The Manufacture of Concrete - Case Study Example

Case Study: Concrete Introduction The urge of humankind to find a strong shelter that can save people from natural disasters like storms and snow or rain, has made concrete attractive for many generations until now. The development of blended mixture for use in buildings dates back to almost 3000 years. Egyptians used this technology, which is evident from the construction of pyramids by them, many years back. Syrians used concrete even before Egyptians did. However, the Roman Empire refined the use of concrete mixture, which involved blending process somewhat similar to the present technique. Concrete remains the most essential part of our daily activity, as people need to take shelter in their homes, which are built with use of concrete. In fact, all buildings, roads, bridges that people use today are constructed with concrete. Some constructions like bridge over the Colorado River, built with strong concrete, are almost 250 yeas old. However, the changes in the manufacturing process, based on the research and developmental efforts have resulted in different types of concrete, being used presently. The use of admixtures and plasticizers to produce concrete of high strength is an example in this direction. History of concrete The background of concrete goes back to the time of Egyptian pyramids, which are almost 5000 years old. Since then, many civilizations have used concrete for building and decorative purposes. The utilization of concrete is now found almost everywhere for projects involving infrastructure, building architecture and decorative construction work. Egyptian technicians used gypsum and lime that served as mortar for the bricks used for building pyramids, Bricks were made of mud and straw. This process continued for many centuries until 300 B.C. when Romans discovered the use of a material that is very near to the modern cement, as they built different architectural structures including Colossuim and Pantheon. This was during the period from 300 B.C. to 476 A. D., when the Romans also mixed animal fats in their cement that served as admixtures. Britisher named Joseph Aspdin, a stonemason discovered Portland cement in 1824. The name Portland came from the mining area, which produced the tough stone required for making such cement, which came from stones quarried on the Isle of Portland, in Great Britain. During the year 1828, Portland cement was used extensively in the construction of tunnel under Thames River Experimenting with the mixture of clay and ground limestone, Aspdin created a product that he named Hydraulic cement, as this mixture became very hard with the addition of water. During the year 1836, cement was tested for the first time in Germany to find its comprehensive and tensile strength. Portland cement was produced, for the first time in America in 1871 at a manufacturing facility in Coplay, Pennsylvania. The Alvord Lake Bridge is a glaring example of use of Portland cement, as it was built in 1889. The bridge is still in use. Later, pathways and streets around the Courthouse in Logan County, Bellefontaine were built in 1891, which witnessed the first time use of concrete in streets. The Ingalls Building came in Cincinnati, Ohio in USA during 1903, when reinforced cement concrete was used for the first time for building high-rise structures. Later this building became the “National Historic Civil Engineering Landmark” in 1974. Kuhlman delivered ready mix concrete in 1928, in the state of Ohio, USA, for the first time. Later the manufacturer expanded the manufacturing facility to build concrete batching plants. Earlier, in 1915, L.M. Scofield was the first company to produce color hardeners, chemical stains and sealers that resulted in the production of colored concrete. While the technical research continued, the damage caused by thawing and freezing was reduced with use of concrete air entraining agents. Hoover Dam on the Colorado River remains the largest concrete project completed, so far, in USA. The necessity to find a hard and durable material for covering the ship decks led John Crossfield to invent a ‘concrete overlay’ in 1938. Latex was added to Portland cement along with other materials to make it suitable for ship deck covering. During the mid 1950s, the Bomanite process came into existence with Brad Bowman developing it, which included textured and colored concrete casting. This was the beginning of using decorative concrete. Assembly hall at the University of Illinois is the first concrete sports dome, which was built in 1967. The era of 1970s witnessed the use of fiber as a concrete reinforcement. During 1980s, Fu Tung Cheng in China and Buddy Rodes in America discovered cast concrete tops, for the first time. The tallest building with 65 floors came in Chicago, Illinois during 1992. This building had wide use of reinforced concrete. During 1999, a warehouse floor with around 40000 square feet area, located in Bellagio, Las Vegas had the polished concrete installation. This was first such extensive use involving polished concrete. As can be seen from the above, concrete has an interesting history spanning almost 5000 years, while no viable replacement of concrete can be found even today.(Concrete History timeline) Definition of concrete Concrete is a mixture involving mainly two compounds. Portland cement is the main component in this mixture, turns into a paste when mixed with water. The other component is the aggregate. The mixture of these two gives the product that is named as concrete. The aggregate mainly consists of the crushed stone, sand and gravel. The paste of Portland cement and water, when mixed with the aggregates creates a hard material, resulting from the binding property of cement. The product of this mixture, called concrete, becomes a rocklike compound, which has tremendous strength. The required hardness comes from the chemical reaction of water and cement. Composition of concrete Water and cement create the paste, required for getting the bond. The other constituent, aggregate, consists of sand and gravel. When only sand is mixed with cement paste, this produces the mortar meant for plastering and other purposes. This ‘mortar’ together with gravel produces “Concrete”. Therefore, concrete is a mixture of cement, sand and gravel, while the mixing and binding agent is water. The gravel and sand characteristics depend on the use of concrete. For example, the concrete required for a basement floor or for sealing the pile-foundation should have bigger size aggregates. However, the manufacture of concrete involves taking due precaution as this product needs a certain period for setting. Thereafter, the concrete become a rocklike hard substance. Hence, if the mixture required for producing concrete is not used at the construction site within the specified period, it will become useless. This is because using the hard concrete, which has already set, is impossible. The concrete composition requires that freshly mixed compounds required to make the concrete should be in the semi-fluid condition, with plasticity. This enables the concrete molding in an effective manner. A wet concrete has the ability of being shaped in a mold, as it can be molded by hand. However, the gravel or stone pebbles and sand particles must bind together to form a good quality concrete. This means that the sand and gravel should remain bound and in suspension, when concrete mixing takes place. It is important that there is no segregation of the ingredients, when cement concrete is being transported from one place to another. However, long distance transportation is not advised, as concrete will set in the required setting time, during transportation. Hence, most of the concrete production is done at the construction site. It requires the use of batching plant at the site. The main characteristic of concrete composition is the required consistency in the prepared concrete mixture. This consistency should ensure that the mixture flows thoroughly, without any segregation. In addition, the composition should ensure that the mixture is not crumbling. Hence, the proper mix ratio is required for a good concrete composition. The water content in the mixture should be around 8 percent of the total mass, while the cement content should be around 12 percent and aggregates around 30 percent of the total mass. Cement paste The paste of Portland cement and water composes of entrained air, while this paste can be 25 to 40 percent of the total volume of concrete. Around 8 percent of the total volume should be the entrapped air content, while cement can be between 7 to 15 percent of the total volume. The water volume must be around 14 to 21 percent of the whole. In addition, the volume of entrained air usually depends on the size of aggregates. (Suez cement 1-4) Aggregates There are mainly two types of aggregates, depending on the sand structure. Fine aggregate has sand particle size up to a maximum of 10 mm, while the course one has the size range going up to 150 mm, using the 1.25 mm sieve for retaining such particles. However, mostly 20 mm size aggregates are used. The selection of proper aggregate size is important, as this material comprises around 60 to 75 percent of the total volume of concrete. The aggregates should have strength and must resist any erosion to concrete due to its exposure to atmosphere. The gradation of particle sizes should be consistently done to retain the plasticity of the mixed paste. Water Water has minerals like chlorine and sulphates that can cause deterioration of concrete, particularly the reinforced concrete, as it causes the formation of rust on the reinforcement. Therefore, clear water, without any salt presence should be used for making the concrete mixture. The cement and water ratio in the concrete mixture is responsible for varying hardness and strength of the concrete mix. However, there are many advantages of adjusting this ratio. These include lower permeability, with lower water absorption and increased tightness. It also results in increased concrete flexibility and comprehensive strength as well lesser weathering affect. The proper water cement ratio also results in a better bond of the reinforcement and concrete along with good bonding of layers. The wetting and drying causes change in the volume of concrete and related shrinkage problems. However, proper cement-water ratio can reduce such defects in the concrete mixture. Usage of less water quantity in the concrete mixture is always recommended. However, proper consolidation of the concrete is an important requirement and water added should be sufficient to bring this property to the mixture. Certain concrete compositions require stiffer mixture, which may use smaller quantities of water. In addition, the use of a cement vibrator is recommended in such cases. Use of vibration while laying the concrete improves the quality of the mixture. The proper composition of a good quality concrete requires cleaning of aggregates before mixing takes place. There should not be any organic impurities in sand and gravel used for this purpose. Sometimes cement concrete composition requires the testing of dry mixture of cement and aggregates for ensuring that the ingredients mix homogenously, before the water is added to the mixture. However, the composition of concrete depends upon the shelf life of cement used for making the mixture. Cement within the middle range of its validity is always preferred for a good concrete composition. The storing instructions of he cement manufacture also help in this direction.(suez cement, pp 1-4) Concrete production The production of concrete takes place in the concrete mixers. The mixing is a process of accurate blending of cement, water, additives and aggregate in proper ratio. Industrial batching plants are used for production of concrete mixture on a large scale. These plants have the storage facilities for storing the required compounds, which are mixed in the required proportion by using the proper equipment for this purpose. Once the concrete mixture is ready, it is loaded in the mixer trucks, which must have the mixing or batching plants installed on them. The mixer rotation during the transportation should be around 4 to 6 revolutions per minute. The important factor for preparation of concrete is the timing between its production and use. The freshly mixed concrete must be used at the construction site within 60 to 80 minutes of its preparation. The concrete starts setting-in after this period and can become hard rock, unfit for use at the site. Hence, the concrete mixing and batching plants are always located within a radius of around 15 miles of the construction site, where use of the concrete mixture is required. Therefore, most of the concrete batching plants are located at the site of construction. Mixing plant Such plant must have storage silos for sand, aggregate and cement. In addition, it should have the storage tanks for plasticizers and other additives. The equipment required for the mixing is the batching plant or concrete mixer. The feeding of thee ingredients must follow the proper ratio as per the formula arrived for this purpose. This ratio depends on various parameters that include the required concrete strength and the precise application area. The mixing should be consistent and continuous, while the addition of water quantity should be precisely as per the calculated formula. Currently automated concrete mixing and batching plants have the facility to choose the materials in required ratio, after the setting is done by the operator. This ensures proper reliability and consistency in maintaining the required water cement ratio, along with other ingredients.(Production Process) (Ready mix Concrete Unit; Source: http://www.vicat.com/en/Activities/Ready-mix-concrete/The-manufacturing-of-concrete) Above is a picture of concrete manufacturing unit with silos for ready mix. Steps involved in manufacturing concrete The concrete plant must have receiving facilities that can enable unloading of goods from trucks directly into the silos through a hopper. The cement is fed into the silos through the conveyor pipes, which work under compressed air pressure. It is necessary to store aggregates for at least a week’s consumption for the smooth operation of the batching plant. These can be stored in hoppers or silos or in an open ground. Cement must be stored in silos to avoid any weather affect on the product, which may cause undue hardening of cement. Different types of cement require separate silos to store the various grades of this product. The silos should have necessary venting system for dust collection. Additives or admixtures can be added during the manufacturing process. The quantity of admixtures and such additives depends on the level of fluidity and plasticity required in the concrete mix. This action also helps in delaying the setting time of concrete. Mixing is the next step in the process of concrete manufacture. As described above, the individual ingredients, which include cement, gravel, admixtures and sand are blended together in a mixer. The mixing or batching plant must have computer aided ‘Programmable Logic Controller’ (PLC), which can decide about the time of mixing, the quantity of each ingredient. These parameters depend on the required quality of the concrete mix. Since the quality and timing of the concrete mixing process is most important to get a good quality concrete, it is essential that the batching or mixing plants are equipped with such PLC devices to ensure that the output is of the good quality. In addition, it eliminates the human error involved in feeding these materials manually. The capacity of mixers should be up to three metric cubes. PLC also helps in controlling the quality of concrete, as it assists the operator in controlling the quality and timing of the whole process to achieve the desired concrete strength, as per customer/user requirement. Using PLC, accurate quantities of cement, sand, admixtures and gravel is fed to the mixing plant. In addition, the required quantity of water depends on the aggregate hygrometric conditions. The use of PLC ensures that proper dose of water goes into the mixer after sensing these conditions. Transportation of concrete Mixer trucks are used to transport fresh concrete from manufacturing unit to construction site. These trucks require the continuous mixing of concrete during transport. Hence, they must have the appropriate mixer installed on them that can operate with a speed range of 2 to 6 revolutions per minute. This helps in the elimination of concrete mix getting harder enough to become unusable at the site. In addition, accurate and precise delivery logistics is essential to ensure that the mix is delivered at the construction site within the required time. This must be within limits of setting time for the cement used in manufacture of the concrete mix. (The manufacturing process) Concrete testing (Stress-Strain relationship for ordinary concrete Source: http://www.theconcreteportal.com/cons_rel.html) Stress-strain behavior The above figure provides details about the relation between stress and strain in a normal strength concrete mix. The curves are linear until the 30 to 40 percent of the applied load. However, with application of more loads, the curve becomes non-linear. During this time, the large strains appear corresponding to small increase in the stress. This is due to presence of micro-cracks, which may occur at the aggregate –paste interface, after the concrete is ready for use. The huge network of cracks results in large stress level. However, strain must be 0.003 percent of the ultimate stress for a normal strength concrete. The micro-cracking and mismatch observed at the level of interface of cement paste and aggregates is responsible for the non-linear stress-strain behavior of concrete, while it is linear for the aggregate-paste stress-strain relationship. This is explained in the following figure, which details about the post peak curve in stress-strain relationship. (Stress-strain behavior) (Source: http://www.theconcreteportal.com/cons_rel.html) Tensile test The possibility of tensile cracking in cement, due to weather and load affects has made it necessary for the concrete to be tested for its tensile strength. Although there are no direct test methods that can apply uniaxial tension to the given specimen of concrete, there are indirect testing methods, done for finding the tensile strength of the concrete. These include split cylinder test and flexure test. The split cylinder test requires placing the concrete specimen between the loading surfaces of compression. The indirect tensile stress developed during the test results in concreter cylinders splitting into two halves, based on the formula represented by poisson’s effect.(tensile test) Advantages and disadvantages of concrete Advantages There are many advantages of concrete, while few of them are briefly described here. As all ingredients to produce concrete are easily available at affordable prices, the product becomes economical for civil engineering constructions. In addition, the economic benefits increase with the requirement of low maintenance and long life of concrete. Concrete does not corrode easily like other building materials lying at the construction site. Concrete can take any desired shape as it can be molded into different shapes and sizes very easily. In addition, the molding and casting can be done at the construction site, which reduces the overall operational costs. Concrete can withstand very high temperatures and is fire-resistant. The use of concrete is very wide as it is resistant to rodents, wind, insects and water. Hence, concrete can be used to build storm shelters and other buildings. Disadvantages The disadvantages of concrete include its low tensile strength as compared to other building materials. In addition, the concrete has very low ductility. The ratio of strength to weight in concrete is low. The major disadvantage of concrete is its property to develop cracks over the period. (Concrete) Usage of concrete The concrete mixture is so extensively used that it leaves a huge carbon footprint, worldwide. Cement manufacturing process produces carbon dioxide as the by-product, due to chemical reactions that take place. Since concrete mix has a large portion of cement, the production of concrete is largely responsible for the release of this byproduct into atmosphere. The reports suggest that the concrete production is responsible for five percent of carbon dioxide production, annually, around the world. However, the addition of admixtures and additives to the mix, during manufacturing process can reduce the carbon release. Efforts are going on to develop special additives for this purpose. In addition, the additives can result in producing the concrete of higher strength. Nevertheless, use of concrete is widespread, globally. For example, new construction projects like Burj Khalifa in Dubai use concrete mix extensively, in varying strengths. The use of concrete can be traced back to thousands of years, when humans used to blend sand and gravel with cement and water to produce a product suitable for construction activities. However, the fall of Roman Empire resulted in the loss of secrets for concrete development and manufacture. The technology reemerged during last few hundred years, which saw concrete usage in a huge manner by all the construction, road and bridge projects, around the world.(Crow) Future of concrete With the emerging technological advances, companies worldwide are making their efforts in research and development of innovations in the production of concrete. One such innovation that will result in a highly strong concrete mix of the future is the development of plasticizers. However, the addition of such plasticizers must meet the quality parameters of these chemical additives. These additives when added in proper quantity and with accurate quality can reduce the usage of water in concrete, during the future manufacturing processes. While the current concrete mixture production is responsible for exhaustive release of carbon in the atmosphere, the efforts are going on to find alternative for reducing the carbon footprint left by the manufacture of concrete. In this direction, an Australian company, TecEco has prodced ‘green’ concrete for this purpose. The technology in developing such products involves addition of ‘magnesium’ to the concrete mix. This results in reducing the release of carbon to the atmosphere with the formation of a porous concrete mix. However, the mass production of green concrete and other innovative types of concrete remains uncertain at present. The chemical technologists and construction experts are certain that use of these products will require change in the building codes. In addition, the manufacture of green and other types of new concrete will result in the price increase for this product. Accordingly, the public shall have to choose between the future safety and current usage, while sacrificing for the increase in cost can reduce the carbon footprint left by the present form of cement manufacturing. Thus, future concrete can turn from being a ‘climate villain’ to a hero, as its advantages are many to make it indispensable for future usage in the building and construction industry. (keim) Conclusion The manufacture of concrete has covered a large footage, with development of improved manufacturing process and additives. The blending of constituents like gravel, sand and cement with precise quantity of water is now being done by computerized programs like PLC, which eliminates the human error involved in feeding the exact quantities of all ingredients to the batching plants. In addition, the mixer-trucks, installed with mobile mixers keep the concrete consistency in the right proportion, while the mixture is transported from manufacturing unit to the construction site. With development of ‘green’ concrete, the future usage of concrete shall aim at reducing the carbon footprint left by the present process for manufacturing this mixture. The addition of plasticizers shall result in reduction of water usage in the mix, while increasing the strength of concrete. Works-cited “Advantages and Disadvantages of Concrete”, civilengineers forum.com, nd, web, 27 Nov. 2014 “Concrete History Timeline” concretenetwork.com , nd, web, 27 Nov. 2014 Crow. M.J, “The Concrete Conundrum” chemistryworld.org , March, 2008, web, 27 Nov. 2014 Keim.B, “Concrete’s Future Looks Lighter, Greener” the csmonitor , 15 Sep. 2005, web, 27 Nov. 2014 “Production process” vicat.com , nd, web, 27 Nov. 2014 “Stress-Strain Behavior of Concrete” the concreteportal , nd, web, 27 Nov. 2014 “Suez Cement” seuzcement.com.eg , nd, web, 27 Nov. 2014 “Tensile Test” buildingresearch.com , nd, web, 27 Nov. 2014 “The Manufacturing of Concrete” vicat.com , nd, web, 27 Nov. 2014 Read More