Effectiveness of Superplasticizers Incorporating Shrinkage-Reducing Admixture in Recycled-Aggregate Concrete

摘要

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.