LS. SNF and LS brought about comparable results. PA was observed to induce flow gain of pastes within 2 hours of rheological measurements, while pastes made with SNF and LS exhibited flow loss as a function of time. The adsorption characteristics were found to depend on the plasticizer type. The plasticizer saturation dosages were, moreover, found to depend on cement surface area (Blaine fineness), amount of cubic C3A and soluble sulfates.

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.