框架结构毕业设计外文翻译

混凝土施工事项

T.Pauly, M.J.N.Priestley

Engineering Structures 59 (2014) 469–483

摘要

根据一般承认的惯例看,巴基斯坦的混凝土结构建筑物在结构上的质量，效用和安全需要上都留下了很多值得关注的问题。当2005 年10月8日发生在巴基斯坦北部的一个7.6级地震的时候，这些施工技术上的缺点袒露无遗了,破坏了数以千计建筑物、桥梁, 而且造成估计79,000人的死亡。在巴基斯坦这种十分低质量的混凝土施工技术是造成这次极广破坏的主要原因。

关键字：混凝土浇筑养护建筑工艺

混凝土浇筑

若混凝土在地面上浇筑，地面应该具有足够的含水量以避免它吸收混凝土中的水分。如果新混凝土将被浇筑在已经凝固的混凝土上或其旁边，已浇混凝土表面应该彻底清除干净，最好使用高压空气、高压水流或钢丝刷。其表应潮湿，但不应有集水，应当在整个范围内刷上少量的水泥浆，然后立即铺上一层厚度为1/2in的砂浆。新混凝土应铺筑在砂浆上或其旁边。

为减少混凝土在浇筑后由于运输导致的离析，混凝土的制备应该尽可能靠近最后浇筑的地方。它应该分层浇筑以使每层可被均压实。每层浇筑间隔的时间应该限制在能够保证新浇筑的混凝土与前面浇筑的有良好的结合。

当向较深的模板中浇筑混凝土时，就应该使用一根导管来限制混凝土的自由降落高度不超过3或4ft，以防止混凝土离析。这种导管是由轻金属制成的管子，具有可调节的长度而且被连接到储存的混凝土的罐的底部。当模板被浇筑满后，就可以抽出管节。

混凝土一浇筑完成，应立即用手提振捣器或机器振捣器进行捣密后就应该移走，否则就会发生骨料离析。一般来说，振捣器不应该集中在先前提起的混凝土上。

振捣主要优点是它允许使用较干的混凝土，由于减少了混凝土的含水量，这种混凝土有较高的强度。振捣混凝土的优点如下：

1、水分的减少允许水泥和细集料的用量，因为这时需要较少的水泥浆。

2、较低的含水量可减少收缩和空隙。

3、较干的混凝土减少混凝土表面处理的费用。

4、机械振捣可代替3—8个手持振捣棒。

5、较低的含水量可提高混凝土的强度。

6、较干的混合料允许较早地拆除一些模板，这样可降低模板的造价。

混凝土的养护

若混凝土想要获得最高强度或其它必要特性，则应该对其用足够的水分和适当的温度进行养护。如果没能提供这些条件，则会导致劣质的混凝土。

混凝土中最初的水分足够所有水泥的水化反应，只要在使用前没有被蒸发。这可以用多种方法来实现，例如把模板留在混凝土中，保持表面潮湿，或在混凝土表面覆盖一层液体养生化合物，它可以形成一种不透水的隔膜来阻止混凝土中的水分遗失。养生化合物可以用刷子或压力喷嘴加到混凝土表面。一加仑可以覆盖200—300ft2。

混凝土应该在40°F以上或80°F以下进行浇筑。低温会降低凝结速度，而高温会降低混凝土的极限强度。

寒冷天气下的混凝土浇筑

当混凝土在寒冷天气下浇筑时，通常有必要预先加热水、骨料或两者同时预热，以便使其初始温度能够保证混凝土的初凝并获得强度。对水进行预热是提供必要的温度最有效的方法。出于这一目的，蓄水池应该安装一个通有蒸气的蛇形管或将蒸气可以直接通到水中，这是可以使用多个排气口以提供良好的热量分布。

当混合料的温度已知，一些专用图可以用来计算混凝土的温度。一条直线穿过所有三种比例，通过任意两个已知温度，就可以确定出第三个温度。如果砂的表面干燥，则应使用实线计算混凝土的温度。然而，如果砂含有3%的水分，则应使用虚线。

规范通常要求新浇筑的混凝土在浇筑后应保持3天温度不低于70°F或5天不低于50°F。当预计有寒冷天气时必须提供一些适当的方法来保持要求的温度。

用于混凝土中的钢筋

与混凝土相比，钢是一种高强度材料。普通钢筋在抗拉和抗压时可以利用的强度，即屈服强度，约为普通的结构混凝土抗压强度的1.5倍，而且超过抗拉强度的100倍。另一方面，与混凝土相比，钢材的成本要高得多。所以，两种材料最好的结合使用是混凝土用于抵抗压应力。因此，在钢筋混凝土梁内，混凝土抵抗压应力，纵向钢筋配置在靠近受拉面处以抵抗拉应力，通常还附加配有一些钢筋，抵抗梁内的剪应力所引起的斜向拉应力。然而，钢材也

可以用于抵抗应力，主要是为了减小受压构件的截面尺寸，例如用于多层建筑的下部楼层柱。即使不存在这种必要性，所有受压构件也要配置最少数量的钢筋，以保证这些构件在偶然出现的小弯矩作用下的安全性，在这情况下，不加钢筋的混凝土构件可能会开裂，甚至破坏。

使配筋最有效的发挥作用的基本条件是钢筋和混凝土的变形要一致，即这两种材料间要有足够强的黏结力，以确保钢筋和其周围混凝土间不发生相对移动。这种黏结力是由钢筋-混凝土结合面上较强的化学粘合作用，热轧钢筋表面层的固有粗糙度，以及间距较小的肋形表面变形等所构成的。钢筋的边面并行为两种材料间提供了很高的咬合作用。

钢材以两种不同方式应用于混凝土中：普通钢筋和预应力钢筋。普通钢筋在浇筑混凝土之前先置于模板内。钢筋中的应力，与硬化混凝土中的应力一样，除了由收缩或类似原因造成的附加应力外，仅仅是由结构上作用的荷载引起的，比较起来，在预应力混凝土结构中，在钢筋与混凝土共同工作承受外部荷载之前，对钢筋已施加了很大的拉力。

最常见的钢筋的形式为圆棒状。现在可以使用的钢筋的直径范围很大，在一般的应用中从10到35毫米，两种大型钢筋的尺寸为44和57毫米。对这些钢筋表面进行了变形处理，其目的是增加钢筋与混凝土之间的抗滑能力。对这些变形的最低要求已经通过实验研究予以确定。不同的钢筋制造厂家采用不同的变形花纹，他们全部都能够满足这些要求。

为了对钢筋进行拼接，或者便于制作置于模板内的钢筋骨架所进行的焊接，可能会引起金相的变化而降低材料的强度和延性，因此，必须对所有钢材的类型和焊接规程加以特殊的限制。ASTM中的A706的条款是专门使用于焊接的。

长期以来，在钢筋混凝土领域明显的趋向于高强度材料，包括钢筋和混凝土。屈服强度为40KSI的钢筋，在20年前几乎是标准的盲目前大部分已由屈服强度为60钢筋所取代。因为后者更为经济，而且使用他们可以减少模板内钢筋的拥挤状况。

ACI规范允许使用强度FY=80KSI的钢筋。这类高强度钢筋通常是逐渐屈服的没有屈服平台。在这种情况下，ACI规范要求在规定的最小屈服强度时的总应变不应超过0.0035。这是将现行的设计方法应用于这类高强度钢筋时必须遵守的。现行的设计方法是按钢材突然屈服，而且有屈服平台的情况而制定的。ASTM规范中没有关于屈服强度高于60KSI的变形钢筋的条款，但是在实际中可能使用这种钢筋，根据ACI规范，他们可以在根据上述的要求的情况下使用。在特殊情况下，例如高层建筑的下部楼层的柱子，使用这一高强度范围内的钢筋就非常适合。

在恶劣的环境田间下，例如受除冰化学剂侵蚀的荞麦内，要求使用镀锌或环氧树脂涂层的钢筋，以便使钢筋的腐蚀和随之发生的混凝土的剥落减至最小。

混凝土结构的修复

钢筋混凝土是非常耐用的结构材料，它需要的修复工作很少。然而，它的耐用性受很多因素的影响。这包括那些设计和施工的失误，使用劣质材料和在侵蚀性环境下暴露。修复的必要性主要取决于对破坏程度的诊断。如果某项工程不只需要作装饰性处理，好的工作技巧很重要。

1、修复系统的功能要求

通过对事故的仔细诊断并找到损坏的原因之后，下一步是考虑修复方法的要求，这样会我到一个解决问题的有效方法。（参看图）

①耐久性

选择有相当耐久性的修复材料非常重要。修复工作所用的材料最起码也要像基层混凝土那样耐用。

②钢筋的保护

对加劲钢筋的保护机理取决于所使用的修复材料的类型。例如，粘性材料能促使混凝土碱性增加，其抑制作用能保护钢筋免受更深的侵蚀，而环氧树脂砂浆可防止氧气、水分和其它有害物的侵人。

③基片粘结

使用基片粘结可进行完整的修复，它可防止水分和大气从界面进人。大多数的修复材料，通过使用合适的粘合手段，其粘合性能已大大地改善了，如环氧树脂系统使用未填充的环氧底漆，普通水泥修复系统使用普通水泥稀浆加上任何一种乳胶添加剂。所要采用的预防措施是：对即将被粘结的表面，必须先清理其上面的疏松和易碎的物质。

④尺寸稳定性

在养护期间，材料的收缩要保持到最小，为了防止修复失败、材料的尺寸变化应与基层材料相当接近。

⑤对环境引起的损伤的早期防护

一些开始就暴露在外的情况会引起修复的过早损坏。例如，热天会阻碍水泥的水合作用从而使局部修复的普通水泥发生变质，为了防止发生这种情况，养护期间应对其作特别的保护。

⑥操作方便

材料应当易混合和方便使用，这样可以随时填到裂缝和孔隙里去，材料最好是不粘工具，用泥刀修平时不剥落，填筑后不坍塌。

⑦外观

修复材料与现有混凝土的匹配程度取决于所修补的结构和业主的要求。当外观很重要或

钢筋的保护层很薄时，要求采用镶面处理。

2、修复方法的选择

适当的修复就是消除与结构使用有关的所有缺陷。认真考虑选择正确的方法和某一特定操作所用的材料，弄清到底是要满足强度、耐用性的特定要求还是满足其它的长期或短期的性能要求。这些因素包括：

(1)损伤的性质

如果活动裂缝里填的是刚性材料，那么不但填补材料会断裂，而且旧裂缝周围也会产生新的裂缝，修复活动裂缝的方法是要么采用柔性材料以适应结构位移，要么在修复之前采取一定措施消除结构位移。

(2)断裂的部位

依靠材料重力填充裂缝技术在水平面上裂缝修补中的成功率较高，但在竖直面上裂缝修补中效果很差。

(3)环境

如果裂缝里有水气、水和杂物时，要先补好漏洞。如果结构还处于工作状态并且环境潮湿时，会使得堵住漏洞这一工作更加困难。

(4)工艺

工人所采用的修复工艺是另一个与修复有关的因素，有时它意味着水久性修复或者修复材料过早损坏。

(5)成本

与为修复工作提供通路、做准备工作和实际劳力的成本相比，所用的修复材料的成本是相当小的。

(6)外观

修复部位的表面可能很难看，特别是当它处于建筑物显眼的地方时。在这种情况下，修复工作系统就要包括对整个表面进行某种处理。

Concrete Construction matter

T. Pauly, M. J. N. Priestley

Abstract

Viewed in terms of accepted practices, concrete construction operations leave much to be desired with respect to the quality, serviceability, and safety of completed structures. The shortcomings of these operations became abundantly clear when a magnitude 7.6 earthquake struck northern Paki-stan on October 8, 2005, destroying thousands of buildings, damaging bridges, and killing an esti-mated 79,000 people. The unusually low quality of construction operations prevalent was a major cause of the immense devastation.

Keywords: Concrete Placing Curing Construction Technology

Placing Concrete

If concrete is placed in the surface, the sur-face should be filled with water sufficiently to prevent it from absorbing the concrete of its water. If fresh concrete is to be placed on or nearby to concrete that has solidified, the surface of the placed concrete should be cleaned absolutely, preferably with a high-pressure air or water jet or steel-wire brushes. The surface should be wet, but there should be no much water. A little quantity of cement grout should be brushed over the whole area, and then followed immediately with the application of a 1/2-in Layer of mortar. The fresh concrete should be placed on or against the mortar.

In order to decrease the disintegration re-sulting from carriage after it is placed. The con-crete should be placed as nearly as probably in its final point. It should be placed in layers to permit uniform compaction. The time interval between the placing of layers should be limited to assure perfect bond between the fresh and previously placed concrete.

In placing concrete in deeper patters, a ves-sel should be used to limit the free fall to not over 3 or 4 ft, in order to prevent concrete disintegra-tion. The vessel is a pipe made of lightweight metal, having adjustable lengths and attached to the bottom of a hopper into which the concrete is deposited. As the patters are filled, sections of the pipe may be removed.

Immediately after the concrete is placed, it should be compacted by hand pudding or a me-chanical vibrator to eliminate voids. The vibrator should be left in one position only long enough to reduce the concrete around it to a plastic mass; then the vibrator should be moved, or

disintegra-tion of the aggregate will occur. In general, the vibrator should not be permitted to penetrate concrete in the prior lift.

The mainly advantage of vibrating is that it permits the use of a drier concrete, which has a higher strength because of the reduced water content. Among the advantages of vibrating con-crete are the following:

1. The decreased water permits a reduction in the cement and fine aggregate because less cement paste is needed.

2. The lower water content decreases shrinkage and voids.

3. The drier concrete decreases the cost of finishing the surface.

4. Mechanical vibration may replace three to eight hand puddles.

5. The lower water content increases the strength of the concrete.

6. The drier mixture permits the removal of some patters more quickly, which may reduce the cost of patters.

Curing Concrete

If concrete is to gain its maximum strength and other desirable properties, it should be cured with adequate moisture and at a favorable tem-perature. Failure to provide these conditions may result in an inferior concrete.

The initial moisture in concrete is adequate to hydrate all the cement, provided it is not should replace the moisture that does evaporate. This may be accomplished by many methods, such as leaving the patters in place, keeping the surface wet, or covering the surface with a liquid curing compound, which comes being to a water-tight membrane that prevents the escape of the initial water. Curing compounds may be applied by brushes or pressure sprayers. A gallon will cover 200 to 300 sq ft.

Concrete should be placed at a temperature not less than 40 or more than 80°F.A lower tem-perature will decrease the rate of setting, while ahigher temperature will decrease the ultimate strength.

Placing Concrete in Cold Weather

When the concrete is placed during cold weather, it is usually necessary to preheat the water, the aggregate, or both in order that the ini-tial temperature will assure an initial set and gain in strength .Preheating the water is the most ef-fective method of providing the necessary

tem-perature. For this purpose a water reservoir should be equipped with pipe coils through which steam can be passed, or steam may be discharged directly into the water, several outlets being used to given better distribution of the heat.

When the temperatures of the mixtures are known, some specific charts may be used to cal-culate the temperature of concrete. A straight line pass all three scales, passing through every two known temperatures, will assure the determina-tion of the third temperature. If the surface of sand is dry, the fact lines of the scales giving the temperature of concrete should be used. However, if the sand contains about 3 percent moisture, the dotted lines should be used.

Specifications usually demand that freshly placed concrete shall be kept at a temperature of not less than 70°F for 3 days or 50°F for 5 days after it is placed. Some proper method must be provided to keep the demanded temperature when the cold weather is estimated.

Reinforcing steels for concrete

Compared with concrete, steel is a high strength material. The useful strength of ordinary reinforcing steels in tension as well as compres-sion, i.e., the yield strength, is about 15 times the compressive strength of common structural con-crete, and well over 100 times its tensile strength. On the other hand, steel is a high-cost material compared with concrete. It follow that the two materials are the best used in combination if the concrete is made to resist the compressive stresses and the compressive force, longitudinal steel reinforcing bars are located close to the ten-sion face to resist the tension force., and usually additional steel bars are so disposed that they re-sist the inclined tension stresses that are caused by the shear force in the beams. However, rein-forcement is also used for resisting compressive forces primarily where it is desired to reduce the cross-sectional dimensions of compression members, as in the lower-floor columns of multi-story buildings. Even if no such necessity exits , a minimum amount of reinforce- ment is placed in all compression members to safeguard them against the effects of small accidental bending moments that might crack and even fail an unre-inforced member.

For most effective reinforcing action, it is essential that steel and concrete deform together, i.

e., that there be a sufficiently strong bond be-tween the two materials to ensure that no relative movements of the steel bars and the surrounding concrete occur. This bond is provided by the rela-tively large chemical adhesion which develops at the steel-concrete interface, by the natural roughness of the mill scale of hot-rolled rein-forcing bars , and by the closely spaced rib-shap- ed

surface deformations with which reinforcing bars are furnished in order to provide a high de-gree of interlocking of the two materials.

Steel is used in two different ways in con-crete structures: as reinforcing steel and as prestressing steel .reinforcing steel is placed in the forms prior to casting of the concrete. Stresses in the steel, as in the hardened concrete, are caused only by the loads on the structure, except for possible parasitic stresses from shrinkage or similar causes. In contrast, in priestesses concrete structures large tension forces are applied to the reinforcement prior to letting it act jointly with the concrete in resisting external.

The most common type of reinforcing steel is in the form of round bars, sometimes called rebars, available in a large range of diameters,from 10 to 35 mm for ordinary applications and in two heavy bar sizes off 44 and 57 mm these bars are furnished with surface deformations for the purpose of increasing resistance to slip be-tween steel and concrete minimum requirements for these deformations have been developed in experimental research. Different bar producers use different patterns, all of which satisfy these requirements.

Welding of rebars in making splices, or for convenience in fabricating reinforcing cages for placement in the forms, may result in metal-lurgical changes that reduce both strength and ductility, and special restrictions must be placed both strength and ductility, and special restric-tions must be placed both on the type of steel used and the welding procedures the provisions of ASTM A706 relate specifically to welding.

In reinforced concrete a long-time trend is evident toward the use of higher strength materi-als, both steel and concrete. Reinforcing bars with 40ksi yield stress , almost standard 20 years ago , have largely been replaced by bars with 60ksi yield stress , both because they are more economical and because their use tends to reduce congestion of steel in the forms .

The ACI Code permits reinforcing steels up to Fy=80ksi. Such high strength steels usually yield gradually but have no yield plateau in this situation the ACI Code requires that at the

speci-fied minimum yield strength the total strain shall not exceed 0.0035 this is necessary to make cur-rent design methods, which were developed for sharp-yielding steels with a yield plateau, appli-cable to such higher strength steels. there is no ASTM specification for deformed bars may be used , according to the ACI Code , providing they meet the requirements stated under special circumstances steel in this higher strength range has its place, e.g., in lower-story columns of high-rise buildings.

In order to minimize corrosion of rein-forcement and consequent spelling of concrete

under sever exposure conditions such as in bridge decks subjected to deicing chemicals , galvanized or epoxy-coated rebars may be specified.

Repair of Concrete Structures

Reinforced concrete is generally a very du-rable structural material and very little repair work is usually needed. However, its durability can be affected by a variety of causes, including those of design and construction faults, use of inferior materials and exposure to aggressive en-vironment. The need for a repair is primarily dic-tated by the severity of the deterioration as de-termined from the diagnosis. Good workmanship is essential if any thing more than just a cosmetic treatment to the creation is required.

1. performance requirements of repair system

Having established the causes of the defect by carefully diagnosing the distress, the next step should be to consider the requirements of the re-pair method that will offer an effective solution to the problem (see fig.).

①Durability

It is important to select repair materials that provide adequate durability. Materials used for the repair job should be at least as durable as the substrate concrete to which it is applied.

②Protection of steel

The mechanism of protection provided to the reinforcing depends on the type of repair ma-terials used. For example, cementations materials can protect the steel from further corrosion by their inhibitive effect of increasing the alkalinity of the concrete, whereas epoxy resin mortars can give protection against the ingress of oxygen,moisture and other harmful agents.

③Bond with substrate

The bond with the substrate must produce an integral repair to prevent entry of moisture and atmospheric gases at the interface. With most re-pair materials, the bond is greatly enhanced with the use of a suitable bonding aid such as an un-filled epoxy resin systems and slurry of Portland cement, plus any latex additives for a Portland cement-based repair system. Precautions should also be taken to remove all loose and friable ma-terials from the surfaces to be bonded.

④Dimensional Stability

Shrinkage of materials during curing should be kept to a minimum. Subsequent dimensional change should be very close in the substrate in order to prevent failure

⑤Initial Resistance to Environmentally In-duced Damage

Some initial exposure conditions may lead to premature damage lo repairs. For example, partially cured Portland cement repairs can dete-riorate from hot weather preventing full hydration of the cement. To prevent this from happening extra protection during curing time may be

nec-essary.

⑥Ease of Application

Materials should be easily mixed and ap-plied so that they can be worked readily into small crevices and voids. Ideally, the material should not stick to tools, and should not shear while being trowel led nor slump after placement.

⑦Appearance

The degree to which the repair material should match the existing concrete will depend on the use of the structure and the client' s re-quirements. A surface coating may be required when appearance is important or when cover to reinforcement is small.

2. Selection of Repair Methods

A suitable repair counteracts all the defi-ciencies which are relevant to the use of the structure. The selection of tile correct method and material for a particular, application requires careful consideration, whether to meet special requirements for placing strength, durability or other short-or long-term properties. These con-siderations include:

1. Nature of the Distress

If alive crack is filled with a rigid material, then either the repair material will eventually fail or some new cracking will occur adjacent to the original crack. Repairs to live cracks must either use flexible materials to accommodate move-ments or else steps must be taken prior to the re-pair to eliminate the movement.

2. Position of the Crack

Techniques which rely on gravity to intro-duce the material into the crack are more

suc-cessfully carried out on horizontal surfaces but are rarely effective on vertical ones.

3. Environment

If moisture, water or contaminants are found in the crack, then it is necessary to rectify the leaks Repair to slop leaks may be further com-plicated by the need to make the repairs while the structure is in service and the environment is damp.

4. Workmanship

The skill the operatives available to carry put the repairs is another relevant factors.

Some-times this can mean the difference between a permanent repair and premature failure of the

re-pair material.

5. Cost

The cost of repair materials is usually small compared with the costs of providing access, preparation and actual labor.

6. Appearance

The repair surface may be unsightly, par-ticularly when it appears on a prominent part of the building. In this case, the repair system will include some form of treatment over the entire surface.