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Sustainable concrete construction is a step towards green and eco friendly concrete construction practices to solve global environmental problems. Concrete is a construction material which has been used substantially all over the world. Regarding the amount of concrete that has produced, used and its impact are considered as an important part of the whole global environmental problems. The effect of concrete is taking place in different stages from extraction of the raw material until the end of structure life. Global warming due to emission of CO2, increasing landfill sizes, and pollutions is the result of these impacts. Rising population density and increasing demand for concrete have exacerbated the situation. Thus, it is required to go for sustainable concrete construction practices.
Contents:
Life Cycle Assessment of Concrete:
Life cycle assessment is employed for better understanding and assessing the concrete environmental impacts. It is necessary take all the life stages of concrete in order to assess the environmental impacts Figure 1 :
Figure 1 Stages considered for estimating environmental impact
1. Concrete Manufacture
Cement, aggregate and water are the essential constituents of concrete. Each of them consists of raw materials with energy consumption and waste production. From this it becomes quite evident that manufacturing phase could be considered as the most crucial stage from environmental point of view and drew attention of both government and public. Cement is produced by adequate blending of limestone with clay and the resultant mixture is heated in a rotary kiln about 1450C. At this temperature chemical transformation take place and the basic material change to clinker and after cooling it ground in a rolling mill to produce cement powder, in all these processes energy is consumed intensively with the emission of CO2. Limestone is mainly calcium carbonate CaCO3 and it is achieved through mining, by heating CaCO3 break down and release emission as a result of both clinker production and combustion of fossil fuel to generate energy required for the production process Figure 2.
Figure 2 percentage of CO2 produced by energy consumption and chemical reaction.
Aggregate and sand are responsible for approximately 30% of all CO2 emission during concrete production. The average concrete utilization is 1 tonne/ year/ single person, and 1 tonne CO2 is released/1tone produced cement which is 7% of global emission.2. Concrete Construction
This stage probably releases less amount of greenhouse emission compare with manufacturing. However, the effectiveness of this stage cannot be neglected. Batching of concrete is one of the affecting factors. Electric power is used to mix components of concrete in a high placed container in addition to small amount of fuel consumed by small trucks used to move virgin materials; 0.0033t CO2-e/m3 emission is produced due to batching/m2. Diesel fuel is used to transport concrete that release 0.009t CO2-e/m3 empty returned journey is included. Onsite activities like pumping, vibrating, and finishing employed fuel is (1.5 L/m3 pumped concrete), sometime concrete is craned instead of pumped so (2x1.5 L/m3) to take other type of pumping into consideration and it emits 0.009t CO2-e/m3. Electric mixing shares largest amount of CO2 emission in this stage. Due to using gigantic amount of concrete all over the world and most of projects using ready mix plant, large amount of water is being consumed to washing machinery and curing concrete to achieving the required strength in harsh situations, as a result large volume of contaminated water is discharged; pure water is used to prevent chemical reactions.
Figure-3: CO2 emission of concrete batching
3. Life of Concrete
The biggest drawback of concrete is deterioration which has been risen in many countries. Not only does it affect the society production, but also a considerable detrimental impact on resources, environment, and safety. Lacking of sufficient and controlled durability in addition to concrete structure maintenance are caused to consume much energy and produce considerable amount of waste. From this it becomes clear that continues maintenance and repair impact both economy and environment.4. Demolition of Concrete Construction
Dismantling of concrete structures and transportation consume about 0.2% of the whole energy consumption of all life cycle assessment. The demolition of concrete structure could be an expensive process.Methods for Achieving Sustainable Concrete Construction
Following are the methods for achieving sustainable concrete construction:Use of Blended cements:
Most of CO2 is being released during clinker production due to calcium calcinations and energy consumption. This emission can be reduced by using blended cement in which industrial by product like slag that replaces part of clinker. Not only does the blended cement reduce greenhouse gas emission in calcinations, but also decrease energy consumption.Energy Efficiency Improvement
Decreasing emission from electricity and fuel utilization can be gained by improving energy efficiency which cause cost production reduction as well. Upgrading, modifying, and replacing of equipments to make them more efficient for producing of cement. Large proportion of energy is used during cement production, and it comes from fuel combustion, so it is possible to obtain large reduction in energy employment by mitigating efficiency of fuel. Changing cement production technology is another way of improving energy efficient.Table 1: Choices of improving energy efficiency for cement production process
Carbon dioxide removal: decreasing CO2 can be achieved by using CO2 removal. In this method, during or after the process of production CO2 is separated and then disposed out of atmosphere. Procedure of this technique is divided into three stages: drying and compressing of CO2, transfer recovered CO2, and storing or disposing it.
1- Low carbon instead of high carbon fuel: using fuel which has lower CO2 content is a method to reduce emission. Therefore, using waste derived fuel or natural gas instead of coal fuel decrease amount of CO2 and minimizing waste disposal.
There are numbers of alternative fuels that could be used for that purpose such as refinery gases, mineral oil, and waste wood.
2- Prolonged service life of concrete: how to extend the operational life of concrete is the most important factor because not only does it decrease the amount of demolition waste of current buildings but also save virgin materials. Durability is very significant which directly related to concrete service life.
3- Waste recycling: it consists of rubble and masonry that can be used as coarse and fine aggregate. This strategy is economical especially in those countries where already raw material is depleted and using recycled waste is better than high cost raw material and expensive hauling.
It is estimated that 1 trillion L of water is used for concrete industry which most of it is drinkable. About 3% water on earth is fresh and it going to be rare in most of continents, so recycling water could be an answer for that problem.
By using textile that has water absorption property for curing, better aggregate grading, increasing the use of superplasticizer and admixtures the amount of utilized water can be reduced to half.
4- Government, companies, and environmental activist responsibilities: clarification dangers of emission are very significant. This might lead to encourage producer and construction companies to implement methods toward lessening greenhouse emission.
Moreover, it is government's duty to enforce environmental rules, put taxes according to CO2 emission and make companies to take environment protection as an important goal, for example using slow planned construction, in this way utilization of supplementary material is increased for instance if 1625 tons of cement was produced, 300 million tons of CO2 emission would have reduced if 18.5% of supplementary material had been used. 
