混凝土毕业设计论文中英文对照资料外文翻译文献

外文文献及译文
目录
•1历史
•2组成
o水泥2.1
o 2.2水
o 2.3骨料
o 2.4化学外加剂
o 2.5掺合料和水泥混合
o 2.6纤维
•3搅拌混凝土
•4个特点
o 4.1和易
o 4.2固化
o 4.3强度
o 4.4弹性
o 4.5扩张和收缩
o 4.6开裂
▪ 4.6.1收缩裂缝
▪ 4.6.2拉裂
o 4.7蠕变
•5损伤模式
o 5.1火灾
o 5.2总量扩张
o 5.3海水效果
o 5.4细菌腐蚀
o 5.5化学武器袭击
▪ 5.5.1碳化
▪ 5.5.2氯化物
▪ 5.5.3硫酸盐
o 5.6浸出
o 5.7人身损害
•6种混凝土
o 6.1普通混凝土
o 6.2高强混凝土
o 6.3高性能混凝土
o 6.4自密实混凝土
o 6.5喷浆
o 6.6透水性混凝土
o 6.7混凝土蜂窝
o 6.8软木复合水泥
o 6.9碾压混凝土
o 6.10玻璃混凝土
o 6.11沥青混凝土
•7混凝土测试
•8混凝土回收
•9使用混凝土结构
o9.1大体积混凝土结构
o9.2钢筋混凝土结构
o9.3预应力混凝土结构
•10参见
•11参考
•12外部链接
历史
在塞尔维亚,仍然是一个小屋追溯到5600bce已经发现,同一个楼层发红色石灰,沙子和砾石。

金字塔陕西中建千多年前,含有石灰和火山灰.或粘土。

碎石
水和泥浆僵硬和发展实力超过时间。

为了确保经济实用的解决方案,既罚款又粗骨料使用,以弥补大部分的混凝土混合物。

砂,天然砾石及碎石,主要用于这一目的。

不过,现在越来越普遍,再生骨料(由建筑,拆卸和挖掘废物)被用作局部代替天然骨料,而一些生产总量包括风冷高炉炉渣和粉煤灰也是不允许的。

装饰石材等石英岩,潆石块或玻璃破碎,有时添加到混凝土表面进行装饰性"的总暴露"完成,流行景观设计师。

化学外加剂
化学外加剂现形式的材料粉末或液体,补充了混凝土给它的某些特性没有可与普通混凝土混合物。

在正常情况下使用,外加剂剂量均低于5%的大量水泥,并补充了混凝土当时的配料/混合.最常见的外加剂有:
加速器加速水化(硬化)的混凝土。

缓速慢水化混凝土主要用在大型或难以倒出地方设置局部面前倒完全是不可取的。

空气夹带放入,并派发小气泡混凝土,从而减少损坏冻融周期,从而提高混凝土的耐久性。

增塑剂(减水剂)提高加工塑料或"新鲜"的混凝土,使其置于更容易,以较少的努力巩固。

减水剂(高量程减水剂)是一类增塑剂已较少有害影响,使用时,大幅度提高关键所在.另外,增塑剂可用于减少水分的混凝土(也被称为减水剂由于这项申请),而维持关键所在.这将提高其强度和耐久性的特点。

颜料,可以用来改变颜色的混凝土,为美学。

缓蚀剂使用,以减少腐蚀钢材和钢筋混凝土。

粘接剂是用来制造一种纽带,对新老混凝土。

抽水艾滋病改善泵,变厚糊,并减少脱水糊的。

矿物外加剂与水泥混纺
有无机材料,也有火山灰或潜在的水力性能。

这些极细粒材料添加到混凝土,以改善混凝土性能(矿物外加剂),或作为替代普通硅酸盐水泥(水泥混合)。

在水化,硬化,具体需要制定一定的物理和化学性能。

在其他品质,机械强度,低透湿性,化学和体积稳定性是必要的。

加工
和易性(或一致性,因为它是已知的是在欧洲),即有能力的新鲜(塑料)混凝土混合填好表格/模具正确处理理想工作(振动),并没有降低混凝土的质量。

和易取决于含水量,骨料(形状和大小分布),水泥含量和年龄(一级水化)并可以修改,加入化学外加剂。

提高水分或添加化学外加剂会增加混凝土的和易性。

水过多会导致更多流血(地表水)和/或隔离料(如水泥,骨料开始分开)并最终导致混凝土具有质量下降。

使用的骨料不良定级结果,可以在非常苛刻的组合设计提供了非常低低迷的情况下,不能轻易作出更为可行的增加,合理用水。

和易性,可衡量的"坍",一个简单衡量的可塑性新一批混凝土以下的ASTM三143或恩12350-2测试标准。

不景气通常是衡量一个填"艾布拉姆斯锥体",以抽样,由新一批混凝土。

锥放在与广末下降到一个水平,非吸收表面.当锥仔细升空,密封材料会大跌一定数量由于重力。

相对干燥样品将大跌甚微具有价值大跌的一个或两英寸(25或50毫米)。

相对湿混凝土样本不景气可能高达六,七吋(150至175毫米)。

不景气,可增加化学外加剂,如中程或高效减水剂代理商(超塑化剂)无改变水/水泥比例。

这是不好的做法加重水在混凝土搅拌机。

高流态混凝土,如自密实混凝土,测试了其他流量测量方法。

其中方法包括配售锥在狭窄的结束和观测如何组合流经锥虽然正在逐步解除。

固化
由于水泥需要时间来充分水合物之前,它具有硬度和强度,混凝土必须治愈一旦它被放置。

固化过程中保持混凝土在一种特定的环境条件,直至水化较为完备。

良好的治疗通常认为是提供一个潮湿的环境,并控制温度。

一股潮湿环境促进水合,因为增加水化降低通透性,提高其强度,从而提高了材料质量。

使混凝土表面干出过多可导致拉应力,但仍水化内政部不能承受,造成混凝土开裂。

另外,发热量产生的放热过程中水化,可问题是大量的存款。

允许混凝土冻结在寒冷的气候条件,然后固化完成后会打断水化过程中,降低混凝土强度,并导致结垢及其他损害或失败。

28天。

大厦使用的似乎是一个总结钢纤维和石英--矿产与抗压强度160,000房,远高于典型的高强度骨料,如花岗岩(15,000-20,000PSI)之中。

弹性
弹性模量混凝土的是一个功能的弹性模量的碎石和水泥矩阵以及它们的相对比例。

弹性模量混凝土相对线性,在低应力水平,而是成为越来越多的非线性作为基体开裂的发展。

弹性模量的硬膏,可在命令10-30GPA和总量约4585个产业。

混凝土复合,然后在射程在30至50岁产业。

扩张和收缩
混凝土具有极低的热膨胀系数。

但是,如果没有作出规定的扩张十分庞大势力,可设立造成裂缝部分的结构不能够抵御武力或重复周期的扩张和收缩。

由于混凝土成熟,它继续萎缩,由于目前反应发生在物质虽然干缩率下降较快,并不断减少的一段时间内(对所有实际用途混凝土通常考虑不惜再经过30年)。

相对收缩与扩张混凝土和砖砌需要仔细住宿时,在两种形式的建筑界面。

开裂
混凝土裂缝是由于应力诱导收缩或外加载荷。

工程师们熟悉的倾向混凝土裂缝,并在适当特别设计的防范措施,以确保裂缝控制。

这就要求把强化中,例如螺纹钢筋,置于期望间距限制裂缝宽度到可接受的水平。

挡水结构和混凝土公路例子结构裂缝控制的行使。

计划的目的是鼓励大批非常细小裂纹,而不是一个小数目庞大,随机发生裂痕。

所有混凝土结构的裂缝会在某种程度上。

一个早期的设计者钢筋混凝土,罗伯特maillart,采用钢筋混凝土在多个拱桥。

他的第一桥很简单,采用大体积混凝土,maillart和发现大面积的结构都很破。

他随后得知,如果混凝土很破,它不能促进结构强度-但结构明显成效。

因此,他的设计,后来干脆取消破获地区,导致苗条,美丽的混凝土拱。

该salginatobel大桥是其中一个例子。

开裂也是一个非常重要的指标结构窘迫,在钢筋混凝土构件。

例如妥善设计的钢筋混凝土梁否则由于负荷过重,将呈现明显增多宽度和裂缝。

这可以修复,修复,或在必要时疏散一个不安全的地区。

收缩裂缝
收缩裂缝发生在混凝土构件进行克制体积变化(收缩),或因干燥收缩,或热效应。

克制是要么提供外部(即支持,墙壁,和其他边界条件)或国内(差干缩,钢筋)。

一旦拉伸强度的混凝土,是超越,有裂缝的发展。

的数量和宽度收缩裂缝的发展,是受量缩发生,数额限制,以及现行的数量和间隔提供增援。

具体是放置在一个潮湿(或塑料)状态,因此可以操纵和塑造的需要。

水化,硬化过程中混凝土的头三天是关键。

异常快速干燥收缩由于种种因素,例如从蒸发风期间安插可能导致增加拉伸讲的时候,还没有取得重大的强度,造成更大的收缩裂缝。

早期的混凝土强度可提高保持潮湿了一段较长时期的固化过程。

最小应力事先固化到最小程度开裂。

早期强度高,混凝土设计水合物更快,往往更多地使用水泥,其中升幅收缩,开裂。

塑性收缩裂缝是立竿见影,但在可见的0-2天内放置,而干燥收缩裂缝,随着时间的推移防范诸如混合选择和联合间距可以采取鼓励裂缝发生在一个共同的审美不要随意。

张力裂缝
混凝土构件,可投入紧张外加载荷。

这是最常见的混凝土梁,当横向载荷将一个表面成压缩及对面的表面张力成(因所致弯曲)。

部分梁,是在紧张,可能裂纹的大小和长度的裂缝是依赖关于震级的弯矩和设计的加固梁在点审议。

钢筋混凝土梁的设计裂缝紧张,而不是压缩这是通过提供钢筋产量中,然后失败的混凝土在压缩过程发生,也使这样提供了一个预警机制。

蠕变
蠕变是一个术语,用来形容永久移动或变形的材料,以减轻应力材料。

具体是受力,很容易走弯路。

蠕变有时可以减少数额裂缝出现在混凝土结构或元素,但也必须加以控制。

数额中小学加固混凝土结构有助于减少数额收缩,蠕变和开裂。

因为它是一种流体,具体可以抽在有需要的地方。

这里有混凝土运输车是喂养具体到一个具体的潜水泵,这是抽水到那里一砖正整齐。

破坏模式
火灾
带和结晶盐由水浸泡成混凝土孔隙,然后干涸。

火山灰水泥及水泥使用超过60%的工业废渣总量较耐海水比纯兰水泥。

细菌腐蚀
细菌本身就没有明显的效果具体化。

然而,厌氧细菌在未经处理的污水,往往能产生硫化氢,然后氧化好氧菌目前在生物膜在混凝土表面以上水位硫酸溶解碳酸盐在水泥固化而导致的强度损失。

混凝土地板躺在地上含黄铁矿也处于危险之中.采用石灰石为骨料,使混凝土更能抵抗酸,及污水可预处理方法,提高pH值或氧化或沉淀的硫化物,以抑制活性硫化物利用细菌。

化学武器袭击
碳化
氯化物
氯化物,尤其是氯化钙,已使用,以缩短混凝土凝结时间的。

然而,氯化钙和(在较小的程度上)氯化钠已显示溶解氢氧化钙并引起化学变化,在普通硅酸盐水泥,导致丧失强度,〔14〕以及攻击钢筋,在当前最为具体。

硫酸盐
硫酸盐溶液接触混凝土可引起化学变化,以水泥,它可造成严重的微观效应导致美元疲软的水泥粘结剂。

类型的混凝土
各类混凝土已制定专门应用,并已成为人所共知的,这些名字。

常规混凝土
普通混凝土是奠定术语,描述具体,是由以下的混合指令,通常刊登包水泥,通常用沙子或其他常见的材料作为骨料,而且往往混杂在简易容器。

这种混凝土可以生产出了屈服强度不同,由约10兆帕至40兆帕,根据不同的用途,从致盲结构混凝土。

许多类型的预拌混凝土复本,其中包括粉末掺入水泥中的一个集合,只需用水。

高强混凝土
高强混凝土的抗压强度一般都大于6000磅/平方英寸(40兆帕)。

高强混凝土,是由降低水灰比(宽/三)的比例为0.35或更低。

常常硅粉是补充,以防止形成自由氢氧化钙晶体在水泥基可能会降低强度,在水泥总量的债券。

低水/碳比例,并利用硅粉,使混凝土搅拌着较可行这一点尤其可能是一个问题,高强混凝土应用中,密钢筋笼,有可能会用。

为弥补因降低和易性,减水剂通常加上高强度的混合物。

总计必须认真选择高强度的混合,实力较弱的总量可能不会强劲到足以抵御荷载对混凝土和事业不开始总结,而不是在矩阵或无效的,因为通常发生在普通混凝土。

Concrete
Contents
• 1 History
• 2 Composition
o 2.1 Cement
o 2.2 Water
o 2.3 Aggregates
o 2.4 Chemical admixtures
o 2.5 Mineral admixtures and blended cements
o 2.6 Fibers
• 3 Mixing concrete
• 4 Characteristics
o 4.1 Workability
o 4.2 Curing
o 4.3 Strength
o 4.4 Elasticity
o 4.5 Expansion and shrinkage
o 4.6 Cracking
▪ 4.6.1 Shrinkage cracking ▪ 4.6.2 Tension cracking o 4.7 Creep
• 5 Damage modes
o 5.1 Fire
o 5.2 Aggregate expansion
o 5.3 Sea water effects
o 5.4 Bacterial corrosion
o 5.5 Chemical attacks
▪ 5.5.1 Carbonation
▪ 5.5.2 Chlorides
▪ 5.5.3 Sulphates
o 5.6 Leaching
o 5.7 Physical damage
• 6 Types of concrete
o 6.1 Regular concrete
o 6.2 High-strength concrete
o 6.3 High-performance concrete o 6.4 Self-compacting concretes o 6.5 Shotcrete
o 6.6 Pervious concrete
o 6.7 Cellular concrete
o 6.8 Cork Cement Composites
o 6.9 Roller-compacted concrete
o 6.10 Glass concrete
o 6.11 Asphalt concrete
•7 Concrete testing
•8 Concrete recycling
•9 Use of concrete in structures
o9.1 Mass concrete structures
o9.2 Reinforced concrete structures o9.3 Prestressed concrete structures •10 See also
•11 References
•12 External links
durable, while a higher w/c ratio yields a concrete with a larger slump, so it may be placed more easily.[3] Cement paste is the material formed by combination of water and cementitious materials; that part of the concrete which is not aggregate or reinforcing. The workability or consistency is affected by the water content, the amount of cement paste in the overall mix and the physical characteristics (maximum size, shape, and grading) of the aggregates.
Aggregates
The water and cement paste hardens and develops strength over time. In order to ensure an economical and practical solution, both fine and coarse aggregates are utilised to make up the bulk of the concrete mixture. Sand, natural gravel and crushed stone are mainly used for this purpose. However, it is increasingly common for recycled aggregates (from construction, demolition and excavation waste) to be used as partial replacements of natural aggregates, whilst a number of manufactured aggregates, including air-cooled blast furnace slag and bottom ash are also permitted.
Decorative stones such as quartzite, small river stones or crushed glass are sometimes added to the surface of concrete for a decorative "exposed aggregate" finish, popular among landscape designers.
Chemical admixtures
"fresh" concrete, allowing it be placed more easily, with less consolidating effort.
Superplasticizers (high-range water-reducing admixtures) are a class of plasticizers which have fewer deleterious effects when used to significantly increase workability. Alternatively, plasticizers can be used to reduce the water content of a concrete (and have been called water reducers due to this application) while maintaining workability. This improves its strength and durability characteristics.
Pigments can be used to change the color of concrete, for aesthetics.
Corrosion inhibitors are used to minimize the corrosion of steel and steel bars in concrete.
Bonding agents are used to create a bond between old and new concrete.
Pumping aids improve pumpability, thicken the paste, and reduce dewatering of the paste.
Mineral admixtures and blended cements
There are inorganic materials that also have pozzolanic or latent hydraulic properties. These very fine-grained materials are added to the concrete mix to improve the properties of concrete (mineral admixtures),[4]or as a replacement for Portland cement (blended cements).[5]
partially replace Portland cement (by up to 60% by mass). The properties of fly ash depend on the type of coal burnt. In general, silicious fly ash is pozzolanic,
strength and durability similar to concrete made with silica fume. While silica fume is usually dark gray or black in color, high reactivity metakaolin is usually bright white in color, making it the preferred choice for architectural concrete where appearance is important.
Fibers
Short fibers of steel, glass, synthetic or natural materials can be incorporated in the concrete during mixing. See Fiber reinforced concrete.
Mixing concrete
Thorough mixing is essential for the production of uniform, high quality concrete. Therefore, equipment and methods should be capable of effectively mixing concrete materials containing the largest specified aggregate to produce uniform mixtures of the lowest slump practical for the work. Separate paste mixing has shown that the mixing of cement and water into a paste before combining these materials with aggregates can increase the compressive strength of the resulting concrete.[9]The paste is generally mixed in a high-speed, shear-type mixer at a w/cm of 0.30 to 0.45 by mass. The premixed paste is then blended with aggregates and any remaining batch water, and final mixing is completed in conventional concrete mixing equipment.[10]
High-Energy Mixed Concrete (HEM concrete) is produced by means of high-speed mixing of cement, water and sand with net specific energy consumption at least 5 kilojoules per kilogram of the mix. It is then added an plasticizer admixture and mixed after that with aggregates in conventional mixer. This paste can be used itself or foamed (expanded) for lightweight concrete.[11] Sand effectively dissipates energy in this mixing process. HEM concrete fast hardens in ordinary and low temperature conditions, and possesses increased volume of gel, drastically reducing capillarity in solid and porous materials. It is recommended for precast concrete in order to reduce quantity of cement, as well as concrete roof and siding tiles, paving stones and lightweight concrete block production.
Characteristics
During hydration and hardening, concrete needs to develop certain physical and chemical properties. Among other qualities, mechanical strength, low moisture permeability, and chemical and volumetric stability are necessary. Workability
Workability (or consistence, as it is known in Europe) is the ability of a fresh (plastic) concrete mix to fill the form / mould properly with the desired work (vibration) and without reducing the concrete's quality. Workability depends on water content, aggregate (shape and size distribution), cementitious content and
age (level of hydration), and can be modified by adding chemical admixtures. Raising the water content or adding chemical admixtures will increase concrete workability. Excessive water will lead to increased bleeding (surface water) and/or segregation of aggregates (when the cement and aggregates start to separate), with the resulting concrete having reduced quality. The use of an aggregate with an undesirable gradation can result in a very harsh mix design with a very low slump, which cannot be readily made more workable by addition of reasonable amounts of water.
Workability can be measured by the "slump test", a simplistic measure of the plasticity of a fresh batch of concrete following the ASTM C 143 or EN 12350-2 test standards. Slump is normally measured by filling an "Abrams cone" with a sample from a fresh batch of concrete. The cone is placed with the wide end down onto a level, non-absorptive surface. When the cone is carefully lifted off, the enclosed material will slump a certain amount due to gravity. A relatively dry sample will slump very little, having a slump value of one or two inches (25 or 50 mm). A relatively wet concrete sample may slump as much as six or seven inches (150 to 175 mm).
Slump can be increased by adding chemical admixtures such as mid-range or high-range water reducing agents (super-plasticizers) without changing the water/cement ratio. It is bad practice to add extra water at the concrete mixer.
High-flow concrete, like self-consolidating concrete, is tested by other flow-measuring methods. One of these methods includes placing the cone on the narrow end and observing how the mix flows through the cone while it is gradually lifted.
Curing
Because the cement requires time to fully hydrate before it acquires strength and hardness, concrete must be cured once it has been placed. Curing is the process of keeping concrete under a specific environmental condition until hydration is relatively complete. Good curing is typically considered to provide a moist environment and control temperature. A moist environment promotes
hydration, since increased hydration lowers permeability and increases strength resulting in a higher quality material. Allowing the concrete surface to dry out excessively can result in tensile stresses, which the still-hydrating interior cannot withstand, causing the concrete to crack.
Also, the amount of heat generated by the exothermic chemical process of hydration can be problematic for very large placements. Allowing the concrete to freeze in cold climates before the curing is complete will interrupt the hydration process, reducing the concrete strength and leading to scaling and other damage or failure.
The effects of curing are primarily a function of geometry (the relation between exposed surface area and volume), the permeability of the concrete, curing time, and curing history.
Improper curing can lead to several serviceability problems including cracking, increased scaling, and reduced abrasion resistance.
Strength
Concrete has relatively high compressive strength, but significantly lower tensile strength (about 10% of the compressive strength). As a result, concrete always fails from tensile stresses —even when loaded in compression. The practical implication of this is that concrete elements subjected to tensile stresses must be reinforced. Concrete is most often constructed with the addition of steel or fiber reinforcement. The reinforcement can be by bars (rebar), mesh, or fibres, producing reinforced concrete. Concrete can also be prestressed (reducing tensile stress) using internal steel cables (tendons), allowing for beams or slabs with a longer span than is practical with reinforced concrete alone.
The ultimate strength of concrete is influenced by the water-cement ratio (w/c) [water-cementitious materials ratio (w/cm)], the design constituents, and the mixing, placement and curing methods employed. All things being equal, concrete with a lower water-cement (cementitious) ratio makes a stronger concrete than a higher ratio. The total quantity of cementitious materials (Portland cement, slag cement, pozzolans) can affect strength, water demand,
shrinkage, abrasion resistance and density. As concrete is a liquid which hydrates to a solid, plastic shrinkage cracks can occur soon after placement; but if the evaporation rate is high, they often can occur during finishing operations (for example in hot weather or a breezy day). Aggregate interlock and steel reinforcement in structural members often negates the effects of plastic shrinkage cracks, rendering them aesthetic in nature. Properly tooled control joints or saw cuts in slabs provide a plane of weakness so that cracks occur unseen inside the joint, making a nice aesthetic presentation. In very high strength concrete mixtures (greater than 10,000 psi), the strength of the aggregate can be a limiting factor to the ultimate compressive strength. In lean concretes (with a high water-cement ratio) the use of coarse aggregate with a round shape may reduce aggregate interlock.
Experimentation with various mix designs begins by specifying desired "workability" as defined by a given slump, durability requirements, and the required 28 day compressive strength. The characteristics of the cementitious content, coarse and fine aggregates, and chemical admixtures determine the water demand of the mix in order to achieve the desired workability. The 28 day compressive strength is obtained by determination of the correct amount of cementitious to achieve the required water-cement ratio. Only with very high strength concrete does the strength and shape of the coarse aggregate become critical in determining ultimate compressive strength.
The internal forces in certain shapes of structure, such as arches and vaults, are predominantly compressive forces, and therefore concrete is the preferred construction material for such structures.
reported on April 13th, 2007, that a team from the University of Tehran, competing in a contest sponsored by the American Concrete Institute, demonstrated several blocks of concretes with abnormally high compressive strengths between 50,000 and 60,000 PSI at 28 days[1]. The blocks appeared to use an aggregate of steel fibres and quartz -- a mineral with a compressive
strength of 160,000 PSI, much higher than typical high-strength aggregates such as granite (15,000-20,000 PSI).
Elasticity
The modulus of elasticity of concrete is a function of the modulus of elasticity of the aggregates and the cement matrix and their relative proportions. The modulus of elasticity of concrete is relatively linear at low stress levels but becomes increasing non-linear as matrix cracking develops. The elastic modulus of the hardened paste may be in the order of 10-30 GPa and aggregates about 45 to 85 GPa. The concrete composite is then in the range of 30 to 50 GPa. Expansion and shrinkage
Concrete has a very low coefficient of thermal expansion. However if no provision is made for expansion very large forces can be created, causing cracks in parts of the structure not capable of withstanding the force or the repeated cycles of expansion and contraction.
As concrete matures it continues to shrink, due to the ongoing reaction taking place in the material, although the rate of shrinkage falls relatively quickly and keeps reducing over time (for all practical purposes concrete is usually considered to not shrink any further after 30 years). The relative shrinkage and expansion of concrete and brickwork require careful accommodation when the two forms of construction interface.
Cracking
Concrete cracks due to tensile stress induced by shrinkage or by applied loading. Engineers are familiar with the tendency of concrete to crack, and where appropriate, special design precautions are taken to ensure crack control. This entails the incorporation of secondary reinforcing, for example deformed steel bars, placed at the desired spacing to limit the crack width to an acceptable level. Water retaining structures and concrete highways are examples of structures where crack control is exercised. The objective is to encourage a large number of very small cracks, rather than a small number of large, randomly-occurring cracks.
All concrete structures will crack to some extent. One of the early designers of reinforced concrete, Robert Maillart, employed reinforced concrete in a number of arched bridges. His first bridge was very simple, using a large volume of concrete, and Maillart noticed that large areas of the structure were very cracked. He then realised that if the concrete was very cracked, it must not be contributing to the strength of the structure - but yet the structure clearly worked. Therefore, his later designs simply removed the cracked areas, leading to slender, beautiful concrete arches. The Salginatobel Bridge is an example of this.
Cracking is also a primary indicator of structural distress in reinforced concrete elements. For example, a properly designed reinforced concrete beam failing as a result of overloading will exhibit a pronounced increase in the number and width of cracks. This can allow remediation, repair, or if necessary, evacuation of an unsafe area.
Shrinkage cracking
Shrinkage cracks occur when concrete members undergo restrained volumetric changes (shrinkage) as a result of either drying, autogenous shrinkage, or thermal effects. Restraint is provided either externally (i.e. supports, walls, and other boundary conditions) or internally (differential drying shrinkage, reinforcement). Once the tensile strength of the concrete is exceeded, a crack will develop. the number and width of shrinkage cracks that develop are influenced by the amount of shrinkage that occurs, the amount of restraint present, and the amount and spacing of reinforcement provided.
Concrete is placed while in a wet (or plastic) state, and therefore can be manipulated and moulded as needed. Hydration and hardening of concrete during the first three days is critical. Abnormally fast drying and shrinkage due to factors such as evaporation from wind during placement may lead to increased tensile stresses at a time when it has not yet gained significant strength, resulting in greater shrinkage cracking. The early strength of the concrete can be increased by keeping it damp for a longer period during the curing process. Minimizing stress prior to curing minimizes cracking. High early-strength concrete is designed to
hydrate faster, often by increased use of cement, which increases shrinkage and cracking.
Plastic-shrinkage cracks are immediately apparent, visible within 0 to 2 days of placement, while drying-shrinkage cracks develop over time. Precautions such as mixture selection and joint spacing can be taken to encourage cracks to occur within an aesthetic joint instead of randomly.
Tension cracking
Concrete members may be put into tension by applied loads. This is most common in concrete beams, where a transversely applied load will put one surface into compression and the opposite surface into tension (due to induced bending). The portion of the beam that is in tension may crack - the size and length of cracks is dependent on the magnitude of the bending moment and the design of the reinforcing in the beam at the point under consideration. Reinforced concrete beams are designed to crack in tension rather than in compression. This is achieved by providing reinforcing steel which yields before failure of the concrete in compression occurs and in so doing provides a warning mechanism. Creep
Creep is the term used to describe the permanent movement or deformation of a material in order to relieve stresses within the material. Concrete which is subjected to forces is prone to creep. Creep can sometimes reduce the amount of cracking that occurs in a concrete structure or element, but it also must be controlled. The amount of primary and secondary reinforcing in concrete structures contributes to a reduction in the amount of shrinkage, creep and cracking.
Because it is a fluid, concrete can be pumped to where it is needed. Here a concrete transport truck is feeding concrete to a concrete pumper, which is pumping it to where a slab is being poured.
Damage modes
Fire
Due to its low thermal conductivity, a layer of concrete is frequently used for fireproofing of steel structures. However, concrete itself may be damaged by fire.
Up to about 300 °C, the concrete undergoes normal thermal expansion. Above that temperature, shrinkage occurs due to water loss; however, the aggregate continues expanding, which causes internal stresses. Up to about 500 °C, the major structural changes are carbonation and coarsening of pores. At 573 °C, quartz undergoes rapid expansion due to Phase transition, and at 900 °C calcite starts shrinking due to decomposition. At 450-550 °C the cement hydrate decomposes, yielding calcium oxide. Calcium carbonate decomposes at about 600 °C. Rehydration of the calcium oxide on cooling of the structure causes expansion, which can cause damage to material which withstood fire without falling apart. Concrete in buildings that experienced a fire and were left standing for several years shows extensive degree of carbonation.
Concrete exposed to up to 100 °C is normally considered as healthy. The parts of a concrete structure that is exposed to temperatures above approximately 300 °C (dependent of water/cement ratio) will most likely get a pink color. Over approximately 600 °C the concrete will turn light grey, and over approximately 1000 °C it turns yellow-brown[12] One rule of the thumb is to consider all pink colored concrete as damaged, and to be removed.
Fire will expose the concrete to gasses and liquids that can be harmful to the concrete, among other salts and acids that occur when fire-gasses get in contact with water.
Aggregate expansion
Various types of aggregate undergo chemical reactions in concrete, leading to damaging expansive phenomena. The most common are those containing reactive silica, that can react (in the presence of water) with the alkalis in concrete (K2O and Na2O, coming principally from cement). Among these: Opal, Chalcedony, Flint and strained Quartz. Following the reaction (Alkali Silica Reaction or ASR), an expansive gel forms, that creates extensive cracks and
to acids, and the sewage may be pretreated by ways increasing pH or oxidizing or precipitating the sulphides in order to inhibit the activity of sulphide utilizing bacteria.
Chemical attacks
Carbonation
Chlorides
Chlorides, particularly calcium chloride, have been used to shorten the setting time of concrete.[13]However, calcium chloride and (to a lesser extent) sodium chloride have been shown to leach calcium hydroxide and cause chemical changes in Portland cement, leading to loss of strength,[14] as well as attacking the steel reinforcement present in most concrete.
Sulphates
Sulphates in solution in contact with concrete can cause chemical changes to the cement, which can cause significant microstructural effects leading to the weakening of the cement binder.
Types of concrete
Various types of concrete have been developed for specialist application and have become known by these names.
Regular concrete
Regular concrete is the lay term describing concrete that is produced by following the mixing instructions that are commonly published on packets of cement, typically using sand or other common material as the aggregate, and often mixed in improvised containers. This concrete can be produced to yield a varying strength from about 10 MPa to about 40 MPa, depending on the purpose, ranging from blinding to structural concrete respectively. Many types of pre-mixed concrete are available which include powdered cement mixed with an aggregate, needing only water.
High-strength concrete
High-strength concrete has a compressive strength generally greater than 6,000 pounds/square inch (40 MPa). High-strength concrete is made by lowering
the water-cement (w/c) ratio to 0.35 or lower. Often silica fume is added to prevent the formation of free calcium hydroxide crystals in the cement matrix, which might reduce the strength at the cement-aggregate bond.
Low w/c ratios and the use of silica fume make concrete mixes significantly less workable, which is particularly likely to be a problem in high-strength concrete applications where dense rebar cages are likely to be used. To compensate for the reduced workability, superplasticizers are commonly added to high-strength mixtures. Aggregate must be selected carefully for high-strength mixes, as weaker aggregates may not be strong enough to resist the loads imposed on the concrete and cause failure to start in the aggregate rather than in the matrix or at a void, as normally occurs in regular concrete.
In some applications of high-strength concrete the design criterion is the elastic modulus rather than the ultimate compressive strength.。