Experimental work on the mechanical properties of concrete using recycled aggregates - Dissertation Example

?Experimental Work on Mechanical Properties of Concrete Using Recycled Aggregates College Introduction The main environmental problem in the world today is the disposal of waste material. In order to protect the environment, the consumption of natural energy is reduced while the consumption of waste energy is increased. This minimizes the amount of natural resources that is used which in turn helps in the conservation of natural resources. This paper is a literature review of the mechanical properties of concrete using recycled aggregates. But for comparison purposes we first take a look at the mechanical properties of cement concrete and make a comparison between it and the recycled aggregates. The main characteristics of the concrete that we look at include the history, hydration of each type of concrete, properties on test and the mechanical strength of each type of concrete. Several articles are used for reference but are limited to their content in the comprehensive strength after a total of 90 days and the static modulus of elasticity is also checked at a maximum of 28 days. With these test, we get that recycled aggregates have a strength averaging 85% to 98% of the natural strength of natural aggregates. This paper is based on works done within the past ten years. Every construction activity needs a lot of materials. Some of these materials include; sand, bricks, timber and mud. However amongst these materials, concrete remains the most important of them all. With respect to the environment and adaptability, the concrete that is used in any construction must satisfy all the environmental and economical needs of any given area. When we use recycled aggregate for construction then we are able to construct the environment while at the same time minimize the cost used in the construction process. The waste and by products of cement contain almost 75% of natural aggregates and so it is of greater importance as natural aggregates. When we talk about recycled aggregates here we mean; slag, waste from power plants, used concrete, quarrying and mining wastes, waste glass and many more. There are lots of demolished concrete that is left without use in demolished sites as well as construction sites. These concrete wastes provide are the main source of recycled waste that we will look at their strength. Cement (Portland cement) History Portland cement is the main type of cement that is used in making concrete in construction site. Its fine powder is produced by grinding clinker and some limited amount of calcium sulfate. It was developed in the early 19th century in Britain from natural cement. The name is derived from Portland stone, a building stone that was used during the same time. The origin of Portland cement is believed to be from Aspdin Joseph. He was a brick layer from Britain Leeds. One of his employees Johnson developed a production technique which resulted in hard and fast drying cement. The cement also had high compressive strength as compared to other concrete. As days passed, people started to prefer the Portland cement to the roman cement which had been in use for many years. In 1843 Aspin’s son William made a version of this cement which was improved. He named it patent Portland cement though it did not have patent in it. He later moved to Germany to improve cement making and came up with Portland cement. In 1878 the German government gave a standard for Portland cement. Hydration When Portland is mixed with water and it reacts with it, a process called hydration, this mixed with sand and some gravel produces a synthetic rock, which we call concrete. This concrete is very essential in today’s world as electricity and water. During the hydration process, four main types of minerals are always present. They include belite, alite, aluminate, and ferrite. The addition of water causes an exothermic reaction to occur, after this reaction, (C3A) reacts with water forming aluminum gel. This gel in turn reacts with any sulphates in the mixture to form ettringite. The reaction is very strong but its only for a short time before ending. After this reaction, a period of low heat production follows and at this time, there is no or minimum reaction. Elite and belite start their reaction soon after this dormant period. In stage three of the reaction process, every reaction that occurs tend to move inwards and this leads to the strengthening of the concrete. The hydration products include; calcium cilicate hydrate, calcium hydroxide, AFM and AFT phases, Ettringite, monosulphate, and monocabohydrate just to mention a few. Mechanical strength of the cement gel Portland cement is high strength cement which can have a compressive strength of up to 19000 psi or (131 Mpa). The strength of the cement depends on the type of cement that is used. ASTM type has up to 120MPa compression strength and it is the one which is mostly used to make very tall buildings as compared to other types of cement. In general Portland cement produces concrete with the highest compression force as compared to any other cement. Test on properties of the concrete One of the properties of Portland cement is its production of heat when water is added to it. The amount of heat that is produced depends on the amount of concrete that is being made. Some tests were done to know the amount of heat that is produced when concrete is made from Portland cement. It was found that an increase in the amount of concrete to be made also increases the amount of heat that is used. Another property that was tested is the fineness of the cement. It is found that the cement is very fine and can pass even in very small sieves. The drying shrinkage of the cement is also a factor that was looked into. The tests showed that sulphates in the cement cause that shrinking in the cement. Split tensile test were also carried out and it was found that the split tensile forces of cement is about 6.0N/nm2. This is in accordance to the standards measured by doing the split test on a truck and spinning the contents of the truck so as to shake the concrete made from the cement. Aggregates In construction aggregate means particles that are used in construction. There are two types of aggregates; these are the natural and the recycled aggregates. The two most commonly used natural aggregates are sand and crushed stones. The two are used in every construction work and can be interchanged with one another. Crushed stone is found from stones that have been exposed onto the surface of the earth by human activities and ten crushed by humans. Sand is always collected from the sides of rivers and lakes and also used in construction. Classification of aggregates Natural aggregates can be classified as sand, gravel and crushed rocks. All these are used for construction purposes. Natural aggregates can also be classified based on their use in the construction industry. Natural rock from which the aggregates are removed are divided into three main divisions, these are igneous, metamorphic and sedimentary rocks. Most of the time sedimentary rocks are used in the construction sites as sand while the metamorphic rocks are used in the making of concrete. While their use is completely the same, it is important to know that some time all types can be used in one construction. Sampling During the research that was taken in most of the articles that were used as references in this paper, stratified sampling was used to get a natural aggregate and use it for experimental purposes. Households are also used by knowing the type of natural aggregates that were used in the construction of different houses. It is always important to obtain results and use the results in the process of finding the strength of each type of natural aggregate. Shape and texture of natural aggregate Sand has a round shape with a very smooth texture. The color differs with some of it being white while some being brown. However the brown sand is preferred due to its strength. Crushed rocks on the other hand has an irregular shape, this is because during the crushing process little attention is given to the shape of the rocks that are made. The color also differs but most of the color that is used is black while some contain different colors. Mechanical properties strength Natural aggregates are very strong. They have very high stretching and shear strength due to their nature of being sticky. The comprehensive forces range from 40 MPa to 60 MPa. This is the strongest compared to other forms of aggregates that have a lower range than this one. The absorption level of the natural aggregates is also high as compared to other aggregates. This means that they absorb water faster and at the same time dry very fast Black (2009, p.54). Recycled aggregates Over past years waste from construction sites was never used for anything rather it was left to scatter and pollutes the environment while natural aggregates were being used and wasted. In the early 20th century, Britain became the first country to think about the use of the recycled aggregates. The largest volume of recycled aggregates used is blast furnace. In 2006 8.7 million tons of waste concrete valued at around $47 million was used in the construction process. There are lots of reasons why recycled aggregates should be used; one is that it helps in the conservation of the environment. Each time that a recycled aggregate is used, a lot of natural aggregates that would have been used is saved. This translates to saving the environment since a lot of natural aggregates would have resulted in the digging of the earth’s crust to obtain the aggregates. This therefore helps in the conservation of the environment. Second use of recycled aggregates is cheap compared to the use of natural aggregates. Most people do not sell destroyed or used concrete and sometimes gives them out for fee. This is very cheap for someone who wants to construct a house. Types of recycled aggregates Recycled aggregates can be grouped according to the materials that are used in making them. These are the natural aggregates that are used in the making of the waste that is recycled. Recycled aggregates can therefore be classified as sand, crushed rocks, rubber, glass and furnace slag Tepordel (2004, p.657). the two main types of recycled aggregates are glass, and crushed rocks. When using recycled glass, the glass is first crushed and then mixed with the concrete material that is to be used in the construction. The glass is only added to t he mixture after thorough screening. Crushed rocks on the other hand does require similar actions before they are added to the final concrete mixture. It is crushed but this time some waste has to be removed by picking process. This is where the large boulders that are not crushed are removed for more crushing. Once the boulder has been removed, it is then mixed with the other concrete mixture as the aggregate of the concrete. Slag is also another type of recycled aggregate but it is not always used due to its mechanical strength which is not as good as glass and crushed rocks aggregate. Methods of recycling There are many methods of recycling aggregates. One method is the use of waste aggregate in the construction of roads. This is the most commonly used method of recycling, in this method, the used aggregates are taken and then crushed after which water is added to them and then the mixture is shaken. Once it is completely mixed, the mixture is then used in the construction process. Another method is the combination of the used aggregate with natural aggregate. In this type, the used aggregate is combined with the unused or natural aggregate and then used to make the new building that is to be made. Mechanical properties of recycled aggregates As mentioned earlier, experiments have showed that the mechanical strength of recycled aggregates is almost the same as the mechanical strength of natural aggregate. In the mechanical strength, the type of aggregate that is used strongly determines the strength of the recycled aggregate. It has however been showed that crushed rocks aggregates when recycled, can produce aggregates which has a comprehensive strength of about 30MPa to around 50Mpa. This is the range of about 75% to 95% f the strength of the natural aggregates. The strength increases with an increase in the amount of natural pure aggregates in the structure of the aggregate that is used. Therefore the recycled aggregate is the same as natural aggregates with differences coming because of the presence of impurities in the recycled aggregate. Concrete Concrete is a very useful construction material. It is mainly made of cement, aggregate, water and some other chemicals. When mixed with water, concrete solidifies and hardens when it dries. It creates something like a stone which is very hard and has high compressive forces. Fresh concrete is a strong paste when it is still wet but as the reaction continues it dries to form a hard stone like substance that cannot be easily Brocken Tepordel (2004, p.657). Concrete mixes Concrete is obtained by mixing Portland cement, sand and gravel in the ration of 1:2:3. Water is then added to the mixture and each time checking if the mixture has turned into thick paste. Once the mixture has turned into thick paste, it is then turned using a shovel until it starts to form a sticky paste. When mixing the cement and the sand as well as the gravel, remember to use a ratio depending on what you intend to use the concrete for. If you want to use the concrete for making the floor which you want to be smooth, then you have to increase the ratio of the Portland cement that is to be used. Workability and measurement of workability of fresh concrete Stress, slump and plastic viscosity are some of the workability measurements that are done on fresh concrete. It is found that fresh concrete, the shear rate of the fresh concrete is high. The plastic viscosity showed that the concrete is able to withstand high levels of stretching and shear. This is in accordance to ICAR rheometer. Values on the stress level of the fresh concrete shows from previous studies that the fresh concrete is able to withstand high levels of stress. Test During this test, fresh concrete was put inside a truck and its properties looked into. The main properties that were being looked into here are the plastic viscosity, stress factor and slump. The fresh concrete that was in the truck was rotated and measurements made on the above properties to check if the properties were the same as those of the ICAR. From the test, the truck was made to act as the ICAR rheometer. Results from the tests showed that the properties of the concrete relating to its plastic viscosity differ with the measurements from the ICAR rheometer. This is due to the nature of the truck that is used in the measuring process. Hardened concrete Hardened concrete has a lot of factors that determine its strength. One of the factors includes the water cement ratio. The law of strength is basically governed by the water cement ration. The less the water cement ratio the greater the strength. Meaning that for strength the water ratio should be reduced. The type of cementing material that is used also affect the strength of the concrete that results from the mixing process. Amount of cement that is used as well as the type and amount of aggregate that is used is also some of the factors to be considered while making the concrete Water cement ratio Water cement ratio has no constant value but keep changing depending on what type of concrete is needed and the strength that is needed. However it is important to note that the lesser the water ratio, the stronger the concrete will be. If a lot of water is used then the concrete will not be as strong. However to less water is also not good for the mixing of the materials. Therefore enough water to be able to mix the materials is necessary Tepordel (2004, p.657). Influence on properties of natural aggregates in concrete The properties of natural concrete have a lot of influence on the nature and strength of the concrete that is formed. One of the main natures is the type of aggregate that is used, if sand is used, then the strength cannot be compared to the strength of crushed rocks. Crushed rocks are made from hard metamorphic rocks and they have a high resistance to stretch. It is very good and its nature adds the quality and strength of a concrete. Influence of natural properties of recycled aggregates on the strength of concrete The properties of recycled aggregate that has influence on the strength of concrete is the type of the original aggregate that was used to make the recycled aggregate. If the aggregate is recycled from slag then it cannot have the same strength as the aggregate that is from used natural aggregate such as gravel and crushed rocks. Therefore when choosing the type of recycled aggregate to use it is advisable to know what type of material was used to make the used aggregate? Different experiments show that natural aggregate is stronger than recycled aggregate. However the range is not that big. It is approximated that the strength of recycled aggregate is as close up to 95% of natural aggregate. The tension that exists in natural aggregate is high as compared to the tension in recycled aggregate. This is due to the presence of impurities and the presence of elements that result into incomplete reaction between water and the elements in the Portland cement. In terms of comprehensive forces, the natural aggregate is very strong as compared to recycled aggregates Black (2009, p.54). Experiments on the mechanical strength of recycled aggregates Many experiments have been carried out to determine the mechanical strength of recycled aggregates. In an experiment done by George in 2009, construction rubbish was collected and the waste in them removed. After this, water was added to the crushed waste and left to mix up. This was used to make concrete and then the concrete was put in a truck to measure the mechanical properties of the recycled aggregates. The test were also done using natural aggregates and the table below summarizes the whole work. Property of aggregate Natural aggregate Aggregates that are recycled Percentage difference Bulk of the aggregate 2.55 2.09 18 Apparent gravity specific 2.63 2.3 12.5 Absorption 1.56 6.4 4.8 Bulk density 1469.8 1325.93 9.8 Moisture content 1.9 4.46 2.6 After this the split tensile strength was checked and it was found that recycled aggregate had a split tensile strength of about 5N/nm2 this is not far from that of natural aggregate which ranged at 5.6N/nm2. All this experiments shows that recycled aggregates have almost similar mechanical strengths as natural aggregates. Therefore they can also be used to make strong buildings. Bibliography TEPORDEI, V. V. (2004). Natural aggregates, foundation of America's future. [Reston, Va.], U.S. Dept. of the Interior, U.S. Geological Survey. BUILDING RESEARCH ESTABLISHMENT. (2007).Recycled aggregates. Garston, Watford, [Eng.], Building Research Establishment. AMERICAN CONCRETE INSTITUTE. (2009). Concrete international: design & construction. Detroit, MI, The Institute. BLACK, C. A. (. (2005). Methods of soil analysis physical and mineralogical properties, including statistics of measurement and sampling. Madison, American Society of Agronomy Read More