عمار دجلة قيد الدّراسة بهدف التّرقية  معلوماتاضافية
انا من : العراق الجنس :  العمر : 40 عدد المواضيع : 242 عدد المساهمات : 293  |  موضوع: بحث تخرج ( للطالب احمد جلال خليل )ج1  الثلاثاء نوفمبر 09, 2010 12:04 pm | |
| بحث تخرج مقدم من قبلاحمد جلال خليل جدوع وقد حاز على المرتبة الاولى في محافظة نينوى بعنوان ( الخرسانة الخلوية عالية المقاومة ) ز. There are three broad methods of producing lightweight concrete, in the first porous lightweight aggregate of law apparent specific gravity is used instead of ordinary aggregate whose specific gravity is approximately 2.6, the resultant concrete is generally known by the name of the lightweight aggregate used. |
The second method of producing lightweight concrete relies on introducing large voids within the concrete or mortar mass, these voids should be clearly distinguished from the extremely fine voids produced by air entraining, this type of concrete is variously known as aerated, cellular, foamed or gas concrete[ 3 J . |
The third means of obtaining lightweight concrete is by simply omitting the fine aggregate from the mix so that a large number of interstitial voids is present. Coarse aggregate of ordinary weight is generally used[ 3 J . |
Cellular concrete is a cementations paste of neat cement or cement and fine sand with a multitude of micro / microscopic discrete air cells uniformly distributed throughout the mixture to create a lightweight concrete .It is commonly manufactured by two different methods :- |
METHOD A: consists of mixing a pre-formed foam [ surfactant J or mix-foaming agents mixture into the cement and water slurry. As the concrete hardens, the bubbles disintegrate leaving air voids of a similar size [ 4 J . |
In concrete construction, self-weight represents a very large proportion of the total load on the structure, and there are clearly considerable advantages in reducing the density of concrete. The chief of these are the use of smaller sections and the corresponding reduction in the size of foundations. Furthermore, with lighter cC!ncrete the form work need withstand a lower pressure than would be the case with ordinary concrete , and also the total weight of materials to be handled is reduced with a consequent increase in productivity, light weight concrete also ~IJ I gives better thermal insulation than ordinary concrete, the practical range of densities of lightweight concrete is between 300 and 1850 kg/m3[ 1 J ,[ 2 J . |
METHOD B :known as Autoclaved Aerated Concrete [ AAC} consist of mix alime , sand, cement, water and an expansion agent. The bubble is made by adding expansion agents [ aluminum powder or hydrogen peroxide] to the mix during the mixing process, this create a chemical reaction that qenerate gas, either as hydrogen or as oxygen to from a gas bubble structure within the concrete, the material is then formed into molds. Each mold fell into one-half of its depth with the slurry. The gasification process begin and the mixture expands to fell the mold above the top, similar to baking a cake . After the initial setting it is then cured under high-pressure-steam [180 to 210 Co /356 to 410 F] " autoclaved "for a specific amount of time to produce thefinal micro/ macro-structure [ 1 ] , [ 4 ] . |
Recently I a direction to concrete composition prepared by using aqueous gels [ aquagels ] is being considered as all or part of the aggregate in a concrete mix. Aquagel spheres, particles or pieces are formed from gelatinized starch and adding into a matrix. Starch modified or unmodified such as wheat corn, rise, potato or a combination of a modified or unmodified starches are examples of aqueous gels. A modified starch is a starch that has been modified by hydrolysis' or dextrinization . A gas is another material that can create a pore or cell in concrete. During the curing process as an aquagelloses moisture, it shrink and eventually dries up to form a dried bead or particle that is a fraction of the size of the original aquagel in the cell or pore in the concrete. This results a cellular, lightweight concrete. These cells may account for up to 80% of the total volume. Weight of the concrete mixtures range from 220 kilograms per cubic meter [ 14/bs. Cubic foot] to 1922 kilograms per cubic meter [ 120 /bs. Cubic foot] and compressive strengths vary from 0.34 megapascals [ 50 pounds per square inch] to 20: 7 mega pascals [ 3000 pounds per square inch] [1]. |
High Performance cellular concrete |
High- performance cellular concrete [ HPCC] has all the properties of cellular concrete and can achieve 20 - 40 Mpa [ 8000 psi] . Higher strengths can be produced with the addition of supplementary cementations materials. Density and strengths can be controlled to meet specific structural and nonstructural design |
requirements. Where as in conventional cellular concrete these cannot be achieved. |
High-performance concrete is defined as " concrete which meets special performance and uniformity requirements that cannot always be achieved routinely by using only conventional materials and normal mixing, placing and curing practices. II The requirements may involve enhancements of characteristics such as ease of placement and compaction without segregation, long term mechanical properties. density, volume, endurance, stability, or severe or hostile environments [ 5 J . |
Density is the best characteristic feature of cellular concrete. The lowest densities being used for fills and insulation; and the higher densities being used for structural application, leading to a substantial reduction in the dead weight of a structure. 0.028 cubic meter [ one cubic foot J of foam in a matrix replaces 28.30 kilograms [ 62.4 Ibs. J of water, or O. 028 cubic meter [ one solid cubic foot J of aggregate weighting 74.84 kilograms [ 165 Ibs. per cubic foot J . HPCC has excellent insulation properties that significantly reduces the transfer of heat through the concrete member. This bubble is accountable for a superior freezing and thawing resistance and thermal reduce conductivity, low water absorption, high tensile strength, high fire resistance and sound retention, and corrects deficiencies in the sand that causes bleeding . Forming, conveyance, placing and finishing systems for cellular concrete are no different than current methods in the construction industry [ 6 J . |
HPCC also has the advantage of being conducive to mobile and remote projects where building materials are difficult to obtain or reach [ 1]. The advantages derived by the use of superelasticizers include production of concrete having high workability for easy placement and production of high strength concrete with normal workability but with a lower water content. To enhance cellular concrete mineral admixtures were added, many properties of concrete can be favorably influenced I some by physical effects associated with small particles which have generally a fine particle size distribution than Portland cement and others by pozzolanic cementations reaction [ 7 J . |
This project aimed to product cellular concrete by using local materials and simple technology . Industrial sand used (they are a small beads J when soaked in water became larger and absorb water I when dried they loss the water J and leave bubbles inside concrete) . Silica fume and superplasticizer used to enhance compressive strength of concrete. |
The literature review will describe the previous investigations concerning the aim of this project: |
The research [1} classify the density of light weight concrete and show that density is the best characteristic feature of cellular concrete. The lowest densities being used for fills and insulation; and the higher densities being used for structural applications, leading to a substantial reduction in the dead weight of a structure. 0.028 cubic meter of foam in a matrix replaces 28.3 kilograms of water. |
In research[ 4 }, lime, cement .silica and sulfonated hydrocarbons were used to produce foamed concrete masonry blocks for insulation with a dry density between 670 - 850 kg 1m3 and having a compressive strength between |
[ 5 ) show a series of concrete mixes was made using various combinations of lightweight and normal weight aggregates. Properties measured include expansion, indirect tensile strength and compressive strength. Test results indicate no significant difference in performance between concrete mixes with lightweight coarse aggregate combined with either reactive, non reactive, or light weight fine aggregate. |
The research [ 6} use a new method of curing was proposed in the early nintis . The main idea consists in the substitution of part of the aggregates by prewetted light weight aggregates ( LWA). The water stored in the light weight aggregates' is on the disposition of the cement paste. During drying and hardening water from the light weight aggregate is absorbed by the cement paste by capillary suction and condensation .As the hydration proceeds newly built hydration products fill micro cracks, improving the density of the cement paste. In order to show the improved properties of the concrete including partially light weight aggregates macroscopical and microscopical investigations were performed. The I most important results is presented. |
The research! 8 J described the use of near-saturated light weight aggregatas a replacement for a portion of the normal weight aggregate in high performance concrete in order to eliminate the autogenous shrinkage can occur. The work show that the addition of saturated light weight aggregate to the concrete does not appear to have any advense effects on the mechanical properties of the concrete andin most instances J small improvement have been noted. |
The focus! 9 J paper is to show that good quality structural light weight concrete with sound engineering properties and high durability characteristics can be produced for a wide range of infrastructure and global applications utilizing structural light weight aggregates and a total binder content of no more than 350 kq/m", the Portland cement contents of this concretes was limited to no less than 250 kg/m3J but the concrete matrix was supplemented by a small amount of a highly reactive pozzolan such as silica fume, and the balance was made up with fly ash and/or slag. |
10 J indicate that wing expanded polystyrene beads will produce air - voids in concrete J and the result indicate that this cellular concrete have a fair compressive strength with a very good thermal insulation and low thermal expansion in addition to high moisture protection. |
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al_katib معلوماتاضافية
انا من : العراق الجنس : العمر : 33 عدد المواضيع : 31 عدد المساهمات : 408 | | موضوع: رد: بحث تخرج ( للطالب احمد جلال خليل )ج1 الأحد نوفمبر 21, 2010 12:05 pm | |
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