17-09-2016, 02:44 PM
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ABSTRACT
The uses of recycled materials like fly ash, slag cement, silica fume, recycled aggregate, recycled materials, recycled plastic, solid waste materials in concrete are explained in this paper through introduction, Concrete incorporates three major types of recycled materials, Fly Ash, Slag Cement, and Silica Fume (SCMs), durability of fly ash concrete, Recycled Aggregates, Recycled Materials in the Cement Plant, Post-consumer recycled material, Use of recycled plastic in concrete, The Use of Solid Waste Materials as Fine Aggregate Substitutes in Cementitious Concrete Composites, Concrete Mixes, Particle Size Distribution, experimental design and sample preparation, experimental results, need of recycling of waste materials, Obstacles in Use of RCA & RAC
INTRODUCTION
Concrete made with Portland cement, water admixtures and aggregates comprises in quantity the largest of all man made material historically whenever new compounds were produced, or waste materials accumulated in industries, they were incorporated as one of ingredients of concrete. Typical examples are fly ash phosphogypsum, blast furnace slag, saw mil waste, rice husk, cotton etc. Recycling of concrete materials also offers some promise. Attempts are already being made to use municipal refuse & waste oil as partial substitutes for the fuel in the production of cement clinker Building products that have a portion of their constituent materials from recycled products reduce the need for virgin materials in new construction.
Concrete incorporates three major types of recycled materials:
1. Fly ash, slag cement, and silica fume are industrial by-products that are used as a partial replacement for Portland cement in concrete. These supplementary cementitious materials (SCMs) are pre-consumer materials.
2. Recycled material or recycled concrete can be used as aggregates in concrete.
3. Spent solvents, used oils, tires, and medical waste are used as fuel in many cement plants. Industrial byproducts are used as ingredients for manufacturing Portland cement.
Fly Ash, Slag Cement, and Silica Fume (SCMs)
Fly ash, slag cement, and silica fume are industrial by-products; their use as a partial replacement for Portland cement does not contribute to the energy and CO2 impacts of cement in concrete. If not used in concrete, these materials would use valuable landfill space. Fly ash is a by-product of the combustion of pulverized coal in electric power generating plants. Slag cement, also called ground granulated blast furnace slag, is made from iron blast-furnace slag. Silica fume is a by-product from the electric arc furnace used in the production of silicon or ferrosilicon alloy.
These SCMs are used as a partial replacement for the Portland cement in concrete. Fly ash is commonly used at replacement levels up to 25%; slag cement up to 60%; and silica fume up to 5% to 7%. When slag cement replaces 50% of the Portland cement in a 7500 psi concrete, greenhouse gas emissions of concrete are reduced by 45%.
SCM frequently enhance the durability of concrete. Some SCMs increase curing times and can affect the construction schedule.
The durability of products with recycled content materials should be carefully researched during the design process to ensure comparable life cycle performance
Recycled Aggregates
One of the major challenges of our present society is the protection of environment. Some of the important elements in this respect are the reduction of the consumption of energy and natural raw materials and consumption of waste materials. It conserves natural resources and reduces the space required for the landfill disposal.
The environmental attributes of concrete can be further improved by using aggregates derived from industrial waste or using recycled concrete as aggregates.
Recycled concrete can be used as aggregate in new concrete, particularly the coarse portion. The Federal Highway Administration (FHWA) reports that eleven states use recycled concrete aggregate in new concrete. These states report that concrete with recycled aggregate performs equal to concrete with natural aggregates. When using the recycled concrete as aggregate, the following should be taken into consideration:
• Recycled concrete as aggregate will typically have higher absorption and lower specific gravity than natural aggregate and will produce concrete with slightly higher drying shrinkage and creep. These differences become greater with increasing amounts of recycled fine aggregates.
• Too many recycled fines can also produce a harsh and unworkable mixture. Many transportation departments have found that using 100% coarse recycled aggregate, but only about 10% to 20% recycled fines, works well. The remaining percentage of fines is natural sand.
• In crushing the concrete, it is difficult to control particle size distribution, meaning that the “aggregate” may fail to meet grading requirements of ASTM C33 – “Standard Specification for Concrete Aggregates
Recycled Materials in the Cement Plant
Cement is made by heating common minerals – typically crushed limestone, clay, iron ore, and sand – to temperatures of about 2700°F (the temperature of molten iron). Achieving these high temperatures requires large quantities of fuel, mainly coal, petroleum coke, and natural gas. Waste materials often have energy value and can be used as fuel.. However, they can be safely burned to destruction as fuel in a cement kiln while reducing the need to use fossil fuels.
The cement industry has steadily increased its use of waste materials to fuel cement kilns, and currently relies on the burning of waste materials to satisfy 10% of these energy needs. Cement plants also burn many industrial wastes, including sludge from the petroleum industry, and agricultural wastes such as almond shells.Common industrial byproducts used include fly ash resulting from the production of electricity, mill scale resulting from steel making, and foundry sand resulting from metal castings. Cement manufacturers even reuse cement kiln dust (CKD), the primary byproduct of cement manufacture, by recycling it back to the kiln as an ingredient for new cement.
Post-consumer recycled material
Post-consumer recycled material is defined as waste generated by households or by commercial, industrial and institutional facilities as the end-users of a product. These are products that are sold and used for a specific purpose and then need to be disposed, such as newspaper, containers, computers, and batteries. Post-consumer materials include crushed concrete and masonry from demolished buildings that are reused as aggregate for concrete in new buildings.
Use of recycled plastic in concrete
The increasing awareness about the environment has tremendously contributed to the concerns related with disposal of the generated wastes. With the scarcity of space for land filling and due to its ever increasing cost, waste utilization has become an attractive alternative to disposal. Research is being carried out on the utilization of waste products in concrete. Such waste products include discarded tires, plastic, glass, steel, burnt foundry sand, and coal combustion by-products (CCBs). Each of these waste products has provided a specific effect on the properties of fresh and hardened concrete.
Reuse of bulky wastes is considered the best environmental alternative for solving the problem of disposal. However, efforts have also been made to explore its use in concrete/asphalt concrete. The development of new construction materials using recycled plastics is important to both the construction and the plastic recycling industries. The effect of recycled and waste plastic on bulk density, air content, workability, compressive strength, splitting tensile strength, modulus of elasticity, impact resistance, permeability, and abrasion resistance is discussed in this paper.