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 steel tensile stresses. Thus in reinforced concrete beams, the concrete resists 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.

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，钢材用来抵抗拉应力。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，在这种情况下，不加钢筋的混凝土构件可能会开裂，甚至破坏。

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 1. 2× /°C for steel vs. an average of 1. 25 × /°C 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.

使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黏合作用、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他特性有：

(1)两种材料的热膨胩系数，钢筋大约为1.2×/°C，而混凝土的平均值为1.25×/°C。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他不利影响。

(2）裸露的钢筋的抗腐住性很差，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。

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.

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，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，对钢筋己施加了很大的拉力。

The most common type of reinforcing steel ( as distinct from prestressing steel) is in the form of round bars, sometimes called bars, available in a large range of diameters, from 10 to 35 mm for ordinary applications and in two heavy bar sizes of 44 and 57 mm. 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.

最常见的钢筋（区别于预应力钢筋）的形式为圆棒状，有时被称为“rebars”。现在可以使用的钢筋的直径范围很大，在一般的应用中从10mm至35mm，两种大型钢筋的尺寸为44mm和57mm。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。已经通过实验研究对这些变形（间距、凸起等）的最低要求进行了确定。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。

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.

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，可能会引起金相组织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。

In reinforced concrete a long-time trend is evident toward 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.

长期以来，在钢筋混凝土 领域明显地趋向于使用高强度材料，包括钢筋和混凝土。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。

The ACI Code permits reinforcing steels up to = 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 should 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.

ACI 规范允许使用强度 =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。

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.

在恶劣的环境条件下，例如受除冰化学剂侵蚀的桥面，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。