General properties of clay bricks

Strength. Strength is a property which is so often used to judge the quality of a brick. In fact, however, strength in itself is not always important factor, since the great majority of bricks are used in positions where they are not required to carry anything like their full safe load. It is true that a really strong brick will nearly always be good in many other respects, but the converse does not by any means follow; for there are some excellent types, quite suitable for many purposes, which are of low strength.

To a large extent the strength is dependent upon the type of clay used and method of making, but with most types of brick there is also a considerable variation according to the temperature at which the bricks are burnt.

Where considerable loads have to be carried a strong brick is needed, but it should be remembered that the strength of a wall is derived from a combination of brick and mortar. Building regulations define the type of mortar to be used with bricks of varying strength to carry varying loads.

Resistance to rain penetration. This again is a property that depends upon the brick and mortar combined rather than on the brick itself. In fact it is only with extremely permeable bricks that rain is ever likely to penetrate a 9 in. or even thicker solid wall, but it does so through cracks (often minute) between the bricks and the mortar.

Weathering and appearance. It is well known that brick are usually extremely durable and that although they may change in appearance after considerable exposure, they usually do so in a manner which is pleasant rather than otherwise. In spite of this there are sufficient examples of brickwork which has deteriorated on exposure to make it necessary to consider how such defects occur. The two chief causes of deterioration are frost and the crystallization of soluble salts.

For an estimate of probable resistance to frost damage, an expert view would be based upon a combination of strength, porosity and saturation coefficient. For a quick judgement by the architect probably the best course is to make sure that no underburnt bricks occur in the normal deliveries.

It is important to differentiate between the conditions of exposure to which the bricks will be subjected. Chimneys, parapet walls and retaining walls are very much more severely exposed to both rain and frost than is the general walling of a building, and they are likely to suffer accordingly. Contrary to general belief, brickwork below damp course level is not especially liable to frost damage.

The effect of soluble salts may vary considerably. They may crystallize on the surface, usually in a spell of dry weather after a wet period. When this occurs they form white crystals which may be very disfiguring to the building, but are unlikely to cause any real damage to the bricks unless the quantity present is abnormal. This crystallization can continue for some years and is usually most obvious in the spring. Normally the crystals are gradually washed away and the trouble gets less each year. There is no cure for such efflorescence once the salts are present in the wall, though dry brushing of the efflorescence may help to get rid of the trouble more quickly.

The subject of the appearance of brickwork cannot be dismissed without referring to the importance of the pattern of jointing – which depends upon the brick bond, and also on the thickness of joints, their color and their type. All these things, together with the type of brick itself, give the overall appearance of color and texture to a wall, and they need careful study on actual examples before they can be handled competently as part of the design technique.

VOCABULARY NOTES

bond связь, соединение

property свойство

clay глина

to derive получать, извлекать; происходить

exposure местоположение, вид; выставление (под дождь и т. п.)

to deteriorate ухудшать(ся); портить(ся)

to define определять

appearance внешний вид

efflorescence продукт кристаллизации

brickwork кирпичная кладка

damage повреждение

damp course изолирующий от сырости слой

joint соединение

texture структура, строение

EXERCISES

I. Read the text and answer the following questions:

1. What are the main three stages in manufacturing bricks?

2. Why is the strength of bricks not always an important factor?

3. What factors does the strength of bricks depend on?

4. What are the two chief causes of the deterioration of bricks?

5. What parts of a building are most severely exposed to rain and frost damage?

6. Describe the effect of soluble salts in a brick wall.

7. What different kinds of bonds do you know?

II. Retell the text. Use the questions above as a plan.

CONCRETE

Concrete is a structural material consisting of a hard substance, and known as aggregate (usually sand and gravel), that is bonded together by cement and water.

Among the ancient Assyrians and Babylonians, the bonding substance most often used was clay. The Egyptians developed a substance using lime and gypsum as binders. Lime derived from limestone, chalk, continued to be the primary cement-forming agent until the early 1800s. In 1824 an English inventor, Joseph Aspdin, burned and ground together a mixture of limestone and clay. This mixture, called Portland cement, has remained the dominant cementing agent used in concrete production.

Aggregates are generally designated as either fine (ranging in size from 0,025 to 6,5 mm) or coarse (from 6,5 to 38 mm or larger). All aggregate materials must be clean and free from admixture because even small quantities of compounds seriously affect the strength of the concrete.

Concrete is characterized by the type of aggregate or cement used, or by the methods used to produce it. In ordinary structural concrete, the character of the concrete is largely determined by a water-to-cement ratio. The lower the water content is, the stronger the concrete. Another durability factor is the amount of cement in relation to the aggregate. Where especially strong concrete is needed, there will be relatively less aggregate. The strength of concrete is measured in pounds per square inch or kilograms per square centimeter of force needed to crush a sample of a given age or hardness. Concrete’s strength is affected by environmental factors, especially temperature and moisture.

In the process known as curing, the concrete is kept damp for some time after pouring to slow the shrinkage that occurs as it hardens. Low temperatures also adversely affect its strength. Concrete that has been hardened onto imbedded metal (usually steel) is called reinforced concrete, or ferroconcrete. Its invention is usually attributed to Joseph Monier, a Parisian gardener who made garden pots and tubs of concrete reinforced with iron mesh; he received a patent in 1867. The reinforcing steel, which may take the form of rods, bars, or mesh, contributes tensile strength. Plain concrete does not easily withstand stresses such as wind action, earthquakes, and vibrations and other bending forces and is therefore unsuitable in many structural applications.

Another innovation in masonry construction is the use of prestressed concrete. It is achieved by either pretensioning or posttensioning processes. In pretensioning, length of steel wire, cables, or ropes are laid in the empty mold and then stretched and anchored. After the concrete has been poured and allowed to set, the anchors are released and, as the steel seeks to return to its original length, it compresses the concrete. In the posttensioning process, the steel is run through ducts formed in the concrete. When the concrete has hardened, the steel is anchored to the exterior of the member by some sort of gripping device. By applying a measured amount of stretching force to the steel, the amount of compression transmitted to the concrete can be carefully regulated. Because it achieves strength without using heavy steel reinforcements, it has been used to great effect to build lighter, and more elegant structures such as bridges and vast roofs.

Concrete is fire resistant and has become one of the most common building materials in the world.

 

VOCABULARY NOTES

strength of concrete прочность бетона

reinforced concrete армированный бетон

ferroconcrete железобетон

prestressed concrete предварит. напряженный бетон

structural material строительные материалы

sand песок

gravel гравий

cement цемент

lime известь

gypsum гипс

aggregate заполнитель

fine aggregate мелкий заполнитель

coarse aggregate крупный заполнитель

binder вяжущее

curing выдерживание

pouring заливка бетона, укладка бетонной смеси

shrinkage усадка

to mold отливать в форму

EXERCISES

I. Read and translate the text.

II. Find synonyms for the following words:

structural material; amount of cement; to affect; construction; essential; aggregate; to erect.

III. Explain the meaning of the following words:

aggregate; mixture; to be free from; water-to-cement ratio; environmental factors; masonry construction.

IV. Form nouns from the following words:

to mix; practical; to produce; to develop; to design; strong; to measure; to shrink; to invent; to contribute; to apply.

V. Put the questions:

Q:...

A: I know that concrete is compound structural material.

Q:...

A: The main aggregates are sand, lime, and clay.

Q:...

A: The character of concrete is determined by a water-to-cement ratio.

Q: …

A: The strength of concrete is measured in pounds per square inch.

VI. Retell the text using words and word combinations from the vocabulary list.