徐新-橋梁混凝土裂縫產(chǎn)生的原因和處理措施(共18頁(yè))

1、精選優(yōu)質(zhì)文檔-傾情為你奉上畢 業(yè) 論 文 論文題目： 橋梁混凝土裂縫產(chǎn)生的原因和處理措施 系 部： 公 路 工 程 系 專(zhuān)業(yè)名稱(chēng)： 道路橋梁工程技術(shù) 班 級(jí)： 08213 學(xué) 號(hào)： 24 姓 名： 徐 新 指導(dǎo)教師： 李 永 成 完成時(shí)間： 2011 年 5 月 12 日專(zhuān)心-專(zhuān)注-專(zhuān)業(yè)目 錄橋梁混凝土裂縫產(chǎn)生的原因和處理措施摘要：在橋梁建設(shè)使用過(guò)程中，因出現(xiàn)裂縫而影響工程質(zhì)量甚至導(dǎo)致橋梁垮塌的現(xiàn)象也屢見(jiàn)不鮮。本文闡述了混凝土橋梁裂縫的種類(lèi)，從荷載、溫度變化、地基變形等5個(gè)方面介紹了混凝土裂縫的成因，提出了相應(yīng)的預(yù)防措施，同時(shí)給出了一些裂縫處理方法。關(guān)鍵詞：橋梁混凝土裂縫、原因、措施前言隨著我

2、國(guó)交通基礎(chǔ)建設(shè)的高速發(fā)展，各地興建了大量的混凝土橋梁。在橋梁建造和使用的過(guò)程中，由于混凝土裂縫而影響工程質(zhì)量甚至造成坍塌事故的案例屢見(jiàn)不鮮。混凝土開(kāi)裂可以說(shuō)是“常發(fā)病”和“多發(fā)病”，嚴(yán)重影響了橋梁的使用性能，也經(jīng)常困擾著橋梁工程技術(shù)人員。要想控制橋梁混凝土裂縫的產(chǎn)生，提高工程質(zhì)量，就必須了解其成因。下面本文就對(duì)橋梁裂縫的產(chǎn)生原因做一分析，并提出一些對(duì)裂縫的預(yù)防措施和處理方法。1 橋梁混凝土裂縫產(chǎn)生的原因1.1 荷載引起的裂縫荷載引起的裂縫可以分為直接應(yīng)力裂縫和次應(yīng)力裂縫。1.1.1直接應(yīng)力裂縫直接應(yīng)力裂縫是指外荷載引起的直接應(yīng)力產(chǎn)生的裂縫。裂縫產(chǎn)生的原因有：(1)設(shè)計(jì)計(jì)算階段的結(jié)構(gòu)設(shè)計(jì)不合理，

3、受力假設(shè)與實(shí)際受力不符，安全系數(shù)不夠，不考慮施工的可能性，構(gòu)造處理不當(dāng)。(2)施工階段中不加限制的堆放施工機(jī)具、材料；隨意翻身、起吊、運(yùn)輸、安裝；不按設(shè)計(jì)圖紙施工，擅自更改結(jié)構(gòu)施工順序等。(3)使用階段時(shí)超出設(shè)計(jì)荷載的重型車(chē)輛過(guò)橋；受車(chē)輛、船舶的接觸、撞擊；發(fā)生地震、爆炸等。1.1.2 次應(yīng)力裂縫次應(yīng)力裂縫是指由外荷載引起的次生應(yīng)力產(chǎn)生的裂縫，其產(chǎn)生的原因可歸納為一下幾個(gè)方面：(1)在設(shè)計(jì)荷載作用下，由于結(jié)構(gòu)物的實(shí)際工作狀態(tài)同常規(guī)計(jì)算有出入或計(jì)算不考慮，從而在某些部位引起次應(yīng)力導(dǎo)致結(jié)構(gòu)開(kāi)裂。例如兩鉸拱橋拱腳設(shè)計(jì)時(shí)常采用布置“X”形鋼筋、同時(shí)消減該處斷面尺寸的辦法設(shè)計(jì)鉸，理論算該處不會(huì)存在彎矩，

4、但實(shí)際該鉸仍然能夠抗彎，以致出現(xiàn)裂縫而導(dǎo)致鋼筋銹蝕。(2)橋梁結(jié)構(gòu)中經(jīng)常需要鑿槽、升洞、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。研究表明，受力構(gòu)件挖空后，力流將產(chǎn)生繞射現(xiàn)象，在空洞附近密集，產(chǎn)生巨大的集中應(yīng)力。在長(zhǎng)跨預(yù)應(yīng)力連續(xù)梁中，經(jīng)常在跨內(nèi)根據(jù)截面內(nèi)力需要截?cái)噤撌?，設(shè)置錨頭，而在錨固斷面附近經(jīng)?？梢钥吹搅芽p。因此，在這些結(jié)構(gòu)的轉(zhuǎn)角處或構(gòu)件形狀突變處、受力鋼筋截面處容易出現(xiàn)裂縫。實(shí)際工程中次應(yīng)力是產(chǎn)生荷載裂縫的最常見(jiàn)原因。次應(yīng)力裂縫多屬于張拉、劈裂、剪切性質(zhì)。在設(shè)計(jì)上，應(yīng)盡量避免結(jié)構(gòu)突變（或截面突變），當(dāng)不能同時(shí)避免時(shí)，應(yīng)作局部處理，如做成圓角或倒角，同

5、時(shí)加強(qiáng)構(gòu)造配筋，轉(zhuǎn)角處配置斜向鋼筋，對(duì)于較大空洞有條件時(shí)可以在周邊設(shè)置護(hù)邊角鋼。1.2 收縮引起的裂縫1.2.1 塑性收縮混凝土澆筑后4h5h左右，水泥水化反應(yīng)劇烈，分子鏈逐漸形成，出現(xiàn)泌水和水分急劇蒸發(fā)，混凝土失水收縮，同時(shí)骨料因自重下沉，而此時(shí)混凝土尚未硬化，稱(chēng)為塑性收縮。在骨料下沉過(guò)程中若受到鋼筋阻擋，便形成沿鋼筋方向的裂縫，在構(gòu)件豎向變截面處如T梁、箱梁腹板與頂板交接處，因硬化前沉實(shí)不均勻?qū)l(fā)生表面的順腹板方向裂縫。(1)干縮：混凝土結(jié)硬以后，隨著表面水分逐漸蒸發(fā)，溫度逐漸降低，混凝土體積減小，稱(chēng)為干縮。因混凝土表面水分損失快，內(nèi)部損失慢，因此產(chǎn)生表面收縮快，內(nèi)部收縮慢的不均勻收縮，導(dǎo)

6、致混凝土表面承受拉力，產(chǎn)生收縮裂縫。(2)自生收縮：混凝土在硬化工程中，水泥和水發(fā)生水化反應(yīng)，這種收縮與外界溫度無(wú)關(guān)，并可以是正的（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混凝土）。(3)碳化收縮：大氣中的二氧化碳和水泥中的水化物發(fā)生化學(xué)反應(yīng)引起的收縮變形。1.3 地基變形引起的裂縫(1)地質(zhì)勘探精度不夠、試驗(yàn)資料不準(zhǔn)。勘察報(bào)告不能充分反映實(shí)際地質(zhì)情況是造成地基不均勻沉降的主要原因。(2)地基地質(zhì)差異太大。(3)結(jié)構(gòu)荷載差異太大。在地質(zhì)情況比較一致的情況下，各部分基礎(chǔ)荷載差異太大時(shí)，有可能引起不均勻沉降。(4)結(jié)構(gòu)基礎(chǔ)類(lèi)型差別大。同一聯(lián)橋梁中混合使用不同

7、基礎(chǔ)，如擴(kuò)大基礎(chǔ)和樁基礎(chǔ)，或雖采用同一種基礎(chǔ)，但是基底標(biāo)高差異太大，也可能引起地基不均勻沉降。(5)分期建造的基礎(chǔ)。在原有的橋梁附近新建橋梁時(shí)，如分期修建的高速公路左右半幅橋梁，新建橋梁荷載或基礎(chǔ)處理時(shí)引起地基土重新固結(jié)，均可能對(duì)原有橋梁基礎(chǔ)造成較大沉降。(6)地基凍脹。(7)橋梁基礎(chǔ)置于滑坡體、溶洞或活動(dòng)斷層等不良地質(zhì)時(shí)，可能造成不均勻沉降。(8)橋梁建成以后，原有地基條件變化。大多數(shù)天然地基和人工地基浸水后，尤其是素填土、黃土、膨脹土等特殊地基土、土體強(qiáng)度遇水下降，壓縮變形加大。1.4 原材料質(zhì)量引起的裂縫混凝土主要由水泥、砂、骨料、拌合用水及外加劑等組成，配置混凝土?xí)r所采用的材料不合格，

8、可能導(dǎo)致結(jié)構(gòu)出現(xiàn)裂縫。(1)水泥質(zhì)量不合格、受潮或過(guò)期會(huì)造成混凝土強(qiáng)度不夠，并導(dǎo)致混凝土開(kāi)裂。(2)砂石含泥量超過(guò)規(guī)定，不僅降低混凝土強(qiáng)度和抗?jié)B性，還會(huì)是混凝土干燥時(shí)產(chǎn)生不規(guī)則的網(wǎng)狀裂縫。砂石的級(jí)配差，或砂石顆粒過(guò)細(xì)，用這種材料拌制的混凝土常造成側(cè)面裂縫。堿骨料反應(yīng)，骨料中含有的活性硅化物質(zhì)與堿性物質(zhì)相遇，水、硅反應(yīng)會(huì)產(chǎn)生膨脹的膠質(zhì)，吸水后體積變大，造成局部膨脹和拉應(yīng)力，則構(gòu)件產(chǎn)生爆裂狀裂縫，在潮濕的地方較為多見(jiàn)。(3)拌合用水及外加劑 拌合用水或外加劑中氯化物等雜志含量較高時(shí)對(duì)鋼筋銹蝕有較大的影響。采用海水或堿泉水拌制混凝土，或采用含堿的外加劑，都可能對(duì)堿骨料反應(yīng)有影響。(4)用于結(jié)構(gòu)的鋼筋

9、已經(jīng)銹蝕 植入結(jié)構(gòu)體中的鋼筋已經(jīng)有銹斑或銹蝕，造成鋼筋表面氧化膜破壞，在保護(hù)層厚度不足的情況下，空氣侵入到混凝土內(nèi)部，鋼筋中的鐵離子與侵入到砼中的氧氣和水發(fā)生反應(yīng)，其銹蝕物體積比原來(lái)增長(zhǎng)越24倍，從而使周?chē)女a(chǎn)生膨脹應(yīng)力，導(dǎo)致保護(hù)層開(kāi)裂、剝離，沿鋼筋縱向產(chǎn)生裂縫，并有銹跡滲透到砼表面。1.5 溫度變化引起的裂縫 (1)年溫差 由于四季溫度不斷變化，但變化相對(duì)緩慢，對(duì)橋梁結(jié)構(gòu)的影響主要是導(dǎo)致橋梁的縱向位移，一般可通過(guò)橋面伸縮縫、支座位移、設(shè)置柔性墩等構(gòu)造措施來(lái)調(diào)節(jié)，只有結(jié)構(gòu)位移受到限制時(shí)才會(huì)引起溫度裂縫的產(chǎn)生，例如拱橋、剛架橋等。(2)日照橋面板、主梁或橋墩側(cè)面受到太陽(yáng)暴曬后，溫度明顯高于其他部

10、位，溫度梯度呈非線(xiàn)性分布。由于受到自身約束作用，導(dǎo)致局部拉應(yīng)力過(guò)大，出現(xiàn)裂縫。日照和下述驟然降溫是導(dǎo)致結(jié)構(gòu)溫度裂縫的最常見(jiàn)原因。(3)驟然降溫 突降大雨、預(yù)冷空氣侵襲、日落等可導(dǎo)致外表面溫度突然下降，但結(jié)構(gòu)內(nèi)部溫度變化相對(duì)較慢而產(chǎn)生溫度梯度。日照和驟然降溫內(nèi)力計(jì)算時(shí)可采用設(shè)計(jì)規(guī)范或參考實(shí)橋資料進(jìn)行，混凝土彈模不考慮折減。(4)水化熱 出現(xiàn)在施工過(guò)程中，大體積混凝土（厚度超過(guò)2m）澆筑之后由于水泥水化放熱，導(dǎo)致內(nèi)部溫度很高，內(nèi)外溫度差太大，導(dǎo)致表面出現(xiàn)裂縫。(5)蒸汽養(yǎng)護(hù)或冬季施工施工措施不當(dāng)，砼驟冷驟熱，內(nèi)外溫度不均，易出現(xiàn)裂縫。(6)預(yù)制T梁之間橫隔板安裝時(shí)，支座預(yù)埋鋼板與調(diào)平鋼板焊接時(shí)，若

11、焊接不當(dāng)，鐵件附近混凝土容易燒傷開(kāi)裂。小結(jié)：水泥混凝土裂縫產(chǎn)生的原因當(dāng)然遠(yuǎn)不止這些，在這里就不一一贅述了。下面我們根據(jù)以上原因給出相應(yīng)的裂縫預(yù)防措施和處理方法。2 預(yù)防裂縫的措施2.1 干縮裂縫的預(yù)防(1)選用收縮量較小的水泥，一般采用中低熱水泥和粉煤灰水泥，降低水泥的用量。(2)混凝土的干縮受水灰比的影響較大，水灰比越大,干縮越大，因此在混凝土配合比設(shè)計(jì)中應(yīng)盡量控制好水灰比的選用，同時(shí)摻加合適的減水劑。(3)嚴(yán)格控制混凝土攪拌和施工中的配合比，混凝土的用水量絕對(duì)不能大于配合比設(shè)計(jì)所給定的用水量。(4)加強(qiáng)混凝土的早期養(yǎng)護(hù)，并適當(dāng)延長(zhǎng)混凝土的養(yǎng)護(hù)時(shí)間。冬季施工時(shí)要適當(dāng)延長(zhǎng)混凝土保溫覆蓋時(shí)間，并

12、涂刷養(yǎng)護(hù)劑養(yǎng)護(hù)。(5)嚴(yán)格按照設(shè)計(jì)要求在混凝土結(jié)構(gòu)中設(shè)置合適的收縮縫。2.2 塑性收縮裂縫的預(yù)防(1)選用干縮值較小早期強(qiáng)度較高的硅酸鹽或普通硅酸鹽水泥。(2)嚴(yán)格控制水灰比，摻加高效減水劑來(lái)增加混凝土的坍落度和和易性，減少水泥及水的用量。(3)澆筑混凝土之前，將基層和模板澆水均勻濕透。(4)及時(shí)覆蓋塑料薄膜或者潮濕的草墊、麻片等，保持混凝土終凝前表面濕潤(rùn)，或者在混凝土表面噴灑養(yǎng)護(hù)劑等進(jìn)行養(yǎng)護(hù)。(5)在高溫和大風(fēng)天氣要設(shè)置遮陽(yáng)和擋風(fēng)設(shè)施，及時(shí)養(yǎng)護(hù)。2.3 沉陷裂縫的預(yù)防(1)對(duì)松軟土、填土地基在上部結(jié)構(gòu)施工前應(yīng)進(jìn)行必要的夯實(shí)和加固。(2)保證模板有足夠的強(qiáng)度和剛度，且支撐牢固，并使地基受力均勻

13、。(3)防止混凝土澆灌過(guò)程中地基被水浸泡。(4)模板拆除的時(shí)間不能太早，且要注意拆模的先后次序。(5)在凍土上搭設(shè)模板時(shí)要注意采取一定的預(yù)防措施。2.4 溫度裂縫的預(yù)防措施(1)盡量選用低熱或中熱水泥，如礦渣水泥、粉煤灰水泥等。(2)減少水泥用量,將水泥用量盡量控制在450kg/m3以下。(3)降低水灰比，一般混凝土的水灰比控制在0.6以下。(4)改善骨料級(jí)配，摻加粉煤灰或高效減水劑等來(lái)減少水泥用量,降低水化熱。(5)改善混凝土的攪拌加工工藝，在傳統(tǒng)的三冷技術(shù)的基礎(chǔ)上采用二次風(fēng)冷新工藝，降低混凝土的澆筑溫度。(6)在混凝土中摻加一定量的具有減水、增塑、緩凝等作用的外加劑，改善混凝土拌合物的流動(dòng)

14、性、保水性，降低水化熱，推遲熱峰的出現(xiàn)時(shí)間。(7)高溫季節(jié)澆筑時(shí)可以采用搭設(shè)遮陽(yáng)板等輔助措施控制混凝土的溫升，降低澆筑混凝土的溫度。(8)大體積混凝土的溫度應(yīng)力與結(jié)構(gòu)尺寸相關(guān)，混凝土結(jié)構(gòu)尺寸越大，溫度應(yīng)力越大，因此要合理安排施工工序，分層、分塊澆筑，以利于散熱，減小約束。(9)在大體積混凝土內(nèi)部設(shè)置冷卻管道，通冷水或者冷氣冷卻，減小混凝土的內(nèi)外溫差。(10)加強(qiáng)混凝土溫度的監(jiān)控，及時(shí)采取冷卻、保護(hù)措施。(11)預(yù)留溫度收縮縫。(12)減小約束，澆筑混凝土前宜在基巖和老混凝土上鋪設(shè)5mm左右的砂墊層或使用瀝青等材料涂刷。(13)加強(qiáng)混凝土養(yǎng)護(hù)，混凝土澆筑后，及時(shí)用濕潤(rùn)的草簾、麻片等覆蓋，并注意灑

15、水養(yǎng)護(hù)，適當(dāng)延長(zhǎng)養(yǎng)護(hù)時(shí)間，保證混凝土表面緩慢冷卻。在寒冷季節(jié)，混凝土表面應(yīng)設(shè)置保溫措施，以防止寒潮襲擊。(14)混凝土中配置少量的鋼筋或者摻入纖維材料將混凝土的溫度裂縫控制在一定的范圍之內(nèi)。3 裂縫的處理措施一般性表面細(xì)小裂縫，可將裂縫部位清洗干凈，干燥后用環(huán)氧漿液灌縫或表面涂刷封閉；裂縫較大時(shí)，可將裂縫鑿成八字形凹槽、洗凈濕潤(rùn)，刷一層水泥漿，用1：2水泥砂漿分層壓實(shí)抹光后用環(huán)氧膠泥嵌補(bǔ)。3.1 墩、柱側(cè)面裂縫墩、柱側(cè)面裂縫一般發(fā)生在距頂面50cm的范圍內(nèi)，當(dāng)裂縫未形成環(huán)狀時(shí)，可用環(huán)氧樹(shù)脂進(jìn)行灌注來(lái)封閉裂縫；當(dāng)裂縫形成環(huán)狀裂縫，且深度達(dá)到箍筋或超過(guò)箍筋時(shí)，應(yīng)將裂縫以上部分鑿除重新澆筑；當(dāng)裂縫深

16、度未達(dá)到箍筋位置時(shí)，可用環(huán)氧樹(shù)脂進(jìn)行灌注封閉裂縫。3.2 梁板及橋面鋪裝出現(xiàn)的裂縫對(duì)于梁、板上表面出現(xiàn)的收縮裂縫，一般均較細(xì)，由于梁、板上還有一層混凝土橋面鋪裝，可不進(jìn)行處理。對(duì)于水泥混凝土橋而鋪裝上出現(xiàn)的裂縫無(wú)需處理，原因是現(xiàn)行設(shè)計(jì)均在水泥混凝土鋪裝上加鋪瀝青混凝土。而在鋪瀝青混凝土之前通常要做防水層或噴灑粘層油，這此措施能夠?qū)⒘芽p封堵住，起到很好的防水作用。對(duì)于梁板側(cè)而出現(xiàn)的裂縫，可以采用“壁可法”（該法是利用注入器橡膠管的壓力300KN/，保持低壓持續(xù)灌注。通過(guò)橡膠管的收縫自動(dòng)完成注漿。該法可灌注的最小裂縫寬度為0 . 02 mm ）灌注來(lái)封閉裂縫，以防止雨水滲入腐蝕鋼筋及發(fā)生堿骨料反應(yīng)

17、而降低橋梁的承載能力。3.3 其他特殊裂縫(1)拆模后發(fā)現(xiàn)的裂縫，一般性表面細(xì)小裂縫，可將裂縫部位清洗干凈，干燥后用環(huán)氧漿液灌縫或表面涂刷封閉；裂縫較大時(shí)，可將裂縫鑿成八字形凹槽、洗凈濕潤(rùn)，刷一層水泥漿，用1：2水泥砂漿分層壓實(shí)抹光后用環(huán)氧膠泥嵌補(bǔ)。(2)對(duì)影響結(jié)構(gòu)整體，防水防滲要求的結(jié)構(gòu)裂縫，應(yīng)根據(jù)裂縫寬度、深度情況，采用水泥壓力灌漿、化學(xué)灌漿的方法修補(bǔ)，或表面封閉與注漿同時(shí)使用；明顯降低結(jié)構(gòu)剛度，承載力和嚴(yán)重裂縫，應(yīng)根據(jù)情況，采用預(yù)應(yīng)力加固或用鋼筋混凝土圍套、鋼套箍或結(jié)構(gòu)膠粘貼劑貼鋼板加固等方法。結(jié)束語(yǔ)鋼筋混凝土構(gòu)造物裂縫往往造成許多困擾，輕則影響結(jié)構(gòu)物外觀，重則危及構(gòu)造物安全。文中分析了

18、產(chǎn)生裂縫的多方面原因，實(shí)際存在的原因當(dāng)然不止這些。因此，嚴(yán)格按照國(guó)家有關(guān)規(guī)范、技術(shù)標(biāo)準(zhǔn)進(jìn)行設(shè)計(jì)、施工和監(jiān)理，是保證橋梁結(jié)構(gòu)安全耐用的前提和基礎(chǔ)。另外，在運(yùn)營(yíng)管理過(guò)程中，進(jìn)一步加強(qiáng)巡查和管理，及時(shí)發(fā)現(xiàn)和處理問(wèn)題，也是相當(dāng)重要的環(huán)節(jié)。參 考 文 獻(xiàn)1侯耀華.混凝土橋梁施工裂縫的成因及防治對(duì)策.陜西建筑 2010/11 2李春鵬，陳宇，高金生.淺談橋梁裂縫產(chǎn)生的原因.吉林交通科技 2008/02 3邱嘉宏.淺議溫度荷載與橋梁混凝土裂縫的控制.汕頭科技 2006/04 4辛登云.常規(guī)混凝土梁裂縫原因的分析.石家莊聯(lián)合技術(shù)職業(yè)學(xué)院學(xué)術(shù)研究 2006/04 5段莉麗.橋梁裂縫產(chǎn)生原因分析及整治方案.太原鐵

19、道科技 2009/02譯文CONTENTS Introduction 1 1 Cracks in Concrete Bridge 1 Loading induced crack 1.1 1 1.1.1 Direct stress fracture 1 2 secondary stress crack 1.1.2 2 1.2 cracks caused by shrinkage 1.2.1 Plastic Shrinkage 2 Cracks caused by foundation deformation 1.3 3 1.4 The quality of raw materials cause

20、d by cracks 4 Cracks caused by temperature changes 1.5 4 Preventive measures 2 5 crack 2.1 Shrinkage Cracks 5 Prevention of plastic shrinkage cracks 2.2 5 2.3 Settlement Cracks 6 2.4 6 preventive measures temperature cracks Treatment measures 3 7 Crack 3.1 pier, column 7, the side cracks 3.2 Beam an

21、d Deck 7 cracks appear 3.3 Other special crack 8 Conclusion 8 8 References   Causes of cracks in concrete bridge And treatment measures Abstract: The use of the bridge construction process, due to cracks and even affect the quality of the project led to bridge collapse phenomenon is also not un

22、common. This paper describes the types of cracks in concrete bridge, from the load, temperature changes, ground deformation 5 introduces the causes of concrete cracks, the corresponding preventive measures, and gives some crack approach. Keywords: bridge concrete cracks, causes, measures Preface Wit

23、h China's rapid development of transport infrastructure, built around a large number of concrete bridges. The bridge construction and use of the process, the concrete cracks caused by affecting the quality of the project or even collapse is not uncommon. Concrete cracking can be said to be "

24、;regular onset" and "frequently-occurring disease," a serious impact on the bridge performance, and often troubled bridge engineering and technical personnel. To control the bridge concrete cracks, improve project quality, we must understand its causes. The following article on the ca

25、uses of cracks in the bridge do an analysis and some of the cracks in the preventive measures and treatment. 1 Bridge Concrete Cracks Loading induced crack 1.1 Load induced cracks can be divided into direct and secondary stress fracture stress fracture. 1.1.1 Direct stress fracture Direct stress fra

26、cture is caused by direct external load stress cracks. Cracks are: (1) design phase of design and calculation of unreasonable force Shouli Bu Fu assumptions and the actual safety factor is not enough, does not consider the possibility of construction, construction handled properly. (2) the construct

27、ion phase of unrestricted stacked construction machinery, materials; free to stand up, lifting, transportation, installation; do not follow the construction design drawings, construction sequence and other unauthorized changes to the structure. (3) the use of load beyond the design stage of heavy ve

28、hicles crossing the bridge; by vehicles, ships of contact, impact; earthquakes, explosions and so on. 1.1.2 secondary stress fracture Secondary stress fracture is caused by the external load secondary stress cracks, the causes can be summarized as following aspects: (1) the design loads, the structu

29、re of the actual working conditions differ with the conventional calculations, or calculations do not take into account, which in some parts of the structure caused by the secondary stress lead to cracking. For example, two-hinged arch bridge design layout often use "X"-shaped bar, where t

30、he section size at the same time reducing design hinge approach, theoretical calculation moment there does not exist, but the actual bending of the hinges still, resulting in cracks and lead to reinforcement corrosion. (2) bridge structures often need to gouges or holes, set the bracket so difficult

31、 in the conventional calculation for an accurate simulation of schema, the general rule of thumb to set the reinforced. The results show that hollowing out the force components, the power flow will produce diffraction phenomenon in the vicinity of hole density, resulting in huge concentration of str

32、ess. In the long-span prestressed concrete continuous, often in a cross-section of internal forces according to need to cut the steel beams, set the anchor head, and in the anchoring section can often be seen near the crack. Therefore, in the corner of these structures or structural mutations at the

33、 shape, the reinforced section was prone to cracks. The secondary stress of the actual project is to produce the most common cause of fracture load. Times are mostly tensile stress cracks, splitting, shear properties. In the design, structural breaks should be avoided (or section mutant), when can n

34、ot be avoided, should be handled locally, such as made of rounded or chamfered, while strengthening the structural reinforcement, diagonal corner reinforcement configuration, for more large hole in the surrounding conditions can be set to protect edge angle. 1.2 contraction cracks 1.2.1 Plastic Shri

35、nkage 4h 5h after pouring concrete around the cement hydration reaction of intense molecular chain gradually formed, there bleeding and rapid evaporation of moisture, concrete shrinkage water loss, and aggregate sinking due to weight, but this time has not yet hardened concrete, known as plasticity

36、contraction. In the aggregate, if the process of being reinforced sink blocked, then the direction of the formation of cracks along the reinforcement in the vertical component, such as T at the variable section beam, box girder webs and roof junction, due to pragmatic and uneven hardening will occur

37、 before the surface The cracks along the web direction. (1) Shrinkage: Concrete Results hard then, as the gradual evaporation of surface water temperature decreased gradually reduce the volume of concrete, known as shrinkage. Water loss due to the concrete surface faster, the internal loss of slow,

38、resulting in rapid contraction of the surface, uneven contraction of the internal contraction of slow, leading to the concrete surface under tension, resulting in shrinkage cracks. (2) autogenous shrinkage: the hardening of concrete engineering, cement and water, hydration occurs, this contraction h

39、as nothing to do with the outside temperature, and can be positive (ie, contraction, such as ordinary portland cement concrete), it can be negative (ie expansion, such as slag cement concrete and fly ash concrete.) (3) carbonation shrinkage: the atmosphere of carbon dioxide and cement hydrates in th

40、e chemical reaction caused by shrinkage. Cracks caused by foundation deformation 1.3 (1) lack of precision of geological exploration, test data are not allowed. Investigation report does not adequately reflect the actual geological conditions are the main reasons causing uneven settlement of foundat

41、ion. (2) are too different geological foundation. (3) structural loads are too different. More consistent in the case of geological conditions, each part of the foundation loads are too different, there is likely to cause differential settlement. (4) Structural basis of type big difference. Bridge i

42、n the same mix together different bases, such as the expansion base and foundation, or while using the same basis, but differences in substrate elevation is too large may also cause uneven settlement of foundation. (5) the construction of the foundation stage. Near the new bridge in the original bri

43、dge, such as stage construction of the highway about half frame bridge, new bridge based on processing load or caused by soil re-consolidation of the existing bridge foundation are likely to lead to a greater settlement. (6) foundation frost heaving. (7) bridge placed on the basis of landslides, cav

44、e or other adverse geological active fault may cause differential settlement. (8) bridges are completed, the original foundation changes. Most of the natural ground and artificial ground after immersion in water, especially in plain fill, loess, expansive soil, and other special soil, soil strength

45、decreased with water, deformation increased. 1.4 The quality of raw materials caused by cracks Concrete mainly by the cement, sand, aggregate, mixing water and additives and other components, configuration, materials used in concrete when the failure may result in structural cracks. (1) substandard

46、quality of cement, damp or expired will cause concrete strength is not enough, and lead to concrete cracking. (2) exceeds the aggregate amount of mud, not only reduces the strength and impermeability of concrete, but also the concrete resulting in irregular mesh dry cracks. Difference between the gr

47、ading of sand and gravel, or sand particles too small, concrete mixing this material often causes cracks in the side. Alkali - aggregate reaction, silica aggregate containing the active material and alkaline substances meet, water, silicon produce expansion of the glial reaction, after absorbing wat

48、er became larger, resulting in local swelling and tensile stress, the components have burst-like cracks are more common in damp places. (3) mixing water and additives or the additives in the mixing water with higher levels of chloride and other magazines have greater when the impact of steel corrosi

49、on. Alkaline spring water mixing with sea water or concrete, or the use of alkaline admixture, are likely to alkali - aggregate reaction affected. (4) has been used for the reinforcement corrosion of steel bars embedded in the structure has rust spots or corrosion, resulting in damage to the steel s

50、urface oxide film, in the case of insufficient thickness of protective layer, the air intrusion into the concrete inside the iron bar and the intrusion into the concrete in the reaction of oxygen and water, the corrosion properties than the original volume growth of 2 to 4 times more, so that the co

51、ncrete around the expansion stress generated, leading to protective layer cracking, peeling, cracks along the longitudinal steel, and rust penetrate into the concrete surface. Cracks caused by temperature changes of 1.5 (1) the temperature in temperature due to changing seasons, but the change is re

52、latively slow, on bridge structure is mainly caused vertical displacement of the bridge, usually through the bridge deck expansion joints, bearing displacement, set flexible measures to regulate piers and other structures, Only limited structural displacement only when the cracks caused by temperatu

53、re, such as bridge, just bridging. (2) sun decks, the main beam or pier side by sun exposure, the temperature was higher than other parts of the distribution of the temperature gradient is nonlinear. Due to its own restraints, resulting in excessive local tensile stress, cracks. The following is a s

54、udden drop in sunshine and temperatures cause cracks in the structure of the most common cause. (3) The sudden drop in dump heavy rain, pre-invasion of cold air, the outer surface of the sun and so can lead to a sudden drop in temperature, but the relatively slow temperature changes within the struc

55、ture resulting from the temperature gradient. Sunshine and the sudden drop in temperature when the internal force calculation can be real bridge design specifications or reference data, without considering the reduction of concrete elastic modulus. (4) heat of hydration in the construction process,

56、mass concrete (thickness of more than 2m) after pouring the cement hydration heat, causing the internal temperature is high, both inside and outside temperature difference is too large, resulting in surface cracks. (5) Construction of steam curing or inappropriate measures winter construction, concr

57、ete cold and heat, the temperature inside and outside the uneven, prone to cracks. (6) pre-T beams installed between the diaphragm, bearing embedded steel plate welded with leveling, if welded properly, iron burns near the concrete easier to crack. Summary: Cracks in concrete of course, far more tha

58、n these, not one by one go into details here. According to the above reasons we have given below the corresponding fracture prevention measures and treatment. 2 measures to prevent fractures 2.1 Prevention of shrinkage cracks (1) use a smaller contraction of cement, commonly used in low-heat cement and fly ash cement, reducing the amount of cement. (2) shrinkage of concrete influenced by the water-cement ratio, the