钢筋混凝土.doc

钢筋混凝土素混凝土是由水泥,水.细骨料.粗骨料(碎石或卵石).空气,通常还有其它外加剂等经过凝固硬化而成.将可塑的混凝土拌合物注入到模板内,并将其捣实,然后进行养护,以加速水泥与水的水化反应,最后获得硬化的混凝土.其最终制成品具有较高的抗压强度和较低的抗拉强度.其抗拉强度约为抗压强度的十分之一.因此,截面的受拉区必须配置抗拉钢筋和抗剪钢筋以增加钢筋混凝土构件中较弱的受拉区的强度.由于钢筋混凝土截面在均质性上标准的木材或钢的截面存在着差异,因此,需要对结构设计的基本原理进行修改.将钢筋混凝土这种非均质截面的两种组成部分按一定比例适当布置,可以最好地利用这两种材料.这一要求是可以达到的,因混凝土有配料搅拌成湿拌合物,经过振捣并凝固硬化,可以做成任何一种需要的形状.如果拌制混凝土的各种材料配合比恰当,则混凝土制成品的强度较高,经久耐用,配置钢筋后,可以作为任何结构体系的主要构件.浇筑混凝土所需要的技术取决于即将浇筑的构件类型,诸如:柱.梁.墙.板.基础,大体积混凝土水坝或者继续延长已浇筑完毕并且已经凝固的混凝土等.对于梁.柱.墙等构件,当模板清理干净后应该在其上涂油,钢筋表面的锈皮及其它有害物质亦应该清除干净.浇筑基础前,应将坑底土夯实并用水浸湿6英寸,以免土壤从新浇筑的混凝土中吸收水份.一般情况下,除使用混凝土泵浇筑外,混凝土都应在水平方向分层浇筑,并使用插入式或表面式高频电动振捣器振实.必须记住,过分的振捣导致骨料分离和混凝土泌浆等现象,因而是有害的.水泥的水化作用发生在有水分存在,而且气温在50F以上的条件下.为了保证水泥的水化作用得以进行,必须具备上述条件.如果干燥过快则会出现表面裂缝,这将有损于混凝土的强度,同时也会影响到水泥水化作用的充分进行.设计钢筋混凝土构件时显然需要处理大量的参数,诸如宽度.高度等几何尺寸,配筋的面积,钢筋的应变和混凝土的应变,钢筋的应力等等.因此,在选择混凝土截面时需要进行试算并作调整,根据施工现场条件.混凝土原材料的供应情况.业主对建筑和净空高度的特殊要求.所用的设计规范以及建筑物周围环境条件等最后确定截面.钢筋混凝土通常是现场浇筑的合成材料,它与在工厂中制造的标准的钢结构梁.柱等不同,因此上述一系列因素必须予以考虑.对结构体系的各个关键部位均需选定试算截面并进行验算,以确定该截面的名义强度是否足以承受所作用的计算荷载.由于经常需要进行多次试算,才能求出所需的截面,因此设计时第一次采用的数值将导致一系列的试算与调整工作.选择混凝土截面时,采用试算与调整过程可以使复核与设计结合在一起.因此当试算截面选定后,每次设计都是对截面进行复核.手册,图表和微型计算机以及专用程序的使用,使这种设计方法更为简捷有效,而传统的方法则是把钢筋混凝土的复核与单纯的设计孤立地加以对待. 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15倍，而且超过其抗拉强度的100倍。另一方面，与混凝土相比，钢材的成本要高很多。所以，两种材料最好的结合使用是，混凝土用于抵抗压应力，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，抵抗梁内剪力所引起的斜向拉应力。然而，钢材也可以用于抵抗压力，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。是配筋最有效地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粘结力，以确保钢筋和其周围混凝土间不发生相对移动。这种粘结力是由钢筋混凝土结合面上较强的化学粘合作用、热扎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。钢筋的表面变形为两种材料间提供了很高的咬合作用。使钢筋和混凝土能够很好地共同工作的其它特性有：1. 两种材料的热膨胀系数，钢筋大约为6.510E-6，而混凝土的平均值为5.510E-6。这两个数值相当接近，足以避免热变形差值引起的混凝土开裂和其它不利影响。2. 裸露的钢筋的抗腐蚀性很差，钢筋周围的混凝土为其提供了优良的防腐蚀保护层，使腐蚀问题及相关的维护费用降至最低。3. 钢材的热传导系数高，而且在高温时其强度会大幅度下降，因而无防护层钢筋的抗火性能较差。相反，混凝土的热传导系数相对较低。因此，即使长期暴露在火焰下，如果发生损坏的话，也仅仅限于混凝土的外层。厚度适当的混凝土保护层，可以为埋置在其内的钢筋提供充分的温度绝缘。钢材以两种不同的方式应用于混凝土结构中：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，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（区别于预应力钢筋）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至35mm，两种大型钢筋的尺寸为44和57mm。对这些钢筋表面进行了处理，其目的是增加钢筋与混凝土之间的抗滑能力。对这些变形（间距、凸起等）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，它们全部都能够满足这些要求。为了对钢筋进行拼接，或者便于制做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，必须对所用钢材的类型和焊接规程加以特殊的限制。ASTM中A706的条款是专门适用于焊接的。长期以来，在钢筋混凝土的领域明显地趋向于高强度材料，包括钢筋忽然混凝土。屈服强度为40ksi（276MPa）的钢筋，在20年前几乎是标准的，目前大部分已由屈服强度为60ksi（414MPa）钢筋所取代。因为后者更为经济，而且使用它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（552MPa）的钢筋。这类高强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超过0.0035。这是将现行的设计方法应用于这类高强钢筋时所必须遵守的。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订的。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，它们可以在满足上述要求的情况下使用。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。在恶劣的环境条件下，例如受除冰化剂侵蚀的桥面，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。Reinforced ConcretePlain concrete is formed from a hardened of cement, water, fine aggregate, coarse aggregate(crushed stone or gravel),air, and often other admixtures. The plastic mix is placed and consolidated in the formwork, then cured to facilitate the acceleration of the chemical hydration reaction of the cement/water mix, resulting in hardened concrete. The finished product has high compressive strength, and low resistance to tension ,such that its tensile strength is approximately one-tenth of its compressive strength. Consequently, tensile and shear reinforcement in the tensile regions of sections has to be provided to compensate for the weak-tension regions in the reinforced concrete element.It is this deviation in the composition of a reinforced concrete section from the homogeneity of standard wood or steel sections that requires a modified approach to the basic principles of structural design. The two components of the heterogeneous reinforced concrete section are to be so arranged and proportioned that optimal use is made of the materials involved. This is possible because concrete can easily be given any desired shape by placing and compacting the wet mixture of the constituent ingredients into suitable forms in which the plastic mass hardens. If the various ingredients are properly proportioned, the finished product becomes strong, durable, and, in combination with the reinforcing bars, adaptable for use as main members of any structural system.The techniques necessary for placing concrete depend on the type of member to be cast: that is, whether it is a column, a bean, a wall, a slab, a foundation, a mass concrete dam, or an extension of previously placed and hardened concrete. For beams, columns, and wall ,the forms should be well oiled after cleaning them ,and the reinforcement should be cleared of rust and other harmful materials In foundations, the earth should be compacted and thoroughly moistened to about 6 in .in depth to avoid absorption of the moisture present in the wet concrete .concrete should always be placed in horizontal layers which are compacted by means of high-frequency power-driven vibrators of either the immersion or external type, as the case requires unless it is placed by pumping .It must be kept in mind ,however ,that over-vibration can be harmful since it could cause segregation of the aggregate and bleeding of the concrete.Hydration of the cement takes place in the presence of moisture at temperatures above50F. It is necessary to maintain such a condition in order that the chemical hydration reaction can take place .If drying is too rapid ,surface cracking takes place .This would result in reduction of concrete strength due to cracking as well as the failure to attain full chemical hydration.It is clear that a large number of parameters have to be dealt with in proportioning a reinforced concrete element, such as geometrical width, depth, area of reinforcement, steel strain, concrete strain, steel stress, and so on .Consequently, trial and adjustment is necessary in the choice of concrete sections ,with assumptions based on conditions at site, availability of the constituent material, particular demands of the owners, architectural and headroom requirements, the applicable codes, and environmental conditions .Such an array of parameters has to be considered because of the fact that reinforced concrete is often a site-constructed composite, in contrast to the standard mill-fabricated beam and column sections in steel structures.A trial section has to be chosen for each critical location in a structural system. The trial section has to be analyzed to determine if its nominal resisting strength is adequate to carry the applied factored load. Since more than one trial is often necessary to carry at the required section, the first design input step generates into a series of trial-and-adjustment analyses.The trial-and-adjustment procedures for the choice of a concrete section lead to the convergence of analysis and design. Hence every design is an analysis once trial section is chosen. The availability of handbooks, charts, and personal computers and programs supports this approach as a more efficient, compact, and speedy instructional method compared with the traditional approach of treating the analysis of reinforced concrete separately from pure design. 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 compression, i.e., the yield strength, is about 15 times the compressive strength of common structural concrete, and well over 100 times its tensile strength. On the other hand, steel is a high-cost material compared with concrete. It follows that the two materials are 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 tension face to resist the tension force, and usually additional steel bars are so disposed that they resist the inclined tension stresses that are caused by the shear force in the beams. However, reinforcement 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 multistory buildings. Even if no such necessity exists, a minimum amount of reinforcement is placed in all compression members to safeguard them against the effects of small accidental bending moments that might crack and even fail an unreinforced member.For most effective reinforcing action, it is essential that steel and concrete deform together, i.e., that there be a sufficiently strong bond between the two materials to ensure that no relative movements of the steel bars and the surrounding concrete occur. This bond is provided by the relatively large chemical adhesion which develops at the steel-concrete interface, by the natural roughness of the mill scale of hot-rolled reinforcing bars, and by the closely spaced rib-shaped surface deformations with which reinforcing bars are furnished in order to provide a high degree of interlocking of the two materials.Additional features which make for the satisfactory joint performance of steel and concrete are the following:1. The thermal expansion coefficients of the two materials, about 0.0000065 for steel vs. an average of 0.0000055 for concrete, are sufficiently close to forestall cracking and other undesirable effects of differential thermal deformations.2. While the corrosion resistance of bare steel is poor, the concrete which surrounds the steel reinforcement provides excellent corrosion protection, minimizing corrosion problems and corresponding maintenance costs.3. The fire resistance of unprotected steel is impaired by its high thermal conductivity and by the fact that its strength decreases sizably at high temperatures. Conversely, the thermal conductivity of concrete is relatively low. Thus damage caused by even prolonged fire exposure, if any , is generally limited to the outer layer of concrete ,and a moderate amount of concrete cover provides sufficient thermal insulation for the embedded reinforcement.Steel is used in two different ways in concrete 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 prestressed concrete structures large tension forces are applied to the reinforcement prior to letting it act jointly with the concrete in resisting external loads.The most common type of reinforcing steel (as distinct from prestressing steel)is in the form of round bars, sometimes called rebars, available in a large range of diameters, from 10 to 35mm for ordinary application and in two heavy bar sizes of 44 and 57mm .These bars are furnished with surface deformations for the purpose of increasing resistance to slip between steel and concrete .Minimum requirements for these deformations(spacing, projection, etc. ) 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 metallurgical changes that reduce both strength and ductility, and special restrictions 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 the use of higher strength materials, both steel and concrete. Reinforcing bars with 40ksi (276MPa) yield stress, almost standard 20 years ago, have largely been replaced by bars with 60ksi (414MPa) 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 (552MPa). Such high strength steels usually yield gradually but have no yield plateau. In this situation the ACI Code requires that at the specified minimum yield strength the total strain shall not exceed 0.0035. This is necessary to make current design methods, which were developed for sharp-yielding steels with a yield plateau, applicable to such higher strength steels. There is no ASTM specification for deformed bars with yield stress above 60ksi, but such 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 reinforcement and consequent spalling of concrete under severe exposure conditions such as in bridge decks subjected to deicing chemicals, galvanized or epoxy-coated rebars may be specified.