Executive summary
This paper looks at concrete as a building material. It examines the process of producing concrete, its various applications, and disposal, looking into its effect on an environmental, social and economic scale. This paper is aimed at increasing the awareness of the environmental, social and economic consequences of decisions made and actions taken in creating the built world. The paper begins by looking at the manufacture of concrete from either one of its sources, which include calcium, chalk, limestone and marl. It also looks at the various kinds of construction cements, based on the blender used. We also get to analyze the two major parts of concrete, namely a paste and mineral aggregate, as well as the significance of trapping air in a concrete mixture, especially in cold climates. In the workability of concrete section, we get to view the various effects of the amount of water added to a concrete mixture. We get to examine the effect of concrete and its manufacture process on the ecosystem; every year about 1.6 billion tons of concrete are manufactured in the world, inducing a large amount of CO2 in the ecosystem. We also get to see the contribution of cement manufacture to global warming. The paper concludes by looking into the ecological footprint of concrete, and the various ways that we can reduce the environmental impact of concrete as a building material
Concrete is a composite material composed of coarse granular materials embedded in a hard matrix of material that fills the space between the aggregate particles and glues them together (Baynes 1). The coarse granular material is composed of either the aggregate or filler, while the cement or binder is the hard matrix material fixing the composition together. These raw materials are then ground and mixed and fed to a rotating cement kiln. This kiln is by far the largest piece of moving industrial equipment in the world. The most common type of kiln used is the dry process kiln, which mixes its ingredients while dry, as most of the older ones use the wet process. Raw materials used in processing concrete are mostly readily available and in great quantities. In case of calcium, chalk, limestone and marl are the most common sources.
There are several kinds of concrete but they all depend on the type of binder used. These may include: Portland cement concrete, which is mostly used in concrete construction; asphalt; and epoxy concrete. Modern concrete has admixtures which are mainly other materials other than water, cement, fiber or aggregate but are added into the concrete batch immediately before or during the mixing. These have several categories.
Transportation
Concrete transportation in early 1900 was done by horse –drawn mixer which had only capacity of ¼ yd. all ingredients mixed into the mixer, turned by the wheels of the vehicle. In 1916, Stephen Stepanian designed a self discharging motorized transit mixer that was similar to modern mixer (White 173). In 1930 a high discharge truck mixer was invented, and in the 1940s, with the help of better engines, the capacities of drums for concrete had been increased, and for the first time hydraulic jacks were used. In modern vehicle with the support of powerful engines the capacity of the mixer added up to 21 cubic yard which is normally 60% of gross volume of the mixer (White 180). The drum rotate according to the axis of mixer with rate of 4 to 18 rpm, the blade is spiral shape that makes it possible for concrete to move out easily.
Recently the truck mixer manufacturers building trucks for special purposes and for big projects such as road paving used portable central mixing plant that can produce concrete in large amount (white, 185).
Nature an workability of concrete
Chemical admixtures help modify the setting and hardening nature of the cement paste by influencing the cement’s rate of hydration, while water reducing admixtures can plasticize fresh concrete mixtures. This is done by reducing the water’s surface tension. Air entraining admixtures mostly improve the concrete’s durability, while mineral admixtures like pozzolans help reduce thermal cracking
Fresh concrete is mainly concrete at the state when its components are fully mixed but its strength has not yet developed thereby making this period to correspond to the cement hydration stages. The properties of fresh concrete are generally defined by the term Workability which defines the amount of mechanical work required for full compaction of the concrete. This type of concrete has water making it possible for solid materials of concrete to be lubricated and move easily. The water is seen as thin layers between materials (Seabrook 1).
Concrete has two major parts; a paste and mineral aggregate (Kelly 91). The paste is produced by mixing water, air and cement that covers the aggregate. Air entrapment is unavoidable in the process of making concrete and this air actually is about 2% of the volume of concrete, which poses no problems. In order to increase resistance of concrete, air is added to it and air voids are as big as a finger size. Having this amount of air during the mixing of concrete prevents it from freezing, when the work must be done in the freezing temperature (Kelly 92). In the process of drying and hardening of the concrete, water that was mixed with paste moves out of the concrete and makes a layer of water on the surface of concrete. The hardening process starts from the bottom of the concrete layer, and moves towards the surface. The final volume of dried concrete is a bit smaller than the fresh concrete, because of the removal of water (Kelly 93).
Several factors affect workability. Water content affects it in that it must fill the spaces among particles, while additional water mainly separates the particles from the water film. Too much water also reduces cohesiveness and concrete strength. Increase in the aggregate/cement ratio decreases the fluidity and the aggregate particles have to be nearly spherical and smoother while for cement, increased fineness reduces fluidity but increases cohesiveness. Workability also decreases with time and with an increase in temperature.
Environmental effect of concrete
Concrete and manufacturing of it has a huge environmental effect on the ecosystem. Every year about 1.6 billion tons of concrete are manufactured in the world, inducing a large amount of CO2 in the ecosystem. Making concrete need energy that mainly came from fossil fuels which increase the pollutants in the air. As concrete made up from 80% aggregate and 20% of cement, the sand and crushed rock that used for is estimated of 10 billion tons yearly. In order to make concrete the manufacturer need large amount of fresh water this would be around 1 trillion liter of fresh water. Mainly making concrete has negatively impact the nature and ecosystem in the world from reducing soil, water and increasing CO2 emissions and global warming. The life expectancy for concrete is about 50 years, but in some areas near the oceans, this is reduced to 20-30 years. The annual cost of repairing concrete bridges is about $80 billion dollars (Seabrook 1).
Global warming is mostly as a result of CO2 emissions from our industries. For every ton of cement produced an estimated ton of CO2 is emitted. This comes from decomposition of limestone and the power plant supplying the electricity to turn the kiln and ball mills for grinding cement. Replacement of the cement with other cementing material not associated CO2 emission may reduce this problem. There is also the problem of visual pollution that often results from quarries that gain raw materials for cement production. Most quarries have a long life and any attempts to sculpture the topography for a visual effect is counter-productive to the quarrying process efficiency. Other factors affecting the economy include traffic congestion, noise pollution, adverse health effects from the chromium content of the cement, and water pollution. However, water pollution has been minimized by stabilizing the concrete retained in the inside of the truck drum and any wash out water (Malhotra 53).
The Intergovernmental Panel on Climate Change (IPCC) meeting in Geneva earlier this year warned that the average temperature is expected to rise between 1.4 and 5.8 C° over the next 100 years. International Centre for Sustainable Development of Cement and Concrete has shown that a 65% replacement of cement by weight with a combination of fly ash and a generous amount of super plasticizer provides excellent mechanical properties, minimizes permeability, and enhances superior durability and low temperature rise during curing. This type of concrete system has been effectively used in high rise office buildings, sidewalks, streets and marine facilities.
Useful life and the ultimate fate of cement
However, there was a good reason leading to concrete being the most widely used construction material. Well made concrete has low maintenance as it resists severe exposure conditions. Also, raw materials making both cement and concrete are evenly distributed thereby easing transportation and the cement kiln is very energy efficient so the energy is mostly used in making the cement. It also has desirable properties in that it has an ability to be cast in the shape of a containment vessel and to be relatively impermeable thus enabling concrete to be effective for domestic water supply and also disposal of domestic liquid washes. Concrete is also a recyclable material.
Ecological Footprint of cement
The eco-footprint is defined as “Areas of biologically productive land and water that people use to produce all the resources they need to consume and absorb the waste products” (Baynes 1). The eco-footprint can be measured by global hectares, it means area of bio-productive lands that use by people and for the world population of around 6 billion in 2003 the global hectare for each person was 1.8 hectares.
In terms of concrete, the cement portion of concrete that is used for construction is a major ecological footprint in construction building. In the modern architectural building and all other constructions, it is almost impossible to avoid the use of concrete as a construction material, but there are different ways and techniques that we can use to reduce the need for more cement. Such methods include using recycled concrete in some components of construction that are not important in terms of structure of building. With reducing use of cement, we can reduce the ecological footprint of concrete. Most of the environmental problems caused by concrete can be effectively managed using the current technology. However more should be done to manage CO2 and nitrous oxide emissions.
Works Cited
Baynes, Tim. Ecological footprint. 2008. Web.
Kelly, J. “Foundation of concrete, ACI Manual of Concrete Inspection, 7th edition.” Aci Publication (1988): 90-103. Print.
Malhotra, V. M. “Making Concrete “Greener” with Fly Ash.” Concrete International (1999): 21(5), 61-66. Print.
Seabrook, Phil. EcoSmart Update Summer 2003. Web.
White, G. “Basic Concrete construction Practices.” John Wiley & Sons publication (1980): 173-191. Print.