25-05-2012, 02:12 PM
Environment impact Of Concrete (Cement)
The environmental impact of concrete is a complex mixture of not entirely negative effects. A major component of concrete is cement, which has its own its own environmental and social impacts.
The cement industry is one of two primary producers of carbon dioxide, a major greenhouse gas. Concrete is used to create hard surfaces which contribute to surface runoff, which can cause heavy soil erosion, water pollution and flooding. Concrete is a primary contributor to the urban heat islandeffect, but is less so than asphalt. Concrete dust released by building demolition and natural disasters can be a major source of dangerous air pollution. The presence of some substances in concrete, including useful and unwanted additives, can cause health concerns due to toxicidity and radioactivity. Wet concrete is highly alkaline and must always be handled with proper protective equipment. Concrete recycling is increasing due to improved environmental awareness, governmental laws and economic benefits.
Carbon Dioxide Emmissions and Climate Change
The cement industry is one of two primary producers of carbon dioxide (CO2), creating up to 5% of worldwide man-made emissions of this gas, of which 50% is from the chemical process and 40% from burning fuel.[1] The carbon dioxide (CO2) produced for the manufacture of one tonne of structural concrete (using ~14% cement) is estimated at 410 kg/m3 (~180 kg/tonne @ density of 2.3 g/cm3) (reduced to 290 kg/m3 with 30% fly ash replacement of cement).[2] The CO2 emission from the concrete production is directly proportional to the cement content used in the concrete mix; 900 kg of CO2 are emitted for the fabrication of every ton of cement.[3] Cement manufacture contributes greenhouse gases both directly through the production of carbon dioxide when calcium carbonate is thermally decomposed, producing lime and carbon dioxide,[4] and also through the use of energy, particularly from the combustion of fossil fuels.
Sun RunOff
Surface runoff, when water runs off impervious surfaces, such as non-porous concrete, can cause heavy soil erosion and flooding. Urban runoff tends to pick up gasoline, motor oil, heavy metals, trashand other pollutants from sidewalks, roadways and parking lots.[5][6] Without attenuation, the impervious cover in a typical urban area limits groundwater percolation and causes five times the amount of runoff generated by a typical woodland of the same size.[7] A 2008 report by the United States National Research Council identified urban runoff as a leading source of water qualityproblems.[8]
Urban Heat
Both concrete and asphalt are the primary contributors to what is known as the urban heat islandeffect.
Using light-colored concrete has proven effective in reflecting up to 50% more light than asphalt and reducing ambient temperature.[9] A low albedo value, characteristic of black asphalt, absorbs a large percentage of solar heat and contributes to the warming of cities. By paving with light colored concrete, in addition to replacing asphalt with light-colored concrete, communities can lower their average temperature.[10]
In many U.S. cities, pavement covers about 30–40% of the surface area.[9] This directly affects the temperature of the city and contributes to the urban heat island effect. Paving with light-colored concrete would lower temperatures of paved areas and improve night-time visibility.[9] The potential of energy saving within an area is also high. With lower temperatures, the demand for air conditioning decreases, saving energy.
Atlanta has tried to mitigate the heat-island effect. City officials noted that when using heat-reflecting concrete, their average city temperature decreased by 6°F (3.3°C).[11] The Design Trust for Public Space found that by slightly raising the albedo value in New York City, beneficial effects such as energy savings could be achieved.[citation needed] It was concluded that this could be accomplished by the replacement of black asphalt with light-colored concrete.
However, in winter this may be a disadvantage as ice will form more easily and remain longer on the light colored surfaces as they will be colder due to less energy absorbed from the reduced amount of sunlight in winter.[10]
Concrete Dust
Building demolition and natural disasters such as earthquakes often release a large amount of concrete dust into the local atmosphere. Concrete dust was concluded to be the major source of dangerous air pollution following the Great Hanshin earthquake.[citation needed]
Toxic and Radioactive contamination
The presence of some substances in concrete, including useful and unwanted additives, can cause health concerns. Natural radioactive elements (K, U and Th) can be present in various concentration in concrete dwellings, depending on the source of the raw materials used.[12] Toxic substances may also be added to the mixture for making concrete by unscrupulous makers. Dust from rubble or broken concrete upon demolition or crumbling may cause serious health concerns depending also on what had been incorporated in the concrete.