Influence of Recycled Aggregates, Foundry Sand and Fly Ash on Compressive Strength of Concrete

Introduction

Concrete is the first and foremost material required in the construction works. It is one of the most versatile, economical and universally used construction material. It is among the few building materials produced directly on the job by the user. The key to use of concrete is the ease with which structural concrete elements can be formed into a variety of shapes and sizes because of the plastic consistency of freshly made concrete which permits the material to flow into prefabricated formwork. It is the second most consumed material after water and is the basis for the urban environment. It is estimated that the present consumption of concrete in the world is of the order of 12 billion tonnes every year. Concrete is a composite material that consists essentially of a binding medium within which fragments of aggregates are embedded. The three basic ingredients of concrete are cement, aggregate and water. Cement binds the ingredients together, the aggregates add bulk to the concrete and water gives viscosity to concrete in order to be moulded and react with the ingredients. Each constituent influences the characteristics of the concrete and must be controlled as to composition and quantity if the end product is to be within acceptable limits of workability and strength.

The utilization of recycled aggregate is particularly very promising as 75% of concrete is made of aggregates. Recycled aggregates comprises of crushed, graded inorganic particles processed from the materials that have been used in the constructions and demolition debris. Most of the waste materials produced by demolished structures is disposed off by dumping into landfills. Further, the user agencies/point out that presently, the Bureau of Indian Standards and other codal provisions do not provide the specifications for use of recycled products in the construction activities. Central Pollution Control Board has estimated current quantum of solid waste generation in India to the tune of 48 million tonnes per annum out of which, waste from construction industry only accounts for more than 25%. It is estimated that approximately, 40% of the generated waste portion globally originates from construction and demolition of buildings. An investigation conducted by the environmental resources limited for European Environmental commission (EEC) envisages that there will be enormous increase in the available quantities of construction and demolition concrete waste from 55 million tonnes in 1980 to 302 million tonnes by the year 2020 in the EEC member countries. There is increasing demand and interest in aggregates from non-traditional sources such as from industrial by-products and recycled construction and demolition (C&D) wastes. Many countries have directed on recycling schemes for construction and demolition wastes to avoid dumping in landfill, as suitable landfill sites are becoming scarce particularly in heavily populated countries. The American Concrete Institute (ACI) has focused on the removal and reuse of hardened concrete whereas the Department of Environment and Water Resources in Australia have developed a guide on the use of recycled concrete and masonry materials. Recycling of Construction and Demolition Wastes has long been accepted to have the possibility to conserve natural resources and to decrease energy used in production. In some countries it is a standard substitute for both construction and maintenance, particularly where there is a scarcity of construction aggregate. Thus, the use of recycled aggregate concrete is need of the hour for the sustainable development of society with savings in natural resources and materials which are depleting at quite an alarming rate.

Foundry sand is high quality silica sand which is a by-product of ferrous and nonferrous metal casting industries. Foundry sand is produced by five different foundry classes. The ferrous foundries produce the most of the percentage of sand while aluminium, copper, brass and bronze produce the rest. Ferrous (iron and steel) industries account for approximately 95% of foundry sand used for castings. The automotive industry and its parts suppliers are the major generators of foundry sand. The physical and chemical characteristics of foundry sand depend on the type of casting process and the industry sector from which it originates. The sand is typically used multiple times within the foundry before it becomes a by-product. There are approximately 4500 units of foundry in India, out of which 80% are small scale units & 10% each are medium & large scale units. Industry estimates that approximately 100 million tonnes of sand is used in production annually & out of this 6-10 million tonnes discarded annually. Used foundry sand is a high volume industrial waste that can be more widely reused as an alternative to landfill disposal. It can be used beneficially in concrete production as a fine aggregate replacement.

Fly ash is finely divided residue resulting from the combustion of coal in thermal power plants and is most used pozzolanic material all over the world. It is one of the supplementary cementitious material that reacts with substances in concrete mix to form cementitious compounds. As a result, fly ash can replace a portion of Portland cement in a concrete mix. Around 110 million tonnes of fly ash get accumulated every year at the thermal power stations in India. It is estimated that around 80 plus thermal power stations across the country would produce more than 175 million tonnes of fly ash by the year 2012 and this would require about 40000 hectares of land for the ash disposal. The Ministry of Power, Govt. of India estimates use of 1800 million tonnes of coal every year and 600 million tonnes of fly ash generated by 2031-2032. Only 15-18% fly ash generated is being used productively. A typical value of the cement for the same volume of concrete is 60% of the cost of all of the raw materials. Thus, the cement substitution with fly ash not only saves the construction cost but also saves natural resources such as limestone and coal, which are used for the manufacturing of cement. Fly ash mixed into concrete accounts for approximately 7% of fly ash that is diverted from landfills each year. The production of Portland cement is estimated to generate about 5% of the world's greenhouse gas (GHG) emissions which is reduced to 3.4%, when cement is substituted by fly ash.

It is expected that present study involving the effect of combination of recycled aggregates, foundry sand as a partial replacement of fine aggregate and fly ash as a partial replacement of cement on compressive strength of concrete will be beneficial to the existing construction methodology.