Looking At The Unique Propertys Of Concrete Construction Essay

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Today concrete due to its unique properties had become most used and non replaceable construction material in the world. It has been a subject of research of last decades and demands further study because of demographic, social and economical development of human.

Meanwhile there are objectives making us reconsider our view of world shaping. Primary point is population. As recent demographic reports show it is growing in geometric progression. Considering that it took thousands years to reach 1.5 billion people by 19th century, it approached 6 billion by third millennium and it is expected that population is going to be 9.1 billion by 2050. Secondly, urbanisation is in high progress and actually it is correlated to population growth. If at the beginning of 20th century 90% of people lived out of cities in year 2000 this ratio was nearly balanced. Lastly, huge changes stated above lead to expansion of energy, industry and logistic sectors of economy during the last century. It is regrettable that our technology development didn`t consider much a rational use of materials and as result sustainability issues of technology choice has became trend now. Today we are receiving a receipt for those mistakes in the form of natural disasters and catastrophes.

It is clear that in not-so-distant future our demands in concrete structures will increase greatly. Ordinary concrete contains 12 percent cement, 8 percent mixing water and 80 percent aggregates. If annually about 1.5 billion tons of cement, 9 billion tons of sand and rock and nearly 1 billion tons of water is used to produce 11.5 billion tons of concrete. By 2050 around 18 billion tons of concrete production is expected. With technology used now enormous quantities of materials and energy will be required. Furthermore, production of concrete is not possible without certain amount of contaminant emission. All these aspects lead us to new conception of concrete production which will consider more sustainable solutions thus preserve natural resources for the future human needs. The development of new technological conceptions of concrete requires holistic approach from most issues related to concrete. Moreover, we should learn lessons from the past and construct a vision for the future of concrete industry.

Concrete materials.

Concrete is made of inorganic material known as cement, which is mixed with water and aggregates as sand (fine aggregate) and rock (coarse aggregate) in crushed or uncrushed condition. So we can say that concretes skeleton is made of aggregates and cement with water is used as a binder material. It is important to notice that cement is most expensive material in concrete. Modern concrete also includes chemical admixtures to modify its properties in both fresh and hardened states. Moreover, mineral admixtures are becoming trend in concrete industry.

Aggregates are classified according to particle size and bulk density. The term coarse aggregate is used for particles larger than 4.75 mm and term fine aggregate is used for particles smaller than 4.75 mm. If aggregates bulk unit is less than 1120 kg/m3 it is lightweight and if more than 2080 kg/m3 it is heavyweight aggregate. Today about 90 % of aggregates used for concrete production are obtained from natural sources. However, there are some uses of synthetic aggregates. Synthetic aggregate is thermally processed materials as clay and shale. Industrial by-products as blast furnace slag and fly ash are also in this category. Besides, recycled concrete from demolished structures is under investigation for use as aggregates.

Crushed and graded blast-furnace slag is used as aggregate. Property of aggregate depends on composition and cooling rate of slag. If acid slag produces denser concrete, basic slag is prone to produce a vesicular with low specific gravity. Colour and durability problems might be caused by high sulphide content in slag.

Fly ash which is a waste of coal fired plants consists of tiny particles of aluminosilicate glass. Certainly it can be used as light aggregate. The fineness and carbon content of fly ash are most important question to deal with.

Recycled aggregates come from construction and demolition waste, asphalt planning, excavation and utility planning. Unfortunately, due to presence of large amounts of hydrated cement paste and gypsum it can be used as coarse aggregate only. Recent studies show that with appropriate methods of processing recycled concrete there is no loss in the quality of concrete. Crushing, sizing, dust control and inappropriate constituent separation are issues demanding investment. It is regrettable that due to long durability and strength affection municipal waste cannot be used in concrete industry.

Cementitious materials include binder for skeleton of concrete. Portland cement is most used cementitious product now and around 1.5 billion tons is manufactured every year producing about 7 percent of global CO2 emission. Besides, production of Portland cement is energy intensive. Sustainability suggests reduction of cement amount in concrete. As a result of many investigations have been made and consequently partial replacements as fly ash, ground granulated furnace slag, limestone fine, silica fume and other industrial waste materials were introduced.

Regular Portland cement contains 95 percent of portland clinker and 5 percent gypsum. As mentioned blended cement is new development in the industry. The term clinker factor was introduced to designate the proportion of the clinker per tonne of cement. According to Jahren`s estimations in 2002 about 240 million tonnes of mineral additions were used to produce 1700 million tonnes of cement thereby clinker factor of 0.86 was obtained.

Among acceptable mineral additions fly ash has a promising future. Recent investigations demonstrate that about 500 million tonnes of fly ash are annually produced. Due to regulations only 15 percent or 75 million tonnes of fly ash can be used for cement manufacture. When mass-for-mass replaced with cement fly ash reduces temperature and thermal cracking is less expected. Comparable standard-cured strength at 28 days and workability of concrete are also achieved with appropriate proportions of fly ash, in fact when correct curing conditions are applied cement replacement demonstrated better performance. Besides, better durability is obtained with the replacement of fly ash.

Ground granulated furnace slag is waste from iron industry and can replace 50-70 percent of Portland cement volume. Proportioning should be carefully chosen since early-age strength development is slower with high proportioning though it gives better durability. Most commonly GGBS is used in ready-mix concrete. Workability, lower early age temperature rise, elimination of the risk of damaging internal reactions as alkali-silica reactions, resistance to chloride, sulphate attack are well known properties of GGBS replaced concrete.

The slow strength gain and low exothermic properties of ggbs have led to its use in the decommissioning of Britain�s older nuclear power plants.

Sellafield Ltd acts as a contractor to the Nuclear Decommissioning Authority, whose objective is to ensure that the 20 civil nuclear sites under its ownership are decommissioned and cleaned up safely. Sellafield first used ggbs for the encapsulation of medium-level radioactive waste in the 1980s.

Since then, with the ?3.2 billion nuclear decommissioning programme gathering pace, it has been using up to 2,000 tons of ggbs a year for the encapsulation of waste streams from its thermal oxide reprocessing plant.

A 90 per cent ggbs to 10 per cent Portland cement ratio is used in the grout for the encapsulation process. This high ratio controls the strength gain, reduces the heat generated, minimizes the risk of thermal cracking and restricts diffusion.

The precisely controlled particle size distribution of the specially produced ggbs produces a grout with high flow and extended working life without the use of chemical admixtures. In this challenging application, encapsulation with ggbs grout provides long term durability, stability and peace of mind.

Limestone fines are obtained from the processing of limestone. There are three different ways limestone fines used today as adding to a Portland cement CEM I; using in the manufacture of Portland limestone cement representing 6-20 percent of CEM II/A-L or A-LL or 21-35 percent CEM II/B-L or B-LL of the cement; Blending with CEM I at the mixer. Fresh concrete with the same water/cement ratio Portland limestone cement has less strength at 28 days than concrete containing CEM I, although it is less prone to bleeding.

Silica fume is waste from silicon metal or silicon alloys. It contains 80 percent silica and very fine particles. Highly pozalanic silica fume can contribute to concrete advantages as reduction of thermal crack, resistance to sulphate attack and acidic water and high ultimate strength.

Finally, water requirements must be considered in sustainability context. Although our planet is reach in water there are still about 1 billion people who still don�t have an access to fresh water and in these areas use of water in industry and particularly in concrete production must be rationalised. In ready mix concrete plants water can be recycled and used as replacement for fresh water.

Concrete properties

Concrete is environmental friendly material. Use of other materials in such quantities as concrete would have lead to further demolishment of environment. Among possible to use materials concrete is the most responsible choice for sustainable development. Resource efficiency, durability, thermal mass, reflectivity, ability to retain storm water and minimal waste make concrete most appropriate sustainable solution.

Raw materials used in concrete are abundant in the world and places less impact on environment to acquire them. Concrete constituents can also be obtained by industrial by-products such as fly ash, GGBS, silica fume etc. which are spread in the world.

Very good condition of ancient concrete structures found in the world demonstrate concrete`s high durability properties. Main characteristics made of concrete are long life span, resistance to rust, rot and fire.

Residences build with concrete have rational energy performance. Insulated concrete walls can make buildings independent from sudden daily temperature changes and as result owner would receive less charges for heating and cooling. Moreover, owner can use smaller capacity equipment i.e. appropriate to his own comfort.

Insulating concrete forms build high-performance, sustainable facility

At first glance, Clearview Elementary School in Hanover, USA is a great example of excellent use of concrete properties. School was built with insulating concrete forms (ICFs). Made from foam and stacked in the shape of the structure, ICFs are filled with reinforced concrete to create a solid wall with excellent thermal mass and structural strength. ICF structures offer energy efficiency, durability and design flexibility at a competitive cost with traditional construction techniques.

Beyond the structural system, the school is designed with features that enhance the learning experience, incorporating daylight and improved ventilation, as well as super-efficient ground source heat pumps and radiant floor heating.

On the Clearview project, approximately 40 percent of the building material was manufactured locally, and about 75 percent (by cost) was manufactured with a high recycled content. These benefits, partnered with the energy efficiency and durability of the concrete structure, will save the school an estimated $34,000 annually on energy costs.

Urban heat island which is caused by solar radiation is one of the problems in cities located in hot regions. Basically, pavements and roofs are most exposed to sun radiation. Due to light colour concrete makes structure absorb less heat and reflect more radiation. Consequently, air conditioning demand decrease.

Due to urbanisation more areas are getting paved over and consequently making impervious surface for water to go through. This creates an imbalance in ecosystem and problems as erosion, flash floods, pollution of natural water sources. At this point pervious concrete is one of the solutions which captures water in network voids and then transmits to underlying soil.

Compared to other materials concrete can be produced in most accurate quantity. After concrete has served its initials purpose it can be recycled and used as aggregates to new concrete making concrete minimum waste material.

Concrete practice. Too related to above.

Concrete design.

The history of structural design didn�t always have written codes. Since 20th century whole picture rapidly changed. Through the progress of methods and technology (computer application) analysis of structures became quite easy. Though concept of durability is relatively new i.e. engineers came to the conclusion that structures can�t be designed for forever. Poor construction practise could cause fast deterioration but on the other hand decrease of service life could happen due to expansion of our needs. Now we have to deal with constructions in severe conditions. It leads us to the durability of concrete.

Today codes are specified by three limit states, these are ultimate limit state, serviceability limit state and fatigue limit state. Five safety factors are introduced as the examination of each limit state, so these are material factor, member factor, load factor, an analysis factor and an important factor. In fact, any of them doesn�t count in deterioration. The code requires specification of service life of structure. But it doesn�t suggest how to guarantee the life span. Only fatigue limit state can be related to service life in repeated loading case. The strength deterioration of concrete is taken not due to chemical reaction during service life.

The serviceability limit is concerned with durability. The code defines how to examine flexural crack width and the difference of environmental conditions is taken into account for the specification of crack width. It is important to mention that allowable crack widths are given as function of cover i.e. not definite numbers.

The final purpose in the durability design is to predict and assure how long structure survives under given condition. For concrete structures lots of researches cover carbonation, penetration and accumulation of chloride. Furthermore, it is known how fast such chemical processes go and how steel reinforcement corrode. Unfortunately, this knowledge is not enough to predict the end of service life. So deterioration process is very time consuming as no acceleration test method exists and we are not able to assure service life of structures.

In order to relate durability design to structural design deterioration process must be studied in terms of stress transfer mechanism and materials. Once deterioration issues are clear durability aspects should be introduced to structural design in following form:

1) Service life: we may indicate the service period without maintenance and extra life with proper maintenance and repair.

2) Material factor: we may choose right numbers with taking into account of strength deterioration

3) Member factor: should be related to bond deterioration

4) Crack control: service life be related to crack width as well as cover thickness

5) Design details: the minimum reinforcement ratio and diameter of reinforcement should be influenced be service life. Cover thickness should be calculated considering service life. Surface coating may be taken into calculation.

Concrete research

Sufficient concrete production for social development and exhaustion of the raw materials demand fast transform of common sense to new scale. Concurrently, new research areas need to be introduced. According to Gunnar M. Idorn the curing of concrete and Alkali Silica Reaction are denominative title for the new era of research.

In the past all the properties of hardened concrete had been evaluated using �room temperature� i.e. for preparation and handling of specimen temperature 20 �C was used. New achievements of these long-term researches became popular and people accepted it as standard condition.

After World War II period ordinary site production of concrete were developed and therefore curing temperatures were in changes:

During first decades, low-heat yielding OPC and 300 kg/m3 cement content were used;

With the increase of C3S and fineness of OPC and cement content curing heat were increased;

In 1970 external heat were used making cycle very fast.

In some curing technologies methods it is possible now to precalculate maturity development in any concrete section at planning phase according to specifications, and identify optimum temperature rise and strength development rates using liquid nitrogen cooling or heated water for fresh concrete, insulation of formwork etc. The recording if maturity development during curing period so curing is complied with the conditions deviate from preset values. So, initial crack-inducing, excessive heat development can be avoided for while, although, advantage of the peculiar heat of hydration is remediable. This monitory system has been subjected to further modifications. Most likely basic clarification of the energetic of cement hydration and optimisation of heat/strength development in concrete under certain conditions are an approach for future investigations.

Alkali silica reaction is still a potential threat to durability of concrete. Investigation of geological/mineralogical survey mapping in developing countries is still in progress. This is long-term issue due to different classification of aggregates available. Cement manufacturers have relevant knowledge the dependence of alkali content in cement. However, when they face different situations of raw materials, regulations and market issues their knowledge is restricted.

Steel and coal manufacturer have enough knowledge about the use of their by products in cement production and thereby reduce harmful ASR but they are too limited production capacity to supply the demands. Furthermore, we don�t have classification studies or surveys of the huge quantities of natural, acid pozzalans from passive or active volcanoes.

There are big advantages from detailed studying harmful ASR in developing countries, in compliance with new research and engineering competence. Materials and financial resource issues suggest that the investigation should be carried out internationally to establish long-term, cost effective solutions.

Market forces

Financial responsibility is important for ecological improvement. Economical concerns should be addressed in efforts made to reduce environmental impact from human activities. Moreover, integration of environmental changes with economic opportunity and fiscal responsibility is the most appropriate solution.

Use of concrete provides advantages on several levels. Long term savings in operating cost and service life are provided by concrete structures. Durability, resistance to harsh weather and disasters, energy performance, low maintenance cost of concrete are beneficial for owners. Moreover, concrete is locally-based industry and therefore manufacture and use of concrete will contribute to the local economy.

Regarding long term concrete is best decision for many applications and economical sustainability. The long term value of concrete comes not only from durability but also to resistance to certain type of loads. For example, concrete roads perform best under rolling loads from heavy truck tyres. Portland Cement Association surveyed structural engineers and contractors and found that concrete buildings take eight weeks less time to complete than steel, so revenue would be increased by putting a building into service sooner.

Concrete buildings are very advantageous on operating. Due to unique properties concrete improves building energy performance from both within and outside of a structure. Within a structure it serves as thermal mass, reduces air filtration and reduction of supporting walls allows daylight to reach farther into the building. Outside it reduces urban heat by reflecting solar radiation due to its light colour.

Legislation

Long-term strategy to integrate economical, social and environmental sustainability development was formulated by European Union. The main objective of the strategy is sustainable improvement of the well-being and standard of living of future generations.

The strategy identifies seven unsustainable trends to take an action on. Strategy specifies a whole range of operational and numerical goals and certain measures at EU level to reach these objectives.

The long-term objective is to limit climate change and its effects by meeting commitments of Kyoto Protocol and European Strategy on Climate Change.

Limiting negative effects from transport and reducing regional disparities, these require remanagment of the link between economic growth and transport growth and develop environmentally friendly transport.

To promote more sustainable modes of production and consumption the link between economic growth and environment degradation needs to be broken and ecosystems toleration should be considered more.

Another objective is sustainable management of natural resources. Overexploitations needs to be avoided, efficiency of natural resource use amended the value of ecosystem services recognized and loss of biodiversity rested.

The rest of issues include public health, social exclusion and poverty and fight against global poverty.

The following standards where developed for the concrete industry in UK regarding sustainability:

BS 7750 (1994) Specifications for Environmental Management Systems now cancelled and replaced by:

ISO 14001 (2000) Environmental Management Systems: Specification with Guidance for Use

ISO 9000 (2004) Quality Management and Quality Assurance Standards Part 1: Guidelines for Selection and Use

BS 8555:2003 Environmental Management Systems. Guide to the phased implementation of an EMS.

Council Regulation (EEC) No. 1836/93 of 29 June 1993allowing participation by companies in the industrial sector in a Community eco-management and audit scheme. This has been replaced by Council Regulation No. 761/01 allowing participation by organizations in a Community eco-management and audit scheme (EMAS).

Public perception.

In order to maximize concrete recycling we need to know the extent to which building codes and green rating schemes identify concrete recycling. In fact there are some restrictions on using recycled concrete in aggregates in projects for filling, asphalt, landscaping and sub-base.

However there are limitations on the quantity that can be used in structural concrete. The main reason for limitations is misguided public perception with regard to the quality of recycled concrete or even non considerations for recycled concrete use. Green building schemes and rating systems are under development to change this perception. Among well-known green building guides BREEAM (UK), LEED (USA), CASBEE (Japan) and HOE (France) are good examples.

Raising awareness of concrete will encourage recycling of concrete by interested parties. Following suggestions are made towards the �zero landfill� of concrete purpose:

Development of reliable and consistent statistics;

Advertise construction and demolition waste data by government and responsible people;

Set goals for application in both road and building constructions;

Develop economic motivation and legislation to allow industry to develop and promote concrete recycling;

Investigation and development to weigh further recovery techniques and applications;

Green building schemes should further promote good construction and demolition waste management and the application of secondary concrete aggregates;

Major stakeholder publicity to change public misconception.

Conclusion

In order to achieve sustainable development in concrete production, we have to seize and value what has happened in the world in last decades. Social changes, instabilities in economy, fast population growth, rapid urbanization advances in technology already put its negative effect on environment. At this point we need to develop a new era of sustainable development and to obtain the purpose of sustainable development we need develop holistic approach to catch a balance between social economic and environmental issues regarding concrete industry.

By using more ecological blended cement in concrete, designing for durability as well as assuring life cycle analyses of construction projects, it is possible to direct construction and in particular concrete industry to higher sustainable solutions. In general it is engineer`s responsibility explore more sustainable solutions and promote sustainability concerns into society.

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