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ФАКУЛЬТЕТ ДИСТАНЦИОННОГО ОБУЧЕНИЯ
СПЕЦИАЛЬНОСТИ ПГС, ГСХ.
ТЕКСТЫ ДЛЯ ЧТЕНИЯ.
I Курс II Семестр.
From the history of concrete
( 3500 печ. зн. )
Mass or plain concrete dates from very early days. It was employed by the Egyptians, Romans and Greeks in the construction of aqueducts and bridges, in the construction of roads and town walls. Romans used it even in under-water structures some of which have survived till our time. A large part of the Great Chinese Wall (the 3d century B.C.) was also built of concrete.
The concrete remains of the foundations of buildings built several thousand years ago have been found in Mexico. As cement was not known at those times, concrete was made of clay and later of gypsum and lime. Nowadays concrete is made in up-to-date machinery with very careful regulation of the proportion of the mix.
The idea of strengthening concrete by a network of small iron rods was developed in the 19th century, and reinforced concrete was introduced into engineering practice.
Concrete (I)
It is difficult to imagine modern structure without concrete. Concrete is the very building material which led to great structural innovations. The most important quality of concrete is its property to be formed into large and strong monolithic units. The basic materials for making concrete are cement, aggregate and water. Cement is the most essential material and the most important one for making concrete of high quality. Cement is made of limestone and clay. It is burnt at high temperature and ground up into powder. Depending on the kind and composition of the raw materials different types of cement are obtained. Portland cement, blast furnace cement are suitable for putting up marine structures.
Concrete is made by mixing cement, water, sand and gravel in the right amount. As soon as it is thoroughly mixed it is poured into forms that hold it in place until it hardens. The crystals forming in the process of making concrete stick together in a very hard artificial stone. Cement starts hardening one hour after the water has been added and the process of hardening lasts for about 28 days. The process is called concrete curing.
The characteristics of concrete depend on the quality of the materials used, grading of the aggregates, proportioning and amount of water. The most important requirements for concrete are: it should be hard, strong, durable, fire-resistant and economical. Concrete can be divided into two classes: mass or plain concrete and reinforced concrete where it is necessary to produce steel. Plain or mass concrete can be used for almost all building purposes. Ferro-concrete is used in building bridges and arches, dams and dock walls, for structures under water, for foundations, columns, girders, beams. The use of concrete and ferro-concrete is almost universal.
Builders now produce two types of new building materials: alkali-slag concrete and silica concrete. In alkali-slag concrete cement is replaced by a mixture of granulated blast-furnace slag and sodium and potassium compounds. The fillers can be sand or sandy loams containing various amounts of clay which usually cannot be used with conventional cement. The new material has been tested successfully and is now being used for irrigation systems, roads, pavements and other structures. Silica concrete is widely used in aviation and in underwater constructions.
Concrete (II)
The term “concrete” is used to describe a dense material composed of cement and aggregate mixed with water. The density of such a material and therefore many of its properties depend upon the density of the aggregate. Therefore there is a broad division of concrete types into:
The aggregates are graded in size from fine to coarse in order to reduce the amount of void spaces to be filled by cement.
There are “cellular” concretes made by using materials which foam or form gas during the mixing of the concrete. These give a product of very light weight because after setting it contains a large number of small voids.
The reduction in weight is accompanied by a considerable decrease in strength. Another type of light-weight concrete is made by “entraining”
air bubbles in the mix to which a substance has been added to keep the bubbles stable during setting.
Gas Concrete
( 1500 печ. зн. )
Lime and silica are ground together to very fine limits. The silicious material can vary considerably in its composition. Much use is made of such waste materials as fly ash from power-stations, blast furnace slag, as well as natural pozzolanas, pumice, etc. The degree of foaming in the gas concrete and thus its specific gravity is determined by the amount of aluminium powder or other agent added. The practical limits of the final density are between 13 and 90 lb. per cu. ft. If the gas concrete is allowed to harden on its own, it usually takes about three weeks before the final strength is achieved. It is more customary to accelerate the setting of the gas concrete by steam hardening it in autoclaves with superheated steam at about 140 lb. per sq. in. The steam hardening process takes about 15-20 hr. Air-cured gas concrete can be used for the manufacture of special components for the refrigeration industry. Such blocks are cast to special dimensions.
Gas concrete can be cast horizontally to form room sized outer wall units.
It is possible to incorporate electric conduit pipes, piping for the cold and hot water systems and also drainage pipes. The units usually include windows and doors and are reinforced by embedding steel mesh in the mix.
Gas concrete can be used as thermally insulating floor screeds or as an additional thermally insulating layer on top of a concrete roof.
Cast gas concrete is often used as the thermally insulating layer in “sandwich” wall units.
Gas concrete is often used as a thermally insulating layer when casting buildings by a continuous technique.
Reinforced Concrete
( 1800 печ. зн. )
Reinforced concrete is a combination of two of the strongest structural materials, concrete and steel.
This term is applied to a construction in which steel bars or heavy steel mesh are properly embedded in concrete. The steel is put in position and concrete is poured around and over it, and then tamped in place so that the steel is completely embedded. When the concrete hardens and sets, the resulting material gains great strength. This new structural concrete came into practical application at the turn of the 19th century. The first results of the tests of the reinforced concrete beams were published in 1887. Since that time the development of reinforced concrete work has made great progress. And the reasons of this progress are quite evident. Concrete has poor elastic and tensional properties, but it is rigid, strong in compression, durable under water and above ground and in the presence or absence of air and water, it increases its strength with age, it is fireproof.
Steel has great tensional, compressive and elastic properties, but it is not durable being exposed to moisture, it loses its strength with age or being subjected to high temperature. So what is the effect of the addition of steel reinforcement to concrete?
Steel does not undergo shrinkage or drying but concrete does and therefore the steel acts as a restraining medium in a reinforced concrete member. Shrinkage causes tensile stresses in the concrete which are balanced by compressive stresses in the steel. For getting the best from reinforced concrete the following considerations should be kept in mind:
Steel constructions with reinforced concrete have become the most important building materials invented in centuries and they have given modern architecture its peculiar features.
Reinforced Concrete (II)
( 2200 печ. зн. )
Plain concrete was used in ancient times by the Egyptians and the Romans and probably by the Mayas in Central America. Sewers, roads, aqueducts, water mains and foundations were constructed of mass concrete by the Romans who also employed it as a filling between the brick and stone ribs of their vaults and arches. The knowledge of the use of natural cements and, consequently, of concrete seems to have been lost during the Middle Ages, and it was not until the 18th century that its value was rediscovered.
The reinforcing of concrete was first introduced in France in 1861 by Joseph Monier who constructed flower pots, tubs and tanks and Francois Coignet who published theories of reinforcing for beams, arches and large pipes. Very little was actually accomplished in building construction until 25 years later when German and Austrian engineers developed formulas for design and Hennebique in France began the use of bent-up bars and stirrups. Between 1880 and 1890 several reinforced concrete buildings were erected in the USA and since 1896 the increase in the amount of construction with this material has been remarkable.
Until recent years there was a tendency among architects to consider reinforced concrete as a method of construction suited only to heavy and massive structures, to foundations, bridges, dams, factories, warehouses and industrial buildings. This feeling was perhaps due to the apparent bulkiness of the material and to the fact that the wooden forms for plain flat surfaces, beams and columns cast less than for curves, arches and domes. The characteristics of the architecture were limited by the economical restrictions of the centering. Much study and experiment have, however, led to vast improvements in the manufacture of the concrete, in the efficiency and the simplicity of formwork and in the development of plastic molds and of self-centering reinforcement such as ribbed fabrics. Indeed, at the present time unlimited possibilities in flexibility, slenderness and aesthetic qualities of design appear to be in the hands of the creators of concrete buildings. The capacity of reinforced concrete is, in the opinion of many architects, not yet realized. The potentialities of a substance which can be poured into any form or shape from delicate ornament to huge cantilevers and parabolic arches and which is monolithic throughout its mass should indeed inspire methods of expression distinctive of its structure and quite different to those called forth by the disjointed elements of steel, wood, brick and stone.
General Properties of Cement
( 1000 печ. зн. )
All types of cement shrink during setting. In a normal concrete the amount of this shrinkage will depend both on the proportion of cement in the mix and the quantity of mixing water employed. Provided enough water is present to enable the chemical action of setting to take place, then the smaller the amount of water the less shrinkage there will be. The type of aggregate used has an appreciable effect upon both the amount of water and the amount of aggregate that can be mixed with given quantity of cement. Strength and durability of concrete are linked properties in that they are both associated with the low water-cement ration. In addition to the proportion of cement and the water cement ratio of a cement product, the method of curing will also affect the amount of shrinkage. Normally, the slower the drying the less shrinkage there will be. All cement products are liable to a considerable shrinkage during setting and hardening.
Strength
The important thing is the strength of the final cement product rather than the strength of the cement itself. The strength of the cement, however, gives some indication of the possible variation in the former, although the strength of the product will also depend upon the type and grading of the aggregate used the proportion of aggregate and other factors such as water cement ratio and quality of workmanship.
ФАКУЛЬТЕТ ДИСТАНЦИОННОГО ОБУЧЕНИЯ
СПЕЦИАЛЬНОСТИ ПГС, ГСХ.
ТЕКСТЫ ДЛЯ ЧТЕНИЯ.
II Курс III Семестр
Classification of Building Mortars
( 1500 печ. зн. )
Building mortar is the name of a mixture containing a binding agent, water and fine aggregate acquiring a stone-like monolithic structure as a result of hardening. Prior to hardening a building mortar is called a building mix. Building mortars often contain different additions – dispersed ( clay ), hydraulic and surface-active.
By composition and properties building mortars are similar to concrete, but they contain no coarse aggregates.
Building mortars are intended for filling joints and as a binder in free-stone and brick masonry, for the preparation of decorative and protective plasters and production of small-size articles ( brick, tile, etc. ).
Building mortars are made with different properties and composition depending on application.
In respect to binding agents and additions there are cement, lime, lime-cement and cement-clay mortars.
In respect to the properties of the binding agent mortars are divided into air-setting, incorporating air-setting binding agents and hydraulic mortars made with hydraulic agents.
In respect to aggregates mortars are classified as heavy, incorporating ordinary sand and light, having porous aggregates ( pumice, sands, etc. ).
By composition mortars are classified as: simple including one binding agent ( cement, lime, etc. ) and combined including two or three binding agents ( cement-lime, lime-gypsum, etc. ).
Air-setting building mortars are used in structures serving in dry environments and hydraulic mortars – in moist environments.
Placing and Curing of Concrete and Quality Checking
( 1500 печ. зн. )
The placing of the concrete mix and its distribution in the form or mould is one of the most labour-consuming operations of concreting.
At present placing and distribution of concrete are mechanized and the operations are carried out with the aid of concrete placers or machines of a simpler construction-concrete distributors. Concrete placers differ from concrete distributors in that they permit the processes of both placing concrete and its distribution to be mechanized in a great measure.
The quality of placing concrete is a very important factor in building durable concrete or reinforced-concrete structures.
The concrete mix must be placed in the form in a manner that no air be entrained in the mass; corners and restrictions in the form must be filled with most care. Placement and leveling of the concrete mix are followed by compacting.
The methods of compacting concrete manually by rodding or with the aid of tampers are almost obsolete now.
Mechanized placing and compacting of the concrete mix by vibrating, vibrostamping, centrifuging, vacuum treatment, rolling and vibrolling are widely practiced.
Vibration consists in uninterrupted positive shaking of the concrete mix by imparting frequent vibratory motion to the entire mass to ensure good compacting.
Vibrostamping. In this method of compacting the treated concrete mass is subjected to the simultaneous action of the oscillatory motion of the vibrator and the load exerted by the stamp, i.e. the method ensures vibration under pressure permitting the outlines of the stamp or dye to be reproduced on the surface of the product being treated.
Brief Information of Reinforced Concrete
( 1300 печ. зн. )
Reinforced concrete is a building material in which the joint functions of concrete and steel are advantageously utilized.
The idea of combining these two materials extremely differing in mechanical properties in one monolith departs from the following premise. Like any other stone material concrete offers a good resistance to compressive loads but it is brittle and poorly withstands, therefore, tensile stresses. The tensile strength of concrete is about 10-15 times inferior to compressive strength. As a result of such anisotropy of mechanical properties concrete cannot be used in structures to be subjected to tensile stresses under load. But if steel possessing a high tensile strength is introduced into concrete, the steel will take over the tensile stresses appearing in the loaded reinforced-concrete element.
It is the most advantageous to employ reinforced concrete in structural elements subjected to bending. In service two oppositely directed stresses appear in such elements – tensile and compressive. In this case the steel reinforcement takes over the first and concrete – the second kind of stress and the entire reinforced-concrete element successfully withstands bending loads.
The figure ( рисунок ) illustrates the stresses appearing in a beam resting on two supports and subjected to a bending load.
It can be seen in the figure that the upper section of the beam arranged above the neutral layer is compressed, as shown by the reduction in its dimensions and the lower zone of the beam is stretched.
Special Properties of Concrete
( 1500 печ. зн. )
Concrete is a porous material. Pores may be formed in concrete due to incomplete evacuation of entrained air in the course of compacting the concrete mix. It is impossible to produce absolutely dense concrete even by practicing dense placement of the concrete mix by vacuum treatment and repeated vibrating. Pores are formed in concrete also as a result of evaporation of water which fails to react with the cement constitution in the course of hardening.
The density of concrete can be increased not only by vacuum treatment, repeated vibrating or by reducing the content of evaporating water which fails to react with cement.
The placement of concrete of a high density can be ensured by the following means:
Admixtures for Concrete
( 1400 печ. зн. )
Concrete can sometimes be improved by an admixture added to the cement, aggregates and water to modify one or more of the properties of the mix. Admixtures are not magic powders that can be added indiscriminately to poor concrete mixes to make good concrete. Neither can it be assumed that they will necessarily make good concrete better. The right admixture for the job must be used if the admixture is to do more good than harm. When a change is made to improve one property of concrete, some other properties will be affected, frequently adversely.
Principal admixtures are: air-entraining agents and water-reducing admixtures. Perhaps the most widely used admixtures are air-entraining agents. Air-entrainment is used to improve the resistance of concrete to damage from freezing and thawing. It also makes concrete slabs much more resistant to scaling where salts are used for deicing. It makes the mix more workable or at least more cohesive. It permits a substantial reduction in the water requirement and consequently the cement content in mass concrete and has helped with the temperature problem by reducing the amount of heat generated during setting of the cement. Air entrainment is generally considered to be the greatest advance in concrete technology in recent years.
Water-Reducing Admixtures
Use of water-reducing admixtures has expanded rapidly in the past few years. The name comes from the ability of these additives to reduce the mixing water required. Also they generally increase strength and they may take it possible to meet a strength requirement that could not otherwise be met with the cement and aggregate at hand.
Corrosion of Concretes
( 3600 печ. зн.)
Many times we know, that the concretes with a various course of corrosion are otherwise the same. They are produced from the same concrete, on the same producing equipment, by the same people, the concretes had the same treatment, static loadings and they have ( from the statistical point of view ) also the same physical/ mechanical parameters ( of strength ). Obviously it is valid, that we do not dimension the concretes correctly until now against corrosion.
The corrosion loading is not given in the concrete structures first of all by the influences of the environment and is not characterized by the present static solution. The environment is a continually changing resultant of the cumulated effect of various variables. Among them certainly belong the changing temperature and the temperature gradients, aggressive materials and further elements.
We are of the opinion, that the problems of the surface corrosion are rooted in deeper connections. We do not know, if it is possible to use various simplifications of the entrance parameters for further treatment, we do not know the laws of the parallel effect of various influences and decisively it is not possible to consider the concrete structures up to the age of 28 days.
Hydration of Concretes
Concrete is a building material which originates by mixing, laying, compacting and treating the mix containing the cement (binders), aggregates, water, ingredients and admixtures. Under certain conditions this mix obtains in time mechanical properties. The set of all chemical, physical and mechanical reactions which are effective at the same time and whose resultant is permeated, is called hydration. There exist a long term efforts to penetrate deep into the knowledge of hydration processes. There are known many theories, many a partial knowledge. It is not the purpose of this paper to give their detailed summary. However, it is possible to generalize that the prevailing part of explanations supposes that the dominant factor is that of the reaction cementing materials and water. This reaction lies in progressive dissolution of the cementing materials ( we do not mean cement only, but possible active aggregates or mixes of various cementing materials, etc. ) and through the complicated process of heterogeneously forming gel and metastabile crystalloid structure.
Considerably differences exist in the opinions about the fact, what sort of gel and metastabile crystalloid stages are continuously created, about their kinetics of origin and perishing, about what may be considered as a final product of hydration and about the period of duration of hydration processes. In this respect most easily are accepted the opinions being possible much general enabling wide interpretation.
Energetic Model of Hydration
Energetic model of hydration is based on the condition that the sufficiently general description including both physical and mechanical acts must be put on the base being common for everybody. One of the possibilities is that of consumption of energies ( work ).
On the base of the laws of quantum physics it is valid that no act is passing in nature unless there is given a reason for it. The reason for it is the imbalance of energies. Energy is meant in a complex way, not only as an outer impulse, but first of all by the so called balanced state. If the environment or its energetic stability are changed, there arises also a newly defined balanced state of the internal energy. The sensitivity of these changes is not continuous, it moves in very small jump values, the so called quanta.
According to these principles the resulting product of hydration may be reached in different ways. The resulting state may theoretically be, but really already need not be identical. E.g. by a chemical reaction there originates a new compound, by the physical reaction the body is cooled or rewarmed and by the mechanical reaction there appears just a small crack.
As far as this process is applied on the hydration processes, then let us have for the entrance condition for the system:
material ( cement ) and water;
created a new stage, i.e. crystalloid or a gel one.
СЛОВАРЬ СТРОИТЕЛЬНЫХ ТЕРМИНОВ
I Семестр
1. Building Materials
structural – строительный, структурный
hard – твердый, тяжелый
durable – прочный, долговечный
fire-resistant – огнеупорный
to fasten – скреплять, соединять
steel – сталь
concrete – бетон
stone – камень
wood – древесина
brick – кирпич
hardness – твердость, тяжесть
durability – прочность, долговечность
fire-resistance – огнеупорность
property – свойство
strength – прочность, крепость
porosity – пористость
artificial – искусственный
cement – цемент
sand – песок
crushed stone – щебень
to combine – соединять, сочетать
to require – требовать
timber – пиломатериал
binding materials – связывающие материалы
lime – известь
secondary (auxiliary) materials – вспомогательные материалы
bearing structures – несущие конструкции
clay – глина
2. Timber
to burn – гореть
to decay – гнить, разлагаться
softwood – мягкая древесина
hardwood – твердая древесина
construction – строительство
frame – рама, ферма
to contain – содержать
to increase – повышать, увеличивать
elimination – исключение, устранение
3.Stone
scarcity – нехватка, дефицит
timber – лесоматериал
masonry – кладка
basement – подвал
base course – слой основания
column – колонна
step – ступенька
sandstone – песчаник
grain – зерно
to depend upon – зависеть от
to select – отбирать
concrete aggregate – заполнитель бетона
marble – мрамор
4.Metals and concrete
ferrous metals – черные металлы
non-ferrous metals – цветные металлы
iron – железо
steel – сталь
alloy – сплав
to possess – обладать
metallic luster – металлический блеск
to forge – ковать
to pull – тянуть, растягивать
mercury – ртуть
to melt – плавить
cast iron – чугун
cheap – дешевый
to design – проектировать
steelwork – стальная конструкция
to consider – учитывать
steel frame – стальная ферма
load – нагрузка
reinforcement – усиление, армирование
metal structure – металлическая конструкция
5.Designing of Concrete Buildings
load-bearing wall – несущая стена
girder – балка
beam – балка, брус
floor – пол
roof – крыша
to rest on – опираться на
to utilize – использовать
exterior – внешний
interior – внутренний
6. Сement: man’s miracle mix
a lot of – множество, много
Portland cement – Портланд цемент
to mix – смешивать
clinker – клинкер
rock – горная порода, природный камень
pre-stressed concrete – предварительно напряженный бетон
to bend – сгибаться
to break – ломаться
to harden – затвердевать
hollow – полый
bubble – пузырь
to foam – пениться
to crack – трескаться
to wet – увлажнять
to cover – покрывать
research – исследование
7. Plastics
basic – основной
application – применение
to manufacture – производить
substitute – заменитель
weight – вес
shape – форма
to absorb – поглощать
design – проект
structure – сооружение, структура
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