Use of Recycled Concrete Powder in Self-Compacting Concrete

Although 37 million tons of concrete wastes have been generated annually in Japan, the use of recycled aggregate for concrete is limited because of low density and high absorption due to adhered cement paste and mortar. A new method to produce high quality recycled aggregate by heating and grinding concrete rubbles to separate cement portions adhering to aggregate was developed recently. In this process, by-product powder with the fineness of 400 m2/kg is generated. By-product recycled concrete powder consists of fine particles of hydrated cement and crushed aggregate. To utilize the recycled concrete powder as concrete additives two series of experiments were performed to make clear the effect of recycled powder. Self-compacting concrete with recycled concrete powder, ground blast-furnace slag and ground limestone were tested for slump-flow, compressive strength, modulus of elasticity and drying shrinkage. Reduction in superplasticizing effect of high-range water reducer was found for concrete with recycled concrete powder. Compressive strength of concrete with recycled concrete powder was the same as those with ground limestone, and lower than those with ground slag. Concrete with recycled concrete powder showed lower elastic modulus and higher drying shrinkage than those with ground slag and ground limestone. The recycled concrete powder is usable for self-compacting concrete without further processing, despite the possible increase in dosage of high-range water reducer for a given slump-flow and in drying shrinkage. The addition of ground blast-furnace slag together with recycled concrete powder to self-compacting concrete improved superplasticizing effect of high-range water reducer and properties of concrete.