Recycled aggregate concrete is the name given to the concrete having coarse aggregates taken from “old concrete” i.e. which obtained from demolition work and that from laboratory waste.
It involves number of operations in manufacturing of recycled aggregates. The basic operations are separation, crushing, separation of ferrous elements, screening, decontamination and removal of impurities (wood, paper, plastics etc.). If required then washing can be done. Now a days manufacturing plants of RAC or C&D waste (construction and demolition waste) treatment plants are available. All the facilities of the required operations in order to prepare the RAC can be provided by these plants.
There are two categories of plants available for recycling of CDW and processing them into RA which are stationary and mobile recycling plants. Stationary facilities uses fixed equipment and they do not have to deal with offsite operations (i.e. they treat all the waste on the site of the plant). On the other hand mobile recycling machineries and equipment are sent to work site to recycle the waste at the source of occurring. Stationary plants are more effective and productive. It has higher capacities and is able to produce better quality of the final product. It have disadvantage of being far from the waste generation site which involve additional cost of transportation. On the other hand Mobile plants have advantages of less transportation cost and working on the site where the waste is being generated. Same operations will takes place in both kinds of plants discussed above.
Crusher reduces the size of input waste. Primary and secondary crushing could be performed with milling operations to achieve the required grading. Generally a jaw crusher or horizontal impact crusher is used to crush the debris. Size classification is performed using screens of various required sizes. At the end washing could be done, which will eliminate fine particles (silt and clay). This step is not so common due to the difficulties in the produced mud disposal. Sometimes washing can be done along with the screening operation. Finally the sorted aggregates are then stored in specific sizes which can be used as aggregates in the new construction work. Considering the facilities in our country, India had started its first CDW recycling plant in 2009 (Burari plant in Jahangirpuri New Delhi). The plant have has an operating capacity of 500 tons per day (TPD) and it is able to recover 95% of incoming waste. The plant has been successfully processing CDW into recycled aggregates which can be used for brick making and building roads.
RECYCLED AGGREGATE CONCRETE AS A STRUCTURAL CONCRETE:
RAC uses RA which is of variable nature. The large use of RAC as a structural concrete requires more technological development in this field. It is difficult to have 100% replacement of NA to achieve a similar mechanical performance of concrete made up of NA. Although, it is considered good to replace the natural aggregates as higher as possible from recycling and economical point of view. Replacing in greater extent and maintaining the mechanical properties of concrete need clever justification. There are many difficulties in doing so, due to the variable characteristics of the RA used. The reason behind its minimal use is lack of evidence to show its economic benefits when used in structural concrete. Regarding to the RAC many research works are consistently reported that it have higher creep and shrinkage, and lower elastic modulus than conventional concrete with similar composition. Lack of technical specification and guidelines for quality control are other reasons. Worldwide more research work has been carried out to enhance the use of RAC as a structural concrete.
Many countries are steel using the RAC in structural applications but, in order to use it on vast level and to replace NA in high performance concrete (HPC) more research work is need to be carried out. Countries those are using the RAC had certain limitations in their specifications in terms of replacement percentages. After careful research and investigations those restrictions should be lower down in order to use it as structural concrete confidently.