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GREEN CONCRETE
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INTRODUCTION
The concrete which is made using wastes which is eco-friendly is called as Green concrete. Green concrete is a revolutionary topic in the history of concrete industry. It was first invented in Denmark in the year 1998.
The CO2 emission related to concrete production, inclusive of cement production, is between 0.1 and 0.2 ton per ton of produced concrete. Since concrete is the second most consumed entity after water it accounts for around 5% of the world’s total CO2 emission (Ernst Worrell, 2001). However, since the total amount of concrete produced is so vast the absolute figures for the environmental impact are quite significant.
The solution to this environmental problem is not to substitute concrete for other materials but to reduce the environmental impact of concrete and cement. Usage of quarry rock dust along with fly ash and micro silica reported satisfactory properties. The potential environmental benefit to society of being able to build with green concrete is huge. It is realistic to assume that technology can be developed, which can have the CO2 emission related to concrete production. With the large consumption of concrete this will potentially reduce the world’s total CO2 emission by 1.5 - 2%. Concrete can also be the solution to environmental problems other than those related to CO2 emission. It may be possible to use residual products from other industries in the concrete production while still maintaining a high concrete quality. During the last few decades, society has become aware of the deposit problems connected with residual products, and demands, restrictions and taxes have been imposed and as it is known that several residual products have properties suited for concrete production, there is a large potential in investigating the possible use of these for concrete production. Well-known residual products such as silica fume and fly ash may be mentioned.[1
GENESIS
Considering the time elapsed since the commencement of the use of concrete, green concrete is very young a material. It was invented in 1998 in Denmark. The increasing awareness and activity to conserve the environment and the realization that concrete production too releases a considerable amount of CO2 in the atmosphere were strong initiatives to catalyze the genesis of Green Concrete.
In 1997, the Kyoto Conference took place, in which several countries, after deliberating over the then environmental conditions laid down several guidelines which would be the directive principles to the participating countries on their environment related practices. The guidelines – Kyoto Protocol, as they are called, needed the countries to cut down their CO2 emissions to a certain degree as assigned. The given goal had to be achieved by the year 2012. Since then several countries started to focus on several available options but Denmark focused on cement and concrete production because approximately 2% of Denmark’s total CO2 emission stems from cement and concrete production.
PROPERTIES OF GREEN CONCRETE
. Materials for Green Concrete
Green construction materials are composed of renewable, rather than non-renewable resources. Green materials are environmentally responsible because impacts are considered over the life of the product. Depending upon project-specific goals, green materials may involve an evaluation of one or more of the following criteria
Fresh Local Aggregate
Many places there are stone quarry available. Though these may not be of high quality stone like granite, basalt, Dolomite etc. but they may be of little lower quality. These can be used in making concrete with the help of appropriate mix design - may be for lower characteristic strength. A typical local aggregate in sand stone is shown in Figure
Recycled Demolition Waste Aggregate
Construction industry produces huge waste called demolition waste or MALWA. It is estimated that per capita waste generation (including Municipal waste) generally range from 0.4 to 0.8 Kg per day per person. A typical waste dump is shown in Figure 2.2. The waste contributes to greenhouse gas emissions and thus waste prevention and/or its recycling will reduce greenhouse gases and methane gas emissions etc.
Therefore, for sustainability of resources, it is necessary that all waste must be scientifically managed. Waste Management is Collection, Transport, Processing, Recycling or disposal of waste materials. When analyzed, a typical waste product distribution in any solid waste dump is shown in Figure 2.3. This waste distribution shows that there is about 50% demolition waste in the dump. In order to have sustainability of resources this demolition waste must be recycled and used
Recycled Concrete Material (RCM)
Recycled Concrete Material (RCM), also known as crushed concrete is similar to demolition waste. It is a reclaimed Concrete material. Primary sources of RCM are demolition of existing concrete pavement, building slabs & foundations, bridge structures, curb and gutter and from commercial/private facilities. This material is crushed by mechanical means into manageable fragments. The resulting material is in the form of Coarse Aggregate. Comprised of highly angular conglomerates of crushed quality aggregate and hardened cement, RCM is rougher and more absorbent than its virgin constituents. The failure pattern of crushed bricks made with RCA is shown in Figure
Blast Furnace Slag (BFS)
In India more than 10 million tones of Blast Furnace Slag is produced every year and it is increasing with the increase in steel production. Blast furnace slag is a waste product from the manufacture of pig iron and obtained through rapid cooling by water or quenching molten slag. Iron ore, as well as scrap iron, is reduced to a molten state by burning coke fuel with fluxing agents of limestone and/or dolomite. Blast furnace slag is a nonmetallic co-product produced in the process of steel production. BFS forms when slagging agents (e.g., iron ore, coke ash, and limestone) are added to the iron ore to remove impurities. In the process of reducing iron ore to iron, a molten slag forms as a nonmetallic liquid (consisting primarily of
Natural Sand Vs Manufactured Sand
Natural sand often contains undesirable minerals and clays, and the effect of these materials on both the fresh and the hardened concrete can be extremely harmful. For example, the effect of clay particles in fresh concrete is obvious, as the particles absorb disproportionate volume of water and hence swell to many times their original size. This swelling occupies a volume in the cement paste in its fresh state. When it hardens, the clay particles contract and leave minute voids which in turn increase the shrinkage and permeability. This in turn reduces the concrete's chemical resistance and compressive strength. Other undesirable materials, ranging from basic chlorides to harmful chemicals, can exist in such fine material fraction. The use of manufactured sand, however, reduces the risk of impurities.
It has been proven that about 20kg of cement can be saved for every cubic meter of concrete that is made by replacing a poorly shaped aggregate with a cubical aggregate. In addition, both compressive strength and flexural strength are improved by using cubical aggregates, which also increases workability and reduces bleeding and shrinkage. The impact of the physical characteristics of the sand used in the concrete mix is even greater than that of the coarse aggregate fractions, both in the concrete's plastic and hardened states
Recycled Glass Aggregate
Glass is formed by super cooling a molten mixture of sand (silicon dioxide), soda ash (sodium carbonate), and/or limestone to form a rigid physical state. Glass aggregate is a waste product of recycled mixed glass from manufacturing and post consumer waste. Glass aggregate, also known as glass cullet, is 100 percent crushed material that is generally angular, flat and elongated in shape. This fragmented material comes in variety of colors or colorless. The size varies depending on the chemical composition and method of production / crushing.
When glass is properly crushed, this material exhibits fineness modulus & coefficient of permeability similar to sand. It has very low water absorption. High angularity of this material, compared to rounded sand, enhances the stability of concrete mixes. Such material can be easily used in concrete construction as fine aggregate and give a better cohesive mix which will save on the consumption of cement
Blast Furnace Slag (BFS)
Blast furnace slag is described above under coarse aggregate. Here if blast furnace slag may be broken down as typical fine aggregate also with the help of processing equipment to meet gradation specifications. Thus it can be available as fine aggregate also as construction materials and acceptable for use in green Concrete. A typical shape of ACBFS fine aggregate is shown in Figure
High Volume Fly Ash Concrete
One of the important milestones in the history of concrete technology is the development of high-volume fly ash concrete (HVFAC). Concrete mixture containing 50% or more fly ash by mass of cementitious material is termed as HVFAC, a term coined by Malhotra in the late 1980s.[9] From theoretical considerations and practical experience of prominent researchers, it was found that, with 50% or more cement replacement by fly ash, it is possible to produce sustainable, high-performance concrete mixtures that show high workability, high ultimate strength, and high durability.[10] It was first developed for mass concrete application where low heat of hydration was of primary consideration.
Subsequent work has demonstrated that this type of concrete has excellent mechanical and durability properties required for structural applications and pavement constructions. Some investigations have also shown the potential use of the high-volume fly ash system for shotcreting, light-weight concrete and roller-compacted concrete. Properly cured HVFAC products are very homogenous in microstructure, virtually crack-free, and are highly durable. The increasing use of high-volume concrete will help to enhance the sustainability of the concrete industry
SILICA FUMES
Silica fume is a byproduct of producing silicon metal or ferrosilicon alloys. One of the most beneficial uses for silica fume is in concrete. Because of its chemical and physical properties, it is a very reactive pozzolan. Concrete containing silica fume can have very high strength and can be very durable. Silica fume is available from suppliers of concrete admixtures and, when specified, is simply added during concrete production. Placing, finishing, and curing silica-fume concrete require special attention on the part of the concrete contractor.
Silicon metal and alloys are produced in electric furnaces as shown in this photo. The raw materials are quartz, coal, and woodchips. The smoke that results from furnace operation is collected and sold as silica fume, rather than being landfilled. Perhaps the most important use of this material is as a mineral admixture in concrete.
Use of Marble powder
Marble as a building material especially in palaces and monuments has been in use for ages. However the use is limited as stone bricks in wall or arches or as lining slabs in walls, roofs or floors, leaving its wastage at quarry or at the sizing industry generally unattended for use in the building industry itself as filler or plasticizer in mortar or concrete. The result is that the mass which is 40% of total marble quarried has reached as high as millions of tons. This huge unattended mass of marble waste consisting of very fine particles is today one of the environmental problems around the world (Corinaldesi et al., 2010). One of the logical means for reduction of the waste marble masses calls for utilizing them in building industry itself. Some attempts have been made to find and assess the possibilities of using waste marble powder in mortars and concretes and results about strength and workability were compared with control samples of conventional cementsand mortar/concrete
ADVANTAGES OF GREEN CONCRETE
Green concrete is part of a movement to create construction materials that have a reduced impact on the environment. It is made from a combination of an inorganic polymer and 25 to 100 percent industrial waste. Here is a list of 4 benefits to using green concrete for your next project
Energy Consumption
If you use less Portland cement and more fly ash when mixing concrete, then you will use less energy. The materials that are used in Portland cement require huge amounts of coal or natural gas to heat it up to the appropriate temperature to turn them into Portland cement. Fly ash already exists as a byproduct of another industrial process so you are not expending much more energy to use it to create green concrete. Another way that green concrete reduces energy consumption is that a building constructed from it is more resistant to temperature changes. An architect can use this and design a green concrete building to use energy for heating and cooling more efficiently
LIMITATIONS OF GREEN CONCRETE
Although Green Concrete seems very promising to an environment friendly sustainable development, the cardinal concern is its durability. Refutations are being made constantly raised regarding the service life of structures made with Green Concrete. Further split tensile strength of Green concrete has been found much less than that of conventional concrete. Another challenge before green Concrete is that of a market until the properties of Green Concrete are at par with the conventional concrete, Green Concrete is unlikely to find many customers.
Several researches have argued that Green Concrete may be durable by using stainless steel reinforcements but prediction is that by using stainless steel, the cost of the concrete increases considerably. Even after this Green Concrete is not as durable as conventional concrete.
APPLICATION AND PROPERTIES
Liquid adhesive
Polyguard green concrete liquid adhesive is a rubber based adhesive in solvent solution which is specifically formulated to provide excellent adhesion with the polyguard waterproofing membrane under many kinds of surface conditions. In addition it is formulated to promote adhesion of the polyguard membranes to green concrete. Green concrete liquid adhesiveis an integral part of the polyguard waterproofing systemand sufficient liquid adhesive must be used on surfaces to condition them to be dust free so that the substrate is suitable for the application of polyguard waterproofing membranes. Polyguard green concrete liquid adhesive will be a green color in appearance
Production of masonry units using green concrete
Masonry units may be manufactured using construction and demolition waste (C&DW). C and DW is used in the manufacture of recycled crushed aggregate (RCA) after crushing. The RCA is then used as the main ingredient in a 14MPa green concrete plaster brick. This has several advantages in terms of reducing embodied energy and preserving the environment:
• The C&DW would normally be dumped in landfill sites, thereby impacting on sensitive areas. Thus using RCA in the manufacture of concrete masonry products eases the pressure on the landfill sites.
• Using RCA means that less virgin materials such as sand and stone have to be quarried, thereby directly lessening the mining impact on the environment.
• The embodied energy involved in quarrying of aggregates and then transporting them to the site of manufacture is saved.
3Light weight insulating fibrous green concrete– Insulitte[/b]
INSULITTE, biogenic opal, is an unique formula composed of naturally occurring puffed silicious material , blended with fiberous polymer liquid and air entraining agents, engineered for light weight aggregates used for low cost and permanent roof insulation ,also it is environmental friendly by eliminating the use of bricks which consume fertile soil required to grow food for our vast population. [18]
HARDENED PROPERTIES
Compressive and split tensile strength
The 150 mm size concrete cubes, concrete cylinder of size 150 mm dia and 300 mm height were used as test specimens to determine the compressive strength and split tensile strength respectively. The results of standard cubes and cylinders are compiled in Table-3.2. The Indian standard method resulted in highly conservative results of compressive and split tensile strengths for the M20 grades of concrete. Compressive strength and split tensile strength were obtained as per IS: 516-1959.. The 7 days and 28 days compressive strength of green concrete is 6.49% and 9.49% higher than controlled concrete respectively. Similarly the 7 days and 28 days split tensile strength of green concrete is 14.62 and 8.66% higher than controlled concrete respectively. The 3 days compressive and split tensile strengths of green concrete were decreasing 12.36% and 10.41% respectively when compared with controlled concrete. The authors suggest that a slightly less strength of concrete at early age, in some degree, is beneficial to the durability of the concrete. Table 3.2 gives a comparison
Resistance to sulphate attack
The resistance to sulphate attack was studied by storage of standard prism specimens were immersed in standard condition for 28 and 90 days and 150 days in testing baths (containing 7.5 percent MgSO4 and 7.5 percent Na2SO4 by weight of water). After 28 and 90 days corrosion the compressive strength of specimens results were compared with those of specimens stored in fresh water. It represents that the durability of Green concrete under sulphate is higher to that of conventional concrete. It can be seen from the test results with storage in 7.5% sodium sulphate solution and 7.5% magnesium sulphate solution for 28 days that the corrosion resistance of mortar specimen with green concrete is much better than that of control specimen, the effect is better for M4 mix (fine aggregate replaced with 50% marble powder and 50% with quarry rock dust).
SCOPE OF GREEN CONCRETE
Green Concrete is a revolutionary topic in the history of concrete industry. Concrete is an indispensible entity for a developing country like India which desperately needs continuously expanding infrastructure. India is the second largest producer of cement in the world. Further India would be facing an exponential growth in the concrete demand by 2011 (Schumacher, 1999).
Being produced in voluminous quantities in India, the concrete industry has a considerable part in the net CO2 emission of the country. The net CO2emissions from the construction agencies are greater than any other industry.In order to act in a responsible manner towards a sustainable development of the nation, Green Concrete is the need of the hour. India being a developing country produces concrete in large quantities which result in huge volume of CO2 being emitted in the atmosphere each year. Thus we can deduce that for a greener future, India needs to adopt Green Concrete into practice as soon as possible. The other advantageous factor is the economy. As Green Concrete is made with concrete wastes and recycled aggregates which are cheaper than the conventional substitutes, and also with most of the industries facing problems with their waste disposal, put it out of question to discard it.
CONCLUSION
The overview of the present state of affairs regarding concrete types with reduced environmental impact has shown that there is considerable knowledge and experience on the subject. The Danish and European environmental policies have motivated the concrete industry to react and will probably also motivate further development of the production and use of concrete with reduced environmental impact. The somewhat vague environmental requirements that exist have resulted in a need for more technical requirements and most important goal is to develop the technology necessary to produce and use resource saving structures i.e. Green Concrete. This applies to structural design, specification, manufacturing, performance, operation and maintenance.
In 1994 cement industry consumed 6.6 EJ of primary energy, corresponding with 2% of world’s energy consumption. Worldwide 1126 Mt CO2 or 5 of the CO2 production original from cement production. The carbon intensity of cement making amounts to 0.81 kg CO2 / kg of cement. In India, North America and china, the carbon intensity is 10% higher than an average. Specific carbon emissions range from 0.36 kg to 1.09 kg CO2 / kg cement mainly depending on type of process, clinker cement ratio and fuel used.
The potential environmental benefit to society of being able to build with Green Concrete is huge. It is realistic to assume that the technology can be developed which can halve the CO2 emission related to concrete production and the large energy consumption of concrete and the following large emission of CO2 this will mean a potential reduction of total CO2 emission by 2% (Obla 2009).
Seventeen different energy efficiency improvement options are identified. The improvement ranges from a small percent to more than 52% per option, depending on the reference case and local situation. The use of waste instead of fossil fuel may reduce CO2 emission by 0.1 – 0.5 kg / kg cement. At the end of pipe technology to reduce CO2 emissions may be CO2 removal. Probably the main technique is combustion under oxygen while recycling CO. However considerably research is required for all unknown aspects of this technique.