土木工程外文翻譯--混凝土施工裂縫原因及防治

1、<p>　　Construction cracks in concrete, causes and prevention</p><p>　　Construction cracks of concrete reason for the external and internal. The initial mainly refers to the internal defects of concrete

2、in early concrete, its internal hardening process will exist numerous micro-pores, air-pocket and micro cracks, External refers to the load reason: temperature crack, dry shrinkage effect etc. In these, can produce vario

3、us forms of concrete cracks. To grasp the construction process of raw materials, in pouring process configuration of progress and thickness, grasp t</p><p>　　A, The cause of the tear</p><p>　　1.

4、The initial defects of concrete</p><p>　　Concrete is composed of cement, sand aggregate, mixed with water and other materials and formation of heterogeneity brittle materials. In the beginning, because harde

5、ned concrete construction and deformation, a series of problems, such as constraint in the concrete shape sclerosis will exist numerous micro-pores, air-pocket and micro cracks of concrete that usually called initial def

6、ects. Micro cracks is usually a cracks in the concrete, bearing, and some other function not harm. But under load,</p><p>　　2.The load factor</p><p>　　Due to the tensile strength of concrete str

7、ucture or structures, small as a part of the external effects of stress than concrete tensile strength, will crack. Therefore, the concrete in the load and the load, can produce a crack. To load the crack, can produce by

8、 reasonable design and construction to prevent. For the load of crack, due to reasons, it is hard to handle and prevent. The more common than load factors have temperature, dry shrinkage, settling contraction deformation

9、, plastic shrinka</p><p>　　3.Big volume concrete crack formation reasons</p><p>　　Massive concrete cracking reasons can be divided into two types: one is the structural cracks, is caused by exte

10、rnal load, 2 it is material of crack is not caused by the changes of deformation and temperature stress and is mainly composed of concrete's shrinkage. This article mainly discussed type materials. One specific reaso

11、ns are as follows:</p><p>　　The temperature stress caused by fracture (temperature crack) temperature cracks in the main reason is caused by temperature difference. Temperature can be divided into three type

12、s: early concrete pouring, produce large amounts of the hydration heat, due to the concrete is a poor conductor of heat accumulation in concrete, the hydration heat to send out, often make internal temperatures rise, con

13、crete and reinforced concrete surface temperature for outdoor environment temperature, it is formed, </p><p>　　Shrinkage crack contraction there are many causes, including drying shrinkage, plastic shrinkage

14、, shrinkage, carbonation contraction, etc. Here mainly introduces drying shrinkage and plastic shrinkage.</p><p>　　After drying shrinkage of concrete sclerosis, in dry environment, the water concrete to the

15、loss caused by inside extroversion, concrete the shrinkage deformation cracks.</p><p>　　The plastic shrinkage of cement and concrete in the activity of higher temperature, or in low water-cement condition ex

16、acerbate cause craze. When the concrete surface water obviously decreased, lactation evaporation of water cannot get the timely supplements, then concrete is still in a state of plastic, by a concrete surface tension, un

17、even distribution will appear the cracks of concrete cracks, and further intensify the moisture inside body, and further expansion crack</p><p>　　Second, the measures to prevent cracks</p><p>　　

18、From the above analysis, the crack is mainly caused by temperature difference and contraction, so in order to prevent the cracks of, will maximize reduce temperature and reduce the shrinkage of concrete, specific measure

19、s as follows.</p><p>　　1. Optimizing raw materials</p><p>　　(1)Cement. The cement should be chosen according to the engineering properties, construction condition, climate and other technical re

20、quirements, comprehensive consideration, to meet the strength and durability of concrete design requirements. When the choice should be labeled with cement concrete design match the label. For full strength of cement, ce

21、ment, cement save from the label for concrete label 1.5 to 2 times as appropriate.</p><p>　　(2) aggregate. (a) coarse aggregate. The coarse aggregate concrete rubble is close-grained, must have the enough st

22、rength, hard to expand the coarse aggregate size. To ensure the concrete pouring dao close-grained, coarse aggregate maximum diameter less than the minimum size of the structural sections 1/4, also not greater than the m

23、inimum distance between steel net for thickness 3/4, in 10 centimeters and concrete, allowing adopts maximum diameter 1 1/2 of the aggregate number, but it shall not</p><p>　　(3) admixtures. (a) water-reduci

24、ng agent. Jianshuiji belongs to the surface active substances, is a kind of cement dispersant. Its main function of improving concrete workability, reduce the consumption of concrete mixture, reduce water-cement ratio in

25、creases, and improve the liquidity of concrete strength of concrete or keep in concrete cement strength must decrease, while the water-cement ratio, the dosage of cement to prevent cracking is reduced. (b) retarder. Reta

26、rder is delaying the role of</p><p>　　(4) mass concrete chooses raw materials should pay attention to the following:</p><p>　?、?The coarse aggregate gradation, appropriate USES continuous fine a

27、ggregate ZhongSha should be adopted.</p><p>　?、?Admixtures appropriate USES retarder, water-reducer, Admixtures appropriate USES fly ash, slag powder etc.</p><p>　?、?In mass concrete strength of

28、 concrete and the slump and the requirements of the premise, should improve mixing and aggregate content, in order to reduce the dosage of cement concrete alone.</p><p>　?、?Should try to choose cement hydra

29、tion heat low, setting time is long, the cement, low thermal Portland cement slag silicate cement, cement, slag dam Portland cement, coal ash Portland cement, volcanic gray Portland cement, etc.</p><p>　　How

30、ever, the slag cement hydration heat low water cement ratio of chromatography in other large, casting lots of water surface layer. This phenomenon is not only affect lactation water, also influence the construction speed

31、 and quality of construction. Because the water gathered in separation between the two casting concrete surface layer, change, and water-cement ratio in the water and took out some of the mortar, thus forming a layer of

32、interlining content, destroyed the cohesive force of conc</p><p>　　2.the reasonable construction method</p><p>　　(1) The concrete mixing and transportation. (a) white, accurate grasp of concrete

33、 proportion, especially the strict control of water consumption, the determination of sand. Records of every block water cement, sand and gravel, specific quantity, the application must be accurate to scale, by mixing. G

34、ood control of concrete slump and general shoulds not be too big, in 120 ± 20 mm. To minimize the mouth 3 concrete temperature. (b) concrete mixing in a storehouse after good transport process, to pr</p><

35、p>　　(2)Concrete pouring and open mode. Pouring process may be close-grained, vibration to vibration time should be uniform in surface functional plasma advisable, uniform spacing, vibrating force scope affected overla

36、p 1/2 advisable, after casting, compaction, surface must, in order to prevent the surface crack. Try to avoid in the solar radiation, high time due to the needs of the project if pouring in summer construction, avoid noo

37、n pouring temperature period, try to arrange at night. Time should </p><p>　　(3) Do surface insulation protection of mass concrete temperature crack, is mainly caused by excessive temperature. Concrete pouri

38、ng, due to internal heat faster, more surface temperature will form the internal and external surface by internal restraint, contraction, but the tensile stress, not more than usually have small tensile strength and the

39、concrete cracks. But if the air raid, or by excessive ventilation cooling, make the surface temperature will easily lead to excessive temperature crac</p><p>　　(4) Of the mass concrete casting:</p>&l

40、t;p>　　Construction scheme, besides should satisfy each place of concrete in early before a new concrete are covered and rammed finished outside, still should consider structure size, steel pipe, buries the bolts and l

41、eave, concrete and hydration heat supply, the influence of factors such as the commonly used method has the following kinds:</p><p>　　Comprehensive stratified:</p><p>　　In the first layer after

42、pouring all comprehensive casting, casting back second, should make the first layer of concrete yet, so early, continuous casting until completion by far. Using this scheme, applicable to the general structure of planar

43、size shoulds not be too big, when construction from the short side began to push along the long side, is more appropriate. When necessary, can be divided into two sections from middle to both ends to or from the middle a

44、nd construction.</p><p>　　Section 2 layer:</p><p>　　Concrete casting, starting from scratch, casting a certain distance to the second floor, so after pouring in other layers on casting. Due to t

45、he total number of layers, therefore, after the first layer top pouring concrete yet at the end of the beginning of the second period, and can turn from layered casting. This scheme is applicable to units within the time

46、 required, the supply of concrete less like the first option on that. This scheme is applicable to structure is too big and thickness of l</p><p>　　Incline stratified:</p><p>　　Requirements of s

47、lope not greater than cant 1/3, applicable to greatly exceed the length of the structure of the thickness of the three times. From the bottom layer of concrete casting, gradually move.</p><p>　　3.maintenance

48、 measures</p><p>　　Maintenance of mass concrete construction is a vital work. Maintain proper maintenance is mainly to control temperature and humidity, temperature, promote concrete outside the normal devel

49、opment of the strength of concrete and prevent the cracks of concrete and development. According to the specific circumstances of should as far as possible much maintenance period of time, after dismantling formwork shou

50、ld immediately, covering protection to prevent recent climate, sudden cold temperature, prev</p><p><b>　　4 Cooled</b></p><p>　　If in the high-temperature seasonal construction, shoul

51、d be in early general refrigeration adopted to reduce the water temperature, but the peak of concrete, water cannot too long, for a long time can cause excessive temperature range of temperature stress caused by large. I

52、n order to cut inside and outside, still should temperature in summer jiamoqiuchu undertake metaphase is cooled by water, river, general middle water lasted about two months. Later, water is made concrete cylindrical blo

53、cks t</p><p>　　Third, the conclusion</p><p>　　Concrete crack, especially large volume concrete crack is currently scholars and engineering is an important problem of attention, through the above

54、 analysis, the material of mass concrete crack is mainly composed of temperature stress and concrete's shrinkage, the author thinks carefully select material, and in the use of reasonable construction method, can eff

55、ectively prevent the cracks.</p><p>　　混凝土施工裂縫原因及防治</p><p>　　混凝土施工裂縫原因不外乎外因和內(nèi)因。內(nèi)因主要指混凝土的初始缺陷：在混凝土初期硬化過程中，其內(nèi)部會存在眾多的微孔隙、氣穴和微裂縫；外因是指非荷載原因：溫度裂縫、干縮作用等。在這些作用下，混凝土會產(chǎn)生各種形式的裂縫。施工過程中要掌握好原材料的配置，在澆筑過程中把握好進度和

56、厚度，及時做好養(yǎng)護工作，這樣就可盡量避免混凝土裂縫了。一、裂縫產(chǎn)生原因1.混凝土的初始缺陷　　混凝土是一種由砂石骨料、水泥、水及其他外加材料混合而形成的非均質脆性材料。在混凝土硬化初期，由于混凝土的施工和本身變形、約束等一系列問題，硬化成型的混凝土中會存在眾多微孔隙、氣穴和微裂縫，通常把這稱作混凝土的初始缺陷。微裂縫通常是一種無害裂縫，對混凝土的承重、防滲及其他一些使用功能不會產(chǎn)生危害。但是混凝土在受到荷載、溫度、收縮等作用下，微

57、裂縫就會不斷擴展和連通，最終形成肉眼可見的宏觀裂縫，也就是混凝土工程中常說的裂縫，這會對混凝土結構的使用性能和耐久性產(chǎn)生不利的影響。2.非荷載因素　　由于混凝土的抗拉強度很小，當結構或構件的某一部分由于外部作用產(chǎn)生的拉應力大于混凝土的抗拉強度時，就會出現(xiàn)裂縫。因此，混凝土在荷載與非荷載作</p><p>　　3. 大體積混凝土裂縫形成的原因</p><p>　　大體積混凝土裂縫產(chǎn)生的原

58、因可分為兩類：一是結構型裂縫，是由外荷載引起的；二是材料型裂縫，是由非受力變形變化引起的，主要是由溫度應力和混凝土的收縮引起的。本文主要探討材料型裂縫。其中具體原因如下：</p><p>　　溫度應力引起裂縫（溫度裂縫） 目前溫度裂縫產(chǎn)生主要原因是由溫差造成的。溫差可分為以下三種：混凝土澆注初期，產(chǎn)生大量的水化熱，由于混凝土是熱的不良導體，水化熱積聚在混凝土內(nèi)部不易散發(fā)，常使混凝土內(nèi)部溫度上升，而混凝土表面溫度為

59、室外環(huán)境溫度，這就形成了內(nèi)外溫差，這種內(nèi)外溫差在混凝土凝結初期產(chǎn)生的拉應力當超過混凝土抗壓強度時，就會導致混凝土裂縫；另外，在拆模前后，表面溫度降低很快，造成了溫度陡降，也會導致裂縫的產(chǎn)生；當混凝土內(nèi)部達到最高溫度后，熱量逐漸散發(fā)而達到使用溫度或最低溫度，它們與最高溫度的差值就是內(nèi)部溫差；這三種溫差都會產(chǎn)生溫度裂縫。在這三種溫差中，較為主要是由水化熱引起的內(nèi)外溫差。</p><p>　　收縮引起裂縫 收縮有很多種

60、，包括干燥收縮、塑性收縮、自身收縮、碳化收縮等等。這里主要介紹干燥收縮和塑性收縮。</p><p>　　燥收縮 混凝土硬化后，在干燥的環(huán)境下，混凝土內(nèi)部的水分不斷向外散失，引起混凝土由外向內(nèi)的干縮變形裂縫。</p><p>　　塑性收縮 在水泥活性大、混凝土溫度較高，或在水灰比較低的條件下會加劇引起開裂。因為這時混凝土的泌水明顯減少，表面蒸發(fā)的水分不能及時得到補充，這時混凝土尚處于塑性狀態(tài)

61、，稍微受到一點拉力，混凝土的表面就會出現(xiàn)分布不均勻的裂縫，出現(xiàn)裂縫以后，混凝土體內(nèi)的水分蒸發(fā)進一步加大，于是裂縫進一步擴展</p><p>　　二、防止裂縫的措施　　由以上分析，裂縫主要是由溫差和收縮引起，所以為了防止裂縫的產(chǎn)生，就要最大限度地降低溫差和減小混凝土的收縮，具體措施如下。1.優(yōu)選原材料(1)水泥。水泥的選用應根據(jù)工程性質、施工條件、氣候環(huán)境及其他技術要求，綜合考慮，以達到滿足混凝土設計強度和耐

62、久性的要求。選擇水泥標號時應與混凝土的設計標號相匹配。為充分發(fā)揮水泥的強度、節(jié)約水泥，一般取水泥標號為混凝土標號的1.5～2倍為宜。(2)骨料。(a)粗骨料。碎石是混凝土中的粗骨料，必須密實堅硬具有足夠的強度，盡量擴大粗骨料的粒徑。為保證混凝土澆搗密實，粗骨料的最大粒徑不大于結構斷面最小尺寸的1/4，同時不得大于鋼筋間最小凈距的3/4，對于厚度在10厘米及以下的混凝土，可允許采用最大粒徑達1/2的骨料，但其數(shù)量不得超過25%。(b)細

63、骨料。砂的顆粒級配影響混凝土的技術性質，既要求空隙率小又要求總表面積小，總表面小需要包裹砂子表面的水泥漿量也就減少，可以減少水泥用量，降低水化熱，對防止裂縫有利。另一方面也要控制砂的含泥量，一般混凝土中其含量<5%，含泥量越大，收縮變形越大，裂縫就越大，因此細骨料盡量用干凈的中粗砂。(</p><p>　　(4)大體積混凝土所選用的原材料應注意以下幾點： ① 粗骨料宜采用連續(xù)級配，細骨料宜采用中砂。

64、② 外加劑宜采用緩凝劑、減水劑；摻合料宜采用粉煤灰、礦渣粉等。 ③ 大體積混凝土在保證混凝土強度及坍落度要求的前提下，應提高摻合料及骨料的含量，以降低單方混凝土的水泥用量。 ④ 水泥應盡量選用水化熱低、凝結時間長的水泥，優(yōu)先采用中熱硅酸鹽水泥、低熱礦渣硅酸鹽水泥、大壩水泥、礦渣硅酸鹽水泥、粉煤灰硅酸鹽水泥、火山灰質硅酸鹽水泥等。 但是，水化熱低的礦渣水泥的析水性比其它水泥大，在澆筑層表面有大量水析出。這種泌水現(xiàn)象，不僅影響

65、施工速度，同時影響施工質量。因析出的水聚集在上下兩澆筑層表面間，使混凝土水灰比改變，而在掏水時又帶走了一些砂漿，這樣便形成了一層含水量多的夾層，破壞了混凝土的粘結力和整體性?；炷撩谒缘拇笮∨c用水量有關，用水量多，泌水性大；且與溫度高低有關，水完全析出的時間隨溫度的提高而縮短；此外，還與水泥的成分和細度有關。所以，在選用礦渣水泥時應盡量選擇泌水性的品種，并應在混凝土中摻入減水劑，以降低用水量。在施工中，應及時排出析水或拌制一些干硬性混

66、凝土均勻澆筑在析水處</p><p>　　2.采用合理的施工方法(1)混凝土的拌制和運輸。(a)拌合混凝土時，要準確掌握配合比，特別是嚴格控制用水量，測定砂的含水量。記錄每一次攔制用水、水泥、砂、碎石的具體數(shù)量，各擋砂石料都必須通過磅秤，準確后才能拌制?？刂坪没炷撂涠?，一般不宜過大在120±20毫米即可。要盡量降低混凝土拌合物出機口溫度。(b)混凝土拌好后在出倉運輸過程中，要防止產(chǎn)生分離、泌水、砂

67、漿流失等現(xiàn)象，應縮短運輸時間，混凝土裝卸不可過滿。(2)混凝土澆筑和拆模。澆注過程中要進行振搗方可密實，振搗時間應均勻一致以表面泛漿為宜，間距要均勻，以振搗力波及范圍重疊二分之一為宜，澆注完畢后，表面要壓實、抹平，以防止表面裂縫。盡量避開在太陽輻射較高的時間澆注，若由于工程需要在夏季施工，則盡量避開正午高溫時段，澆注盡量安排在夜間進行。拆模時間應根據(jù)氣溫和強度情況而定?；炷猎趯嶋H溫度養(yǎng)護的條件下，強度達到設計強度的75%以上，混凝土

68、中心與表面最低溫度控制在25℃以內(nèi)，預計拆模后混凝土表面溫降不超過9℃以上允許拆模。</p><p>　　(3)做好表面隔熱保護 大體積混凝土的溫度裂縫，主要是由內(nèi)外溫差過大引起的?；炷翝沧⒑?，由于內(nèi)部較表面散熱快，會形成內(nèi)外溫差，表面收縮受內(nèi)部約束產(chǎn)生拉應力，但是這種拉應力通常很小，不至于超過混凝土的抗拉強度而產(chǎn)生裂縫。但是如果此時受到冷空氣的襲擊，或者過分通風散熱，使表面溫度降溫過大就很容易導致裂縫的產(chǎn)生，

69、所以在混凝土在拆模后，特別是低溫季節(jié)，在拆模后立即采取表面保護。防止表面降溫過大，引起裂縫。另外，當日平均氣溫在2～3d內(nèi)連續(xù)下降不小于6～8℃時，28d齡期內(nèi)混凝土表面必須進行表面保護。</p><p>　?。?）體積混凝土的澆筑： 澆筑方案，除應滿足每一處混凝土在初凝以前就被上一層新混凝土覆蓋并搗實完畢外，還應考慮結構大小、鋼筋疏密、預埋管道和地腳螺栓的留設、混凝土供應情況以及水化熱等因素的影響，常采用的方

70、法有以下幾種： ①全面分層： 即在第一層全面澆筑全部澆筑完畢后，再回頭澆筑第二層，此時應使第一層混凝土還未初凝，如此逐層連續(xù)澆筑，直至完工為止。采用這種方案，適用于結構的平面尺寸一般不宜太大，施工時從短邊開始，沿長邊推進比較合適。必要時可分成兩段，從中間向兩端或從兩端向中間同時進行澆筑。 ②分段分層： 混凝土澆筑時，先從底層開始，澆筑至一定距離后澆筑第二層，如此依次向前澆筑其他各層。由于總的層數(shù)較多，所以澆筑到頂后，第一層末端的

71、混凝土還未初凝，又可以從第二段依次分層澆筑。這種方案適用于單位時間內(nèi)要求供應的混凝土較少，不象第一種方案那樣集中。這種方案適用于結構物厚度不太大而面積或長度較大的工程。 ③斜面分層： 要求斜面的坡度不大于1/3，適用于結構的長度大大超過厚度3倍的情況?；炷翉臐仓酉露碎_始，逐漸上移。</p><p>　　3.養(yǎng)護措施　　養(yǎng)護是大體積混凝土施工中一項十分關鍵的工作。養(yǎng)護主要是保持適宜的溫度和濕度，以便控制混

72、凝土內(nèi)外溫差，促進混凝土強度的正常發(fā)展及防止混凝土裂縫的產(chǎn)生和發(fā)展。根據(jù)工程的具體情況，應盡可能多養(yǎng)護一段時間，拆模后應立即覆蓋保護，同時預防近期驟冷氣候影響，以控制內(nèi)外溫差，防止混凝土早期和中期裂縫。大體積混凝土的養(yǎng)護，不僅要滿足增長的需要，還應通過人工的溫度控制，防止因溫度變形引起混凝土的開裂。 養(yǎng)護 混凝土澆注完畢后，應及時灑水養(yǎng)護以保持混凝土表面經(jīng)常濕潤，這樣既減少外界高溫倒罐，又防止干縮裂縫的發(fā)生，促進混凝土強度的穩(wěn)定增長。一

73、般在澆注完畢后12～18h內(nèi)立即開始養(yǎng)護，連續(xù)養(yǎng)護時間不少于28d或設計齡期。</p><p><b>　　4. 通水冷卻 </b></p><p>　　若是在高溫季節(jié)施工，則要在初期采用通制冷水來降低混凝土最高溫度峰值，但注意，通水時間不能過長，因為時間過長會造成降溫幅度過大而引起較大的溫度應力。為了削減內(nèi)外溫差，還應在夏末秋初進行中期通水冷卻，中期通水一般采用河水

74、，通水歷時兩個月左右。后期通水是使混凝土柱狀塊達到接縫灌漿的必要措施，一般采用通河水和通制冷水相結合的方案。</p><p><b>　　三、結束語</b></p><p>　　混凝土裂縫特別是大體積混凝土的開裂是目前學者和工程界關注的一個重要問題，通過以上分析可知，大體積混凝土的材料型裂縫主要是由溫度應力和混凝土的收縮引起的，筆者認為精心選擇原材料，并在施工中采用合