Sustainability Issues In Concrete And Self Compacting Concrete Construction Essay

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As global population and affluence increases, the use of various materials also increases in volume, diversity and distance transported. Sustainability in broad sense means ability to preserve a certain process or state over a long period in future. Sustainable use of materials has targeted the idea of dematerialization, converting the linear path of materials (extraction, use, disposal in landfill) to a circular material flow that reuses the materials as much as possible - concept similar to recycling and reuse of materials. This approach would help to minimize ecological imbalances, preserve environment, would have no impact on human health and the process can continue in this manner for a long period of time into future without compromising with the productivity.

Concrete is the most widely used construction material which contains 12% of cementitious material and 80% of aggregate material (Coarse aggregate plus fine aggregate). The manufacture of cementitious material involves heating of pulverized limestone, clay and sand. Production of 1 metric tonne of cement releases 650 to 950 kg of carbon di oxide. In 2009, 2.8 billion metric tonnes of cement was produced worldwide and it contributed to about 5% of total carbon di oxide emissions. Similarly the embodied energy potential for crushed aggregates is around 100-110 MJ/kg. Also iron-steel which are the preferred construction materials after concrete consume about 19% of final energy use and CO2 emissions from this industry constitute 25% of industrial emissions.

Self compacting concrete (SCC) is an innovative concrete that does not require vibration for placing and compaction. It offers rapid rate of concrete placement, with faster rate of construction times and ease of flow around congested reinforcement. The elimination of vibrating equipment improves the environment on and near the construction and precast site where concrete is being placed thus reducing the exposure of workers to the ill effects of noise and vibration.

Key Words: Sustainability, Concrete, Self Compacting Concrete.

INTRODUCTION:

Sustainable development, green buildings and in particular climate change, are now realities of life. Corporations in every industry are increasingly becoming sensitive towards their environmental responsibility. Government regulations to limit environmental impact of processes will continue to place pressures on corporations to improve environmental performance and this will be increasingly monitored and regulated. In the near future individual companies will compete on lowest environmental impact and most likely on carbon footprint. Companies who do not adapt to these changes will not survive. Those that do will have the opportunity to innovate build brand trust and minimize business risk, while behaving ethically and responsibly.

Construction of built environment and manufacture of building products also have significant impacts on the environment. These processes consume natural resources and require significant amounts of energy. The manufacture, transport of important construction materials along with operation of on-site machinery for construction projects emits green house gases and other emissions. Again at the end of project life cycle much of the demolished material is currently land filled as opposed to being reused or recycled into new products.

SUSTAINABILITY:

Change is inevitable. But it is the rapid rate of change that often becomes disruptive. This is why all of a sudden we are confronted with the present situation that our current ways of economic and industrial development seem unsustainable. Population growth, urbanization, technology choices and their environmental impact are unquestionably among the key forces that are shaping the new world.

At the start of 20th century, the world population was 1.5 billion1, by the end of which it had risen to 6 billion. It took about 10000 years after the ice age for population to rise to the 1.5 billion mark. The rate of growth from 1.5 to 6 billion people during the short span of 100 years has been indeed explosive.

Statistics show a direct correlation between population growth and urbanisation of the planet. At the beginning of the 20th century approximately 10 percent of the people lived in cities and in the year 2001 nearly 3 of the 6 billion people live in and around the cities. According to the statistics published by United Nations in the year 20001 the population distribution in various urban centres on this planet is as under:

Sr. No.

Population Size

Number of Cities

1.

10 million or more

19

2.

5 million to 10 million

22

3.

1 million to 5 million

370

4.

0.5 to 1 million

430

Population growth and urbanization have played a great part in the enormous expansion of energy, manufacturing and transportation sectors of economy during the 20th century. Unfortunately our technology choices have turned out to be wasteful because decisions are based on short term and narrow goals of the enterprise rather than a holistic view of the full range of consequences from the use of a technology. According to Hawken et al. only 6 per cent of the total global flow of materials, some 500 billion tonnes a year, actually ends up in consumer products, whereas much of the virgin materials are being returned to the environment in the form of harmful solid, liquid and gaseous waste.

Environmental StewardshipSustainability in a broad sense is the ability to preserve a certain process or state over a long period in future. Sustainability is the characteristic of a process or state that can be maintained at a certain level indefinitely. The term in its environmental usage refers to the potential longevity of vital human ecological support systems such as planets climatic system, systems of agriculture, industry, forestry, fisheries and human communities. Sustainable Development focuses on more than environmental issues. Sustainable Development means meeting the present needs without compromising the ability of future generations to meet their needs. Sustainable Development policies encompass three general policy areas; economic sustainability, environmental sustainability and social sustainability. According to United Nations the three mutually dependent and reinforcing pillars of Sustainable Development are Economic Development, Social Development and Environmental Development.

governance

Social Responsibility

Economic prosperity

Fig. No. 1: The Sustainability Balance2

Materials that are already present in nature would not require any production and hence such materials can be used from sustainability point of view. Manufactured material on the other hand is produced with great expense of energy and therefore their sustainability needs are to be carefully analyzed and evaluated both at the production as well as application stage. Ideally considering the use of locally available material for construction purposes leads to sustainable green activity. However the buildings and structures need to satisfy various functional requirements along with various design loads thus calling for the use of engineered construction materials like concrete. Besides site constraints, land availability, durability requirements and construction speed make it impossible to avoid using the conventional engineered construction material in structures.

SUSTAINABLE CONCRETE LIFE CYCLE:

It is obvious that lack of holistic approach in meeting our socio-economic needs is the primary cause of environmental problems. The holistic approach has its roots in the idea that the whole exists before the parts. For instance the holistic approach would consider society as a whole and the concrete industry a part of it. Therefore in addition to providing a low cost building material, the concrete industry must also consider societal needs like conservation of earths natural resources and safe disposal of polluting wastes produced by other industries. As far as developing a sustainable concrete life cycle is considered a balanced approach cannot be achieved by studying the disciplines in pieces but rather through pursuit of consilience among them. Consilience is defined as unification of knowledge by linking together facts and insights across disciplines to create a common ground for action.

The figure below describes the Sustainable Concrete life cycle2 which focuses not only on concrete technology, materials, applications and techniques but also focuses on legislations, codes, norms, standard specifications etc.

Fig. No. 2: Sustainable Concrete Life Cycle2

PUBLIC POLICIES, CODES, LEGISLATIONS, SPECS.

PUBLIC POLICIES, CODES, LEGISLATIONS, SPECS.

PUBLIC POLICY, CODES, LEGISLATIONS, SPECS.

PUBLIC POLICY, CODES, LEGISLATIONS, SPECS.

INCENTIVES & DISINCENTIVES

SITE SELCTION, ENV. MGT. CARBON GAPS, CARBON TRADE

INCENTIVES & DISINCENTIVES

ENERGY MANDATES, GREEN REGULATIONS

INCENTIVES & DISINCENTIVES

INCENTIVES & DISINCENTIVES

PERFORMANCE CODES, IS 14000 FAMILY

CARBON CREDITS, LEED, TAXATION

CONSTRUCTION

RAW MATERIAL ACQUISITION

REUSE, RECYCLE, DISPOSAL

OPERATION & MAINTENANCE

CONCRETE PRODUCTION

MATERIAL PROCESSING

Reinforcement

Cement & Binders

Admixtures

Waste Materials

On-site construction Assembly

Operate & Maintain

Rehabilitate

Recycle, Reclaim, Reuse

Dispose

Concrete Mix Proportioning

Concrete Products

Concreting Machines

Sustainable Concrete Life Cycle

Industrial By Products

Recycled, Reclaimed Material

New Raw Materials

Complexity, Number of Actors, Collaboration, Integration

Global Sustainability

Sustainable Communities

Sustainable Built Environment

Sustainable Construction Industry.

HIGH

Long time frame and wide spatial boundaries.

LOW HIGH

Fig. No. 3: Part to Whole Relation of Concrete Industry with Society2

CONCRETE:

As per C. E. Kesler, Concrete as a construction material has been important in the past, is more useful now, and is confidently forecast to be indispensable in the future3.

Concrete is a widely used material world wide required for construction of housing, industrial and commercial buildings, drinking water and sanitation facilities, dams and canals, roads, bridges, tunnels and other infrastructure. The principal material of construction is Portland cement. India is the second largest manufacturer of Cement with production of 224 metric tonne (MT) till May 20104.

Concrete may appear to be a simple product to put together, but it requires a highly engineered approach. In an increasingly competitive design and construction environment, where high performance requirements, such as longer life cycles, more durable concrete and value engineering are expected, careful consideration must be paid to basic requirement such as water cement ratio, cementing materials and more sophisticated chemical admixtures.

Ordinary concrete typically contains about 12 percent cement, 8 percent mixing water and 80 percent aggregate by mass. This means that in addition to 1.5 billion tonnes of cement that is being consumed today, the concrete industry is consuming annually 9 billion tonnes of sand and rock together with 1 billion tonnes of mixing water. The 11.5 billion tonne a year industry is thus the largest user of natural resources in the world. The demand for concrete is expected to grow to approximately 16 billion tonnes by year by 2050. The mining, processing and transport of huge quantities of aggregates in addition to billions of tonnes of raw materials needed for the cement manufacture consume considerable amount of energy and adversely affect the ecology of virgin lands.

As far as Indian scenario is concerned, considering the rapid rise in population there is huge demand for housing and infrastructure facilities leading to large scale demand for engineered materials. Concrete is the most popular construction material in India considering its versatility as well as availability at low cost. The main advantages of concrete are,

Its relative low cost as a finished product.

Its mouldability to any desired shape.

High range of workability, from zero slump for Roller Compacted Concrete to flowable slump in case of Self Compacting Concrete.

Robustness it imparts to a structure.

Versatility with respect to strength and density (from light weight concrete to high density concrete).

PERFORMANCE OF CONCRETE WITH RESPECT TO WOOD & STEEL vis-à-vis CO2 EMISSIONS:

From the earlier discussion it can be said that concrete is the safest, most durable and sustainable building material. It provides superior fire resistance, gains strength over time and has an extremely long service life. With a service life of nearly 100 years concrete conserves resources by reducing the need for reconstruction. Its ingredients (except cement) are naturally available. As compared to lumbar constructions, concrete constructions would be preferred as it would not require cutting down of carbon di oxide (CO2­) absorbing trees. The land required to extract ingredients of concrete is fractional as compared to land required for harvest of forest wood. Due to massive size and cost factor timber is seldom used as a construction material in urban areas.

So concrete faces stiff competition from steel as an alternative to building construction. However studies undertaken by various investigators have highlighted the usefulness of concrete as a sustainable construction material over steel. As per studies undertaken by Gjerde5 et. al. on use of prestressed concrete gravity platforms and steel jacket structures as offshore oil field structures in North Sea following points would justify the use of concrete over steel,

Control of Deflections: as compared to steel sections of same slenderness, the deflection of prestressed concrete girders is only 35%. Also by prestressing it is possible to give a girder a positive camber (upward deflection) under self weight, and zero camber for the total payload.

Explosion resistance: owing to very high elastic limit of tendons commonly used in prestressed concrete beams, their explosion resistance is better than the normal steel girders. As per Internationale de la Precontrainte report (FIP), considering explosions, fire, sabotage and missile attack, the structures of reinforced concrete have less residual risks than alternative materials.

Resistance to cryogenic temperatures6: the ductile behaviour of concrete under impact at sub-zero temperatures favours it over steel. At low temperatures normal structural steel becomes brittle and loses its impact resistance. Also the testing of use of prestressed concrete tanks for storage of liquefied natural gas (LNG) at temperatures as low as -260oC has opened up new avenues for exploring and possible expanding the scope of use of concrete under cryogenic conditions.

Environmental considerations: it is a well known fact that cement, a major constituent of concrete is one of the major source of emission of green house gases (GHGs). As per various reports, production of one tonne of ordinary Portland cement (OPC) emits approximately one tonne of carbon di oxide. The CO­2 emissions from cement manufacture account to about 5% to 7% of global CO2 emissions. Also the manufacture of iron steel which accounts to 25% of global CO­2 emissions.

As per various studies concrete absorbs CO2 throughout its life time through carbonation process helping to reduce its carbon footprint. For a 100 year service life of concrete structure approximately 86% of concrete is carbonated. During this time concrete will absorb 57% of CO2 emitted during the original calcination process.

CONCRETE AND GLOBAL WARMING:

The worlds energy production is largely dependent on burning of fossil fuels that releases CO2. For manufacture of any good or material fossil fuels are burnt. It is most relevant to mention the term Embodied Energy. Embodied energy is defined as the available energy that was used in the work of making the product. Higher embodied energy thus contributes to global warming and hence acts against the preservation of environment and ecological balance. A sustainable material thus shall have low embodied energy. Embodied energy is the sum of energy consumed during production and transportation.

As compared to steel the embodied energy of cement is less and concrete has still smaller embodied energy. The table listed below indicated comparative embodied energy4 of major concrete ingredients:

Sr. No.

Material

Energy (MJ/kg)

Production

Transportation

50km

100km

1.

Sand (River)

0

87.5

175

2.

Crushed Aggregates

20.5

87.5

175

3.

Burnt Clay Bricks

2550

100

200

4.

Ordinary Portland Cement

5850

50

100

5.

Steel

42000

50

100

Of all the materials listed above the embodied energy of cement can be reduced in production stage.

INDUSTRIAL ECOLOGY:

Blended Portland cements containing high volume fly ash from coal fired power plants and granulated slag from the blast furnace of iron industry provide excellent examples of industrial ecology because they offer a holistic solution to reduce the environmental impact of several industries. Concrete mixes containing 15% to 20% flyash or 30% to 40% slag by mass are already being manufactured worldwide. For same grade of concrete and for similar workability conditions, addition of flyash increases the volume of overall cementitious material by 10% to 15%4. (Specific weight of flyash is lower than cement by 25%). Further the perfectly spherical shape of flyash particles ensures a good lubricant effect in the cement matrix. The pozzolanic reaction of flyash in cement matrix helps in reducing the development of heat of hydration. Use of flyash helps in improving the durability properties of concrete. For addition of 30% flyash of the cement content it has been observed that there is reduction in7:

Green House Gas emission by 17%.

Acidification by 15%.

Winter smog by 5%.

Eutrophication by 13%.

Further with the ban by government on mining or collection of sand from pits and river beds, the problem of aggregates can be solved by use of demolished material and construction waste. Worldwide it has been found that a billion tonne of construction and demolition wastes are being disposed in road bases and land fills every year, inspite of the fact that cost effective technologies are available to recycle most of the waste as a partial replacement for coarse aggregates in concrete mixes.

Phase 1: Raw Material Processing and Transportation to the Plant.

Phase 2: Concrete Plant mix & Site Transport

Phase 3: Building site casting, curing, installation

Phase 4: Service Life Repairs O & M.

Phase 5: Secondary Life, Demolition, Recycle, Reuse

Fig. No. 4: Life Cycle of Concrete from Cradle to Grave8

SELF COMPACTING CONCRETE:

Self Compacting Concrete (SCC) offers a rapid rate of concrete placement due to faster construction times. SCC is able to flow under its own weight, completely filling the formwork and achieving full compaction even in presence of congested reinforcement. The hardened concrete is dense, homogenous and has same engineering properties and durability as traditionally vibrated concrete9.

The concept of SCC was first proposed in 1986 by Prof. Hajime Okamura, but the first prototype was first developed by Prof. Ozawa in 1988 at University of Tokyo10. The concept of SCC gained momentum due to lack of availability of skilled workforce especially during concrete placement and also from the point of view of durability considerations.

The fluidity and segregation resistance of SCC ensures a high level of homogeneity, minimal concrete voids and uniform concrete strength providing a potential for superior level of finish and durability to the structure. SCC is often produced with lower water cement ratio providing the potential for high early strength, earlier demoulding and faster use of elements and structures. The improved construction practice and performance combined with the health and safety benefits make SCC an attractive solution for both precast concrete and conventional cast in situ construction practices.

A typical SCC mix comprises of cement, coarse and fine aggregate, mineral and chemical admixtures. A limiting value of coarse aggregates as 50% of solid volume of the concrete, and of fine aggregate as 40% of the solid volume of the mortar fraction in the SCC mix proportion is suggested for achieving good self compact ability. Commonly used mineral admixtures include flyash, silica fume, ground blast furnace slag. Chemical admixtures consist of super plasticizer and a viscosity modifying admixture. The use of one or more mineral admixture having different morphology and particle size distribution improves deformability, self compact ability and stability of SCC. Use of super plasticizers help to improve self compact ability at low water cement ratio while the Viscosity Modifying Admixtures (VMA) helps to increase the cohesion and segregation resistance.

CONCLUSION AND DISCUSSION:

Concrete industry should try to adopt a four point sustainability programme of

Sustainable Development: Development of industry along with development of society.

Sustainable Construction: construction of built environment and infrastructure with minimum harm to environment.

Sustainable Consumption: Consumption of resources with focus on reducing the carbon footprint of concrete.

Sustainable Concrete Industry: sustainable technology and business model (combination of socio-economic needs, use of database from physical and life sciences and human values from holistic view.)

Phase wise implementation of Sustainable Concrete Life Cycle by identifying key performance indicators and setting realistic targets of continuous improvement.

Achievement of identified goals is possible with the help of information mining, stakeholder interactions, periodic review of status of achieving the life cycle objectives, development of symbiotic relationships amongst various agencies involved, joint studies on formulation of codes, specifications and standards for sustainable concrete constructions, education and sensitising of organisational personnel about sustainability objectives decided at industry level.

The main contributor for increasing carbon footprint for concrete is cement manufacturing process and application of supplementary cementitious materials has great potential for emission savings.

The concept of Industrial Ecology has to widely publicised and practiced.

Use of SCC helps in overcoming the drawbacks of poorly engineered concrete.

Use of SCC ensures advantages of easy placement in thin walled elements, densely reinforced sections, quality, durability and reliability of concrete structures and faster construction.

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