【正文】 of constructing , concrete happens, the cement water response is fierce at this moment, the strand takes shape gradually, secrete water and moisture to evaporate sharply, the concrete desiccates and shrinks, it is at the same time conduct oneself with dignity not sinking because aggregate,so when harden concrete yet,it call plasticity shrink. The plasticity shrink producing amount grade is very big, can be up to about 1%. If stopped by the reinforcing bar while the aggregate sinks, form the crack along the reinforcing bar direction. If web , roof beam of T and roof beam of case and carry baseplate hand over office in ponent vertical to bee sectional place, because sink too really to superficial obeying the web direction crack will happen evenly before hardenning. For reducing concrete plasticity shrink,it should control by water dust when being construct than,last longtime mixing, unloading should not too quick, is it is it take closely knit to smash to shake, vertical to bee sectional place should divide layer build. Shrink and shrink (do and contract). After the concrete is formed hard , as the top layer moisture is evaporated progressively , the humidity is reduced progressively , the volume of concrete is reduced, is called and shrunk to shrink (do and contract). Because concrete top layer moisture loss soon, it is slow for inside to lose, produce surface shrink heavy , inside 西南交通大學(xué)本科 畢業(yè)設(shè)計(jì) 外文翻譯 第 7 頁(yè) shrink a light one even to shrink, it is out of shape to restrain from by the inside concrete for surface to shrink, cause the surface concrete to bear pulling force, when the surface concrete bears pulling force to exceed its tensile strength, produce and shrink the crack. The concrete hardens aftercontraction to just shrink and shrink mainly .Such as mix muscle rate heavy ponent (exceed 3% ), between reinforcing bar and more obvious restraints relatively that concrete shrink, the concrete surface is apt to appear in the full of cracks crackle. Shrink spontaneously. Spontaneous to it shrinks to be concrete in the course of hardenning , cement and water take place ink react, the shrink with have nothing to do by external humidity, and can positive (whether shrink, such as ordinary portland cement concrete), can negative too (whether expand, such as concrete, concrete of slag cement and cement of fly ash). The char shrinks. Between carbon dioxide and hyrate of cement of atmosphere take place out of shape shrink that chemical reaction cause. The char shrinks and could happen only about 50% of humidity, and accelerate with increase of the density of the carbon dioxide. The char shrinks and seldom calculates . The characteristic that the concrete shrinks the crack is that the majority belongs to the surface crack, the crack is relatively detailed in width , and crisscross, bee the full of cracks form , the form does not have any law . Studies have shown , influence concrete shrink main factor of crack as follows, 1, Variety of cement , grade and consumption. Slag cement , quickhardening cement , lowheat cement concrete contractivity are relatively high, ordinary cement , volcanic ash cement , alumina cement concrete contractivity are relatively low. Cement grade low in addition, unit volume consumption heavy rubing detailed degree heavy, then the concrete shrinks the more greatly, and shrink time is the longer. For example, in order to improve the intensity of the concrete , often adopt and increase the cement consumption method by force while constructing, the result shrinks the stress to obviously strengthen . 2, Variety of aggregate. Such absorbing water rates as the quartz , limestone , cloud rock , granite , feldspar ,etc. are smaller, contractivity is relatively low in the aggregate。 Once temperature goes up , the frozen soil is melted, the setting of ground. So the ground is icy or melts causes and does not subside evenly . 7, Bridge foundation put on body, cave with stalactites and stalagmites, activity fault,etc. of ing down at the bad geology, may cause and does not subside evenly . 8, After the bridge is built up , the condition change of original ground . After most natural grounds and artificial grounds are soaked with water, especially usually fill out such soil of special ground as the soil , loess , expanding in the land ,etc., soil body intensity meet water drop, press out of shape to strengthen. In the soft soil ground , season causes the water table to drop to draw water or arid artificially, the ground soil layer consolidates and sinks again, reduce the buoyancy on the foundation at the same time , shouldering the obstruction of 西南交通大學(xué)本科 畢業(yè)設(shè)計(jì) 外文翻譯 第 10 頁(yè) rubing to increase, the foundation is carried on one39。其實(shí)，如果采取一定的設(shè)計(jì)和施工措施，很多裂縫是可以克服和控制的。直接應(yīng)力裂縫是指外荷載引起的直接應(yīng)力產(chǎn)生的裂縫。 使用階段，超出設(shè)計(jì)載荷的重型車輛過(guò)橋；受車輛、船舶的接觸、撞擊；發(fā)生大風(fēng)、大雪、地震、爆炸等。 橋梁結(jié)構(gòu)中經(jīng)常需要鑿槽、開(kāi)洞、設(shè)置牛腿等，在常規(guī)計(jì)算中難以用準(zhǔn)確的圖式進(jìn)行模擬計(jì)算，一般根據(jù)經(jīng)驗(yàn)設(shè)置受力鋼筋。 實(shí)際工程中，次應(yīng)西南交通大學(xué)本科 畢業(yè)設(shè)計(jì) 外文翻譯 第 12 頁(yè) 力裂縫是產(chǎn)生荷載裂縫的最常見(jiàn)原因。在設(shè)計(jì)上，應(yīng)注意避免結(jié)構(gòu)突變（或斷面突變），當(dāng)不能回避時(shí)，應(yīng)做局部處理，如轉(zhuǎn)角處做圓角，突變處做成漸變過(guò)渡，同時(shí)加強(qiáng)構(gòu)造配筋，轉(zhuǎn)角處增配斜向鋼筋，對(duì)于較大孔洞有條件時(shí)可在周邊設(shè)置護(hù)邊角鋼。根據(jù)結(jié)構(gòu)不同受力方式，產(chǎn)生的裂縫特征如下： 中心受拉。沿構(gòu)件出現(xiàn)平行于受力方向的短而密的平行裂縫。當(dāng)結(jié)構(gòu)配筋較少時(shí)，裂縫少而寬，結(jié)構(gòu)可能發(fā)生脆性破壞。小偏心受壓和受拉區(qū)配筋較多的大偏心受壓構(gòu)件，類似于中心受壓構(gòu)件。方向相互平行的斜裂縫。 受沖切。在局部受壓區(qū)出現(xiàn)與壓力方向大致平行的多條短裂縫。引起溫度變化主要因素有： 年溫差。 日照。 驟然降溫。出現(xiàn)在施工過(guò)程中，大體積混凝土（厚度超過(guò) ）澆筑之后由于水泥水化放熱，致使內(nèi)部溫度很高，內(nèi)外溫差太大，致使表面出現(xiàn)裂縫。采用電熱張拉法張拉預(yù)應(yīng)力構(gòu)件時(shí)，預(yù)應(yīng)力鋼材溫度可升高至 350℃，混凝土構(gòu)件也容易開(kāi)裂。 塑性收縮。在構(gòu)件豎向變截面處如 T梁、箱梁 腹板與頂?shù)装褰唤犹?，因硬化前沉?shí)不均勻?qū)l(fā)生表面的順腹板方向裂縫。因混凝土表層水分損失快，內(nèi)部損失慢，因此產(chǎn)生表面收縮大、內(nèi)部收縮小的不均勻收縮，表面收縮變形受到內(nèi)部混凝土的約束，致使表面混凝土承受拉力，當(dāng)表面混凝土承受拉力超過(guò)其抗拉強(qiáng)度時(shí)，便產(chǎn)生收縮裂縫。自生收縮是混凝土在硬化過(guò)程中，水泥與水發(fā)生水化反應(yīng)，這種收縮與外界濕度無(wú)關(guān)，且可以是正的（即收縮，如普通硅酸鹽水泥混凝土），也可以是負(fù)的（即膨脹，如礦渣水泥混凝土與粉煤灰水泥混凝土）。炭化收縮一般不做計(jì)算。另外水泥標(biāo)號(hào)越低、單位體積用量越大、磨細(xì)度越大，則混凝土收縮越大，且發(fā)生收縮時(shí)間越長(zhǎng)。另外骨料粒徑 大收縮小，含水量大收縮越大。外摻劑保水性越好，則混凝土收縮越小。蒸汽養(yǎng)護(hù)方式比自然養(yǎng)護(hù)方式混凝土收縮要小。機(jī)械振搗方式比手工搗固方式混凝土收縮性要小。構(gòu)造上配筋宜優(yōu)先采用小直徑鋼筋 (φ 8~φ 14)、小間距布置 (10~15cm)，全截面構(gòu)造配筋率不宜低于 %，一般可采用 %~%。比如丘陵