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Concrete has long been relied upon as structurally stalwart. While many products are being modified and marketed to meet emerging green building standards, concrete has always been robust, durable, versatile, and recyclable. The use of concrete can bolster the environmental benefits of the structure, while contributing additional sustainable solutions .The idea of green concrete came from eco friendly construction material for building various structures .The concrete which uses less energy in its production & produces less carbon dioxide than normal concrete is known as green concrete. According to the American Concrete Institute (ACI), the type of concrete that has set but not appreciably hardened is referred to as green concrete.
The purpose of introducing green concrete are
To reduce greenhouse gas emission
To reduce use of natural resources like limestone, shale ,Clays ,natural river sand ,natural rocks etc
Use of waste materials in concrete that also prevents the large area of land that is used for the storage of waste materials that result in the air, land and water pollution.
And the Characteristics of green concrete are always inspiring for a structural engineer .Those are-
• Optimizes use of available materials
• Better Performance
• Enhanced consistency
• Reduced shrinkage / creep.
• More durable
• Less permeable
• Reduced carbon footprint
• Cost-effective
• Heat of hydration is much lower
More over, green concrete Can be used in construction where waste disposal and its recycling are in order. Utilization of micro silica in high rise buildings , Manufacturing cement with less environmental impact using mineralizer , performance improvers ,Use of waste derived fuel and byproducts are the great prospects of green concrete.
MATERIALS
Green construction materials are composed of renewable, rather than non-renewable resources. Material choice depends on several factors including first cost, life cycle cost and performance for a specific application.
Materials used for green concrete are:
• Recycled Demolition Waste Aggregate
• Recycled Concrete Aggregate
• Blast furnace Slag
• Manufactured Sand
• Glass Aggregate
• Fly ash
They are divided in cement, cementitious material, coarse and fine aggregate. Their definitions are as usual.
The Recycled Demolition Waste Aggregate ,The construction industry produces huge waste called demolition waste or MALWA,MALWA, can be converted to Course aggregate . The waste distribution shows that there is about 50% demolition waste in the dump.
The Recycled Concrete Material (RCM),The Primary sources of RCM are demolition of existing concrete pavement, building slabs & foundations, bridge structures, curb and gutter and from commercial/private facilities.
The Blast Furnace Slag (BFS), The Blast furnace slag is a waste product from the manufacture of pig iron and obtained through rapid cooling by water or quenching molten slag. Thus, blast furnace slag can be available as an aggregate as construction materials and acceptable as coarse or fine aggregate for use in green Concrete.
The Manufactured Sand For Concrete, Cubical sand manufactured from crushed rock is the most desirable fine material for concrete production.
MIX DESIGN
Same as for conventional concrete and Constituent materials must pack themselves giving minimum voids in concrete .
Mix can result in a reduced paste volume within the concrete structure resulting in a higher level of protection against concrete deterioration.
ADVANTAGES
•Uses local and recycled materials
•Higher Modulus of elasticity
•30% CO2reduction
•20% residual products as aggregate
•Concrete industry to use own waste
•Eco-friendly
•Easier handling and easier finishing
•Higher strength per kilogram of cement
•Higher level of protection against concrete
In other hand there are some BARRIERS like
Crushed oyster shell improves compressive strength, decreases workability.
Plastic aggregate decreases compressive and tensile strengths
Recycling of wastes requires extra labor and energy input
Existing industries prefer to go with the traditional concrete over the green one.
Geo-polymer Green Concrete:
The Geo-polymerisation process comprises of a chemical reaction under highly alkaline conditions on Al-Si minerals in slag or fly ash yielding polymeric Si-O-Al-O bonds.Geopolymer is used as the binder, instead of Cement paste, to produce concrete. Geopolymer Concrete:
Low calcium flyash + Alkali solution + water = Polymerisation gets hard product on curing
Conventional concrete: Conventional Cement + water = Hydration process gets hard product on curing
Nano Tech in Green Concrete
Nano-materials include Nano- particles, Nano-chemical additives, Nano Silica, Nano Al2O3, Nano TiO2, Nano Fe2O3, Nano clays, Nano limestone and highly dispersed carbon nano tubes .Can introduce new properties such as photocatalysis, conductivity, and piezo -resistivity.
Fig: Stapleton International Airport , Denver ,Colorado(example of green structure)
Therefore, it is necessary to advance the understanding of concrete properties when using “green” raw materials, reduce potential cost in the recycling and reuse process, improve industry standards, and educate the industry about new technologies.