病历翻译样例-入院记录

泛瑞翻译

病历翻译作为出国看病的基本依据，应当引起医学翻译工作者的严肃对待。译者不但要完全明白病历意思，更要以合理的逻辑思维及语言表达来表述病历内容。病历翻译主要涉及CT、MRI检查、生化检查、出院小结、入院记录等。所有这些内容都要求准确翻译，不能出现乱译的现象。但有时候，会因为中文与英文的表述习惯，出现一定的字面偏颇。根据知情同意的原则，翻译公司有必要进行书面描述，以避免不必要的理解错误。下面列举一个病历翻译样例：

History of present illness: On October 23, 2012, physical examinations revealed that the patient’s serum creatinine was 278umol / L, hematuria was + +, proteinuria was + +. Subsequently, she was treated at Peking University First Hospital on November 14, 2012, and her blood pressure was 140/90mmHg. Blood IgA was 3.93g / L, and 24-hour urinary protein was 2.97 g (urine volume 1500 mL). Microscopy for urine red blood cell phase difference revealed a high red blood cell distortion rate. Bilateral renal B ultrasound showed a slightly smaller right kidney, and the renal parenchyma was slightly thin. She was admitted to hospital for treatment, and carried out a renal biopsy. The pathologic report revealed crescentic IgA nephropathy (moderate to advanced).

During the hospitalization, the serum creatinine, blood uric acid, and hemoglobin were 390.6 umol/L, 499 umol/L, and 93 g/L, respectively. She was diagnosed with chronic glomerulonephritis, crescentic IgA nephropathy (moderate to advanced), renal anemia, renal hypertension, and hyperuricemia. Thus, treatments including hypertension-relieving and anemia-correction were given. The patient's condition was stable after discharge. Oral administration of allopurinol was recommended (2 tablets each time, t.i.d.). After 2 weeks, the recheck of blood biochemical indexes was performed, which showed the levels of alanine aminotransferase, aspartate aminotransferase, albumin, and serum creatinine were 52 IU/L, 71 IU/L, 33.2g/L, and 55 umol L, respectively. Meanwhile, systemic red rash was reported. Subsequently, she was admitted to our department. As she was speculated to suffer from acute exacerbation of chronic renal

insufficiency, acute drug-induced liver injury, drug-induced dermatitis, symptomatic treatments including administration of hormones, liver-protection, renal function protection were performed accordingly. She was discharged after her condition was improved. At this time, she was admitted to our department to adjust the amount of hormone. Presently, the patient's condition is stable with satisfactory mental condition and appetite. The patient reported no cough or expectoration. Additionally, no painful swelling of joint or fever was reported. No abnormality was observed in the defecation and urination. No significant changes were noted in her body weight.

第二篇：翻译样例

附录 A

Bridges

Bridges are great symbols of mankind’s conquest of space. That day when you fall in the Pacific Ocean to see deep red grid of the golden gate bridge, or glide on the seas in the deep canyons, you will feel surprise and admiration for them the art of builders. They are the enduring expressions of mankind’s determination to remove all barriers in its pursuit of a better and freer world. Their design and building schemes are conceived in dream-like bison. But vision and determination are not enough. All the physical forces of nature and gravity must be understood with mathematical precision and such forces have to be resisted by manipulating the right materials in the right pattern. This requires both the inspiration of an artist and the skill of an artisan.

Scientific knowledge about materials and structural behavior has expanded tremendously, and computing techniques are now widely available to manipulate complex theories in innumerable ways very quickly. Engineers have virtually revolutionized bridge design and construction methods in the past decade. The advances apply to short-medium and long-span bridges.

For permanent bridge，the most commonly used materials are steel and concrete. Many different types of bridges are built with these materials, used singly or in combination. Timber may be used for temporary above-water construction, for the elements of a structure that lie below the waterline (particularly timber pile s), or for short-span bridges located on secondary roads. A few short-span aluminum bridges have been built in the United States on an experimental basis.

The principal portions of a bridge may be said to be the “substructure” and the “superstructure.” This division is used here simply for convenience, since in many bridges there is no clear dividing lint between the two.

Common elements of the substructure are abutments (usually at the bridge ends) and piers (between the abutments).Piers and abutments often rest on separately constructed foundations such as concrete spread footings or groups of bearing piles; these foundations are part of the substructure. Occasionally a bridge substructure comprises a series of pile bents in which the piles extend above the waterline and are topped by a pile cap that, in turn, supports the major structural elements of the superstructure. Such bents often are used in a repetitive fashion as part of along, low,

over-water crossing.

In recent years, the dividing lines between short-medium and long-span bridge have blurred somewhat. Currently, spans of 20 to 100 ft (6.1 to 30.5m) are regarded as short by many designers, who have developed many standardized designs to handle these spans economically. Medium spans range up to, per-haps, 400ft (121.9m) in modern bridge practice, depending on the organization involved and the materials used. Long spans range up to 4000ft (1219.2m) or more, but a clear span above 1000ft (304.8m) is comparatively rare.

In the United States, highway bridges generally must meet loading, design, and construction requirements of the AASHTO Specification. The design and construction of railway bridges are governed by provisions of the AREA Manual for Railway Engineering. Design requirements for pedestrian crossings and bridges serving other purposes may be established by local or regional codes and specifications. ACI Code provisions are often incorporated by reference and in most cases serve as model provisions for other governing documents.

Bridge spans to about 100 ft often consist of pre-cast integral-deck units. These units offer low initial cost, minimum maintenance, and fast easy construction, with traffic interruption. Such girders are generally pretensioned; the units are placed side by side, and are often post-tensioned laterally at intermediate diaphragm. After which shear keys between adjacent units are filled with non-shrinking mortar. For highway spans, an asphalt wearing surface may be applied directly to the top of the pre-cast concrete. In some cases, a cast-in-place slab is placed to provide composite action.

For medium-span highway bridges, to about 120 ft, AASHTO standard I beams are generally used. He is intended for use with a composite cast-in-place roadway slab. Such girders often combine pre-tensioning of the pre-cast member with post-tensioning of the composite beam after the deck is placed.

Pre-cast girders may not be used for spans much in excess of 120 ft because of the problems of transporting and erecting large, heavy units. On the other hand, there is a clear trend toward the use of longer spans for bridges. Highway safety is improved by eliminating central piers and moving outer piers away from the edge of divided highways. For elevated urban expressways, long spans facilitate access and minimize obstruction to activities below. Concern for environmental damage has led to the choice of long spans for continuous viaducts. For river crossing, intermediate piers may be impossible because of requirements of navigational clearance.

Such requirements have led to the development in Europe, and more recently in the western hemisphere, of long span segmental pre-stressed concrete box girder bridges. In typical construction of this type, piers are cast-in-place, often using the slip-forming technique. A “hammerhead” section of box girder is then cast at the top of the pier, and construction proceeds in each direction by the balanced cantilever method. The construction is advanced using either cast-in-place or pre-cast segments, each post-tensioned to the previously completed construction. Finally, after the closing cast-in-place joint is made at mid-span, the structure is further post-tensioned for full continuity.

Bridge may also be classed as “deck” or “through” types. In the deck type of bridge, the roadway is above the supporting structure; that is, the load-carrying elements of the superstructure are below the roadway. In the through type of bridge, the roadway passes between the elements of the super-structure, as in a through steel-truss bridge. Deck structures predominate: they have a clean appearance, provide the motorist with a better view of the surrounding area, and are easier to widen if future traffic requires it.

Examples of short-span concrete bridges include cast-in-lace, reinforced concrete beam (and slab); simple-span, pre-stressed (this type incorporates pre-cast, pre-stressed I-girders or box girders topped by a cast-in-place deck); and cast-in place box girder.

The designer of each medium-and long-span bridge tries to devise a structure that is best suited to the conditions encountered at that particular location. The result is an almost bewildering variety of structures that differ either in basic design principles or in design details.

General categories of steel bridge are briefly described in the following paragraphs.

Girder bridges come in two basic varieties-plate and box girders.

Plate girders are used in the United States for medium spans. They generally are continuous structures with maximum depth of girder over the piers and minimum depth at mid-span. The plate girders generally have an across section; they are arranged in lines that support stringers, floor-beams, and, generally, a cast-in-lace concrete deck. The girders are shop-fabricated by welding; field connections generally are by high-strength bolts.

Welded-steel box girder structures are generally similar to plate girder spans except for the configuration of the bridge cross section.

Rigid frames are used occasionally, most often for spans in the range of 75 to 100 ft (22.9 to 30.5 m) and for grade0separation structures.

Arch bridges are used for longer spans at locations where intermediate piers cannot be used and where good rock is available to withstand the thrusts at the arch abutments.

Variations in the arch bridge are specially suited in the span range of 200 to 500m and thus provide a transition between the continuous box girder bridge and the stiffened suspension cable. The cables provided above the deck and connected to the towers would permit elimination of intermediate piers facilitating a larger width for purposes of navigation. Because of the damping effect of inclined cables, the cable-stayed decks are less prone to wind-induced oscillations than suspension bridges.

Suspension bridges are used for very long spans or for shorter spans where intermediate piers cannot be built. An example is the Verrazano Narrows Bridge which was completed in 1964.The $305 million,4260ft(1298.5m)structure spans the entrance to New York Harbor to join Staten Island and Brooklyn.

Concrete bridges come in nearly as great a variety as do steel bridges.

The bridge construction in France benefits by a strong growth in rail and highway infrastructures. For the time being the competition with other material turns to the advantage of composite bridge solutions. Before presenting any features concerning the recent trends in composite bridge design it is important to clarify, the bridge market, through the analysis of some statistical data.

In France, there is a very limited market for long span bridges. In the recent construction, the demand for bridges of span length higher than 200m is rather exceptional. The main market is for bridges of span length (or multi span length) less than 100m.

In France 800 to 1200 bridges are built every year, which represent about 300,000m to 500,000m of deck surface. However the majority of bridges being erected each year are of small span length. Less than 10% of the bridge patrimony have span. Length greater than 30m and deck surface greater than 1000 ㎡. Now that the market has been identified lets have an idea, in term of competitiveness, of the French market situation between several bridge types. In 1977，it less than 2.5%.

Of bridges were steel or composite bridges. The steel-concrete composite construction has continued to grow steadily over the last 15 years. This trend is mainly attributable to the gain in competitiveness of composite bridges against reinforced and prestressed concrete bridges.

For short span length the majority of steel bridges are of concrete type. Bridges composed of steel beams encased in concrete are very often used for railway bridges of small span length in order to meet stiffness requirements.

The recent statistical evaluation, performed by SETRA [1] on the bridges recently built in France between 1990 to 1993 by various owners (State, Highway concession companies, Departments and Communities, SNCF) shows that the competitive span length range for steel and concrete composite bridges is between 30 and 110 m with a very distinctive peak for the interval 60 to 80 m. In that range of spans length it is noticed that 85% of bridges being built belong to the composite category (Fig. 4).

The statistical analysis of the deck cost per square meters of surface confirms that the average price for a composite bridge is less than the price for a concrete bridge for spans length within intervals of 40 to 60 m and 60 to 80 m. The difference being of 1 500 FF/㎡over a total cost of 8 200 FF/㎡ (VAT excluded) in favor of the composite bridge. It means that an 18% cost difference represents a great shift in terms of competition.

The last 15 years have seen a great simplification of composite bridges for both roadway and railway bridges, which have made them, as previously indicated, very competitive compared to prestressed and reinforced bridges. These composite bridges, that we will name them as classical, have however several features which are described hereafter. Then, from these classical features, improvements have been constantly brought to the design and execution of composite bridges, which will be depicted later on.

The traditional composite roadway bridge is composed of two longitudinal girders which are connected to the concrete slab by shear connectors (usually welded stud are mostly met; however steel angle connectors are still used). A limited number of transverse cross beams joining the two longitudinal girders, usually not connected to the slab — see half cross section (a) are welded to the vertical stiffeners. The main girders have a few numbers of horizontal stiffeners, if any which are mostly needed to resist the stress state in the girder webs occurring at the launching phase.

Plain concrete is formed form a hardened mixture of cement, water, fine aggregate, coarse aggregate (crushed stone or gravel), air, and often other admixtures. The plastic mix is placed and consolidated in the formwork, then cured to facilitate the acceleration of the chemical hydration reaction of the cement/water mix, resulting in hardened concrete. The finished product has high compressive strength, and low resistance to tension, such that its tensile strength is approximately one-tenth of its compressive strength. Consequently, tensile and shear reinforcement in the tensile regions of sections has to be provided to compensate for the weak-tension regions in the reinforced concrete element.

It is this deviation in the composition of a reinforced concrete section from the homogeneity of standard wood or steel sections that requires a modified approach to the basic principles of structural design. The two components f the heterogeneous reinforced concrete section are to be so arranged and proportioned that optimal use is made of the materials involved. That is possible because concrete can easily be given any desired shape by placing and compaction the wet mixture of the constituent ingredients into suitable forms in which the plastic mass hardens. If the various ingredients are properly proportioned, the finished product becomes strong, durable, and, in combination with the reinforcing bars, adaptable for use as main members of any structural system..

The techniques necessary for placing concrete depend on the type of member to be cast: that is, whether it is a column, a beam, a wall, a slab, a foundation, a mass concrete dam, or an extension of previously placed and hardened concrete. For beams, columns, and walls, the forms should be well oiled after cleaning them, and the reinforcement earth should be compacted and thoroughly moistened to about 6 in. in depth to avoid absorption of the moisture present in the wet concrete. Concrete should always be placed in horizontal layers which are compacted by means of high power-driven vibrators of either the immersion or external type, as the case requires, unless it is placed by pumping. It must be kept in mind, however, that over vibration can be harmful since it could cause segregation of the aggregate and bleeding of the concrete.

Hydration of the cement takes place in the presence of moisture at temperatures above 50°F . It is necessary to maintain such a condition in order that the chemical hydration reaction can take place. If drying is too rapid, surface cracking takes place. This would result in reduction of concrete strength due to cracking as well as the

failure to attain full chemical hydration.

It is clear that a large number of parameters have to be dealt with in proportioning a reinforced concrete element, such as geometrical width, depth, area of reinforcement, steel strain, concrete strain, and so on. Consequently, trial and adjustment is necessary in the choice of concrete sections, with assumptions based on conditions at site, availability of the constituent materials, particular demands of the owners, architectural and headroom requirements, the applicable codes, and environmental conditions. Such an array of parameters has to be considered because of the fact that reinforced concrete is often a site-constructed composite, in contrast to the standard mill-fabricate beam and column sections in steel structures.

A trial section has to be chosen for each critical location in a structural system. The trial section has to be analyzed to determine if its nominal resisting strength is adequate to carry the applied factored load. Since more than one trial is often necessary to arrive at the required section, the first design input step generates into a series of trial-and-adjustment analyses.

The trial-and-adjustment procedures for the choice of a concrete section lead to the convergence of analysis and design. Hence every design is an analysis once a trial section is chosen. The availability of approach as a more efficient, compact, and speedy instructional method compared with the traditional approach of treating the analysis of reinforced concrete separately from pure design.

The rapid growth from 1945 onwards in the prestressing of concrete shows that there was a real need for this high-quality material. The quality must be high because the worst conditions of loading normally occur at the beginning of the life of the member, at the transfer of stress later, when the concrete has become stronger and the stress in the steel has decreased because of creep in the steel and the concrete, and shrinkage of the concrete. Faulty members are therefore observed and thrown out early, before they enter the structure, or at least before it becomes inconvenient and expensive to remove them.

The main advantages of prestressed concrete in comparison with reinforced concrete are:

(a) The whole concrete cross-section resists load. In reinforced concrete about half the section, the cracked area below the Neutral layer does no useful work. Working deflections are smaller.

(b) High working stresses are possible. In reinforced concrete they are not

usually possible because hey result in severe cracking which is always ugly and may be dangerous if it causes rusting of the steel.

(c) Cracking is almost completely avoided in prestressed concrete.

The main disadvantage of prestressed concrete is that much more care is needed to make it than reinforced concrete and it is therefore more expensive, but because it is of higher quality less of it needs to be used.

It can therefore happen that a solution of a structural problem may be cheaper in prestressed concrete than in reinforced concrete, and it does often happen that a solution is possible with prestressing but impossible without it.

Prestressing of concrete is used in concrete under any load in the past, to make it by compression. This means that the section can be designed so that it takes no tension or very little under the full design load. It therefore has theoretically no cracks and in practice the concrete in which it is embedded has hardened. After the concrete has hardened enough to take the stress from the steel, some of the stress is transferred from the steel to the concrete. In a bridge with abutments able to resist thrust, the prestress can be applied without steel in the concrete. It is applied by jacks forcing the bridge inwards from the abutments. This method has the advantage that the jacking force, or prestress, can be varied during the life of the structure as required.

In the ten years from 1950 to 1960 prestressed concrete ceased to be an experimental material and engineers won confidence in its use. With this confidence came an increase in the use of precast prestressed concrete particularly for long-span floors or the decks of motorways. Wherever the 500 m long, provided that most of the spans could be made the same and not much longer than 18 m, it became economical to use factory-precast prestressed beams, at least in industrial areas near a precasting factory. Most of these beams are heat-cured so as to free the forms quickly or reuse.

In this period also, in the United States, precast prestressed roof beams and floor beams were used in many school buildings, occasionally 32 m long or more. Such long beams over a single span could not possibly be successful in reinforced concrete unless they were cast on site because they would have to be much deeper and much heavier than prestressed concrete beams. They would certainly be less pleasing to the eye and often more expensive than the prestressed concrete beams. These school buildings have a strong, simple architectural appeal and will be a pleasure to look at for many years.

The most important parts of a precast prestressed concrete beam are the tendons

and the concrete. The tendons, as the name implies, are the cables, rods or wires of steel which are under tension in the concrete. Before the concrete has hardened (before transfer of stress), the tendons are either unstressed (post-tensioned prestressing) or are stressed and held by abutments outside the concrete (pre-tensioned prestressing). While the concrete is hardening it grips each tendon more and more tightly by bond along its full length. End anchorages consisting of plates or blocks are placed on the ends of the tendons of post-tensioned prestressed units, and such tendons are stressed up at the time of transfer, when the concrete has hardened sufficiently. In the other type of presstressing, with pre-tensioned tondons, the tendons are released from external abutments at the moment of transfer, and act on the concrete through bond or anchorage or both, shortening it by compression, and themselves also shortening and losing some tension.

Further shortening of the concrete (and therefore of the steel) takes place with time. The concrete is said to creep. This means that it shortens permanently under load and spreads the stresses more uniformly and thus more safely across its section. Steel also creeps, but rather less. The result of these two effects (and of the concrete shrinking when it dries) is that prestressed concrete beams are never more highly stressed than at the moment of transfer.

The factory precasting of long prestressed concrete beams is likely to become more and more popular in the future, but one difficulty will be road transport. As the length of the beam increases, the lorry becomes less and less manoeuvrable until eventually the only suitable time for it to travel is in the middle of the night when traffic is at a minimum. The limit of length for road transport varies the traffic in the district and the route, whether the roads are straight or curved. Precasting at the site avoids these difficulties; it may be expensive, but it has often been used for large bridge beams.

Materials for building must have certain physical properties to be structurally useful. Primarily, they must be able to carry a load, 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 .All 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 time as stiff as aluminum, 10 times as stiff as concrete, and 15 times as stiff as wood.

Masonry consists of natural materials, such as stone or manufactured products, 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 modern 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 kg/sq cm), depending on the particular combination of masonry unit and mortar used.

Timber is one of the earliest construction materials and 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, and redwood. The ultimate tensile strength of these species varies from 5 000 to 8 000 psi (350 to 560 kg/sq 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 truss, however, because the tensile strength of a truss member depends upon connections between members. It is difficult to devise connections which do not depend on the shear or tearing strength along the grain, although numerous metal connectors have been produced to utilize the tensile strength of timbers.

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°C), steel rapidly loses its strength, and there- fore 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 (17 500 kg/sq cm). These steels are used where the size of a structural member becomes critical, as in the case of columns in a skyscraper.

Aluminum 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 as steel and therefore deform three times as much as steel under the same load. The unit weight of an aluminum 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 4 200 kg/sq cm).

Aluminum can be formed into a variety of shapes; it can be extruded to form 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.

Concrete is a mixture of water, sand and gravel, and Portland cement. Crushed stone, manufactured lightweight stone, and seashells are often used in lieu of natural 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 the 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 shrink- age, 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 in- creases 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 the modulus of elasticity of concrete is 1/10 of the steel, but also is about 1/10 of the steel due to its strength, so they have similar shape .Concrete is basically a compressive material and has negligible tensile strength.

Reinforced 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 (0.64

cm) to 2.25 inches (5.7 cm), 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 Monnier, a French gardener, who used a wire network to reinforce concrete tubes in 1868. This process is workable because steel and concrete expand and contract equally when the temperature changes. 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 therefore easily adapted to a particular form of building. Reinforced concrete with ultimate tensile strengths in excess of 10000 psi (700 kg/sq cm) is possible, although most commercial concrete is produced with strengths under 6 000 psi (420 kg/sq cm).

Concrete is strong in compression, but weak in tension: its tensile strength varies from 8 to 14 percent of its compressive strength. Due to such a low tensile capacity, flexural cracks develop at early stages of loading. In order to reduce or prevent such cracks from developing, a concentric or eccentric force is imposed in the longitudinal direction of the structural element. This force prevents the cracks from developing by eliminating or considerably reducing the tensile stresses at the critical mid-span and support sections at service load, thereby rising the bending, shear, and almost the full capacity of the concrete in compression can be efficiently utilized across the entire depth of the concrete sections when all loads act on the structure.

附录 B

桥梁

桥梁是人类征服空间的象征。当日落时你在太平洋中看到深红色网格的金门大桥时，或者耀武扬威的在深谷上滑翔，你会感到惊奇并钦佩它们建造者的艺术。它们是人类在追求更好更自由的世界，移除堡垒的决心的持久的证明。他们的设计和建筑计划是像在梦里一样的想象，但是构想和决心都还不够，各种力如重力等的具体数据必须靠数学模型精确计算，这需要艺术的灵感和工匠的技术。

科学关于材料和结构的行为的知识已经明显地扩大，并且计算技术现在可广泛便捷地应用在操作复杂的理论，工程师们实际上在过去十年已经彻底改革了桥设计和建设方法。这些发展应用于短、中、长跨桥。

对于耐久的桥，最普通的材料就是用钢筋和混凝土。不同种类的桥都是用这些材料的。 木材都是用于水上的暂时的建筑，木制短跨度桥不可设置在水平面下，现在美国已经在实验一些铝制的短跨度桥。

一座桥的主要部分可以据说是“基础”和“上层建筑”。因为在很多桥里没有清楚的在这两个之间划分，这个划分被仅仅为了方便起见在这里使用。

基础的一般要素是桥台和墩台，它们经常分别建造基础，例如混凝土扩展基础，这些基础是基础的一部分。偶尔有些基础是桩基础，桩伸到水平线以上，顶 部有桩帽，这样可以支持主要的上部结构。这种形式经常被用来做长的、低的、跨水的结构。

在美国，公路桥梁通常必须满足美国州际公路及运输工作者协会（AASHTO）规范的荷载、设计和施工要求。铁道桥梁的设计和施工以美国铁道工程师协会（AREA）的铁道工程手册的规定为准绳。对于人行立交桥以及用语其它目的的桥梁的设计要求，可由当地的或地区性的规范和规程来确定。美国混凝土学会 （ACI）规范的条款常常为参考文献所采纳，而且在大多数情况下可以被用作其它指导性文件的标准规定。

跨度在100英尺左右的桥梁，常常由整体式桥面板的预制构件所组成。这些构件的生产成本低，养护费用最少，并且便于快速施工，从而使交通中断的时间 最短。这种梁一般都采用先张法预加应力。将构件并排安放，通常在中间各班位置处用后张法预加应力，之后就用抗缩砂浆填充相邻构件之间的剪力键。对于公路桥梁，可以直接在预制混凝土的顶面敷设一层现浇混凝土。

对于中等跨度的公路桥，跨度在 120 英尺左右，通常采用 AASHTO 的标准 工字梁。 这些梁是准备与组合的现浇行车道板一起使用的。它们往往使预制构件 的先张法预加应力与设置桥面板后的后张法预加应力相结合。

由于运输和安装大而且重的构件会遇到许多问题，预制梁不可能用于超出120英尺很多的跨度。另一方面，桥梁有采用较大跨度的明显倾向。通过取消中央桥墩和使边墩从分隔行驶的公路的边缘向外移出可以改善公路的安全。对于城市内的高甲快速公路，大跨度可以简化引道，并且使桥下活动的障碍物减至最小。出自对损坏周围环境的担心，也促使选用大跨度来建造连续的高架桥。对于跨河桥梁，由于通航宽度的要求，多半不能设置中间桥墩。

在欧洲，以及近年来在西半球，这些要求导致了分段施工的大跨度预应力混凝土箱形梁桥的发展。在这种桥型的典型施工中，往往应用滑模技术就地浇筑桥墩。然后在桥墩顶部浇筑“榔头”形梁节段，进而在每一方向用平衡悬臂梁法进行施工。施工或者采用现浇节段，或者采用预制节段向前推进，每一节段都与前面 已完成的结构用后张法联结。待跨中闭合的现浇接头做好以后，再用后张法张拉

整个结构以达到完全的整体性。

桥还可以分为上承式桥和下承式桥两种，上承式类型的桥，公路桥是支柱构造，那就是说上部结构的主要部分在车道以下。下承式类型的桥，车道穿过上部 结构的要素，就想钢筋桁架桥。上承式结构占优势，这很明显，它可以提供给司机更好的视觉效果，更大的事业，更容易躲避交通事故。

例如，短跨度混凝土桥，包括现浇，预应力混凝土T型钢构桥，预应力简支跨桥（这种桥属于预浇）和现浇箱梁。

每个中跨和长跨桥的设计者，都尽力设计一个可以适应所有特殊情况的桥，结果都迷惑于结构的多样化，区别于设计的原则和设计的细节。

下面的段落描述钢架桥的种类。

梁桥由两部分组成，板和箱梁。在美国，杆梁桥应用于中跨度桥，它们一般是连续结构，它们在墩台处有最大深度，在中跨处有最小深度。板梁桥一般有一个T形状部分，它被安排支撑纵梁，板的一般是现浇混凝土板。主梁是预制的焊接梁，用高强螺栓连接。

钢筋焊接箱梁结构与板桥结构，除了穿越形式不同之外，剩下的部分都相同。 钢架桥很少被应用，最常用的长度是在75 到100英尺分离层结构。

拱桥被用于更长跨度的桥，中间的支柱不能用，岩石可以抵抗拱桥的压力拱桥方面的一个变化是系杆拱。它的承受路面的水平系杆承受水平拱的压力。

斜拉桥尤其适用于200 到500m 的桥型，它给梁桥和悬索桥之间提供了个过渡。缆绳在桥面的上方，直接连在塔上。它删除桥面下的部分，可以为海上运输提供更广阔的空间。由于斜索的阻尼作用，斜拉桥桥面的摆动比悬索桥小。

悬索桥被应用于非常长的桥及中间不能建桥墩的短跨度桥。1964 年建成的Verrazano Narrows 桥就是个例子，耗资30.5亿美元， 跨度4260英尺 （1298.5m），它连接了纽约港与布鲁克林。

钢筋混凝土桥和钢桥一样种类繁多。

法国的桥梁建设得益于在公路基础设施方面的坚固的发展。 目前用其他材料的竞争转向合成桥解决办法的优势。近期，在合成桥里设计提出任何特征之前， 验证是重要的，通过一些统计资料的来分析桥梁市场。 在法国，长跨度桥是一个非常有限的市场。在新近建设的桥里，长度比200米长的跨度的桥是非常少的主要市场，是跨度 (或者多跨)不到100米的桥。

在法国每年建造800到1200座桥，意味着大约有300000㎡到500000㎡的桥面。但是，每年被建造的多数桥是小跨度的。少于10%的桥是长度超过30m和桥面大1000㎡的。尽管市场上已经有相应的鉴别方法，以竞争的形式，法国市场上有多种桥型。在19xx年，少于2.5%的桥是钢或者合成桥。钢混合成建设

已经继续稳定在过去的15年增长。这个趋势给预应力混凝土桥的压力主要由于在合成桥竞争的增加所带来的。

对短的跨度距离来说多数钢桥具有混凝土类型。为了满足刚度要求，由在混凝土里装箱的钢梁组成的桥经常用于小的跨度距离的铁路桥。

在新近SETRA进行的对1990到19xx年之间法国建造的各种桥统计评估中，有数据显示钢筋混凝土桥的具有竞争性的跨度距离范围是，30和110米之间，顶峰在60到80米之间。在跨度距离的范围内我们注意到建造的85%的桥属于合成 桥。

对桥面每平方米的数据统计说明，合成桥的价格比混凝土桥少的距离是40 米到80米。这不同是具有1500 /㎡通过总8200岁的费用/米2支付给那些合成桥(增 值税排除)。它意味着在竞争中有18%的变化。

过去15年已经看见车行道和铁路桥的合成桥的大简化，象以前表明的那样，这与预加压力并且加强的桥相比非常具有竞争性。这些合成桥，我们将命名他们古典的，但是后来有描述的几个特征。然后，从这些古典特征，这些描述以后经 常被对合成桥的设计和实行带来改进。

传统的合成车行道桥由两根长度主梁组成，它们用剪联编杆连接(通常使用 焊接，但是钢角连接器仍然被使用)。有一部分连接处是用横梁连接两条主梁的， 通常不连到板上的，见图：6 横断面-焊接在垂直加劲杆上。

主梁有一些水平的加劲杆，主梁顶推过程中产生的应力主要是由它们承受的。

下面介绍下钢筋混凝土和预应力混凝土，素混凝土是由水泥、水、细骨料(碎 石或卵石)、空气，通常还有其它外加剂等经过凝固石化而成。将可塑的混凝土拌合物注入到模板内,并将其捣实，然后进行养护，以加速水泥与水的水化反应，最后获得硬化的混凝土。其最终制成品具有较高的抗压强度得较低的抗拉强度。 其抗拉强度约为抗压强度的十分之一。因此，截面的受拉区必须配置抗拉钢筋和抗剪钢筋以增加钢筋混凝土构件中较弱的受拉区的强度。

由于钢筋混凝土截面在均质性上与标准的木材或钢的截面存在着差异，因此，需要对结构设计的基本原理进行修改。将钢筋混凝土这种非均质截面的两种 组成部分按一定比例适当布置，可以最好地利用这两种材料。这一要求是可以达 到的，因混凝土由配料搅拌成湿拌合物，经过振捣并凝固硬化，可以做成任何一种需要的形状。如果拌制混凝土的各种材料配合比恰当，则混凝土制成品强度较高，经久耐用，配置钢筋后，可以作为任何结构体系的主要构件。

浇筑混凝土所需的技术取决于即将浇筑的构件类型，诸如：柱、梁、墙、板、 基础、 大体积混凝土水坝或者继续延长已浇筑完毕并已经凝固的混凝土等。对

于 梁、柱、墙等构件，当模板清理干净后应该在其上涂油，钢筋表面及其它有害物 质亦应清除干净。浇筑基础前，应将坑底土夯实并用水浸湿 6 英寸，以免土壤从 新浇筑的混凝土中吸收水份。一般情况下，除使用混凝土泵浇筑外，混凝土都应在水平方向分层浇筑，并使用插入式或表面式高频电动振捣器振实。必须记住，过分的振捣将导致骨料分离和混凝土泌浆等现象，因而是有害的。

水泥的水化作用发生在有水分存在，而且气温在 50°F 以上的条件下。为了保证水泥的水化作用得以进行，必须具备上述条件。如果干燥过快则会出现表面 裂缝，这将有损于混凝土的强度，同时也会影响到水泥水化作用的充分进行。

设计钢筋混凝土构件时显然需要处理大量的参数，诸如宽度、高度等几何尺 寸，配筋的面积，钢筋的应变和混凝土的应变，钢筋的应力等等。因此，在选择 混凝土截面时需要进行试算并作调整，根据施工现场条件、混凝土原材料的供应 情况、业主对建筑的净空高度的特殊要求、所用的设计规范以及建筑物周围环境 条件等最后确定截面。 钢筋混凝土通常是现场浇筑的合成材料，它与在工厂中制造的标准的钢结构梁、柱等不同，因此上述一系列因素必须予以考虑。

对结构体系的各个关键部位均需选定试算截面并进行验算， 以确定该截面的 名义强度是否足以承受所作用的计算荷载。 因此设计时第一次采用的数值将导致 一系列的试算与调整工作。

选择混凝土截面时，采用试算与调整过程可以使复核与设计结合在一起。因此，当试算截面选定后，每次设计都是对截面进行复核。手册、图表和微型计算 机以及专用程序的使用， 使这种设计方法更为简捷有效，而传统的方法则是把钢筋混凝土的复核与设计孤立加以对待。

19xx年以来，预应力混凝土的迅速发展表明了对对于高质量的结构材料的实际需要。 结构必须具有高质量，这是因为结构构件承受荷载的最不利情况通常发生在它的寿命的初始阶段，即发生在将钢筋应力传递给混凝土的时候。因而破坏更易于在这时发生，而不是发生在以后混凝土强度有所提高，同时由于钢筋和混凝土的徐变以及混凝土的收缩而使钢筋应力有所降低的时候。所以，应该在构件成为结构物的组成部分之前，或者至少在撤换它已很不方便而且耗资较高之 前，及早检查出不合格的构件并予以报废。

与钢筋混凝土相比，预尖力混凝土的主要优点是：

（1）整个混凝土截面承受荷载。而在钢筋混凝土中约有一半的截面，即中性层一下的开裂面,不起作用。使用阶段的挠度较小。

（2）可以承受很高的工作应力。而在钢筋混凝土中，这通常是不可能的，因为这将导致构件严重开裂，开裂会影响美观，而且如果造成钢筋生锈的话，还会发生危险。

（3）预应力混凝土几乎可以完全避免出现裂缝。

预应力混凝土的主要缺点是，制造时需要比钢筋混凝土更加细心地操作和管理，因而成本较高；但由于其性能较好，所以预应力混凝土构件的尺寸较小。因此会遇到这样的情况： 一个结构问题如果采用预应力混凝土就可能比采用钢筋混 凝土所花费的成本更低； 而县城通常还会出现这样的情况：只有采用预应力才能得到解决，而不加预应力就无法解决。

对混凝土施加预应力， 指的是在混凝土承受任何使用荷载以前，使它受到压缩。这意味着混凝土截面可以设计得使它在全部设计荷载作用下，不承受拉应力，或只承受很小的拉应力。因此，从理论上说，预应力混凝土构件没有裂缝，而实 际上也只有非常少的裂缝。预应力一般是在埋置钢筋的混凝土完全硬化之前，通过张拉钢筋来施加的。在混凝土已经硬化得足以承受由钢筋传来的应力以后，此应力的一部分就可以由钢筋传递到混凝土上去。在桥台能承受推力的桥梁中，桥体混凝土中没有钢筋也能施加预应力。这种预应力是用千斤顶施加的，千斤顶迫使桥体受到自桥台向中心的力。这种方法的优点是在结构的使用寿命期间，千斤 顶的作用力，或者说预应力的大小可以按照需要进行调整。

在1950 年至1960 年的10年中，预应力混凝土不再是一种试验性材料，工程师们在应用这种材料方面获得了人们的信任。 这种信任使预制的预应力构件的应用日益广泛，尤其是在大跨度楼板或公路桥面构件方面。凡是所需制伏的构件的数量达到一定程度，例如在一座 500m 长的公路桥上，如果大多数跨度可以做成同样大小，并县城不大于18m 的话，采用在工厂中预制的预应力梁是经济的，至少在预制厂附近的工业区内是如此。

这类梁大多数是用加热法养护，以便能够很快脱模，使模板能够尽快地重复使用。

在这个时期内，美国的许多学校建筑也采用预制的预应力屋顶梁和楼板梁，它们有时可达 32m 或者更长一些。这样长的单跨梁除非现场浇筑，否则用钢筋混凝土是不可能取得成功的， 因为钢筋混凝土梁的梁高和重量都会远远大于预应力混凝土梁。与预应力混凝土梁相比，它们当然会显得不够美观，而且成本也往 高一些。这些学校建筑要求具备坚固和简朴的建筑感染力，并且在许多年的时间 内都会使人们看上去觉得赏心悦目。

预制的预应力混凝土梁最重要的部分是预应力筋和混凝土。预应力筋，顾名 思义就是在混凝土中承受拉力的钢缆、钢筋或钢丝束。在混凝土硬化以前（即在传递应力以前）预应力筋或者是未张拉（后张法），或者昌已经进行了张拉并固 定在混凝土外的台座上（先张法） ，随着混凝土的逐步硬化，它通过沿其全长的 粘结力将预应力筋握得越来越紧。端部锚具，包括锚板或锚块，设置在后张预应力

构件的预应力筋的两端。在混凝土已经硬化到一定程度时，对这些预应力筋进行张拉，将这些预应力筋进行张拉，将预应力传递给混凝土。在另外一种预加应力的方法中，先张拉预应力筋，待预应力筋从外部台座上放松时，通过粘结力或锚固力，或者两者兼而有之地将应力传递给混凝土，使混凝土受压变短，同时钢筋本身也缩短，因而损失一部分拉力。

随着时间的增加，混凝土（同时还有钢筋）还会产生进一步的缩短。混凝土的这种现象被称为徐变。徐变意味着在荷载作用下，混凝土会永久性地缩短，应 力在截面上会分布得更加均匀， 因而也更安全。 钢筋也产生徐变， 但徐变量很小。 这两种作用（还有混凝土干燥时的收缩）的结果是；预应力混凝土梁中的应力永 远不会高于传递预加应力时所受到的应力。

将来， 在工厂里预制预应力混凝土长梁可能会越来越普遍，但是路途运输将成业个难题。随着梁的长度的增加，运输车辆将变得越来越不容易操纵，以致于不得不将运输构件的最适宜时间安排在半夜交通量最少的时候。不管道路的曲直如何，道路运输对梁的长度的限制是随地区交通量和道路交通量而有所不同的。现场预制可以避免这些困难；尽管这样做成本可能高一些，但是这种方法已经常应用于制造大跨度桥的梁。

建筑材料必须具有对结构有用的某些物理性质。首先，建筑材料必须能够承担荷载或重量，而不会永久改变其原有的形状。当荷载施加到结构单元上时，材料将发生变形，也就是说，线材将伸长或梁将会弯曲。然而卸载后，线材和梁将恢复原状。材料的这种性质称为弹性。如果某种材料是非弹性的，在卸载后结构 将残留变形，重复加载和卸载，结构的变形将持续增加，直至最后结构失效。用于建筑结构的所有材料，诸如砖石、木材、钢材、铝材、钢筋混凝土和塑料等， 在一定范围的荷载作用下均表现出弹性。如果荷载增加超过了这个范围，材料将 表现出两种类型的性质： 脆性和塑性。 若为前者，材料将会突然断裂；若为后者，材料在达到某一荷载（屈服强度）开始塑性流动，最后破坏。例如，钢材表现出 塑性，石材则是脆性的。材料的最终强度用材料破坏时的极限应力来表示。

建筑材料第二个重要性质是刚度。这一性质用弹性模量来表示，弹性模量是应力（单位面积上的力）和应变（单位长度上的变形）的比值。因而弹性模量是衡量材料在荷载作用下抵抗变形能力的指标。 对于相同荷载作用下相同面积的两 种 材 料 ， 弹 性 模 量 越 高 者 变 形 越 小。结 构 钢 材，其 弹 性 模 量 是 3×10^8或 2100000kg/㎝^2,是铝材刚度的 3 倍、混凝土刚度的10倍、木材刚度的15倍。

砌体包含天然材料，如石材、人造产品如砖和混凝土砌块。砌块出现在远古时期。在古巴比伦城市，泥土砖用于建造非宗教性建筑物，而石材被广泛用于尼

罗河流域雄伟的寺庙。高及 481ft（147m）的埃及大金字塔是最为壮观的石工建筑。最初，砌块的叠砌是不用胶结剂的，但所有现代圬工建筑都使用水泥砂浆做 胶结材料。这一类现代建筑材料包括石材、 烧结粘土砖或页岩砖以及混凝土砌块。

砌体材料基本上属于受压材料，不能承受张拉力，亦即拉力。砌体的极限抗压强度取决于块体和砂浆。极限强度在1000到 4000(70～280kg/cm^2之间变化，其值取决于所有块体和砂浆的具体结合。

木材是一种最古老的建筑材料，是少数具有极好抗拉性能的天然材料之一。全世界已经发现的木材种类有数百种，每一类都表现出不同的物理特性。只有少数木材在建筑中被用作结构构件。例如，在美国，600 多种木材中仅有 20 种被 用于结构。这些木材通常是一些针叶树或软木材，主要因为这两类木材资源丰富 以及易于成型。在美国建筑中较为普遍使用的木材树种是花旗松、南方松、云杉和红木。这些木材的极限抗拉强度变化范围5000～8000Ib/in^2（350～ 560kg/cm^2） 。硬木材主要用于细木工或用于铺地板之类的室内装修。

由于木材本身有细胞状构造，其顺纹强度要大于其横纹强度，木材顺纹的抗 拉强度和抗压强度尤其高，并且有很好的抗弯强度。这些性质使得木材成为建筑 结构中柱和梁的理想材料。但是，由于桁架杆件中抗拉强度取决于各杆件间的连接，所以木材不能有效地在桁架中用作受拉构件。尽管为了利用木材的抗拉强度制造出许多金属节点，但很难设计出与顺纹剪切强度或抗裂强度关系不大的接头。

钢材是一种优异的结构材料。与其他材料相比，钢材有高强度质量比（单位 质量的强度） 即在相同体积条件下其质量是木材的10 倍以上。钢材具有较高的，弹性模量，这就使得钢材在荷载作用下变形较小。钢材可被轧制成各种不同的结构形状，如工字型梁、钢板和压型钢板，还能被铸造成复杂形状，也能用以生产 出钢丝和钢绞线，用作悬索桥和悬索屋面的钢缆，电梯升运机缆索，或用作预应 力混凝土的钢丝绞线。钢制构件可以用多种方法进行连接，如螺栓连接、铆接或焊接。碳素钢易遭氧化导致腐蚀，必须防止其与大气的接触，可采用在其上刷防锈漆或将其埋入混凝土的办法。当温度高于 700F（371°C）时，钢材将迅速丧失 其强度，因而必须在其外包裹上放火材料（通常为混凝土）对其加以保护。

合金元素如硅或锰的加入使钢材强度变得更高，其抗拉强度可达 250000 Ib/in^2（17 500 kg/cm^2）。当结构构件的尺寸变得重要时，如摩天大楼的柱子，就要使用这类合金钢。

当轻质、强度和防腐蚀能力成为建筑考虑的重要因素时，铝材作为一种建筑

材料就显得特别有用。因为纯铝极软，易延展，必须在其中加入锰、硅、锌和铜 这些合金元素，使其获得结构所要求的强度。建筑用铝合金表现出弹性，其弹性 模量是钢材的 1/3，因而在相同荷载作用下，其变形为钢材的3倍。铝合金的密度为钢材的1/3，因而在相似强度条件下，铝合金构件比钢材构件轻。铝合金的极限抗拉强度范围在 20000～60000Ib/in^2 (1400～4200 kg/cm^2)。铝材能被加工成各种形状，可以被挤压成工字型梁，拔成线材和杆件，辊压成铝箔和板材。铝 构件可以像钢材一样采用铆接、螺钉连接以及（较少地）焊接等方式进行连接。 铝除了用作建筑和预制房屋的框架构件以外，还被广泛地用作窗框，以及幕墙建 筑物的幕墙材料。

混凝土是水、砂、石子和波特兰水泥的混合物。碎石、人造轻骨料、贝壳经常被用以代替天然石料。 波特兰水泥，是将由钙质材料和粘土质材料形成的混合物在窑中进行煅烧然后进行粉磨而形成的。混凝土强度即源于磨细的水泥与水混合时经水化而硬化的过程。在理想的混合状态下，混凝土由占其体积大约 3/4 的砂、石子和占其体积 1/4 的水泥浆组成。混凝土的物理特性对其组成成分变化 是极其敏感的，所以为了获得混凝土在强度和收缩等方面特定的效果，必须对这些组成材料的配料进行特定的设计。当往模具或模板中浇注时，混凝土中含有大 量并非用于水化而是要蒸发掉的水。混凝土硬化时，经过一段时间将蒸发掉多余 的水而产生收缩，这种收缩通常将导致细裂缝的发展。为了将这些裂缝减至最少，混凝土硬化时必须保持潮湿状态至少在5天以上。 因为混凝土的水化过程能持续进行多年，故其强度能够持续增长。事实上，常把混凝土28 天的强度视为标准 强度。

混凝土在荷载作用下会发生弹性变形。尽管混凝土的弹性模量是钢材的1/10，但由于其强度也大约是钢材的1/10，所以它们有相似的变形。混凝土主要用作抗压材料，其抗拉强度可不予考虑。

钢筋混凝土中配有钢筋，用以承受混凝土构件中的拉力。这些钢筋的直径范围在 0.25in（0.64cm）～2.25in（5.7cm） ，其表面带肋，以保证与混凝土的粘结。尽管钢筋混凝土在很多国家得到发展，但其发现一般归功于约瑟夫·蒙约 （Joseph Monnier） ，一位法国园丁，他在 1868 年曾使用钢筋网片来加强混凝土管，因为温度变化时，钢材与混凝土胀缩系数相同，所以这种做法是可行的，如若不然，钢材与混凝土的粘结会因温度的变化导致两者变形不一致而破坏。钢筋混凝土可以浇注成各种形状， 如梁、柱、 板和拱，因而适用于特殊形态的建筑物。钢筋混凝土的极限抗拉强度可能会超过 10000Ib/in^2（700 kg/cm^2），尽管生产的 大部分商品混凝土的强度低于 6000Ib/in2（420kg/cm^2） 。

混凝土抗压但不抗拉，其抗拉强度是其抗压强度的 8%～14%。由于混凝土

抗拉能力如此低，在荷载作用早期，混凝土内部即出现弯曲裂缝。为减缓或防止裂 缝发展，可沿结构构件纵向施加轴心力或偏心力。这种力通过消除或尽可能地减 少使用荷载作用下跨中临界截面的拉应力，以防止裂缝发展，从而增大了截面抵抗弯曲、剪切和扭转的能力。当全部荷载作用在结构上时，混凝土截面表现出弹性，混凝土的极限抗拉能力几乎在混凝土全截面高度得到有效利用。