The Special Properties Of Concrete Construction Essay

The Special Properties Of Concrete Construction Essay Admixtures are used to give special properties to fresh or hardened concrete. Admixtures may enhance the durability, workability and characteristics of a given concrete mixture. Admixtures are used to overcome difficult construction situations, such as hot or cold weather placements, pumping requirements. The use of admixtures to increase the workability or to accelerate the hardening of the concrete will be permitted when approved by the Engineer. Admixture dosages shall result in the mixture meeting the specified plastic and hardened properties. The major reasons for using admixtures are 1. To reduce the cost of concrete construction. 2. To achieve certain properties in concrete more effectively than by other means. 3. To maintain the quality of concrete during the stages of mixing, transporting, placing, and curing in adverse weather conditions. 4. To overcome certain emergencies during concreting operations. CHAPTER 2 Set Retarding Admixtures Set retarding admixtures are water soluble chemicals that have little or no other effect than to delay the setting of the cement. They do not plasticize significantly and have little or no effect on the water demand or other properties of the concrete. Set retarding water-reducing admixtures not only delay the setting of the cement but are also efficient in plasticizing concrete or reducing its water demand. Most commercially available retarders are of this type. Retarding water-reducers and retarding high range water reducers are used to: Give workability retention to the concrete Delay the setting time of concrete Prevent the formation of cold joints Increase initial workability Increase ultimate strength Produce economies in mix designs Materials Used The main types of chemical used for retarding admixtures are: Sucrose and other polysaccharides Citric acid Tartaric acid Salts of boric acid Salts of phosphoric, poly-phosphoric and phosphonic acids. The main types of chemical used for retarding water reducing admixtures are: Hydroxy carboxylic acid salts Hydroxylated polymers Lignosulphonic acid salts These may be also be used in conjunction with sulphonated naphthalene/melamine-formaldehyde condensates or polycarboxylates to produce retarding high range water reducing admixtures. The retarder molecule chemically adsorbs onto the cement particle in a mechanism similar to that described for water reducers. The main difference is the strength of the chemical bond that is formed. This strongly links the retarder molecule onto the cement surface, blocking and slowing down the rate of initial water penetration into the cement. Retarder molecules also chelate calcium ions in solution, slowing the crystallization of portlandite. These two mechanisms slow the growth of hydration products, delaying the stiffening and setting of the cement but once initial hydration starts, the retarder molecules are swamped and normal hydration proceeds. Uses 5.1 Admixture Selection Where predictable set retardation is required, a retarding admixture is the best choice. Where set needs to be continually amended, a graph of dosage rate versus setting time can be determined for a given mix, at a given temperature. For phosphate based products, the retardation time graph may often be linear, and this is a major advantage for this type of retarder. Sucrose and other polysaccharides are more efficient, but a dosage versus setting time graph may often be exponential, making accurate prediction more difficult. These types are often blended with lignosulphonate to produce cost effective, retarding/water reducing admixtures. Hydroxy carboxylic acid salts will often reduce cohesion in the mix potentially enhancing bleed and segregation. Polysaccharides, especially if blended with a lignosulphonate, tend to stabilise some air and may enhance cohesion.The choice between the different types of retarding water reducing admixtures is often determined by other properties such as mix cohesion. Selection may therefore be based on the particular mix characteristics of the concrete. 5.2 Dosage Retarding admixtures based on phosphates and phosphonates are designed to have a linear effect of dosage upon setting time. The dosage rate used may be quite high, as they do not have any effect other than retardation. Depending on the molecule chosen, typical dosages are 0.1% to 3.0% by weight of cement to yield a delay of set of 1 hour to 35 hours. Retarding admixtures based on sucrose and other similar polysaccharides are very powerful, and their retarding effects are rarely linear so that small increases on the intended dosage can lead to large increases in retardation. They are very cost effective, but are more difficult to control than the higher dosage phosphate based types. Typical dosages are 0.1% to 1.5% by weight of cement to yield a delay of set of 3 hours to 50 hours. Retarding water reducing admixtures are very commonly used at a low dose, typically 0.2% by weight of cement, to reduce the water content of the concrete by 7 to 10% with only a small delay in setting. They can be used at higher dosages; typically 0.3 to 0.6% to reduce water content, but simultaneously to delay the setting time. Greater water reductions may be achieved by using retarding high range water reducers, generally at 0.5 to 1.5% dose, depending on the type. Retarders are quite sensitive to temperature. At low temperatures retardation will be further extended. At very high temperatures, the converse is true and it may be difficult to achieve the required workability retention and extension of stiffening time. 5.3 Cement type Retarding admixtures and water reducing retarding admixtures can be used with all types of Portland cement, including all those covered by EN197-1. However, it is very important to note that their effectiveness in terms of retardation of set is very dependent upon the type of cement. Cements such as CEM1 require the highest level of retarder to achieve a given level of retardation. However, as the proportion of slag or pulverised fuel ash, as used in CEM 2 and CEM3 types increases, then the level of retarder will decrease in order to achieve a given level of retardation. The chemistry of the cement is also important in determining the effect of retarders. Cements low in tricalcium aluminate (C3A) require significantly less retarder for a given degree of retardation than normal cements. 5.4 Yield Retarding admixtures do not have any significant effect upon the yield of concrete. Retarding water reducing admixtures, when used to reduce the water content of concrete, will reduce the yield in direct proportion to the water reduction made. This needs to be taken into account when modifying the mix design. 5.5 Overdosing The level of retardation achieved is related to the dosage used. Any overdose will result in an increase in setting time. Large overdoses of retarders can produce very long setting times and even small overdoses can have this effect if the initial dose is high. Provided the overdose is no more than double that which was intended, and the concrete is well cured to prevent it from desiccation, accidentally retarded concrete will normally set and recover strength within two to three days. Where a dosage range is given, the normal dose should be taken as the bottom of the range. Where very large, accidental overdoses occur or where large overdoses of a water reducing retarders have been used without a correspondingly large water reduction, the concrete may not recover its strength in a reasonable time. As a general rule, if concrete contains an overdose of a retarding admixture and has not set hard in 5 days, then it may not gain useful mechanical strength within a reasonable time. Effects upon properties of concrete 6.1 Strength As with water reducing admixtures, ultimate strength gain is increased with increasing water reduction Retardation of set allows the slower formation of a more ordered, smaller, denser cementitous matrix. This has the effect of increasing ultimate strength relative to an unretarded mix with the same water cement ratio. Acceleration of strength by heat produces the opposite effect, with the rapid formation of a coarse matrix. This explains why steam cured precast concrete rarely produces the same ultimate strength as concrete cured at normal temperatures and produced from the same concrete. 6.2 Workability Retarding admixtures do not have a significant effect upon initial workability. However, they generally have a beneficial effect upon workability retention, particularly at elevated temperature. Retarding water reducing admixtures, have a pronounced effect upon workability. Typically, an increase in slump of 60-100mm results from the addition of a dosage of 0.25% by weight cement. Set retarding high range water reducing/plasticizing admixtures may be used to enable workability to be increased to a greater extent, at a typical dosage level of 0.3 to 1.0%. 6.3 Slump loss Retarding admixtures are useful for helping to reduce slump loss, particularly at elevated temperature but it is still important to have a high initial workability. Retarding water reducing admixtures are very effective at reducing slump loss when used to increase the initial workability of the mix, but less so when used as a water reducer. Indeed, if water reduction is taken at the expense of high initial workability, initial slump loss may be slightly faster and will slow when about half the initial slump is reached. 6.4 Setting time The prime function of a retarder is to extend the setting (stiffening) time of concrete, usually in order to prevent the formation of cold joints between deliveries of concrete. Even if workability has fallen to almost zero slump, fresh concrete can be vibrated into, and will bond with, a preceding, older pour. In hot weather, even a small delay in deliveries or a short breakdown of the pump can result in the first concrete pours setting before subsequent pours can be placed and vibrated to form a monolithic joint. In deep pours, if concrete placed early starts to set, the heat generated can cause faster setting of concrete above it and again lead to cold joints. In this situation, retarder dosage can be progressively reduced as the pour proceeds. 6.5 Air entrainment Retarding admixtures do not normally entrain air, and some types, especially those based on hydroxycarboxylic acid, may actually reduce air content. This may cause these retarded mixes to feel harsher and have more tendency to bleed. Most types of retarder can be used effectively in combination with an air entraining agent. 6.6 Bleeding The total volume of bleed water arising from concrete is often related to its setting time because once setting starts, bleeding stops. Thus retarded concretes are always more prone to bleed. Any reduction in air tends to aggravate this potential problem. The plasticising component of a retarding water reducing admixture may help to offset this effect and some types are formulated to slightly air entrain in order to reduce bleed. 6.7 Heat of hydration Retarding admixtures do not reduce the heat output of concrete but do serve to delay the time of peak temperature rise by exactly the same time interval by which it was retarded. In small sections this may allow slightly more heat dissipation and so peak temperature may be a little lower. In thick sections there will be no reduction in peak temperature and there is evidence that the peak temperature may even be increased slightly. 6.8 Volume deformation Creep and drying shrinkage are not significantly affected by the inclusion of retarding admixtures. If the concrete is water reduced by the use of a retarding water reducing admixture, then drying shrinkage will be reduced. 6.9 Durability Provided that the concrete is correctly cured, then retarded concrete should be stronger and just as durable as equivalent plain concrete. However, because of the extended plastic stage, more attention needs to be paid to protecting the concrete before it sets. Retarded water reduced concrete will have a lower water content than the equivalent plain concrete, and will be correspondingly more durable. MECHENISM OF RETARDING ADMIXTURES Retarding admixture is an admixture that retards the setting of concrete. A retarding admixture causes cement set retardation by one or more offollowing mechanisms: (1) Adsorption of the retarding compound on the surface of cement particles, forming a protective skin which slows down hydration; (2) Adsorption of the retarding compound on to nuclei of calcium hydroxide, poisoning their growth, which is essential for continued hydration of cement after the end of induction period; (3) Formation of complexes with calcium ions in solution, increasing their solubility and discouraging the formation of the nuclei of calciumhydroxide . (4) Precipitation around cement particles of insoluble derivatives of the retarding compounds formed by reaction with the highly alkaline aqueous solution, forming a protective skin . Detailed Explanation According to the first mechanism, a retarding admixture is adsorbed on the surface of cement particles. This layer of retarding admixture around the cement particles acts as a diffusion barrier. Due to this diffusion barrier, it becomes difficult for the water molecules to reach the surface of the unhydrated cement grains and hence the hydration slows down, and the dormant period (period of relatively inactivity) is lengthened. Due to the slow hydration, no considerable amount of the hydration products giving rigidity to the cement paste will be formed and thus the paste remains plastic for a longer time. Later, when the admixture is removed from solution by reaction with C3A from cement or by some other way it is removed and incorporated into the hydrated material, further hydration is eliminated. On first contact of water with cement grains (C3S and C2S) calcium ions and hydroxyl ions are rapidly released from the surface of the cement grains. When concentration of these ions reach es a critical value (at which the solution becomes saturated), the hydration products calcium hydroxide and calcium silicate hydrate start to crystallize from the solution and then hydration proceeds rapidly. According to the second mechanism, a retarding admixture incorporated into cement paste is adsorbed on the calcium hydroxide nuclei and prevents its growth until some level of super saturation is reached during the induction period of hydration. Thus, retarder lengthens the induction period by causing an increase in the level of calcium hydroxide super saturation before crystallization begins. This is analogous to the poisoning of crystal growth of calcium hydroxide by the retarding admixture as both calcium and hydroxyl ions are present in the solution but unable to precipitate as a result of poisoning of the calcium hydroxide nuclei. According to the third mechanism, a retarding admixture incorporated into cement paste forms some kind of complexes with calcium ions released by the cement grains during the first few minutes. Formation of the complexes increase the solubility of cement, i.e., increased concentration of Ca2+, OH, Si, Al and Fe in the aqueous phase of the cement pastes will occur when hydrated in the presence of the retarding admixture. Thus the calcium ions and hydroxyl ions will accumulate in solution and will be unable to precipitate to form calcium hydroxide. For example, when ordinary Portland cement is hydrated in sucrose solution, lime is solubilised and a sucrose calcium complex (R -O à ¢Ãƒ ¢Ã¢â‚¬Å¡Ã‚ ¬Ca+ -OH) is formed in which Ca+ -OH group is attached to the five membered ring (R) of the sucrose molecule. Such sucroseà ¢Ãƒ ¢Ã¢â‚¬Å¡Ã‚ ¬calcium complex will be able to become absorbed on the growing calcium hydroxide nucleus. The adsorption of the complex on the calcium hydroxide nucleu s will inhibit its growth as the calcium and hydroxyl ions will not be able to precipitate. In this way, hydration is retarded. The fourth mechanism is similar to the first but here some kind of insoluble derivatives of retarder are formed by reaction with the highly alkaline solution as pH of the solution rises to over 12 within few minutes after first contact of water with cement. For example, inorganic salt admixtures (borates, phosphates, zinc and lead salts etc.) give insoluble hydroxides in alkaline solution. The cement hydration is suppressed through the precipitation of protective coatings of these insoluble derivatives around the cement grains.

Critical Review of Carn Essay -- Essays Papers

Critical Review of Carn The novel Carn, by Patrick McCabe, is a thought-provoking tale of people from a town in Ireland. The town, Carn, goes through economic failure, complete industrialization and commercial revival, back to total desolation. As the town changes, so do the main characters, Josie Keenan and Sadie Rooney. Although they do not know each other at the beginning of the novel, after the indulstrialization of the town, their lives eventually intersect. All they want from life is to lead normal lives -- outside of Carn. Even though Carn is now an industrial town, it holds bad memories and a sense of imprisonment for Josie and Sadie. Both their lives become tied to the town of Carn. Sadie plans to move to England, but when she becomes pregnant she must stay in Carn and raise a family. A lack of options forces Josie to remain in Carn, the only home she knows. Eventually, Josie's destructive lifestyle and the political conflicts between England and Ireland result in tragedy for both characters. McCabe does an excellent job at developing the characters of Josie, Sadie, and the town of Carn itself. He shows the futility of their hopes, which ultimately results in tragedy and despair. The reader can relate to the characters, and by the conclusion of the novel, the reader will feel as if s/he knows the characters personally. Josie Keenan lives a life without hope. The author does a good job of providing insight into Josie's life by informing the reader of everything that effects Josie from the beginning of her life, right until the end. Living with an abusive father has made Josie believe that no one is good, and everyone is only looking for what they can gain from others. The only kind words she has ever received are from her mother. Even this source of happiness is taken from her though, because her mother dies when Josie is young. She moves from an orphanage right into the working world, and into a world of men. Because she does not have a strong father figure in her life, she looks for love else where. Men love her body, and she loves the control this gives her. "Josie [sees] now that there [is] nothing she [can't] do with [men] (49)." She takes men's money and does with it as she pleases. She "[takes] the bus to a town across the border where she [sits] on her own in a cafe listening to a jukebox and eating ice-creams (... ...e able to give an inanimate object its own character, and to do it well. Carn is an excellent example of a well-developed character. McCabe's views of life are demonstrated by the lives of Sadie and Josie, and the town of Carn. McCabe's characters' lives are replete with suffering and sorrow, and they all react with bitter resignation. Sadie, Josie, and Carn are resigned to accept their fates - Sadie's, involuntary life in Carn; Josie's, debauchery and death; and Carn's, economic desolation. Carn is very well written and is a wonderful book. However, not everyone will enjoy it. There is a considerable amount of history involved in it, so people who enjoy history will benefit from it. It is also very depressing, so readers who love happy endings will be disappointed with the conclusion of the story. For these reasons I did not enjoy reading this book, however I do understand the literary merit that is displayed by McCabe. The story line and characters are well developed, and it is an interesting story. I recommend it to anyone who is learning about the conflicts in Ireland, or to anyone who likes to get to know fictional characters in a personal way.

Good And Bad Manners Essay

Good Manners have a deep Affect on our minds. As bad manners produce bad results in society, good manners produce good results. Good manners are unconsciously acquired in our childhood, but they can also be cultivated later on. Every human being should be treated with due respect and dignity. The animal instinct in us makes us rough and vulgar, but a cultured and civilized man is gentle and polite. By good manners we understand the way in which a person should behave in society. One is expected to show respect one’s elders. Good manners are necessary for everyone, in every walk of life. It is good manners to thank anybooy who does anything for you. When a waiter brings you a glass of water, you may thank him. â€Å"Friends and good manners will carry you where money won’t go.† Good manners are vital for success in life. Nobody likes a rude person, but good manners endear a man to other. A shopkeeper who behaves rudely with his customers can never flourish in his trade, but one who behaves politely and pleasantly with others attracts a large number of customers. Patients always rush to a doctor who is sympathetic towards them. A leader cannot have a large follower unless he is polite to others. In fact good manners have a magnetic influence over the people. Good manner are not, however, born with us. They have to be cultivated with great care. Childhood is the best period to cultivate good manners and so both parents and teachers have a great role to play. Bad Manners: The Height of Bad Manners is Way too Much To begin with, Bad Manners is defined by lack of polite or well-bred social behavior resulting by ignorance, this may be true, but I don ´t think so, this is because nowadays people are adopting this reprehensive conducts no just by ignorance, but for doing whatever they want, in this bad manner conducts we can see: not respecting adults, farting in front of anybody, not saluting, people that don ´t respect the third age privileges in public places/transport. In the world we can see many of this awful behavior but each day we can see it more in our community its becoming part of our daily life to be disrespectful with others and don ´t minding about if the other  person would feel bad about it. As your parents probably told you not so many years ago this â€Å"Bad Manners Acts† where severally punished with very strict sanction but now we can see a teenager threatening and old man so they give him the spot in the bus.

The Case Of The United States Constitution - 1354 Words

When relating a historical document to a contemporary situation, context is integral to proper analyzation. In the case of the United States Constitution, many political theorists share their opinion, on what they believe to be, the optimal interpretation of the document, however, only Antonin Scalia’s originalist ideology repels personal and moral views during analysis and encourages understanding the Constitution as originally intended. It is not the job of a United States judge, nor should it be the job of one, to intervene their personal beliefs and morals with the law when making a judicial decision. Instilling inappropriate factors in legal decisions causes potential inconsistency and, occasionally, bias, both of which would set†¦show more content†¦Heller allow for the examination of originalism being successfully implemented during supreme court trials. A judge remaining impartial is crucial towards decisions made by a court. Although it may not be apparent, n on-originalists tend not to be as truthful when enforcing the law than originalists since they often involve outside morals or beliefs that do not pertain to the document itself. According to Antonin Scalia, â€Å"Non-originalist opinions have almost always had the decency to lie, or at least to dissemble, about what they were doing--either ignoring strong evidence of original intent that contradicted the minimal recited evidence of an original intent congenial to the court’s desires, or else not discussing the original intent at all, speaking in terms of broad constitutional generalities with no pretense of historical support.† The acquisition of a bias can sway the court’s decision towards an incorrect stance, thus, making it very important that non-originalist views aren’t involved in Constitutional readings. Consistency in court ruling is also essential to a constitutional theory. Capital punishment, although currently controversial, was a frequent co nsequence of murder in the late 1700s. It is indirectly mentioned twice in the Constitution. First, it is mentioned in the 8th amendment of article 7: â€Å"...nor cruel and unusual punishments inflicted†. Secondly, it is mentioned in the 14th amendment of article 7:†...nor shall any State deprive any person