Plain concrete is formed from a hardened mixture 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.

素混凝土是由水泥、水、細骨料、粗骨料（碎石或卵石）、空氣，通常還有其他外加劑等經(jīng)過凝固硬化而成。將可塑的混凝土拌合物注入模板內(nèi)，并將其搗實，然后進行養(yǎng)護，以加速水泥與水的水化反應(yīng)，最后獲得硬化的混凝土。其最終制成品具有較高的抗壓強度和較低的抗拉強度。其抗拉強度約為抗壓強度的十分之一。因此，截面的受拉區(qū)必須配置抗拉鋼筋和抗剪鋼筋以增加鋼筋混凝土構(gòu)件中較弱的受拉區(qū)的強度。

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.

由于鋼筋混凝土截面在均質(zhì)性上與標準的木材或鋼的截面存在著差異，因此，需要對結(jié)構(gòu)設(shè)計的基本原理進行修改。將鋼筋混凝凝土這種非均質(zhì)截面的兩種組成部分按一定比例適當布置，可以最好地利用這兩種材料。這一要求是可以達到的，因混凝土由配料攪拌成濕拌合物，經(jīng)過振搗并凝固硬化，可以做成任何一種需要的形狀。如果拌制混凝土的各種材料配合比較恰當，則混凝土制成品的強度較高，經(jīng)久耐用，配置鋼筋后，可以作為任何結(jié)構(gòu)體系的主要構(gòu)件。

The techniques necessary for placing concrete depend on the type?of member to be cast: that is, whether it is a column, a beam, a wall, a slab, a foundation, a mass concrete dam, or an extension of previously placed and hardened concrete. For beams, columns, and walls, 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 overvibration can be harmful since it could cause segregation of the aggregate and bleeding of the concrete.

澆筑混凝土所需的技術(shù)取決于即將澆筑的構(gòu)件類型，諸如：柱、梁、墻、板、基礎(chǔ)，大體積混凝土水壩或者繼續(xù)延長已澆筑完畢并且已經(jīng)凝固的混凝土等。對于梁、柱、墻等構(gòu)件，當模板清理干凈后應(yīng)該在其上涂油，鋼筋表面的銹皮及其他有害物質(zhì)亦應(yīng)清除干凈。澆筑基礎(chǔ)前，應(yīng)將坑底土夯實并用水浸濕6英寸，以免土壤從重新澆筑的混凝土中吸收水分。一般情況下，除使用混凝土泵澆筑外，混凝土都應(yīng)在水平方向分層澆筑，并使用插入式或表面式高頻電動振搗器振實。必須記住，過分的振搗將導致骨料分離和混凝土泌漿等現(xiàn)象，因而是有害的。

Hydration of the cement takes place in the presence of moisture at temperatures above 50°F. 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.

水泥的水化作用發(fā)生在有水分存在，而且氣溫在50°F以上的條件下。為了保證水泥的水化作用得以進行，必須具備上述條件。如果干燥過快則會出現(xiàn)表面裂縫，這將有損于混凝土的強度，同時也會影響到水泥水化作用的充分進行。

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 materials, 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.

設(shè)計鋼筋混凝凝土構(gòu)件時顯然需要處理大量的參數(shù)，諸如寬度、高度等幾何尺寸，配筋的面積，鋼筋的應(yīng)變和混凝土的應(yīng)變，鋼筋的應(yīng)力等等。因此，在選擇混凝土截面時需要進行試算并作調(diào)整，根據(jù)施工現(xiàn)場條件、混凝土原材料的供應(yīng)情況、業(yè)主對建筑和凈空高度的特殊要求、所用的設(shè)計規(guī)范以及建筑物周圍環(huán)境條件等最后確定截面。鋼筋混凝土通常是現(xiàn)場澆筑的合成材料，它與在工廠中制造的標準的鋼結(jié)構(gòu)梁、柱等不同，因此上述一系列因素必須以考慮。

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 arrive at the required section, the first design input step generates into a series of trial-and-adjustment analyses.

對結(jié)構(gòu)體系的各個關(guān)鍵部位均需選定試算截面并進行驗算，以確定該截面的名義強度是否足以承受所作用的計算荷載。由于經(jīng)常需要進行多次試算，才能求出所需的截面，因此設(shè)計時第一次采用的數(shù)值將導致一系列的試算與調(diào)整工作。

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 a 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.

選擇混凝土截面時，采用試算與調(diào)整過程可以使復核與設(shè)計結(jié)合在一起。因此，當試算截面選定后，每次設(shè)計都是對截面進行復核。手冊、圖表和微型計算機以及專用程序的使用，使這種設(shè)計方法更為簡捷有效，而傳統(tǒng)的方法則是把鋼筋混凝凝土的復核與單純的設(shè)計分別地進行處理。