混凝土工程中常见裂缝问题的预防与处理措施.doc

混凝土工程中常见裂缝问题的预防与处理措施一、前言混凝土是一种由砂石骨料、水泥、水及其他外加材料混合而形成的非均质脆性材料。由于混凝土施工和本身变形、约束等一系列问题，硬化成型的混凝土中存在着众多的微孔隙、气穴和微裂缝，正是由于这些初混凝土建筑和构件通常都是带缝工作的，由于裂缝的存在和发展通常会使内部的钢筋等材料产生腐蚀，降低钢筋混凝土材料的承载能力、耐久性及抗渗能力，影响建筑物的外观、使用寿命，严重者将会威胁到人混凝土裂缝产生的原因很多，有变形引起的裂缝：如温度变化、收缩、膨胀、不均匀沉陷等原因引起的裂缝；有外载作用引起的裂缝；有养护环境不当和化学作用引起的裂缝等等。在实际工程中要区别对待，根据实际情况解决问题。二、 凝土工程中常见裂缝及预防1、干缩裂缝及预防 干缩裂缝多出现在混凝土养护结束后的一段时间或是混凝土浇筑完毕后的一周左右。水泥浆中水分的蒸发会产生干缩，且这种收缩是不可逆的。干缩裂缝的产生主要是由于混凝土内外水分蒸发程度不同而导致变形不同的结果：混凝土受外部条件的影响，表面水分损失过快，变形较大，内部湿度变化较小变形较小，较大的表面干缩变形受到混凝土内部约束，产生较大拉应力而产生裂缝。相对湿度越低，水泥浆体干缩越大，干缩裂缝越易产生。干缩裂缝多为表面性的平行线状或网状浅细裂缝，宽度多在0.050.2mm之间，大体积混凝土中平面部位多见，较薄的梁板中多沿其短向分布。干缩裂缝通常会影响混凝土的抗渗性，引起钢筋的锈蚀影响混凝土的耐久性，在水压力的作用下会产生水力劈裂影响混凝土的承载力等等。混凝土干缩主要和混凝土的水灰比、水泥的成分、水泥的用量、集料的性质和用量、外加剂的用量等有关。 主要预防措施：一是选用收缩量较小的水泥，一般采用中低热水泥和粉煤灰水泥，降低水泥的用量。二是混凝土的干缩受水灰比的影响较大，水灰比越大,干缩越大，因此在混凝土配合比设计中应尽量控制好水灰比的选用，同时掺加合适的减水剂。三是严格控制混凝土搅拌和施工中的配合比，混凝土的用水量绝对不能大于配合比设计所给定的用水量。四是加强混凝土的早期养护，并适当延长混凝土的养护时间。冬季施工时要适当延长混凝土保温覆盖时间，并涂刷养护剂养护。五是在混凝土结构中设置合适的收缩缝。2、塑性收缩裂缝及预防塑性收缩是指混凝土在凝结之前，表面因失水较快而产生的收缩。塑性收缩裂缝一般在干热或大风天气出现，裂缝多呈中间宽、两端细且长短不一，互不连贯状态。较短的裂缝一般长2030cm，较长的裂缝可达23，宽15mm。其产生的主要原因为：混凝土在终凝前几乎没有强度或强度很小，或者混凝土刚刚终凝而强度很小时，受高温或较大风力的影响，混凝土表面失水过快，造成毛细管中产生较大的负压而使混凝土体积急剧收缩，而此时混凝土的强度又无法抵抗其本身收缩，因此产生龟裂。影响混凝土塑性收缩开裂的主要因素有水灰比、混凝土的凝结时间、环境温度、风速、相对湿度等等。主要预防措施：一是选用干缩值较小早期强度较高的硅酸盐或普通硅酸盐水泥。二是严格控制水灰比，掺加高效减水剂来增加混凝土的坍落度和和易性，减少水泥及水的用量。三是浇筑混凝土之前，将基层和模板浇水均匀湿透。四是及时覆盖塑料薄膜或者潮湿的草垫、麻片等，保持混凝土终凝前表面湿润，或者在混凝土表面喷洒养护剂等进行养护。五是在高温和大风天气要设置遮阳和挡风设施，及时养护。3、沉陷裂缝及预防 沉陷裂缝的产生是由于结构地基土质不匀、松软，或回填土不实或浸水而造成不均匀沉降所致；或者因为模板刚度不足，模板支撑间距过大或支撑底部松动等导致，特别是在冬季，模板支撑在冻土上，冻土化冻后产生不均匀沉降，致使混凝土结构产生裂缝。此类裂缝多为深进或贯穿性裂缝，其走向与沉陷情况有关，一般沿与地面垂直或呈3045角方向发展，较大的沉陷裂缝，往往有一定的错位，裂缝宽度往往与沉降量成正比关系。裂缝宽度受温度变化的影响较小。地基变形稳定之后，沉陷裂缝也基本趋于稳定。主要预防措施：一是对松软土、填土地基在上部结构施工前应进行必要的夯实和加固。二是保证模板有足够的强度和刚度，且支撑牢固，并使地基受力均匀。三是防止混凝土浇灌过程中地基被水浸泡。四是模板拆除的时间不能太早，且要注意拆模的先后次序。五是在冻土上搭设模板时要注意采取一定的预防措施。4、温度裂缝及预防 温度裂缝多发生在大体积混凝土表面或温差变化较大地区的混凝土结构中。混凝土浇筑后,在硬化过程中,水泥水化产生大量的水化热，（当水泥用量在350550 kg/m3，每立方米混凝土将释放出1750027500kJ的热量，从而使混凝土内部温度升达70左右甚至更高）。由于混凝土的体积较大，大量的水化热聚积在混凝土内部而不易散发，导致内部温度急剧上升，而混凝土表面散热较快，这样就形成内外的较大温差，较大的温差造成内部与外部热胀冷缩的程度不同，使混凝土表面产生一定的拉应力（实践证明当混凝土本身温差达到2526时，混凝土内便会产生大致在10MPa左右的拉应力）。当拉应力超过混凝土的抗拉强度极限时，混凝土表面就会产生裂缝，这种裂缝多发生在混凝土施工中后期。在混凝土的施工中当温差变化较大，或者是混凝土受到寒潮的袭击等，会导致混凝土表面温度急剧下降，而产生收缩，表面收缩的混凝土受内部混凝土的约束，将产生很大的拉应力而产生裂缝，这种裂缝通常只在混凝土表面较浅的范围内产生。温度裂缝的走向通常无一定规律，大面积结构裂缝常纵横交错；梁板类长度尺寸较大的结构，裂缝多平行于短边；深入和贯穿性的温度裂缝一般与短边方向平行或接近平行，裂缝沿着长边分段出现，中间较密。裂缝宽度大小不一，受温度变化影响较为明显，冬季较宽，夏季较窄。高温膨胀引起的混凝土温度裂缝是通常中间粗两端细，而冷缩裂缝的粗细变化不太明显。此种裂缝的出现会引起钢筋的锈蚀，混凝土的碳化，降低混凝土的抗冻融、抗疲劳及抗渗能力等。 主要预防措施：一是尽量选用低热或中热水泥，如矿渣水泥、粉煤灰水泥等。二是减少水泥用量,将水泥用量尽量控制在450kg/m3以下。三是降低水灰比，一般混凝土的水灰比控制在0.6以下。四是改善骨料级配，掺加粉煤灰或高效减水剂等来减少水泥用量,降低水化热。五是改善混凝土的搅拌加工工艺，在传统的三冷技术的基础上采用二次风冷新工艺，降低混凝土的浇筑温度。六是在混凝土中掺加一定量的具有减水、增塑、缓凝等作用的外加剂，改善混凝土拌合物的流动性、保水性，降低水化热，推迟热峰的出现时间。七是高温季节浇筑时可以采用搭设遮阳板等辅助措施控制混凝土的温升，降低浇筑混凝土的温度。八是大体积混凝土的温度应力与结构尺寸相关，混凝土结构尺寸越大，温度应力越大，因此要合理安排施工工序，分层、分块浇筑，以利于散热，减小约束。九是在大体积混凝土内部设置冷却管道，通冷水或者冷气冷却，减小混凝土的内外温差。十是加强混凝土温度的监控，及时采取冷却、保护措施。十一是预留温度收缩缝。十二是减小约束，浇筑混凝土前宜在基岩和老混凝土上铺设5mm左右的砂垫层或使用沥青等材料涂刷。十三是加强混凝土养护，混凝土浇筑后，及时用湿润的草帘、麻片等覆盖，并注意洒水养护，适当延长养护时间，保证混凝土表面缓慢冷却。在寒冷季节，混凝土表面应设置保温措施，以防止寒潮袭击。十四是混凝土中配置少量的钢筋或者掺入纤维材料将混凝土的温度裂缝控制在一定的范围之内。5、化学反应引起的裂缝及预防 碱骨料反应裂缝和钢筋锈蚀引起的裂缝是钢筋混凝土结构中最常见的由于化学反应而引起的裂缝。混凝土拌和后会产生一些碱性离子，这些离子与某些活性骨料产生化学反应并吸收周围环境中的水而体积增大，造成混凝土酥松、膨胀开裂。这种裂缝一般出现中混凝土结构使用期间，一旦出现很难补救，因此应在施工中采取有效措施进行预防。主要的预防措施：一是选用碱活性小的砂石骨料。二是选用低碱水泥和低碱或无碱的外加剂。三是选用合适的掺和料抑制碱骨料反应。由于混凝土浇筑、振捣不良或者是钢筋保护层较薄，有害物质进入混凝土使钢筋产生锈蚀，锈蚀的钢筋体积膨胀，导致混凝土胀裂，此种类型的裂缝多为纵向裂缝，沿钢筋的位置出现。三、裂缝处理裂缝的出现不但会影响结构的整体性和刚度，还会引起钢筋的锈蚀、加速混凝土的碳化、降低混凝土的耐久性和抗疲劳、抗渗能力。因此根据裂缝的性质和具体情况我们要区别对待、及时处理，以保证建筑物的混凝土裂缝的修补措施主要有以下一些方法：表面修补法，灌浆、嵌逢封堵法，结构加固法，混凝土置换法，电化学防护法以及仿生自愈合法。1、表面修补法 表面修补法是一种简单、常见的修补方法，它主要适用于稳定和对结构承载能力没有影响的表面裂缝以及深进裂缝的处理。通常的处理措施是在裂缝的表面涂抹水泥浆、环氧胶泥或在混凝土表面涂刷油漆、沥青等防腐材料，在防护的同时为了防止混凝土受各种作用的影响继续开裂，通常可以采用在裂缝的表面粘贴玻璃纤维布等措施。2、灌浆、嵌逢封堵法 灌浆法主要适用于对结构整体性有影响或有防渗要求的混凝土裂缝的修补，它是利用压力设备将胶结材料压入混凝土的裂缝中，胶结材料硬化后与混凝土形成一个整体，从而起到封堵加固的目的。常用的胶结材料有水泥浆、环氧树脂、甲基丙烯酸酯、聚氨酯等化学材料。嵌缝法是裂缝封堵中最常用的一种方法，它通常是沿裂缝凿槽，在槽中嵌填塑性或刚性止水材料，以达到封闭裂缝的目的。常用的塑性材料有聚氯乙烯胶泥、塑料油膏、丁基橡胶等等；常用的刚性止水材料为聚合物水泥砂浆。3、结构加固法 当裂缝影响到混凝土结构的性能时，就要考虑采取加固法对混凝土结构进行处理。结构加固中常用的主要有以下几种方法：加大混凝土结构的截面面积，在构件的角部外包型钢、采用预应力法加固、粘贴钢板加固、增设支点加固以及喷射混凝土补强加固。4、混凝土置换法 混凝土置换法是处理严重损坏混凝土的一种有效方法，此方法是先将损坏的混凝土剔除，然后再置换入新的混凝土或其他材料。常用的置换材料有：普通混凝土或水泥砂浆、聚合物或改性聚合物混凝土或砂浆。5、电化学防护法电化学防腐是利用施加电场在介质中的电化学作用，改变混凝土或钢筋混凝土所处的环境状态，钝化钢筋，以达到防腐的目的。阴极防护法、氯盐提取法、碱性复原法是化学防护法中常用而有效的三种方法。6、仿生自愈合法 仿生自愈合法是一种新的裂缝处理方法，它模仿生物组织对受创伤部位自动分泌某种物质，而使创伤部位得到愈合的机能，在混凝土的传统组分中加入某些特殊组分(如含粘结剂的液芯纤维或胶囊)，在混凝土内部形成智能型仿生自愈合神经网络系统，当混凝土出现裂缝时分泌出部分液芯纤维可使裂缝重新愈合4。四、结 论 裂缝是混凝土结构中普遍存在的一种现象，它的出现不仅会降低建筑物的抗渗能力，影响建筑物的使用功能，而且会引起钢筋的锈蚀，混凝土的碳化，降低材料的耐久性，影响建筑物的承载能力，因此要对混凝土裂缝进行认真研究、区别对待，采用合理的方法进行处理，并在施工中采取各种有效的预防措施来预防裂缝的出现和发展，保证建筑物和构件安全、稳定地工作。参考文献：1 钢筋混凝土结构设计规范.中国建筑工业出版社，1999.2.2 鞠丽艳.混凝土裂缝抑制措施的研究进展.混凝土，2002. 5.3 郭仕万，肖欣，赵和平.混凝土施工中的裂缝控制.山西水利科技，2000.11.4 鞠丽艳，张雄.混凝土裂缝防治的两种新方法.施工技术，2002. 7.Common cracks in concrete engineering problems prevention and treatment measures I. Introduction Concrete is an aggregate from sand and gravel, cement, water and other materials, plus the formation of a mixture of heterogeneous brittle material. As concrete construction and its deformation, constraints, and a series of problems, hardening of concrete forming the existence of a large number of micro-pores, cavitation and micro-cracks, it is precisely because of these early concrete buildings and structures are usually working with a slit, due to cracks The existence and development is generally within the steel and other materials will produce corrosion, reducing the carrying capacity of reinforced concrete materials, durability and impermeability capacity, affecting the appearance of the building, life would be serious threat to human Concrete Cracks There are many reasons, there is distortion caused by the cracks: If the temperature changes, shrinkage, expansion, uneven subsidence causes such as cracks; a crack caused by external loads; the conservation of the environment and chemical effects caused by improper cracks and so on. In practical engineering should be treated differently, according to the actual situation to solve the problem. 2, condensate soil engineering and prevention of common cracks 1, shrinkage cracks and prevention of Shrinkage cracks in concrete curing more after the end of a period of time, or pouring concrete around the week after. The evaporation of water slurry will produce shrinkage, and this contraction was not reversible. Shrinkage cracks in concrete outside was mainly due to evaporation caused by different levels of deformation of different results: the impact of concrete affected by external conditions, a rapid loss of surface moisture, deformation large, small changes in the internal humidity, little distortion, the larger shrinkage deformation of concrete surfaces subject to internal constraints, resulting in greater tensile stress and cracks. The lower the relative humidity, the greater the shrinkage of cement paste, shrinkage cracks in the more easy to produce. Shrinkage cracks are mostly parallel to the surface of linear or a mesh of fine cracks in shallow, the width of 0.05 0.2mm more in between the flat parts of the mass concrete in the more common, thin flat-slab in the distribution of more than along its short. Shrinkage cracks will usually affect the impermeability of concrete, causing corrosion affected reinforced concrete durability, water pressure will produce hydraulic fracturing under the impact of the bearing capacity of concrete and so on. Shrinkage of concrete and concrete water-cement ratio the main, cement composition, cement consumption, the nature and amount of aggregates, additives and other relevant amount. The main preventive measures: First, choose a smaller contraction of cement, commonly used in low-heat cement and fly ash cement, reduce the amount of cement. Second, the shrinkage of concrete influenced by the water-cement ratio, the greater the water-cement ratio, the greater the shrinkage, so in the concrete mix proportion design should try to control the selection of a good water-cement ratio, while a suitable water-reducing admixture agent. Third, strict control of concrete mixing and construction of the mixing ratio, water must not be larger than the concrete mix design for a given amount of water used. The fourth is to strengthen the concrete early conservation and, where appropriate to extend the concrete curing time. Construction in winter when the appropriate time to extend the coverage of thermal insulation of concrete and the curing agent brushing conservation. 5 is set in the concrete structure an appropriate contraction joints. 2, plastic shrinkage cracks and prevention of Plastic shrinkage refers to the concrete in the condensed prior to the surface due to dehydration resulting from faster shrinkage. Plastic shrinkage cracks is generally hot and dry, or windy weather occurs, cracks Duocheng the middle of wide and thin at both ends of varying lengths, each inconsistent state. The shorter the length of cracks in 20 30cm, long cracks up to 2 3m, width 1 5mm. The main reasons for its formation: concrete in the final setting with virtually no strength or intensity is very small, or the final setting of concrete and the strength is just hours by high winds or large the impact of excessive water loss of concrete surface, resulting in capillary generated a large negative pressure Ershi sharp contraction in the volume of concrete, while the strength of concrete at this time is unable to resist its own contraction, resulting in cracks. Plastic shrinkage cracking of concrete affected the main factors water-cement ratio, concrete setting time, ambient temperature, wind speed, relative humidity and so on. The main preventive measures: First, choose a relatively small shrinkage of high early strength of Portland or ordinary portland cement. Second, strict control of water-cement ratio, admixture superplasticizer to increase the slump and workability of concrete, reduce the dosage of cement and water. Third, prior to pouring of concrete will be watered evenly wet the grass roots and the template. Fourth, timely coverage of plastic film or a wet straw, hemp films and so on, to keep the surface wet before the final setting of concrete, or concrete surface spraying of the conservation conservation agents. 5 is a high temperature and strong wind to set the sun and wind facilities, and timely maintenance. 3, subsidence cracks and prevention of Subsidence is due to structural cracks in the foundation soil is uneven, soft, or backfill is not false or due to flooding caused by uneven settlement; or because of lack of rigidity of the template, template support spacing is too large or support the bottom of the loose and so lead to, especially in the In winter, the template support in the permafrost, the permafrost thaw resulting from uneven settlement, resulting in cracks in concrete structure. Such cracks are mostly or penetrating deep into the cracks, its direction is related with the subsidence, generally along the vertical or the ground was 30 45 angle direction, a large subsidence cracks, tend to have a certain degree of dislocation, crack width is often proportional relationship with the settlement. Crack width less affected by temperature changes. Ground deformation and stability, the subsidence cracks has been largely stabilized. The main preventive measures: First, on the soft soil, filling in the upper structure of the foundation should be the necessary pre-construction compaction and consolidation. Second, the template is to ensure that there is sufficient strength and rigidity, and support firm, and to force the foundation evenly. 3 is to prevent watering during the foundation of concrete by water immersion. Fourth, the time the template can not be removed too early, and pay attention to the order form removal. 5 puts up in the frozen earth on the template should pay attention to take certain preventive measures. 4, temperature, and prevention of cracks Multi-temperature cracks occurred in the large volume of concrete surface or temperature changes in a larger area of concrete structures. After pouring concrete in the hardening process, generate a lot of hydration of cement hydration heat (when the cement content in the 350 550 kg/m3, per cubic meter of concrete will be released out of 17500 27500kJ heat, so that the internal temperature rise of concrete up to about 70 or higher). As the larger concrete, a large number of hydration heat accumulation in the concrete house and easy to distribute, resulting in a sharp rise in internal temperature, while the concrete surface heat faster, so that the formation of large temperature difference between inside and outside the larger temperature difference caused by internal and external different degrees of thermal expansion and contraction, so that concrete surfaces have a certain tensile stress (proven itself when the concrete temperature reached 25 26 , the concrete will be produced within about roughly 10MPa tensile stress). When the tensile stress exceeds the tensile strength of concrete limits, the concrete surface will produce cracks, such cracks will occur in the latter part of concrete construction. In concrete construction, when the temperature changed greatly, or by the cold concrete of the attacks will lead to a sharp decline in temperature of the concrete surface, while the contraction, the surface of concrete shrinkage of concrete by internal constraints, will have a significant tensile stress and the cracks, such cracks in the concrete surface is usually generated within shallow. The trend of temperature cracks are usually no fixed rules, large-scale structural cracks often criss-cross; flat-slab type length of the large size of the structure, cracks, more parallel to the short side; in-depth and cross-cutting of the temperature cracks generally associated with the short side parallel to the direction or nearly parallel to , cracks occur along the long side of the section, in the middle more dense. Crack width varies, affected by the temperature change is more obvious in winter wider, narrower in summer. High temperature expansion of concrete caused by temperature cracks are usually fine at both ends of the middle of thick, while the shrinkage cracks in the thickness of the changes were less pronounced. The emergence of such cracks can cause corrosion of steel bars, concrete carbonation, reducing freeze-thaw resistance of concrete, anti-fatigue and anti-permeability capability. The main preventive measures: First, try to use low heat or medium heat cement, such as slag cement, fly ash and cement. Second, is to reduce the amount of cement, cement consumption will try to control the 450kg/m3 below. 3 is to reduce the water-cement ratio, the general control of the concrete water-cement ratio below 0.6. 4 is to improve the aggregate gradation, or superplasticizer admixture such as fly ash to reduce cement consumption, lower heat of hydration. 5 is to improve the mixing of concrete processing technology, in the traditional three cold technology based on the use of secondary air-cooled new technology, reduce the temperature of concrete pouring. 6 is a certain amount of admixture in concrete that has reduced water, plasticized, such as the role of retarding admixture to improve the mobility of concrete mixture, water retention, reduce heat of hydration to delay the emergence of hot-peak hours. 7 is a high-temperature season, can be used when pouring puts up sun visor and other auxiliary measures to control the concrete temperature rise, reducing the temperature of pouring concrete. 8 is the temperature of mass concrete and structural dimensions of stress related to the larger size of concrete structure, the greater the thermal stress and therefore, reasonable arrangements for the construction process, layered, sub-block casting, to facilitate heat dissipation, reducing constraints. 9 is the internal settings of large concrete cooling pipes, pass cold water or air cooling, reducing the temperature difference between inside and outside the concrete. 10 is to strengthen the concrete temperature monitoring, timely cooling, protective measures. 11 are reserved for temperature contraction joints. 12 is to decrease the constraints appropriate to the bedrock before pouring of concrete and old concrete laid on the sand cushion of about 5mm or use of asphalt materials such as brushing. 13 is to strengthen the concrete curing, concrete pouring, the timely and wet straw mat, hemp films and so on coverage and pay attention to watering conservation, appropriate to extend the curing time to ensure the concrete surface slow cooling. In the cold season, the concrete surface should be set up thermal insulation measures to prevent the cold wave attack. 14 is configured in a small number of reinforced concrete or fiber material will be mixed with the temperature of concrete crack control in a certain range. 5, chemical reaction caused by cracks and prevention of Alkali-aggregate reaction caused by cracks and steel corrosion cracks in reinforced concrete structure is the most common due to cracks caused by chemical reaction. After the concrete mixing will produce a number of alkaline ions, these ions with certain activities of aggregate chemical reaction and to absorb the surrounding environment in the water volume increase, resulting in concrete crisp, expansion cracks. Such cracks in concrete structures are usually used during the event is difficult to remedy, it should take effective measures in the construction for prevention. The main preventive measures: First, selection of alkali-aggregate activity of a small sand and gravel. Second, use low-alkali cement and low-alkali or alkali admixtures. Third, choose the appropriate admixture inhibiting alkali-aggregate reaction. As the concrete pouring, vibrating bad or reinforced protecti