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1、2007届港海专业毕业设计译文 A、Concrete a、PIain Concrete Con Crete is a ston e-like material obta ined by p ermitti ng a carefully prop orti oned mixture of ceme nt, sand and gravel or other aggregate nd water to harde n in forms of the sha peand dime nsions of the desired structure. The bulk of the material con

2、 sists of fine and coarse aggregate. Ceme nt an water in teract chemically to bi nd the aggregate p articlesi nto a solid mass. Additional water, over and above that needed for this chemical reaction, is necessary to give the mixture the workability that enables it to fill the forms prior to harde n

3、ing. Con cretes in a wide range of stre ngth prop erties can be obta ined by approp riate adjustment of the proportions of the constituent materials. Special cements (such as high-early-strength cements),special aggregates (such asvarious lightweight or heavyweight aggregates), and sp ecial curi ngm

4、ethods (such as cream-curi ng) p ermit an evenwider variety of prop erties to be obta ined. These properties dependto a very substantial degree onthe proportions of the mix, on the thorough ness with which the various con stitue nts are in termixed, and on the con diti ons of humidity and temp eratu

5、re in which the mix is mai ntai ned from the mome nt it is p laced i n the forms un til it is fully harde ned. The p rocess of con trolli ng these con diti ons is known as curi ng. To p rotect aga inst the uninten ti onal p roducti on of substa ndard con crete, a high degree of skillful control and

6、supervision is necessarythroughout the process, from the proportioning by weight of the in dividual componen ts, throughmix ing and placing, un til the comp leti on of curi ng. The factors which make con crete auni versal buildi ng material are so pronoun cedthat it has been used, in more primitive

7、kinds and ways than at present, for thousands of years, p robably beg inning in Egyp tia n an tiquity. The facility with which, while p lastic, it can be dep osited and made to fill forms or molds of almost any p ractical sha pe is one of these factors. Its high fire and weather resista nee are evid

8、e nt adva ntages. Most ohe con stitue nt materials, with the po ssible exce pti on of ceme nt, are usually available at low cost locally or at small dista nces from the con struct ion site. Its comp ressive stre ngth, like that of n atural ston es, is high, which makes it suitable for members p rima

9、rily subject to comp ressi on, such as colu mns and arches. On the other hand, again as in natural stones, it is a relatively brittle material whose ten sile stre ngthis small comp ared with its comp ressive stre ngth. Hence p lain con crete is usedoniy for footings and concrete slabs laid on the gr

10、ound, and for massive structures such as retai ning walls; eve n the n rei nforceme nt is freque ntly empio yed. b、Properties of Concrete and reinforced Concrete Con crete is a man-madec on glomerate stone comp osed of esse ntially four in gredie nts: P ortla nd ceme nt, water, sand, and coarse aggr

11、egate. The ceme nt and water comb ine to make a p aste that binds the sand and stones together. Ideally, the aggregatesare graded so that the volume of paste is at a minimum, merely surrounding every piece with a thin layer. Most structural con crete Is stonecon crete, but structural lightweight con

12、 crete (roughly two-thirds the den sity of stone con crete) is beco ming in creas in gly popu lar. Con crete is esse ntially a comp ressive material hav ing almost no ten sile stre ngth, so con crete s weak ness in tension also causes it to be weak in shear. These deficie ncies are overcome by using

13、 steel bars for rein forceme nt at the pl aces where ten sile and sheari ng stresses are gen erated. Un der load, rein forced con crete beams actually have nu merous minute cracks which run at right an gles to the direct ion of major ten sile stresses. The ten sile forces at such locations are being

14、 taken compietely by the steel -bars. ”“re The comp ressive stre ngth of a give n con crete is a fun ctio n of the quality and prop orti ons of its con stitue nts and thema nner in which the fresh con creteis cured. (Curi ng is the p rocess of harderi ng duri ng which time the con crete must be p re

15、ve nted from “ dryin gout ” as the p rese nee of water is n ecessaryfor the chemical acti on to p rogress.) Coarseaggregate thatis hard and well graded is p articularly esse ntial for quality con crete. The most imp orta nt factor governing the strength, however, is the percentage of water used in t

16、he mix. S minimum amount of water is n eededfor proper hydrati on of the ceme nt. Additi onal water is n eededfor han dli ng and p laci ng the con crete, but excess amounts cause the stre ngth to drop marke dly These and other top ics are fully covered in the booklet,“ Desig n and Con trol of C Mixt

17、ures, published by the P ortla nd Ceme nt Associati on. This is an excelle nt refere nee, treati ng both con crete mix desig n and proper con structi on p ractices. The America n Con crete In stitute p ublishes a widely adop ted code sp ecify ing the structural requireme nts for rein forced con cret

18、e. Concrete is known as the “ formable or “ moldable structural material. Compared to other materials, it is easy to make curvili near members and surfaces with con crete. Ithas no in here nt texture but adopts the texture of the formi ng material, so it can range widely in surface appearancet is re

19、latively inexpensive to make, both in terms of raw materials and labor, and the basic in gredie nts of P ortla nd ceme nt are available the world over. (It should be no ted, however, that the n ecessary rein forci ng barsfor con crete may not be readily available in lessdevel oped coun tries.) Concr

20、ete is naturally fireproof and needs no separateprotection system. Becauseof its mass, it can also serve as an effective barrier to sound tran smissi on. In viewing the negative aspects, concrete is unfortunately quite heavy and it is often no ted that a con crete structure expends a large p orti on

21、 of its cap acity merely carry ing itself. Atte mp ts to make con crete less den se, while mai ntai ning high quality levels, have gen erally resulted in in creased costs. Neverthelessu se of lightweight con crete can sometimes result in overall econo mies. Concrete requires more quality control tha

22、n most other building materials. Modern tran sit-mixed con crete supp liers ate available to all U. S. urba n areas and the mix is usually of a uniformly high quality. Field-or-job-mixed concrete requires knowledgeable supervision, however. I n any ty pe of con crete work, miss ing or mislocated rei

23、n forci ng bars can result in eleme nts with reduced load cap acities. Poorhan dli ng an d/or curi ng con diti ons can seriously weaken any concrete. For these and other reasons, most building codes require independent field inspections at various stages of con struct ion. Proper concrete placement

24、is also somewhat dependent upon the ambient weather con diti ons. Extremely high temp eratures an d,more imp orta nt, those below (or n ear) freez ing can make con crete work very difficult. Concrete derives its strength from the fact that pulverized portland cement when mixed with water, hardens by

25、 a process called hydration In an ideal mixture, concrete consists of about three fourths sand and grav( aggregate by volume and one fourth cement paste The physical properties of concrete are highly sensitive to variations in a mixture of the components so a p articular comb in ati on of these in g

26、redie nts must be custom-desig ned 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 shrink

27、sAs 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 in creases in time because the hydrati on p rocess continues for yearas a p ractical matte, the stre ngt

28、h at 28 days is con sidered sta ndard Concrete deforms under load in an elastic manner. Although its elastic modulus is one tenth that of steel similar deformations will result since its strength is also is one tenth that of steel Concrete is basically a compressive material and has negligible tensi

29、le streng.th c、Advantages and Disadvantages of Concrete and Its Water-Cement Ratio Con crete is a mixture of P ortla nd ceme nt, water, sand, and crushedgravel or stone. The water and ceme ntform a ceme nt p astei n which the sand and stone orgravel are mixed. The sand and stone or gravel together m

30、ake up the aggregate of a con crete mixture. The aggregate serves no structural function .It is merely a filler that adds low-cost bulk to the ceme nt p aste; it usually makes up about 75 p erce nt of a give n mass of con crete, by volume, although a poor aggregateca n reduce the4 stre ngth of a bat

31、ch of con crete con siderably,good aggregate adds only slightly to the stre ngth of the ceme nt. The two principal advantages of concrete as a construction material are its relative chea pn ess and the ease with which it can be han dled and pl aced while it is in the pl astic state. The principal st

32、ructural advantages of concrete are its great compressive strength and its durability. Con crete can withsta nd very high comp ressive loads. This is what makes con crete so suitable for the foun dati ons, walls, and colu mns of build in gs, and for driveways and walks as well. The principal structu

33、ral disadva ntage of con crete is its poor ten sile stre ngth. That is, it cannot withsta nd p ulli ng or bending loads without crack ing or break ing. For this reas on, steel rods, or reinforcement steel, are often embedded in concrete, the reinforcement steel p rovid ing the ten sile stre ngth the

34、 con crete lacks. Con crete with rein forceme nt steel embedded in it is rein forced con crete. In addition to its poor tensile strength, concrete, like most construction materials, expands in hot weather and whe n wet and con tracts in cold weather and as it dries out. Uni ess these moveme nts are

35、allowed for duri ng con struct ion, the con crete will crack. And, con trary to com mon belief, solid con crete is not imp ervious to water. Some moisture will migrate into the best-made concrete.But if the concrete should be excessively po rous, which can happen if too much water has bee nu sed in

36、mix ing it, moisture can easily enter the concrete after it has cured. If this moisture should be present within the concrete whe n cold weather comes, the moisture may freeze, which may result in serious frost damage to the structure. Desp ite these limitati ons, con crete is an in here ntly stro n

37、g and durable con struct ion material. If the prop orti ons of water, ceme nt, and aggregate are carefully calculated an(if the con crete is pl aced and allowed to cure accord ing to sim pie but defi nite rules, it is p ossible to obta in from the con crete all the stre ngth and durability that is i

38、n here nt in it. The ratio of water to ceme nt in a batch of con crete is the principal determ inant of the con crete ? final stre ngth. As one time the in structi ons for preparing a batch of con crete would have contained prop orti ons such as 1:2:4,i ndicati ng that 1 part of P ortla nd ceme nt t

39、o 2 p arts of sand to 4 p arts of gravel by volume were to be mixed together, after which sufficie nt water was to be added to obta in a workable mixture. This p rocedure ignored en tirely the importance of the water-cement ratio. It also resulted very often in the preparation of a very weak con cre

40、te, since the n atural tendency is to add eno ugh water to make pl aceme nt of the con crete as easy as p ossible-the slopp ier the better, afar as the workme n are concern ed.This manner of sp ecify ing the prop orti ons of con crete is obsolete and should n ever be followed. In theory, it takes on

41、ly 3 gal of water to hydrate comp letely 1 cu ft of ceme nt. (A sack of cement contains 1 cu ft exactly, and the sack weighs 94 1b). But this water-cement ratio produces a mixture that is too stiff to be worked. In practice, therefore, additional water, betwee n 4 and 8 gal per sack of ceme nt, is u

42、sed to obta in a workable mixture. But the greater the proportion of water in a water-cement ratio, the weaker the final con crete will be. The additi onal water that is n ecessary to achieve a workable batchwill only evaporate from the concrete as the concrete sets, and it will leave behind in the

43、concrete innumerable voids. This is the reason there will always be some porosity in the concrete. When an excessive amount of water has been used, there will be an excessive number of voids, which may cause the concrete to leak badly. If these voids should be filled with moisture whe n cold weather

44、 comes, they will cause the frost damage alluded to above. As a gen eral rule, therefore, 6 gal of water per sack of ceme nt should be the maximum amount used for making con crete; and the less the amount of water that is used,the stron ger the con crete will be. Also in cluded in the 6 gal is whate

45、ver surface moisture is contained is the sand that is part of the aggregate. B、Reinforced Concrete To offset the limitati on of plain con crete, it has bee nfound p ossible, in the sec on dhalf of the nin etee nth cen tury to use steel with high ten sile stre ngth to rei nforce con crete, chiefly in

46、 those places where its small tensile strength would limit the carrying capacity of the member. The rein forceme nt, usually round steel rods with approp riate surface deformati ons to pro vide in terlock ing, is p laced in the forms in adva nee of the con crete. Whe n compi etely surr oun ded by th

47、e harde ned con crete mass, it forms an in tegral part of the member. The result ing comb in ati on of two materials, known as rein forced con crete, comb in esma ny of the adva ntages of each: the relatively low cost, good weather and fire resista nee, good comp ressive stre ngth, and excelle ntfor

48、mability of con crete and the high ten sile stre ngth and much greater ductility and tough ness of steel. It is this comb in ati on which allows the almost un limited range of uses and p ossibilities of rein forced con crete in the con structi on of buildi ngs, bridges, dams, tan ks, reservoirs, and

49、 a host of other structure. The steel bars in con crete take the ten sile component of the bending mome nt. But they do not p reve nt the con crete from crack ing (Fig. 6-1) The p rese nee of fine cracks in rei nforced con crete is in evitable. The stress in the lowest-grade reinforcing steel under

50、the normal working loads is of the order of 140 Mpa. Taking the modulus of elasticity of steel as 200 Gpa, this amounts to an elastic strain of -4 7X10 , which is more than the ultimate tensile strain of concrete. Cracks are thus produced in the concrete by the mere process of the reinforcing steel

51、being stressed under the normal work ing loads. It is p erha psfort un ate that this was not un derstoodwhe n rein forced con crete was first empio yed more tha n a cen tury ago, otherwise buildi ng authorities with a reas on able concern for p ublic safety would p robably have forbidde n its use. B

52、ecauseif the cracks are kept very small and are bridged by tension steelthey have no adverse effect on the safety or durability of the structure, the safety of rein forced con crete structures depends on thewidth of the cracks being kept below a permissible minimum and this has become a more serious

53、 problem in recent years because the use of higher steel stressesalso in creases the stra in of the con crete. Cracks would not only be un sightly but would expose the steel bars to corrosion by moisture and other chemical action. Thus crack con trol is a more serous matter i n rein forced con crete

54、 desig n as comp ared with, say, twenty years ago. Rein forced con crete has steel barsthat are p laced in a con crete member to carry ten sile 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 t

55、he concreteAlthough rein forced con crete was devel oped in many coun tries, its discovery usually is attributed to Jose ph Monnier, a French garde ne, who used a wire n etwork to rein force con crete tubes in 1868. This p rocess is workable becausesteel and con crete expand and con tract whe n the

56、temperature changes If this were not 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

57、 adapted to a particular form of building. Reinforced concrete with ultimate tensile strengths in excess of 10,000 psi (700 kg/sqcm) is possible, although most commercial concrete is produced with strengths under 6,000 psi (420 kg/sqcm). The best structural use of rein forced con crete, in terms of

58、the characteristics of the material, is in those structures requiri ng continu ity and or rigidity. It has a mono lithic quality which automatically makes fixed or continuous connections. These moment-resistant joints are such thatma ny low-rise con crete build ings do not require a sec on dary brac

59、i ng systemfor lateral loads. In essence,a concrete beam joins a concrete column very differently from the way steel and wood pieces join, and the sensitive designer will not ignore this differenee. (These remarks do not apply to p recast structural eleme nts, which are usually not joined in a con t

60、i nu ous mann er.) C、Prestressing Concrete Methods of inducing compression in concrete member before it is loaded is known as p restressi ng. The con struct ion which uses steels and con crete of very high stre ngth in comb in ati on is known as p restressed con crete (Fig. 6-2). The steels, mostly