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Concrete has a substantial environmental impact. Various methods to reduce environmental impact of concrete such as material conservation is discussed. For example, it is estimated that 1.6 billion ton of concrete is produced annually which is responsible for about 7% carbon dioxide in addition to depletion of natural sources and dumping considerable volume of waste material. Concrete is made of three major materials includes cement, aggregate, and water. Concrete production need huge energy consumption and release large amount of green house emission. Utilization of aggregate and water lead to decrease natural resources and probably lead to water contamination. Additionally, there are various materials which are used as admixture to improve concrete properties and these admixtures may cause detrimental environmental impact. Finally, lack of highly durable material is another source of concrete environmental issue and concrete is commonly designed for service life of 50 years. If concrete durability were higher, its environmental impact would be lesser for instance the rate of using natural resources would be smaller. There are number of ways by which the environmental impact of concrete may be decreased and this article will shed light on these techniques. 
Fig.1: Landscape Degradation due to Production of Concrete
Methods to Reduce Environmental Impact of Concrete
Following are the various methods which can considered to reduce the environmental impact of concrete:- Cement conservation
- Aggregate conservation
- Water conservation
- Concrete durability
Cement Conservation to Reduce Environmental Impact of Concrete
The conservation of cement is the first and most important step in decreasing both energy utilization and greenhouse gas emission. Resource productivity consideration stipulates the deduction of the utilization of Portland cement while meeting the future demands for more concrete. This needs to be the first agenda of successful concrete industry. The use of Portland cement containing pozzolanic materials for example ground granulated blast furnace slag, fly ash, and silica fume is increasing considerably. However, these by product admixture which are used as cement replacement material are employed in minor or low value applications for instance landfills and road sub-bases. The use of cement in structural applications can be decreased by using by product cementitious or pozzolanic material as a cement replacement. This leads to decrease the demand for cement production. It is reported that, replacing cement with slag or fly ash by 50% will provide better durable product compare with that of Portland cement with zero replacement and consequently natural resource application is decreased. Furthermore, the setting and hardening of concrete containing large percent of cementitious material is low but this can be tackled to a certain extent by using superplasticizer. It is possible that slow pace construction process be approved in the future when resource maximization is become the most important industry purpose instead of labor productivity.Aggregate Conservation to Reduce Environmental Impact of Concrete
It is claimed that in North America, Japan, and Europe, around two thirds of construction and demolition waste are composed of old broken concrete and masonry. If these waste materials are reused as a coarse aggregate, material productivity will improve greatly. Moreover, dredge sand and mining waste which are present in number of countries around the world, can be processed and applied as fine aggregate. Even though the processing of these waste material require a budget but it can be considerably economical particularly in those countries where the cost of damping waste material is substantial and land is rare. So significant is the recycling and reusing waste material that solve natural resource depletion problem in many regions and avoid high cost of transporting virgin aggregate over long distances. It is claimed by Lauritzen that, 1 billion tons of concrete and masonry rubble are produced per year and small amount of concrete and masonry waste are reused again. Expensive waste disposal and environmental considerations have motivated the majority of European country to set short term goals to recycle between 50 to 90% of demolition and construction waste. Lastly, recycled aggregate in general and specifically masonry aggregate possess large porosity compare with natural aggregate. So, for the same workability, water demand to produce fresh concrete is higher compare the case of using natural aggregate and the mechanical properties of hardened concrete are influenced detrimentally. To tackle this problem, combination of natural and recycle aggregate may be used or fly ash and water reducing admixture can be employed in concrete.Water Conservation to Reduce Environmental Impact of Concrete
It is reported by Hawken et al that, the availability of fresh and clean water decreases continuously and only 3% of all water on earth are fresh water which most of it either located deep beneath earth surface of trapped fast melting glaciers. As a result of increasing industrial, agricultural, and urban demand for water, water table is lowering in addition to the increase of water contamination. It is recommended that, the only practical and reasonable solution to this problem is the utilization of available resources more efficiently. Concrete producers consume water in large scale and these producers and other fresh large consumers should be forced to use water efficiently. It is estimated that 100L/m3 is used to clean ready mix trucks and large amount of water is employed for mixing. It is believed that, 1 trillion L of water is used for mixing annually and this huge quantity can be decreased to half by the increase of mineral admixture and superplasticizer application and better grading of aggregate. Moreover, with the approval of test results, the use of brackish water and industrial recycled water must be enforced instead of clean water, and this must be entirely imperative in the case of washing equipments. Furthermore, it is reported that considerable amount of water saved when retarder used for fresh returned concrete. Finally, during concrete curing, the application of textile, which has exterior impermeable membrane and water absorbent fabric at interior face, cut water utilization.Concrete Durability to Reduce Environmental Impact of Concrete
In addition to those measures discussed in the above sections, enhancing concrete durability offer long term solution and exceptional breakthrough for improving productivity of concrete industry and hence decrease environmental impact on concrete production, for instance if concrete structure is constructed for a service life of 500 year rather than 50 year, the resource productivity of concrete industry will increased by factor of 10. The durability of modern structures is questionable because deterioration begins after around 20 years whereas there are buildings and seawalls constructed from unreinforced roman concrete which maintain their good condition after nearly 2000 years. This might mainly because of considerably crack prone Portland cement concrete which consequently became permeable during its service life. Moreover, steel reinforcement in permeable concrete corrodes and leads to progressive damage of the structure. In modern times, construction practice is controlled by culture of accelerating construction speed in which large amount of high early strength Portland cement is employed. Consequently, weak crack resistance concrete structure is constructed because of large drying shrinkage and thermal contraction and small creep relaxation. Furthermore, roman concrete made with mixture of volcanic ash and hydrated lime, and created homogenous hydrated product that set and hardened in a slow pace however better than hydrated Portland cement product thermodynamically. Added to that, less amount of water used in the roman concrete and were not subjected to cracks at the same degree as Portland cement concrete. Therefore, if concrete durability is major concern or purpose, producing less crack-prone concrete rather than high speed construction should be focused on and hence construction practice need to undergo paradigm change toward that direction. It is demonstrated that, entire of most of cracking and shrinkage in concrete can be prevented and consequently high durable concrete can be produced provided that water to cementitious material in concrete is deduced through superplasticizer application. A large free crack monolithic concrete foundation of a temple that is in Kauai which is an island in Pacific Ocean is described by Mehta and Langley. The foundation made of two parallel of unreinforced concrete slab. To produce concrete with considerably small shrinkage stresses, it was necessary to decrease to limit both thermal and drying shrinkage through substantial reduction of Portland cement and water in concrete. Slump of concrete used for the foundation was 125±25 mm and its compressive strength after 90 day was 20 MPa with 13 C rise of temperature. The exposed surface of the foundation was carefully and properly examine after almost two year and there was no any crack evidence. Investigation on the core samples taken from the slab demonstrated that, not only did the homogeneity of the hydration product of high fly ash system were better compare with conventional concrete but also the bond between aggregate and hydration product was very well. This is a precondition for achieving crack resistant and high durable concrete. Figure 2 shows thin section of core sample taken from the slab and neither interfacial zone of micro-cracking are shown between coarse aggregate and adjacent cement mortar. In contrary, Figure 3 illustrates the how interfacial aggregate-paste micro-cracks connect and allow the ingression of fluid from outside Finally, the mixture proportion employed to produce concrete in the construction of foundation of the temple in Kauai are shown in Table-1.
Fig.2: Photomicrograph of Thin Section from Concrete Core obtained from High Volume Fly Ash System
Fig.3: Photomicrograph of a Thin Section taken from Conventional Portland Cement
Table-1: Mixture Proportions of Crack Resistant High Volume Fly Ash Concrete| Constituents of the mixture | Proportions by weight (Kg/m3) |
| Type I Portland cement | 106 |
| Class F fly ash | 142 |
| Water | 100 |
| Crushed calcareous sand | 944 |
| Crushed basalt rock 25mm maximum size | 1120 |
| Superplasticizer | 3.5 L/m3 |
