Research On Asessment Of Recycled Coarse Aggregates And Early Performance Of Recycled Concrete

As a kind of new building material, recycled aggregate concrete (RAC) has been applied in non-bearing structures in industrial and civil buildings, pavement engineering. The technology of RAC can not only reduce the environmental pollution brought by the waste concrete, but also can preserve the natural aggregate resources. It can bring us social benefit, economic benefit and environmental benefit, according with the requirement of sustainable development strategy.A lot of studies on the behavior of recycled aggregates concrete have been going, especially for the performances of recycled aggregates, research results aren't agreement due to the recycled aggregates. It is of vital significance to research the early performances of recycled aggregates concrete for construction safety and quality requirements. Early performance mainly includ early concrete strength and drying shrinkage, some qualitative study at home and abroad has been made, but quantitative study is not reported as yet. Based on the test, the distribution and random of recycled aggregates are studied in the paper. Besides, classification methods of recycled aggregates are made to provide the basis for practical engineering. The early strength of recycled aggregates concrete are investigated,and test parameters are water-cement ratio, replacement ratio and water amount. The laws of the early strength with the age are found, and mathematical models of early strength are established. In addition, the early shrinkage of recycled aggregates concrete has been researched, some methods to prevent drying shrinkage are put forward, and the model for drying shrinkage of recycled concrete with 100% replacement of the recycled coarse aggregate is presented. The main results are summarized as follows:1.The water absorption, bulk density, apparent density, crush index of recycled coarse aggregate and compressive strength of recycled coarse aggregates all accord with the normal distribution law. The dispersion degrees of the water absorption and crush index are lager than other index. The quality of recycled aggregate is mainly affected by service life, strength class, service environment, utility of the original concrete and so on.2. The strength of RAC increases quickly in the early period and then gradually decreases. The compressive strength and the flexural strength reach 50%,70%,85% of the 28 days'strength at the age of 3,7,14 days. The early strength at different ages decreases with the water-cement ratio and replacement ratio, and the smaller the water usage per unit volume is, the lager the strength. is.3. The shrinkage ratio and the water loss ratio of recycled concrete increase with the water-cement ratio and replacement ratio, the rates of the shrinkage and water loss reach to the Maximum in the first 7 days, then gradually decreased. At the age of 28 days, the shrinkage rate fell off sharply. At about 45 days, the water loss ratio no longer increased, but the shrinkage ratio continued to develop slowly. Adding fly ash or superplasticizer into the recycled concrete could effectively reduced the shrinkage of recycled concrete.