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Abstract
There are many choices in regard to selection of materials in any type of constructions. Due to growing interest in sustainable construction, engineers and architects are motivated to choose the materials which are more sustainable. Green concrete capable for sustainable construction is characterized by application of industrial wastes to reduce consumption of natural resources and energy and pollution of the environment. Replacement of materials over nominal concrete is what makes green concrete more environmental friendly concrete. Marble sludge powder, quarry rocks, crushed concrete and fly ashes are some of the materials used for making green concrete, a sustainable construction.
INTRODUCTION
Color has nothing to do with green concrete. It is a concept of thinking environment into every aspect of the raw materials manufacture over construction, mixture design to structural design, and durability. Green concrete is very often considered to be cheap to produce due to the use of recycled material whereby avoiding the charges for the disposal of waste, less energy consumption and greater durability.
While a normal construction practices are guided by short term economic considerations, sustainable construction is focused on best practices which emphasize on long term affordability, durability and effectiveness. At each stage of the life cycle of the construction, it increases ease and quality of life, while minimizing the negative environmental impacts and increasing the economic sustainability of the construction. Any infrastructure designed and constructed in a sustainable way minimizes the use of resources through the whole life cycle of the construction process in which the green concrete play a vital role in achieving the sustainable construction. Having so much of advantageous has lead to popularity in construction world and one of the emerging technology in sustainable construction. Green concrete is a miracle of present and tool for the future when the natural resources are on the verge of extinction.
2. WHAT IS SUSTAINABLE CONSTRUCTION?
Sustainable constructions are those constructions which are concern with the minimizing of environmental impact, while optimizing it economically capability
GREEN CONCRETE FOR SUSTAINABLE CONSTRUCTION: THE CHALLENGE FOR THE CONSTRUCTION INDUSTRY
The turnout of the construction industry, in any infrastructure like public and commercial building has a major drawback on our ability to maintain a sustainable economy overall and has a major burden on our environment. Furthermore, it is clear that we cannot have a sustainable construction without bringing changes in the concrete technology as it is a major technology that is used by the construction industry.
This paper will discuss how the green concrete would be able to achieve sustainable construction. Conventional concrete with well known advantages has made huge popularity and is widely used material by the construction industry. Yet this popularity of concrete comes with huge impact on environment as well as making the construction unsustainable [1].
More than 5 billion cubic yards of concrete are produced globally. Such quantity require huge amount of natural resources for the aggregate and cement production
Cement is one of the major components of the concrete and contributes to the urban heat island effect when used in concrete. The production of one ton of cement releases one ton of CO2 into the atmosphere. CO2 is known to be greenhouse gas that contributes to the global warming.
Normal concrete are usually produce with the poor quality which results in a corrosion of reinforced concrete, alkali-aggregate reaction, sulfate attack etc…
The demolition and disposal of concrete structures, pavements, etc., contributes to the solid waste disposal problem, and concrete constitutes the largest single component.
Lastly, requirement of water is so high that the concrete industry uses over one trillion gallon of water each year globally without including wash water and curing water, which becomes a problem in those region where the fresh water is not easily available.
The above mention points seem to indicate that the concrete industry has become a victim of its own success and therefore is now faced with tremendous challenges but the scenario is not as bad as it might seem, because concrete is inherently an environmentally friendly material, as can be demonstrated readily with study of life-cycle . The challenges, therefore lies in primarily to reduce impact of Portland cement’s on the environment. It means, we should use as much as concrete yet with as little Portland cement as possible.
4. TOOLS AND STRATEGIES
There are number of strategies whereby green concrete can help in achieving the sustainable construction [2], [3]:
• Increased dependence on recycled materials: Effective use of recycle material can help in reducing the dependence on virgin material.
• Effective use of supplementary cementitious material: Partial replacement of cement can be done by the byproducts of industrial processes, such as fly ash and slag as the production of Portland of cement is responsible for generation of CO2 and huge energy is consumed.
• Improved mechanical properties: Proper use of recycle material can help in improving the mechanical properties.
• Reuse of wash water: The recycling of wash water can be seen practice by the most of the construction industry and required by law in some countries.
• The above mention points clearly indicate that there are means to achieve sustainable construction with the help of green concrete. The means will be discussed in detailed under the following points
Use Geo-Polymer Concrete
An interesting new innovation in concrete is the use of a variety of Geopolymers. These materials can be combined with materials such as ground granulated slag, fly ash, and natural pozzolanas, to produce concretes without the need to use Portland cement. The materials are said to be strong and
durable. Geopolymer has been found to be having high resistance to acid attack. Further concrete exhibits zero alkali-aggregate expansion which is an important property in areas with potentially reactive aggregates. However a major advantage of some types of geopolymer concrete is their greatly improved fire resistance in comparison with traditional Portland cement concretes. Geopolymer concretes produce only about 7% of the carbon dioxide generated in the production of traditional OPC concretes, giving the material the potential to earn valued carbon credits [4].
4.2 Partial Replacement of Cement
The reduction in the use of Portland cement can be achieved with partial replacement of cement by the various cementitious materials, such as fly ash, ground granulated blast furnace slag, metakaoilin, wood ash and limestone powder. The high-strength concrete which was known in the late 1970s is now referred to as high-performance concrete (HPC) because it has been found to be much more than simply strong. Use of this cementitious material has resulted in an improvement of the properties of concrete. These include a lower heat of hydration, which minimizes the risk of thermal contraction cracking, providing the concrete is insulated in order to minimize temperature differentials between the core and surface temperature at early ages. Secondly there is an increased resistance to chemical attack, including that from sulphates and salt water. In addition there is increased resistance and often elimination of alkali – aggreg ate reaction. There is also increased resistance to chloride induced corrosion of reinforcing steel, which h is especially important in structures, in or adjacent, to the marine environment[4].
4.2.1 Fly-ash
The use of fly ash has a number of advantages. It is theoretically possible to replace 100% of Portland cement by fly ash, but replacement levels above 80% generally require a chemical activator. Moreover, fly ash can improve certain properties of concrete, such as strength. Since it generates less heat of hydration, it is particularly well suited for mass concrete applications. Fly ash is also widely available, namely wherever coal is being burned. Another advantage is the fact that fly ash is still less expensive than Portland cement. Maybe most important, as a byproduct of coal combustion fly ash would be a waste product to be disposed of at great cost, if we don’t make good use of it. By utilizing its cementitious properties of fly ash, we are making best use of its value [1], [2], [3], [4].
Ground Granulated Blast-Furnace Slag
Ground granulated blast furnace slag (GGBFS) is another excellent cementitious material. It is obtained by quenching molten iron slag (a by-product of iron and steel making) from a blast furnace in water or steam, to produce a glassy, granular
product that is then dried and grounded into a fine powder. Here the optimum cement replacement level is somewhere between 70 and 80%. Like fly ash, also GGBFS can improve many mechanical and durability properties of concrete and it generates less heat of hydration. The use of granulated blast furnace slag in concrete has increased considerably in recent years, and this trend is expected to continue. The worldwide production of granulated blast-furnace slag, however, is only about 25 million tones per year. Yet, slag is not as widely available as fly ash. Generally, the comparison of ground granulated blast furnace slag (GGBFS) with Portland cement concrete can be summarized as follows[4]:
• Concrete with Type IS cement (Pozzolana cement) or with higher dosages of GGBFS added at the mixer usually will have lower heat of hydration.
• Concretes containing slag may show somewhat longe r time of setting than straight Portland cement mixtures, particularly for moderate and higher dosages and at lower ambient temperatures.
• Concrete with Type IS cement gains strength more slowly, tending to have lower strength at early ages and equal or higher strength at later ages.
• Increasing slag dosage is associated with lower p ermeability in concrete.
• Concrete containing GGBFS dosages greater than 35 % by mass of cementitious material, have demonstrated an improvement in the resistance to sulfate attack, as well as suppression of alkali-aggregate expansion