МУ англ ПГС

upper end of a rotary kiln heated to 2600 to 3000F by burning oil, gas, or powdered coal. Because cement production is an energy-intensive process, reheaters and the use of alternative fuel sources, such as old tires, are used to reduce the fuel cost. (Burning tires provide heat to produce the clinker and the steel belts provide the iron constituent.) Exposure to the elevated temperature chemically fuses the raw materials together into hard nodules called cement clinker. After cooling, the clinker is passed through a ball mill and ground to a fineness where essentially all of it will pass a No. 200 sieve (75 m). During the grinding, gypsum is added in small amounts to control the temperature and regulate the cement setting time. The material that exits the ball mill is portland cement. It is normally sold in bags containing 94 lb of cement. Concrete, the most common use for Portland cement, is a complex material consisting of Portland cement, aggregates, water, and possibly chemical and mineral admixtures. Only rarely is Portland cement used alone, such as for a cement slurry for filling well holes or for a fine grout. Therefore, it is important to examine the relationship between the various Portland cement properties and their potential effect upon the finished concrete. Portland cement concrete is generally selected for structural use because of its strength and durability. Strength is easily measured and can be used as a general directly proportional indicator of overall durability. Specific durability cannot be easily measured but can be specified by controlling the cement chemistry and aggregate properties.

ALUMINOUS CEMENTS

These are prepared by fusing a mixture of aluminous and calcareous materials (usually bauxite and limestone) and grinding the resultant product to a fine powder. These cements are characterized by their rapid-hardening properties and the high strength developed at early ages. Table 4.3 shows the relative strengths of 4-in cubes of 1:2:4 concrete made with normal Portland, high-early-strength Portland, and aluminous cements. Since a large amount of heat is liberated with rapidly by aluminous cement during hydration, care must be taken not to use the cement in places where this heat cannot be dissipated. It is usually not desirable to place aluminous-cement concretes in lifts of over 12 in; otherwise the temperature rise may cause serious weakening of the concrete.

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Aluminous cements are much more resistant to the action of sulfate waters than are Portland cements. They also appear to be much more resistant to attack by water containing aggressive carbon dioxide or weak mineral acids than the silicate cements. Their principal use is in concretes where advantage may be taken of their very high early strength or of their sulfate resistance, and where the extra cost of the cement is not an important factor. Another use of aluminous cements is in combination with firebrick to make refractory concrete. As temperatures are increased, dehydration of the hydration products occurs. Ultimately, these compounds create a ceramic bond with the aggregates.

NATURAL CEMENTS

Natural cements are formed by calcining a naturally occurring mixture of calcareous and argillaceous substances at a temperature below that at which sintering takes place. The ‘‘Specification for Natural Cement’’ requires that the temperature be no higher than necessary to drive off the carbonic acid gas. Since natural cements are derived from naturally occurring materials and no particular effort is made to adjust the composition, both the composition and properties vary rather widely. Some natural cement may be almost the equivalent of Portland cement in properties; others are much weaker. Natural cements are principally used in masonry mortars and as an admixture in Portland-cement concretes.

LIMES

These are made principally of calcium oxide (CaO), occurring naturally in limestone, marble, chalk, coral, and shell. For building purposes, they are used chiefly in mortars.

Hydraulic Limes

These are made by calcining a limestone containing silica and alumina to a temperature short of incipient fusion so as to form sufficient free lime to permit hydration and at the same time leave unhydrated sufficient calcium silicates to give the dry powder its hydraulic properties.

Because of the low silicate and high lime contents, hydraulic limes are relatively weak. They find their principal use in masonry mortars. A hydraulic lime with more than 10% silica will set under water.

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1. Put questions the words given on bold type

1. The water used for mixing cements, limes and plasters must be reasonably clean.2. Pond, river and canal waters often contain vegetable and other organic impurities and should not be used without examination. 3. Water fit for drinking can be assumed to be free from harmful ingredients. 4. Only sufficient water should be used to enable the hydration to take place and to make the mixture easily workable; any access is detrimental to the ultimate strength of the concrete.

2. Ask and answer the questions about building materials used:

The sand should be clean and free from clay and vegetable matter because when it is mixed with water and cement a chemical action takes place. Therefore, if impurities are present, the binding or adhesion is affected.

3. Give a written translation of the following:

1.Портландцемент в основном употребляется для изготовления наземных и подземных бетонных и железобетонных конструкций. 2. Бетон применяется в строительстве с глубокой древности. 3. В качестве вяжущих в древности использовали известь, глину, гипс и асфальт. 4. Егор Челиев впервые приготовил цемент в начале XIX в. 5. Изготовление цементов в Англии и Германии началось на несколько лет позже.

4. Insert the needed words and groups of words:

Portland cement is a … product. It is made of …, … or … . They are … and … with water to form a paste. The mixture is then … in a kiln. The clinker is ground to … .

5. Discussion: Your ideas about the material.

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UNIT 4

TEXT A. MORTARS

Mortars are composed of a cementitious material, fine aggregate, sand, and water. They are used for bedding unit masonry, for plasters and stuccoes, and with the addition of coarse aggregate, for concrete. Here consideration is given primarily to those mortars used for unit masonry and plasters. Properties of mortars vary greatly, being dependent on the properties of the cementitious material used, ratio of cementitious material to sand, characteristics and grading of the sand, and ratio of water to solids.

Mortar is the matrix used in the beds and side joints of brickwork and for plastering walls and floors. Its functions are as follows:

1)to distribute the pressure throughout the brickwork,

2)to adhere and bind together the bricks,

3)to act as a non-conductor and prevent the transmission of heat, sound, and moisture from one side of wall to the other. The factors governing the choice of mortars for various purposes are:

a)strength as being a main factor determining the strength of the wall

b)porosity and capillary characteristics as affecting the rain-excluding properties and durability of the wall,

c)content of soluble safts which determines the possibility to destruction of the masonry or brickwork.

Mortar consists of an inert aggregate bound by a cementing material. The cementing material is most important in determining the characteristics of the mortar. The usual cementing materials used for constructional work are hydraulic limes or Portland cement.

Clean, sharp pit sand is the best aggregate. Old bricks, burnt ballast or stories ground in a mortar mill may be used as substitutes for sand.

Mortars may be classified as follows:

1.Cement mortars

2.Cement-lime mortars

3.Lime mortars

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Lime Mortar – This is a mixture of quick lime and sand, in the proportion of a part lime to 2 or 3 parts sand in addition to water. It is the principal material used for bedding and jointing bricks, stones, etc. The slaked lime is mixed with the sand and water either by hand or in a mortar mill. The period of slaking, composition and strength of mortar depends upon the class of lime used.

Non-hydraulic Lime Mortars must be well slaked before use. This type can be stored in a heap for several days after mixing, provided the surface is smoothed over with a shovel to, minimize carbonation by the exclusion of as much air as possible. As such mortars can only harden when exposed to the atmosphere, a relatively large proportion of sand must be added to the lime to assist in the penetration of air for this reason the proportion of sand may be as high as 4 parts by volume of sand to 1 part lime. These mortars are not suitable for work below ground level, especially if the ground is water-logged.

Hydraulic Lime Mortars should be used within an hour after being mixed. Any mortar which has stiffened and cannot be knocked up by means of a trowel to a sufficiently plastic condition should never be used. The proportion of lime to aggregate ranges from 1 part lime to from 2 to 3.5 parts sand, and common mixture being 1:3. These are excellent mortars for all purposes and are particularly suited for work below the ground level and in exposed positions.

Magnesian and Dolomitic Lime Mortars have a slow-setting an action and should therefore be slaked for several hours before use. Their properties and uses are somewhat similar those of hydraulic lime mortars.

Black Mortars. - Ashes or clinkers from furnaces are crushed finely and ground with the lime in a mortar mill to produce a cheap and strong mortar, known as black mortar. The ashes should be free from unburnt coal and dust. A common proportion is 1 part lime to 3 parts ashes or clinker. They are hardsetting mortars and are suitable for internal walls and for brickwork and masonry where the color is not objected to.

Cement Mortars. - It is stronger than lime mortar and is used in the construction for external walls. Cement mortar is now extensively used during winter, owing to as relatively quick-setting property. It must be used

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immediately after mixing. The usual, composition is 1 part cement to 3 parts sand. A dense cement mortar should not be used for bedding and jointing lowstrength bricks.

Lime-Cement or Compo Mortars. - Compo is a mixture of lime, cement and sand. It is usual to mix the lime mortar and then to gauge this mixture with the necessary proportion of Portland cement immediately before the mortar is required for use. Only non-hydraulic lime should be used for this class of mortar. The addition of cement increases the hydraulicity of the mortar, besides, increasing its strength and the rate of hardening is therefore accelerated. The gauging also increases the workability of the mortars. The proportions vary from 1 part cement: 2 to 3 part lime: 9 to 12 parts sand. Eminently hydraulicand magnesian limes should not be ganged with cement.

Strength of Mortar. - Cement mortar produces the strongest brickwork, non-hydraulic lime mortar is approximately half the strength of that in cement mortar, and the strength of eminently hydraulic mortars is intermediate between that of cement and non-hydraulic lime mortars. The strength of compo mortars depends upon the cement content and may be very little less than that of cement mortar.

Properties of Mortars

Workability is an important property of mortars, particularly of those used in conjunction with unit masonry of high absorption. Workability is controlled by the character of the cement and amount of sand. For example, a mortar made from 3 parts sand and 1 part slaked lime putty will be more workable than one made from 2 parts sand and 1 part Portland cement. But the 3:1 mortar has lower strength. By proper selection or mixing of cementitious materials, a satisfactory compromise may usually be obtained, producing a mortar of adequate strength and workability.

Water retention — the ratio of the flow after 1-min standard suction to the flow before suction — is used as an index of the workability of mortars. A high value of water retention is considered desirable for most purposes. There is, however, a wide variation in water retention of mortars made with varying

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proportions of cement and lime and with varying limes. The ‘‘Standard Specification for Mortar for Unit Masonry’’ requires mortar mixed to an initial flow of 100 to 115, as determined by the test method, to have a flow after suction of at least 75%.

Strength of mortar is frequently used as a specification requirement, even though it has little relation to the strength of masonry. The strength of mortar is affected primarily by the amount of cement in the matrix. Other factors of importance are the ratio of sand to cementing material, curing conditions, and age when tested.

Volume change of mortars constitutes another important property. Normal volume change (as distinguished from unsoundness) may be considered as the shrinkage during early hardening, shrinkage on drying, expansion on wetting, and changes due to temperature. After drying, mortars expand again when wetted. Alternate wetting and drying produces alternate expansion and contraction, which apparently continues indefinitely with Portland-cement mortars.

PORTLAND-CEMENT CONCRETE

Portland-cement concrete is a mixture of Portland cement, water, coarse and fine aggregates, and admixtures proportioned to form a plastic mass capable of being cast, placed, or molded into forms that will harden to a solid mass. The desirable properties of plastic concrete are that it is workable, placeable and no segregating, and that it set in the desired time. The hardened concrete should provide the desired service properties:

1.Strength (compressive and flexural)

2.Durability (lack of cracks, resistance to freezing and thawing and to chemical attacks, abrasion resistance, and air content)

3.Appearance (color, lack of surface imperfections)

Each of these properties affects the final cost of the mix design and the cost of the in-place concrete. These properties are available from normalweight, lightweight, and heavyweight concretes.

1. Discussion: Your ideas about the material.

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UNIT 5

TEXT A. FROM THE HISTORY OF CONCRETE.

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 3rd century before our era) 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 in 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 on the 19th century, and ferro-concrete was introduced into engineering practice.

Text B. Concrete.

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 (calcined) 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 is 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

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after the water has been added and the process of hardening lasts for about twenty-eight days. The process is called concrete curing. The characteristics of concrete depend upon 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 (ferro-concrete) where it is necessary to introduce steel. Plain of mass concrete can be used for almost all building purposes. Ferro-concrete us used in buildings bridges and arches, dams and dock-walls, for structured 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 slags 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 light, fireresistant and acid-proof. It contains no cement whatever. Silica concrete is widely used in aviation and in under water constructions.

The term “Concrete” is used to describe a dense material composed of cement and aggregate mixed with water.

The density of the aggregate. Therefore there is a broad division of concrete types into:

a)Dense concretes – composed of heavy aggregates.

b)Light-weight concretes – composed of light aggregates.

The aggregates are graded in size from fine to coarse in order to reduce the amount of void space to be filled by cement. There are “cellular” concretes made by using materials which foam of 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.

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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.

1.Answer the following questions:

1.How is concrete made? 2. What takes place when water is added to the cement? 3. Does the whole mixture set and harden when hydration takes place?

4.A solid mass is formed, right? 5. Do you know what is termed “aggregate”?

6.Is sand known as “fine aggregate”? 7. And what is meant by “coarse aggregate”? 8. Can concrete be made on a building site and poured into position as a wet mix? 9. Are you able to explain what is meant by “in-situ” concrete?

2.Read the following and then describe the behavior of concrete:

1. Curing. Concrete becomes hard by the chemical combination of cement and water, during which process it is necessary to prevent as far as possible evaporation of the water from the surface of the concrete; this is called “curing”, and is accomplished by covering the concrete, as soon as it can be done without damaging the surface, with damp cloths, wet straw, wet sand, etc., kept wet by sprinkling, or by immersing in water.

2. Hardening. The strength of concrete under favorable curing conditions increases with age. Hardening is very rapid in the early stages, but continues more slowly for an indefinite period amounting to years.

3. Insert the needed words and groups of words:

Portland cement is a … product. It is made of …, … or … . They are … and … with water to form a paste. The mixture is then … in a kiln. The clinker is ground to … .

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