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1、【精品文档】如有侵权，请联系网站删除，仅供学习与交流英文翻译-混凝土裂缝成因及控制.精品文档.山东交通学院2015届毕业生毕业论文外文翻译院 别 交通土建工程学院 专 业 土木工程 班 级 土木111 学 号 姓 名 指导教师 二一五年六月混凝土裂缝成因及控制摘要: 针对目前建筑混凝土的裂缝问题, 分别从温度应力、原材料质量、收缩形变、钢筋锈蚀以及施工工艺质量几个方面论述了开裂原因, 并提出了相应的控制措施。关键词: 混凝土; 裂缝; 原因; 控制混凝土出现裂缝的原因多种多样, 通常情况下,裂缝的存在不会影响构件的正常使用( 宽度 0. 05 mm ) , 但如果裂缝过大就会降低结构的安全性和
2、耐久性, 本文主要针对后者进行讨论。1混凝土裂缝成因 1. 1温度裂缝 当外部温度或结构内部温度发生变化, 混凝土将发生形变, 若变形受到约束, 结构内将产生应力,当应力超过混凝土抗拉强度时将产生温度裂缝。通常情况下, 温度应力主要有3 种形成方式: 1) 大体积混凝土(厚度超过2 m ) 浇注之后由于水泥水化放热产生温度应力; 此外, 由于混凝土弹性模量的急剧变化也会在其内部形成残余应力; 2) 当水泥放热基本结束后, 由于混凝土的冷却以及外界气温变化引起温度应力; 3) 在使用过程中, 由于突发降水、冷空气侵袭或日落等致使混凝土结构外表面温度突然下降, 内部温降相对较慢而产生温度应力。 1
3、. 2原材料质量引起的裂缝 配制混凝土时所采用原材料质量不合格, 也可能导致结构出现裂缝。 1. 2. 1水泥 如果水泥安定性不合格, 其中的游离氧化钙含量超标, 则由于氧化钙在凝结过程中水化很慢, 在混凝土凝结后仍然继续起水化作用, 可破坏已硬化的水泥石, 使混凝土抗拉强度下降。 若水泥出厂时强度不足, 水泥受潮或过期, 亦可能使混凝土强度不足, 导致混凝土开裂。 1. 2. 2砂、石集料 1) 砂石的粒径、级配、杂质含量 如果砂石粒径太小、级配不良, 将导致水泥和拌和水用量加大, 使混凝土收缩加大, 影响混凝土的强度; 如果使用超出规定的特细砂, 后果将更加严重。当砂石中含泥量高时, 将造
4、成水泥和拌和水用量加大, 降低混凝土强度以及抗冻性和抗渗性。 2) 碱骨料反应 混凝土中的碱与集料中的某些成份发生碱骨料反应, 其生成物容易吸水膨胀, 导致混凝土开裂。 碱骨料反应裂缝的分布与钢筋限制有关。当限制力小时, 常出现地图状裂缝, 并在缝中有白色或透明的浸出物; 当限制力强时则出现顺筋裂缝。 1. 2. 3拌和水及外加剂 拌和水或外加剂中氯化物含量较高时对钢筋锈蚀有较大影响。采用海水或含碱泉水拌制混凝土, 或采用含碱的外加剂, 可能对碱骨料反应有影响。 1. 3收缩裂缝 在混凝土收缩种类中, 塑性收缩和缩水收缩是造成混凝土体积变形的主要原因, 另外还有自生收缩和碳化收缩。 塑性收缩主
5、要发生在施工过程中, 混凝土浇注后4 15 h, 水泥水化反应激烈, 分子链逐渐形成, 出现泌水和水分急剧蒸发, 混凝土失水收缩, 同时骨料因自重下沉, 此时混凝土尚未硬化, 称为塑性收缩。塑性收缩产生的量级很大, 可达1% 左右。 缩水收缩是因为混凝土表层水份损失快, 内部损失慢, 从而产生表面收缩大、内部收缩小的不均匀收缩。表面收缩受到内部约束, 使表面混凝土承受拉应力, 当拉应力较大时, 容易产生收缩裂缝. 混凝土收缩还包括由于混凝土的体积变形不能自由伸缩而产生的自生收缩; 由大气中的二氧化碳与水泥水化物发生化学反应而引起的碳化收缩。1. 4钢筋锈蚀引起的裂缝 由于混凝土质量较差或保护层
6、厚度不足, 混凝土保护层受二氧化碳侵蚀碳化至钢筋表面, 或由于氯化物介入, 钢筋周围氯离子含量较高, 均可引起钢筋表面氧化膜破坏, 钢筋中铁离子与混凝土中的氧气和水份发生锈蚀反应, 其锈蚀物氢氧化铁体积比原来增长2 4 倍, 对混凝土产生膨胀应力, 导致保护层开裂、剥离, 沿钢筋纵向产生裂缝, 并有锈迹渗到混凝土表面。由于锈蚀, 使得钢筋有效断面面积减小, 钢筋与混凝土握裹力削弱, 结构承载力下降, 并将诱发其它形式的裂缝, 加剧钢筋锈蚀, 导致结构破坏。1. 5施工工艺质量引起的裂缝 在混凝土结构浇注过程中, 如果施工工艺不合理、施工质量低劣, 容易产生各种形式的裂缝, 特别是细长薄壁结构。
7、裂缝出现的部位和走向、裂缝宽度因产生的原因而异, 比较常见的有: 1) 混凝土浇注过快, 混凝土流动性较低, 容易在浇注数小时后发生前述的塑性收缩裂缝; 2) 混凝土搅拌、运输时间过长, 使水分蒸发过多, 引起混凝土塌落度过低; 3) 混凝土初期养护时急剧干燥, 使得混凝土与大气接触的表面上出现不规则的收缩裂缝; 4) 拆模过早, 混凝土强度不足, 构件在自重或施工荷载作用下产生裂缝; 5) 施工质量控制差, 如任意套用配合比, 水、砂石、水泥材料计量不准, 造成混凝土强度不足和其他性能(和易性、密实度) 下降, 导致结构开裂。2混凝土裂缝控制 2. 1原材料控制 1) 选用中低水化热的水泥,
8、 如矿渣硅酸盐水泥、火山灰硅酸盐水泥或粉煤灰水泥等。充分利用混凝土的后期强度, 减少水泥用量。在满足使用的条件下, 大体积混凝土设计强度应就“能低不高”的原则。同时应注意水泥的质量, 特别是安定性达标。 2) 尽量选用粒径较大(不超过40 mm ) 且级配良好的粗集料; 砂和碎石的含泥量不超过1% , 针片状颗粒含量不得大于15%。 3) 在混凝土中掺加超细矿粉。超细矿粉具有微珠润滑效应, 有明显的减水作用, 并能够提高混凝土的和易性、体积稳定性、密实性以及抗化学侵蚀性能, 同时还具有增加混凝土强度, 减少塌落度损失、降低水化热等性能。 4) 适当加大活性细掺料的用量, 以替代部分水泥, 从而
9、降低水化热, 增强硬化前后混凝土的体积稳定性。如使用大掺量粉煤灰不仅能提高混凝土的和易性, 而且改善了混凝土的工作性能和可靠性。 5) 拌和水中不得有较高含量的氯化物等杂质,禁止使用海水和含碱泉水拌制混凝土。 6) 选用合适的外加剂。混凝土中掺入水泥重量0. 25%的木钙减水剂, 不仅使混凝土工作性能有了明显的改善, 同时又减少10% 拌和水, 节约10% 左右的水泥, 降低了水泥水化热。又如利用膨胀剂置换等量的水泥, 不仅降低了水化热, 同时微膨胀剂吸收部分水化热后发生化学反应, 在混凝土中产生自应力而使结构处于受压状态, 提高了混凝土的抗渗和抗拉能力, 避免了裂缝的产生。 2. 2改进设计
10、与施工技术措施 从技术层面讲, 混凝土裂缝控制主要体现在设计和施工中的技术措施改进上。 2. 2. 1设计技术措施 1) 优化钢筋配置, 根据“细而密”的原则, 减小水平布筋间距, 将混凝土中可能产生的收缩应力分散。 2) 由于挖孔、转角和形状突变处等部位应力集中容易产生裂缝, 所以应采取有效的构造措施, 例如在转角、孔边作构造筋加强; 转角处增配斜向钢筋或网片; 突变处做成渐变过渡等。 2. 2. 2施工技术措施1) 混凝土用水量控制在170 kg/m 3 以下;2 ) 控制胶凝材料总量在410 kg/m 3 以下, 同时增大优质 级粉煤灰的掺量; 3) 控制混凝土中胶骨比在1. 3 以下;
11、 4) 在保证和易性和泵送的前提下降低砂率; 5) 在混凝土中掺入适量的复合纤维, 提高基体的抗裂形变能力, 阻止原始缺陷(裂纹等) 的扩展; 6) 在温度较高的季节, 通过对原材料和粗骨料进行遮阳、覆盖, 以及采用低于5的拌和水喷淋、冷却降温来降低混凝土的浇注温度。 2. 3提高施工工艺质量 由于评价混凝土抗裂能力是以极限拉伸率、施工强度保证率或施工均匀性为指标(离差系数)。因此, 在施工过程中, 必须严格按照相关的国家或行业规范, 以及技术标准进行, 加强现场管理监督和质量控制, 从而确保混凝土结构或构件符合设计要求的抗裂能力。 1) 在混凝土浇捣前, 应先将基层和模板浇水湿透, 避免混凝
12、土失水; 振捣时, 应组织好振捣棒的走向, 保证混凝土振捣密实并防止漏振。同时也应避免过度振捣(以混凝土不再下沉、不冒气泡为准)。 2) 混凝土浇注完毕后可进行二次抹压, 必要时利用吸水泵排出表面泌水。但表面刮抹应限制到最小程度, 避免在其表面洒水泥干粉刮抹, 加强混凝土早期养护。 3) 严格施工操作程序, 杜绝过早施加荷载和过早拆模, 当混凝土温度高于气温时应适当考虑拆模时间, 过早拆模, 会在表面引起很大的拉应力, 出现“温度冲击”现象; 在浇捣过程中需派专人护筋, 以免踩弯板面负筋的现象发生。 4) 确定合理的搅拌和运输时间, 避免因时间过长而使水分过多蒸发, 降低混凝土的塌落度, 使得
13、在混凝土中出现不规则的收缩裂缝。 5) 采用分层连续浇注并合理设置施工缝, 能够尽快散发水化热, 并放松约束条件的聚集。由于施工技术和施工组织上的原因, 不能连续将结构整体浇完, 并且间歇时间过长, 应预先选定适当的部位设置施工缝, 以减少裂缝和确保结构安全。 2. 4改善养护措施 在大体积混凝土施工中, 良好的保温保湿养护对于减少混凝土的收缩, 控制内外温差, 降低约束应力, 充分利用其松弛效应具有重要的意义。根据混凝土结构工程施工质量验收规范可知, 大体积混凝土内外温差不应高于25。内外温差越大, 裂缝发生可能性越大, 程度越严重。在混凝土浇注完毕后12h内, 应先盖塑料薄膜1 层, 然后
14、盖草包或麻袋2 层, 再盖塑薄膜1 层进行养护。遇气温突变, 温度急剧下降的情况, 养护措施必须随时跟上。值得一提的是,混凝土养护应使其浇筑的内外温差及降温速度满足温控指标的要求; 保温养护的持续时间, 应根据温度应力(包括混凝土收缩产生的应力) 加以控制和确定, 但不得少于15 d。 2. 5表面防护 为了提高钢筋混凝土结构的耐久性, 避免因钢筋锈蚀而使混凝土开裂, 对混凝土抗裂性能有较高要求的特殊工程, 也可采用在其表面涂覆有机涂层的措施。采用外覆有机涂层的方法能够有效阻止或降低诸如水、二氧化碳以及氯化物等腐蚀性介质向混凝土内部的渗透, 从而有效地延长了混凝土的使用寿命。目前, 国内常用的
15、防护体系是: 环氧封闭漆+ 环氧云铁中间漆+ 丙烯酸聚氨酯面漆。经过近10年的实践检验, 该有机涂层能够对混凝土进行腐蚀防护, 且效果良好, 但造价较高。 此外, 对于已产生较宽裂缝的混凝土, 为了避免开裂处的钢筋发生腐蚀, 或已腐蚀的钢筋继续腐蚀,必须对其进行灌浆处理。如果因开裂导致其结构承载力下降, 应进行加固处理以保证其承载力要求。3结语 混凝土裂缝的控制从设计、施工、养护到运营等各方面是一个有机的整体, 它们不仅相互联系, 而且相互制约。因此在实际工程中, 应综合考虑各种影响因素, 抓住主要矛盾, 全面剖析, 重点施制, 才能收到实效。Reason and Control Concre
16、te Cracking Abstract: aim at cracking problems of concrete for construction, the cracking reasons are expounded in the paper from the several aspects such as the temperature stress, raw material quality, contraction deformation, and rust corrosion as well as quality of construct ion technology. The
17、relative control measures are also proposed in it.Key words: concrete; crack; reason; control The reasons of concrete cracks is varied . In general , the existence of cracks will not affect the normal use of component ( width 0.05 mm ) . But if the crack is too wide , it will lower the safety and du
18、rability of construction . This article will mainly discuss the later .1 . The reason of temperature crack 1.1 Temperature cracking When the external temperature or structure of internal temperature change , the concrete deformation will occur . If the deformation is restrained , the structure will
19、produce stress . When the stress is larger than concrete tensile strength will produce temperature crack . In general , the temperature crack will happen in three kinds of form . 1) Mass concrete ( thickness exceed 2 m ) , after pouring the cement hydration heat will produce temperature stress . Thu
20、s , due to the dramatic changes of concrete elastic modulus in its internal , the residual stress will be formed .2) When the cement exothermic basically ended, because of the concrete cooling and external temperature change will produce temperature stress .3) In the course of , because of sudden ra
21、in , the cold air invasion or sunset and so on , will make the surface of concrete temperature drop suddenly . However the temperature in internal drop relatively slow which will result from temperature stress . 1. 2crack cause because of raw material qualityThe raw material quality is not qualified
22、 when compound concrete , will also lead to structural cracks .1. 2. 1 cement If the cement stability is unqualified , the free calcium oxide content exceeds bid . Because of in the condensation process ,the calcium oxide hydration is slow . When the concrete setting , it continue to hydration and w
23、ill damage the hardened cement stone . At last it will drop the tensile strength of concrete . If the cement strength is not insufficient when it was sold . Cement can be affected with damp or expired , it may also lead to the insufficient strength of concrete and finally make concrete crack . 1. 2.
24、 2 sand stone aggregate 1) The size of sand particle、gradation 、impurity content If the size of sand stone is too small or gradation is not good , which Will lead to the dosage of cement, and mixing water . Make the shrinkage of concrete , affect the concrete strength . If you use sand is beyond the
25、 prescribed , the consequence will be even worse .When the soil in sand stone is high , which Will lead to the dosage of cement, and mixing water .Lower concrete strength、frost resistance and seepage resistance . 2) alkali-aggregate reaction The alkali in concrete will react with certain ingredients
26、 in aggregate , their products easy to swell , make the concrete crack . Alkali aggregate reaction crack distribution is related to steel limited . When limited is small , Often appear crack , in the seam we can see white or transparent extract . When restrictions on power appeared cracks along reba
27、r . 1. 2. 3 Mixing water and admixture When chloride content in mixing or admixture is at high level , it has a larger effect on reinforcement corrosion . When use the sea water or alkali water mixing concrete 、Using alkali admixture ，Can have an impact on alkali aggregate reaction . 1. 3 shrinkage
28、crack In the type of shrinkage crack , plastic shrinkage and shrinkage mark are the main reason to lead to a concrete volume deformation , otherwise there also have autogenous shrinkage and carbonization contraction .Plastic shrinkage mainly occurs in the construction process , after concrete pourin
29、g 4 - 15 hours , the cement hydration reaction . Molecular chain is gradually formed , a secrete water and moisture evaporation sharply . The concrete will lost water and shrink . At the same time the aggregate sink because of gravity .Meanwhile concrete has not yet been hardened which is called pla
30、stic shrinkage . The range of plastic shrinkage is very large , can be about 1% .Because of the concrete surface layer moisture loss rapidly can lead to shrink shrinkage , the internal loss slow, which result in a large in the surface and a small in the internal lead to uneven contraction .The surfa
31、ce shrinkage can be constraint by internal , make the surface of concrete under tensile stress . When the tensile stress is larger , it is easy to produce shrink crack .The shrinkage of concrete also include due to the volume deformation of concrete can not free expansion lead to the spontaneous con
32、traction; By carbon dioxide in the atmosphere and react with the cement hydrate cause carbonization contraction . 1. 4 Crack s caused by reinforcement corrosion Due to the poor quality of concrete or insufficient thickness of protective layer , the protective layer of concrete Corroded by carbon dio
33、xide carbon to the surface of steel or the Chloride in .Surrounding the steel chloride ions content is higher , all can destroy oxide film on reinforced surface ,the iron ions of steel react with oxygen and water in the concrete cause Corrosion reactions .The rust iron hydroxide content volume 2 4 t
34、imes higher than the original ,make expansion stress of concrete and lead to the protective layer crack 、stripping , along the reinforcement produce Longitudinal cracks and rusty infiltration to the concrete surface . Because of corrosion , make the effective section area of steel is reduced . Grip
35、of reinforcement and concrete weaken , the bearing capacity of structure is down and bring about other forms of crack , make the reinforcement corrosion even worse and eventually the structure destroy . 1. 5 Crack caused by Construction process quality In the pouring process of concrete structure ,i
36、f the construction technology is not reasonable and poor quality of construction . Make it is easy to produce all kinds of crack ,especially the slender thin-walled structures . The reason of crack appear in the area and width is different .The following we can often see: 1) Concrete pouring is too
37、quick , concrete liquidity is slow , make it easy to produce plastic shrinkage after pouring a few hours later.2) The mixing and transmit time is too long , make the water evaporate too much ,cause the concrete loss of slump . 3) At the beginning of concrete curing age is rapid dry ,make the surface
38、 of concrete contact with the atmosphere appears irregular contraction crack . 4) To early to dismantle ,the strength of concrete is not enough , cracks caused by component weight or the action of construction loads . 5) Poor control of construction quality , for example adopt any form of mix design
39、 .Water , sand and cement is not accurate measure . Make the strength of concrete is not enough ,Other performance (workability compactness) decline , lead to the structure crack .2 The control of concrete crack 2.1 The control of raw material 1) Use low hydration heat of cement , such as slag portl
40、and cement , the ash portland cement and fly ash cement and so on .Make full use of the later strength of concrete ,reduce the use of cement . Under the condition of use , mass concrete design strength should abide by the principle of low not high . At the same time pay attention to the quality of c
41、ement , especially reach to aim of qualitative standard . 2) As far as possible choose the large size ( no more than 40 mm ) and good gradation of coarse aggregate ,the mud content in sand and gravel is less than 1%, the content of needle flake particle should no more than 15% . 3) Mixed superfine m
42、ineral powder in the concrete ,ultrafine mineral power have the function of beads lubrication effect , have obvious water reducing effect . Can improve the workability , volume stability of concrete ,the compactness and chemical erosion resistance . At the same time also can increase the strength of
43、 concrete, decrease slump loss properties, and reducing heat of hydration and so on. 4) Increase the use of active fine admixture appropriately , to replace part of cement ,so as to reduce the hydration heat ,enhance the volume stability of concrete before and after hardening . Such as using large d
44、osage of fly ash not only can improve the workability of concrete, but also improve the working performance and reliability of the concrete. 5) Mixing water can not have higher content of impurities such as chloride , banning the use of sea water and alkaline water to mix concrete . 6) Choose suitab
45、le additive . Mixed with 0.25% of the wood calcium water reducing agent in the concrete weight , can not only improve the concrete working performance , at the same time reduce 10% mixing water ,save about 10% cement ,reduce the cement hydration heat . What is more , use expansion agent to replace t
46、he same amount of cement , not only reduce the cement hydration heat , micro-expanding agent absorb part of the hydration heat after chemical reaction .Produce stress in the concrete make the construction in compression state , improve concrete of impermeability and tensile ability , and avoid the g
47、eneration of the cracks . 2.2 Improve the design and construction technical measures From the respect of technology , the control of concrete crack is mainly reflected in the improvement of design and construction. 2. 2. 1 Design technical measures1) Optimize configuration of reinforced , based on the principle of fine and dense ,reduce the horizontal spacing , scatter possible shrinkage stress in the concrete .2) Due to dig a hole 、angle and shape ,which is easy to cause stress concentration and lead to cracks , so we should take