建筑材料毕业论文外文翻译.doc

外文原文building materialsMaterials for building must have certain physical properties to be structurally useful. Primarily, they must be able to carry a local or weight, without changing shape permanently. When a load is applied to a structure member, it will deform: that is a wire will stretch or a beam will bend. However, when the load is removed，the wire and the beam come back to the original positions. This material property is called elasticity. If a material were not elastic and a deformation were present in the structure after removal of the load ，repeated loading and unloading eventually would increase the deformation to the point where the structure would become useless. Materials used in architectural structures，such as stone and brick, wood, steel, aluminum, reinforced concrete, and plastics，behave elastically within a certain defined range of loading. If the loading is increased above the range，two types of behavior can occur: brittle and plastic. In the former, the material will break suddenly. In the latter, the material begins to flow at a certain load (yield strength), ultimately leading to fracture. As examples，steel exhibits plastic behavior，and stone is brittle. The ultimate strength of a material is measured by the stress at which failure (fracture) occurs.A second important property of a building material is its stiffness. This property is defined by the elastic modulus，which is the ratio of the stress(force per unit area)，to the strain (deformation per unit length). The elastic modulus, therefore, is a measure of the resistance of a material to deformation under load. For two materials of equal area under the same load .the one with the higher elastic modulus has the smaller deformation. Structural steel, which has an elastic modulus of 30 million pounds per square inch(psi)，or 2，100,000 kilograms per square centimeter, is 3 times as stiff as aluminum , 10 times as stiff as concrete , and 15 times as stiff as wood.MasonryMasonry consists of natural materials，such as stone or manufactured product，such as brick and concrete blocks. Masonry has been used since ancient times; mud bricks were used in the city of Babylon for secular buildings，and stone was used for the great temples of the Nile Valley. The Great Pyramid in Egypt，standing 481 feet (147 meters) high，is the most spectacular masonry construction. Masonry units originally were stacked without using any bonding agent，but all modem masonry construction uses a cement mortar as a bonding material. Modern structural materials include stone，brick of burnt clay or slate，and concrete blocks.Masonry is essentially a compressive material: it cannot withstand a tensile force，that is，a pull. The ultimate compressive strength of bonded masonry depends on the strength of the masonry unit and the mortar. The ultimate strength will vary from 1，000 to 4，000 psi(70 to 280 k/cm2)，depending on the particular combination of masonry unit and mortar used.TimberTimber is one of the earliest construction materials and I one of the few natural materials with good tensile properties. Hundreds of different species of wood are found throughout the world，and each species exhibits different physical characteristics. Only a few species are used structurally as framing members in building construction .In the United States，for instance .out of more than 600 species of wood，only 20 species are used structurally. These are generally the conifers，or softwoods，both because of their abundance and because of the ease with which their wood can be shaped. The species of timber more commonly used in the United States for construction are Douglas fir，Southern pine，spruce，and redwood. The ultimate tensile strength of these species varies from 5，000 to 8，000 psi (350 to 560 kg/cm'). Hardwoods are used primarily for cabinetwork and for interior finishes such as floors.Because of the cellular nature of wood，it is stronger along the grain than across the grain. Wood is particularly strong in tension and compression parallel to the grain. And it has great bending strength. These properties make it ideally suited for columns and beams in structures. Wood is not effectively used as a tensile member in a tress, however because the tensile strength of a truss member depends up connections between members . It is difficult to devise connections which do not depends on the shear or tearing strength along the grain，although numerous metal connectors have been produced to utilize the tensile strength of timbers.Steel Steel is an outstanding structural material. It has a high strength on a pound-for-pound basis when Compared to other materials，even though its volume-for-volume weight is more than ten times that of wood. It has a high elastic modulus，which results in small deformations under load. It can be formed by rolling into various structural shapes such as I-beams, plates, and sheets; it also can be cast into complex shapes; and it is also produced in the form of wire strands and ropes for use as cables in suspension bridges and suspended roofs, as elevator ropes, and as wires for prestressing concrete. Steel elements can be joined together by various means，such as bolting，riveting，or welding. Carbon steels are subject to corrosion through oxidation and must be protected from contact with the atmosphere by painting them or embedding them in concrete. Above temperatures of about 700F(371)，steel rapidly loses its strength，and therefore it must be covered in a jacket of a fireproof material (usually concrete) to increase its fire resistance.The addition of alloying elements，such as silicon or manganese，results in higher strength steels with tensile strengths up to 250,000 psi(17,500kg/cm2).These steels are used where the size of a structural member becomes critical，as in case of columns in a skyscraper.AluminumAluminum is especially useful as a building material when lightweight, strength，and corrosion resistance are all important factors. Because pure aluminum is extremely soft and ductile alloying elements such as magnesium silicon，zinc and copper，must be added to it to impart the strength required for structural use. Structural aluminum alloys behave elastically. They have an elastic modulus one third as great its steel and therefore deform three times as much as steal under the same load. The unit weight of an aluminum iinum alloy is one third that of steel，and therefore an aluminum member will be lighter than a steel member of comparable strength. The ultimate tensile strength of aluminum alloys ranges from 20,000 to 60,000 psi (1,400 to 4,200kg/cm2).Aluminum can be formed into a variety of shapes; it can be extruded to form I-beams drawn to form wire and rods，and rolled to form foil and plates. Aluminum members can be put together in the same way as steel by riveting，bolting，and(to a lesser extent) by welding. Apart from its use for framing members in buildings and prefabricated housing，aluminum also finds extensive use for window frames and for the skin of the building in curtain-wall construction.ConcreteConcrete is a mixture of water，sand and gravel，and portland cement，Crushed stone, manufactured lightweight stone，and seashells are often used in lieu of mural gravel. Portland cement. Which is a mixture of materials containing calcium and clay，is heated in a kiln and then pulverized. Concrete derives its strength from the fact that pulverized portland cement，when mixed with water，hardens by a process called hydration. In an ideal mixture, concrete consists of about three fourths sand and gravel(aggregate)by volume and one fourth cement paste. The physical properties of concrete are highly sensitive to variations in mixture of the components，so a particular combination of these ingredients must be custom-designed to achieve specified results in terms of strength or shrinkage. When concrete is poured into a mold or form, it contains free water, not required for hydration，which evaporates. As the concrete hardens, it releases this excess water over a period of time and shrinks. As a result of this shrinkage，fine cracks often develop. In order to minimize these shrinkage cracks, concrete must be hardened by keeping it moist for at least 5 days. The strength of concrete increases in time because the hydration process continues for years; as a practical matter，the strength at 28 days is considered standard.Concrete deforms under load in an elastic manner. Although its elastic modulus is one tenth that of steel，similar deformation will result since its strength is also about one tenth that of steel. Concrete is basically a compressive material and has negligible tensile strength. Reinforced concreteReinforced concrete has steel bars that are placed in a concrete member to carry tensile forces. These reinforcing bars，which range in diameter from 0. 25 inch (0. 64cm) to 2.25 inches (5 .7cm), have wrinkles on the surfaces to ensure a bond with the concrete. Although Reinforced concrete was developed in many countries，its discovery usually is attributed to Joseph Mourner, a workable because steel and concrete expand and contract equally when the temperature change. If this were not the case, the bond between the steel and concrete would be broken by a change in temperature since the two materials would respond differently. Reinforced concrete can be molded into innumerable shapes, such as beams, columns slabs .and arches，and is the therefore easily adapted to a particular form of building. Reinforced concrete with ultimate tensile strengths in excess of 10，000 psi (700 kg/cm2) is possible，although most commercial concrete is produced with strengths under 6,000 psi(420 kg/cm2).PlasticsPlastics are rapidly becoming important construction materials because of the great variety, strength，durability, and lightness. A plastic is a synthetic material or resin which can be molded into any desired shape and which uses an organic substance as a binder. Organic plastics are divided into two general groups: thermosetting and thermoplastic. The thermosetting group becomes rigid through a chemical change that occurs when heat is applied: once set，these plastics cannot be remolded. The thermoplastic group remains soft at high temperatures and must be cooled before becoming rigid; this group is not used generally as a structural material. The ultimate strength of most plastic materials is from 7，000 to 12，000 psi(490 to 840 kg/cm2)，although nylon has a tensile strength up to 60,900 psi (4，200 kg/cm2).Text B Texting of MaterialsThe most common test of building materials is the strength test to destruction. This is partly because strength is a very important property of a building material，even a material in a "non-load-bearing" part of the building; partly because strength tests are comparatively simple to carry out; and partly because they offer a guide to other properties，such as durability.The strength of a ductile material such as steel，aluminum，or plastics is usually determined by applying a tensile load. A compression test is used for brittle materials such as concrete, stone, and brick because their tensile strength is low and thus harder to measure accurately.The method of testing and the dimensions of the test pieces are laid down in the appropriate standards published by the American Society for Testing Materials (ASTM)，the British Standards Institution (BSI)，the Standards Association of Australia (SAA)，etc.The size and shape of the test specimen are particularly important for brittle materials because they influence the number of flaws that are likely to occur in the test specimen. For concrete tests, the standard American and Australian test specimen-a cylinder 150 mm in diameter and 300 mm long-gives a lower result than the standard British test specimen-a 150 mm cube-because the former contains more concrete.The speed of testing is also specified. A passage of time is required for both plastic deformation and the formation of cracks，and a faster rate of testing thus gives a higher result. and the formation of cracks，and a faster rate of testing thus gives a higher result.For tests on concrete and timber，it is necessary to specify moisture content because this the strength.A test on a single specimen is unreliable because we do not know whether it is an average specimen or whether it has fewer or more than the average number of minute flaws. Standard specifications lay down how many specimens shall be tested and how they are to be selected.Tests of factory-made materials carried out by the manufacturer are usually accepted by the user of the building material unless he has reason to doubt their veracity. Since concrete is made on the building site or brought from a ready-mix concrete plant，its testing becomes the responsibility of the building contractor. This is therefore a more frequent testing activity than that for other material Concrete cylinders or cubes are normally tested in a hydraulic press，which may be used exclusively for this purpose. A universal machine based on the same principle.Timber differs from other building materials in that it is produced from growing trees and is thus move variable. Cut timber from virgin foresters may consist of a variety of different species. Even timber cut from a planted forest containing trees of the same species all planted at the same time may show appreciable variation between pieces because of knots，or other flaws.A substantial proportion of timber is used on domestic construction where it is not highly stressed; in such cases, "visual grading" (that is merely looking at it) may be sufficient. Because of the imperfections in individual pieces, "stress grading" is usually more reliable than even accurate testing of selected test pieces. A stress grading machine tests every individual piece of timber by a method that is very fast and relatively cheap. The machine is based on an empirical relation between the strength and the deflection of timber. Each piece of timber is deflected (but not stressed to its limit) at several points along its length，and the deflection category marked by means of a spot of dye. The timber is then classified visually by its color markings.The strength of metals is reduced if they are repeatedly loaded alternately in tension and in compression. This is called repeated loading if it is applied several hundred or thousands of times, and fatigue loading if it is applied millions of times. Fatigue loading is a major problem in machines but rarely in buildings. Wind loads，however，can cause repeated loading in roof structures. There are special machines for testing the strength of materials under repeated loading.Other special tests are for ductility and for hardness. Ductility is tested by bending a bar around a pin over a wide angle. Hardness is tested by indentation with a diamond or a hardened steel ball. The hardness test is car