混凝土的施工過(guò)程中溫度與裂縫控制中英文

混凝土的施工過(guò)程中溫度與裂縫控制為了防止業(yè)主對(duì)混凝土工程的索賠,就必須做好施工過(guò)程中溫度與裂縫控制。本文通過(guò)多年的現(xiàn)場(chǎng)觀察，通過(guò)查閱有關(guān)混凝土內(nèi)部應(yīng)力方面的專著，對(duì)混凝土溫度裂縫產(chǎn)生的原因、現(xiàn)場(chǎng)混凝土溫度的控制和預(yù)防裂縫的措施進(jìn)行等進(jìn)行闡述?；炷猎诂F(xiàn)代工程建設(shè)中占有重要地位。而在今天，混凝土的裂縫較為普遍，在橋梁工程中裂縫幾乎無(wú)所不在。盡管我們?cè)谑┕ぶ胁扇「鞣N措施，小心謹(jǐn)慎，但裂縫仍然時(shí)有出現(xiàn)。究其原因，我們對(duì)混凝土溫度應(yīng)力的變化注意不夠是其中之一。在大體積混凝土中，溫度應(yīng)力及溫度控制具有重要意義。這主要是由于兩方面的原因。首先，在施工中混凝土常常出現(xiàn)溫度裂縫，影響到結(jié)構(gòu)的整體性和耐久性。其次，在運(yùn)轉(zhuǎn)過(guò)程中，溫度變化對(duì)結(jié)構(gòu)的應(yīng)力狀態(tài)具有顯著的不容忽視的影響。我們遇到的主要是施工中的溫度裂縫，因此本文僅對(duì)施工中混凝土裂縫的成因和處理措施做一探討。1、裂縫的原因混凝土中產(chǎn)生裂縫有多種原因，主要是溫度和濕度的變化，混凝土的脆性和不均勻性，以及結(jié)構(gòu)不合理，原材料不合格（如堿骨料反應(yīng)），模板變形，基礎(chǔ)不均勻沉降等?；炷劣不陂g水泥放出大量水化熱，內(nèi)部溫度不斷上升，在表面引起拉應(yīng)力。后期在降溫過(guò)程中，由于受到基礎(chǔ)或老混凝上的約束，又會(huì)在混凝土內(nèi)部出現(xiàn)拉應(yīng)力。氣溫的降低也會(huì)在混凝土表面引起很大的拉應(yīng)力。當(dāng)這些拉應(yīng)力超出混凝土的抗裂能力時(shí)，即會(huì)出現(xiàn)裂縫。許多混凝土的內(nèi)部濕度變化很小或變化較慢，但表面濕度可能變化較大或發(fā)生劇烈變化。如養(yǎng)護(hù)不周、時(shí)干時(shí)濕，表面干縮形變受到內(nèi)部混凝土的約束，也往往導(dǎo)致裂縫?；炷潦且环N脆性材料，抗拉強(qiáng)度是抗壓強(qiáng)度的1／10左右。由于原材料不均勻，水灰比不穩(wěn)定，及運(yùn)輸和澆筑過(guò)程中的離析現(xiàn)象，在同一塊混凝土中其抗拉強(qiáng)度又是不均勻的，存在著許多抗拉能力很低，易于出現(xiàn)裂縫的薄弱部位。在鋼筋混凝土中，拉應(yīng)力主要是由鋼筋承擔(dān)，混凝土只是承受壓應(yīng)力。在素混凝土內(nèi)或鋼筋混凝上的邊緣部位如果結(jié)構(gòu)內(nèi)出現(xiàn)了拉應(yīng)力，則須依靠混凝土自身承擔(dān)。一般設(shè)計(jì)中均要求不出現(xiàn)拉應(yīng)力或者只出現(xiàn)很小的拉應(yīng)力。但是在施工中混凝土由最高溫度冷卻到運(yùn)轉(zhuǎn)時(shí)期的穩(wěn)定溫度，往往在混凝土內(nèi)部引起相當(dāng)大的拉應(yīng)力。有時(shí)溫度應(yīng)力可超過(guò)其它外荷載所引起的應(yīng)力，因此掌握溫度應(yīng)力的變化規(guī)律對(duì)于進(jìn)行合理的結(jié)構(gòu)設(shè)計(jì)和施工極為重要。2、溫度應(yīng)力的分析根據(jù)溫度應(yīng)力的形成過(guò)程可分為以下三個(gè)階段：早期：自澆筑混凝土開(kāi)始至水泥放熱基本結(jié)束，一般30天。這個(gè)階段的兩個(gè)特征，一是水泥放出大量的水化熱，二是混凝上彈性模量的急劇變化。由于彈性模量的變化，這一時(shí)期在混凝土內(nèi)形成殘余應(yīng)力。中期：自水泥放熱作用基本結(jié)束時(shí)起至混凝土冷卻到穩(wěn)定溫度時(shí)止，這個(gè)時(shí)期中，溫度應(yīng)力主要是由于混凝土的冷卻及外界氣溫變化所引起，這些應(yīng)力與早期形成的殘余應(yīng)力相疊加，在此期間混凝上的彈性模量變化不大。晚期：混凝土完全冷卻以后的運(yùn)轉(zhuǎn)時(shí)期。溫度應(yīng)力主要是外界氣溫變化所引起，這些應(yīng)力與前兩種的殘余應(yīng)力相迭加。根據(jù)溫度應(yīng)力引起的原因可分為兩類：自生應(yīng)力：邊界上沒(méi)有任何約束或完全靜止的結(jié)構(gòu)，如果內(nèi)部溫度是非線性分布的，由于結(jié)構(gòu)本身互相約束而出現(xiàn)的溫度應(yīng)力。例如，橋梁墩身，結(jié)構(gòu)尺寸相對(duì)較大，混凝土冷卻時(shí)表面溫度低，內(nèi)部溫度高，在表面出現(xiàn)拉應(yīng)力，在中間出現(xiàn)壓應(yīng)力。約束應(yīng)力：結(jié)構(gòu)的全部或部分邊界受到外界的約束，不能自由變形而引起的應(yīng)力，如箱梁頂板混凝土和護(hù)欄混凝土。這兩種溫度應(yīng)力往往和混凝土的干縮所引起的應(yīng)力共同作用。要想根據(jù)已知的溫度準(zhǔn)確分析出溫度應(yīng)力的分布、大小是一項(xiàng)比較復(fù)雜的工作。在大多數(shù)情況下，需要依靠模型試驗(yàn)或數(shù)值計(jì)算?；炷恋男熳兪箿囟葢?yīng)力有相當(dāng)大的松馳，計(jì)算溫度應(yīng)力時(shí)，必須考慮徐變的影響，具體計(jì)算這里就不再細(xì)述。3、溫度的控制和防止裂縫的措施為了防止裂縫，減輕溫度應(yīng)力可以從控制溫度和改善約束條件兩個(gè)方面著手?？刂茰囟鹊拇胧┤缦拢海?）采用改善骨料級(jí)配，用干硬性混凝土，摻混合料，加引氣劑或塑化劑等措施以減少混凝土中的水泥用量；（2）拌合混凝土?xí)r加水或用水將碎石冷卻以降低混凝土的澆筑溫度；（3）熱天澆筑混凝土?xí)r減少澆筑厚度，利用澆筑層面散熱；（4）在混凝土中埋設(shè)水管，通入冷水降溫；（5）規(guī)定合理的拆模時(shí)間，氣溫驟降時(shí)進(jìn)行表面保溫，以免混凝土表面發(fā)生急劇的溫度梯度；（6）施工中長(zhǎng)期暴露的混凝土澆筑塊表面或薄壁結(jié)構(gòu)，在寒冷季節(jié)采取保溫措施；改善約束條件的措施是：合理地分縫分塊；避免基礎(chǔ)過(guò)大起伏；合理的安排施工工序，避免過(guò)大的高差和側(cè)面長(zhǎng)期暴露；此外，改善混凝土的性能，提高抗裂能力，加強(qiáng)養(yǎng)護(hù)，防止表面干縮，特別是保證混凝土的質(zhì)量對(duì)防止裂縫是十分重要，應(yīng)特別注意避免產(chǎn)生貫穿裂縫，出現(xiàn)后要恢復(fù)其結(jié)構(gòu)的整體性是十分困難的，因此施工中應(yīng)以預(yù)防貫穿性裂縫的發(fā)生為主。在混凝土的施工中，為了提高模板的周轉(zhuǎn)率，往往要求新澆筑的混凝土盡早拆模。當(dāng)混凝土溫度高于氣溫時(shí)應(yīng)適當(dāng)考慮拆模時(shí)間，以免引起混凝土表面的早期裂縫。新澆筑早期拆模，在表面引起很大的拉應(yīng)力，出現(xiàn)“溫度沖擊”現(xiàn)象。在混凝土澆筑初期由于水化熱的散發(fā)，表面引起相當(dāng)大的拉應(yīng)力，此時(shí)表面溫度亦較氣溫為高，此時(shí)拆除模板，表面溫度驟降，必然引起溫度梯度，從而在表面附加一拉應(yīng)力，與水化熱應(yīng)力迭加，再加上混凝土干縮，表面的拉應(yīng)力達(dá)到很大的數(shù)值，就有導(dǎo)致裂縫的危險(xiǎn)，但如果加，再加上混凝土干縮，表面的拉應(yīng)力達(dá)到很大的數(shù)值，就有導(dǎo)致裂縫的危險(xiǎn)，但如果在拆除模板后及時(shí)在表面覆蓋一輕型保溫材料，如泡沫海棉等，對(duì)于防止混凝土表面產(chǎn)生過(guò)大的拉應(yīng)力，具有顯著的效果。加筋對(duì)大體積混凝土的溫度應(yīng)力影響很小，因?yàn)榇篌w積混凝土的含筋率極低。只是對(duì)一般鋼筋混凝土有影響。在溫度不太高及應(yīng)力低于屈服極限的條件下，鋼的各項(xiàng)性能是穩(wěn)定的，而與應(yīng)力狀態(tài)、時(shí)間及溫度無(wú)關(guān)。鋼的線脹系數(shù)與混凝土線脹系數(shù)相差很小，在溫度變化時(shí)兩者間只發(fā)生很小的內(nèi)應(yīng)力。由于鋼的彈性模量為混凝土彈性模量的7~15倍，當(dāng)內(nèi)混凝土應(yīng)力達(dá)到抗拉強(qiáng)度而開(kāi)裂時(shí)，鋼筋的應(yīng)力將不超過(guò)100~200kg／cm，因此，在混凝土中想要利用鋼筋來(lái)防止細(xì)小裂縫的出現(xiàn)很困難。但加筋后結(jié)構(gòu)內(nèi)的裂縫一般就變得數(shù)目多、間距小、寬度與深度較小了。而且如果鋼筋的直徑細(xì)而間距密時(shí)，對(duì)提高混凝土抗裂性的效果較好?；炷梁弯摻罨炷两Y(jié)構(gòu)的表面常常會(huì)發(fā)生細(xì)而淺的裂縫，其中大多數(shù)屬于干縮裂縫。雖然這種裂縫一般都較淺，但它對(duì)結(jié)構(gòu)的強(qiáng)度和耐久性仍有一定的影響。為保證混凝土工程質(zhì)量，防止開(kāi)裂，提高混凝土的耐久性，正確使用外加劑也是減少開(kāi)裂的措施之一。例如使用減水防裂劑，筆者在實(shí)踐中總結(jié)出其主要作用為：(1)混凝土中存在大量毛細(xì)孔道，水蒸發(fā)后毛細(xì)管中產(chǎn)生毛細(xì)管張力，使混凝土干縮變形。增大毛細(xì)孔徑可降低毛細(xì)管表面張力，但會(huì)使混凝土強(qiáng)度降低。這個(gè)表面張力理論早在六十年代就已被國(guó)際上所確認(rèn)。(2)水灰比是影響混凝土收縮的重要因素，使用減水防裂劑可使混凝土用水量減少25％。(3)水泥用量也是混凝土收縮率的重要因素，摻加減水防裂劑的混凝土在保持混凝土強(qiáng)度的條件下可減少15％的水泥用量，其體積用增加骨料用量來(lái)補(bǔ)充。(4)減水防裂劑可以改善水泥漿的稠度，減少混凝土泌水，減少沉縮變形。(5)提高水泥漿與骨料的粘結(jié)力，提高的混凝土抗裂性能。(6)混凝土在收縮時(shí)受到約束產(chǎn)生拉應(yīng)力，當(dāng)拉應(yīng)力大于混凝土抗拉強(qiáng)度時(shí)裂縫就會(huì)產(chǎn)生。減水防裂劑可有效的提高的混凝土抗拉強(qiáng)度，大幅提高混凝土的抗裂性能。(7)摻加外加劑可使混凝土密實(shí)性好，可有效地提高混凝土的抗碳化性，減少碳化收縮。(8)摻減水防裂劑后混凝土緩凝時(shí)間適當(dāng)，在有效防止水泥迅速水化放熱基礎(chǔ)上，避免因水泥長(zhǎng)期不凝而帶來(lái)的塑性收縮增加。(9)摻外加劑混凝土和易性好，表面易抹平，形成微膜，減少水分蒸發(fā)，減少干燥收縮。許多外加劑都有緩凝、增加和易性、改善塑性的功能，我們?cè)诠こ虒?shí)踐中應(yīng)多進(jìn)行這方面的實(shí)驗(yàn)對(duì)比和研究，比單純的靠改善外部條件，可能會(huì)更加簡(jiǎn)捷、經(jīng)濟(jì)。4、混凝土的早期養(yǎng)護(hù)實(shí)踐證明，混凝土常見(jiàn)的裂縫，大多數(shù)是不同深度的表面裂縫，其主要原因是溫度梯度造成寒冷地區(qū)的溫度驟降也容易形成裂縫。因此說(shuō)混凝土的保溫對(duì)防止表面早期裂縫尤其重要。從溫度應(yīng)力觀點(diǎn)出發(fā)，保溫應(yīng)達(dá)到下述要求：(1)防止混凝土內(nèi)外溫度差及混凝土表面梯度，防止表面裂縫。(2)防止混凝土超冷，應(yīng)該盡量設(shè)法使混凝土的施工期最低溫度不低于混凝土使用期的穩(wěn)定溫度。(3)防止老混凝土過(guò)冷，以減少新老混凝土間的約束?；炷恋脑缙陴B(yǎng)護(hù)，主要目的在于保持適宜的溫濕條件，以達(dá)到兩個(gè)方面的效果，一方面使混凝土免受不利溫、濕度變形的侵襲，防止有害的冷縮和干縮。一方面使水泥水化作用順利進(jìn)行，以期達(dá)到設(shè)計(jì)的強(qiáng)度和抗裂能力。適宜的溫濕度條件是相互關(guān)聯(lián)的。混凝上的保溫措施常常也有保濕的效果。從理論上分析，新澆混凝土中所含水分完全可以滿足水泥水化的要求而有余。但由于蒸發(fā)等原因常引起水分損失，從而推遲或妨礙水泥的水化，表面混凝土最容易而且直接受到這種不利影響。因此混凝土澆筑后的最初幾天是養(yǎng)護(hù)的關(guān)鍵時(shí)期，在施工中應(yīng)切實(shí)重視起來(lái)。防水混凝土施工質(zhì)量控制結(jié)合經(jīng)驗(yàn)，對(duì)模板的設(shè)計(jì)、制作安裝、鋼筋綁扎、混凝土澆筑及養(yǎng)護(hù)等關(guān)鍵工序的操作要點(diǎn)進(jìn)行了闡述，并介紹了成型后混凝土的質(zhì)量控制標(biāo)準(zhǔn)，以降低工程造價(jià)，具有很好的經(jīng)濟(jì)和社會(huì)效益。影響防水混凝土的質(zhì)量因素很多。任何一個(gè)環(huán)節(jié)不注意均會(huì)不同程度地?fù)p失防水混凝土阻水功能，在地下室工程施工中，采取了從模板制作與安裝、鋼筋制作與綁扎、混凝土的拌制和運(yùn)輸、混凝土的澆筑和振搗、施工縫作法、混凝土的養(yǎng)護(hù)與拆模以及做好回填土等方面嚴(yán)把質(zhì)量關(guān)，確保防水混凝土自防水，并通過(guò)實(shí)踐獲得成功。施工方法2.1模板制作與安裝由于防水混凝土特別強(qiáng)調(diào)密實(shí)性，所以對(duì)模板的制作安裝也相應(yīng)的提高了要求，即要求密實(shí)不漏漿，堅(jiān)固不變形，吸水性要小，宜優(yōu)先選用竹膠板模板或鋼模板。嚴(yán)格控制模板間的縫隙大小，超過(guò)2mm的必須用海綿條塞縫，對(duì)于板面多孔的模板一律不用，同時(shí)做好墻柱爛根防治工作，采取根部貼海綿條，底部抹水泥砂漿，澆筑混凝土前，先注入同標(biāo)號(hào)的水泥砂漿由于防水混凝土結(jié)構(gòu)壁厚大多較薄，為保證構(gòu)件的幾何尺寸，常采用內(nèi)外螺栓拉接的措施，需要注意的是，應(yīng)在拉接螺栓中間設(shè)置止水鐵板，以防止水沿著螺栓滲漏，形成引水通路。2.2鋼筋制作與綁扎防水混凝土結(jié)構(gòu)對(duì)鋼筋有如下要求:鋼筋接頭應(yīng)盡可能采用焊接接頭，盡量少用或不用綁扎接頭。鋼筋應(yīng)盡可能采用螺紋鋼筋，增加握裹力和止水能力。嚴(yán)格控制鋼筋保護(hù)層厚度。防水混凝土鋼筋保護(hù)層最小厚度不小于25mm，特別是迎水面的保護(hù)層不允許有負(fù)誤差，用砂漿墊塊作保護(hù)層時(shí)，應(yīng)用預(yù)埋在砂漿塊內(nèi)的鐵絲與鋼筋綁扎牢固。若采用鋼筋鐵馬架固定鋼筋時(shí)，也應(yīng)加焊止水鐵板或固定在墊塊上，以增強(qiáng)防水效果,本工程用新型材料尼龍固定片有效的防止了鋼筋保護(hù)層偏差過(guò)大的質(zhì)量通病。2.3混凝土的拌制和運(yùn)輸由于防水混凝土要求較高的密實(shí)性，所以拌制也要有較好的均勻性，為此應(yīng)做好如下幾點(diǎn)：確保攪拌時(shí)間，每次攪拌從投料到出料一般不少于2min。當(dāng)使用外加劑，應(yīng)將外加劑制成一定濃度的溶液后加入攪拌機(jī)內(nèi)，不得將外加劑的干粉或高濃度溶液直接加入攪拌機(jī)內(nèi)，防止攪拌不均勻而局部集中，既失去外加劑作用，又影響混凝土質(zhì)量。施工過(guò)程中應(yīng)定期抽查測(cè)定混凝土坍落度，施工中如遇下雨或其他原因，砂石含水量發(fā)生變化時(shí)，調(diào)整施工配合比。本工程使用商品混凝土，有效的提高了混凝土攪拌質(zhì)量及各類外加劑的摻量，嚴(yán)格控制了混凝土水灰比及坍落度?；炷恋臐仓捎帽盟凸に?，有效的避免了混凝土在運(yùn)輸過(guò)程中產(chǎn)生離析泌水及漏漿現(xiàn)象。2.4施工中的注意事項(xiàng)(1)施工作業(yè)面的合理劃分。防水混凝土澆筑應(yīng)嚴(yán)格分層進(jìn)行，并一次連續(xù)施工完成前后及上下層之間接頭應(yīng)嚴(yán)格控制在水泥的初凝時(shí)間內(nèi)為此，可將操作工人分成幾個(gè)作業(yè)組同時(shí)相向或相背而行。(2)嚴(yán)格做到定點(diǎn)定量下料。根據(jù)分層澆筑高度和運(yùn)輸工具的車容量，嚴(yán)格進(jìn)行定點(diǎn)定量方可下料，這是提高防水混凝土澆筑質(zhì)量的重要一環(huán)。(3)堅(jiān)持開(kāi)門子下料或用串桶(溜槽)下料。為防止水泥漿與骨料分離，防水混凝土下料時(shí)自由高度應(yīng)不超過(guò)防水結(jié)構(gòu)鋼筋密集，當(dāng)澆筑高度較高時(shí)，應(yīng)按規(guī)定在模板上開(kāi)門子下料或用串桶(溜槽)下料，防止水泥漿與骨料分離，同時(shí)也避免水泥漿或混凝土松散地堆積于鋼筋交叉處和水泥漿濺粘于模板表面，既造成后面下料困難，又影響結(jié)構(gòu)表面光滑，成為日后滲漏隱患。(4)混凝土的振搗是保證混凝土密實(shí)、防滲的重要一環(huán)，點(diǎn)定量均勻下料的基礎(chǔ)上，也應(yīng)采取定點(diǎn)振搗，精心操作注意上下層的銜接，既要防止漏振、欠振，又要防止過(guò)振，特別要禁止用撬動(dòng)鋼筋或振動(dòng)鋼筋來(lái)助振的方法，這樣將使前面已澆筑振實(shí)部分的鋼筋與混凝土分離，造成嚴(yán)重后果。(5)認(rèn)真做好接漿措施，及時(shí)調(diào)整坍落度。防水混凝土結(jié)構(gòu)施工中，常常是底板與豎壁分別施工，即在豎壁上做施工縫處理。在澆筑豎壁時(shí)應(yīng)在接縫處先澆一層5cm～10cm厚的同標(biāo)號(hào)水泥砂漿銜接層，以保證接縫處振搗密實(shí)。同時(shí)，隨著澆筑高度的逐步提高，應(yīng)逐步減少用水量，減小坍落度防止?jié)仓缴喜砍霈F(xiàn)骨料偏少而水泥漿偏多現(xiàn)象。2.5施工縫做法施工縫是防水結(jié)構(gòu)的薄弱環(huán)節(jié)之一。應(yīng)盡量少留或不留，當(dāng)混凝土澆筑受到設(shè)備或支模條件的限制時(shí)，則可在征得設(shè)計(jì)單位的同意后，在規(guī)定部位留置水平施工縫，一般留在底板以上200mm處的豎壁上工程中采用新工藝——膨脹性橡膠止水帶處理施工縫的方法，取代常用方法，諸如企口式施工縫、高低式施工縫、平口施工縫等。防水混凝土一般在垂直方向不留置施工縫，在主樓與裙樓之間設(shè)了后澆帶，垂直方向的施工縫在接縫部位設(shè)置膨脹性橡膠止水帶或鋼板止水帶。由于施工縫是防水結(jié)構(gòu)的一個(gè)薄弱環(huán)節(jié)，施工中應(yīng)專人負(fù)責(zé)，精心操作，確保施工質(zhì)量。采用止水鋼板時(shí)不得有漏焊，采用橡膠止水帶在接長(zhǎng)時(shí)務(wù)必要將接頭用專用膠水粘貼牢固可靠。2.6混凝土的養(yǎng)護(hù)與拆模防水混凝土澆筑后養(yǎng)護(hù)工作的好壞，不僅影響到混凝土的強(qiáng)度，而且影響到混凝土的抗?jié)B性能。試驗(yàn)證明，防水混凝土早期脫水或養(yǎng)護(hù)過(guò)程缺水，將大幅度降低其抗?jié)B性能。防水混凝土養(yǎng)護(hù)應(yīng)不少于14d防水混凝土拆模要求也比較高，拆模時(shí)間不宜過(guò)早，拆模時(shí)應(yīng)精心操作，防止用鐵棍撬，大錘猛敲硬撬，使混凝土遭到破壞。2.7做好回填土地下防水結(jié)構(gòu)的回填土是最后一道工序，它往往被忽視，這實(shí)在是一個(gè)很大的失誤。當(dāng)構(gòu)筑物防水工作結(jié)束后應(yīng)立即進(jìn)行回填土施工。首先應(yīng)重視回填土本身的質(zhì)量；其次回填前應(yīng)清理坑底的雜物和積水，回填時(shí)應(yīng)分層夯實(shí)。地下防水結(jié)構(gòu)周圍一層夯實(shí)的回填土，本身是一道可靠而又經(jīng)濟(jì)的防水帷幕。在工程地下室工程施工中，由于切切實(shí)實(shí)嚴(yán)把以上七道關(guān)口，組織防水混凝土施工，效果比較理想，沒(méi)有出現(xiàn)滲漏水現(xiàn)象。結(jié)束語(yǔ)以上對(duì)混凝土的施工溫度與裂縫之間的關(guān)系進(jìn)行了理論和實(shí)踐上的初步探討，雖然學(xué)術(shù)界對(duì)于混凝土裂縫的成因和計(jì)算方法有不同的理論，但對(duì)于具體的預(yù)防和改善措施意見(jiàn)還是比較統(tǒng)一，同時(shí)在實(shí)踐中的應(yīng)用效果也是比較好的，具體施工中要靠我們多觀察、多比較，出現(xiàn)問(wèn)題后多分析、多總結(jié)，結(jié)合多種預(yù)防處理措施，混凝土的裂縫是完全可以避免的。DiscusstheconstructiontemperatureandcrackoftheconcretelightlyByG.K.Kululanga,W.Kuotcha,R.McCaffer,Member,ASCE,andF.Edum-Fotwe,TheAmericanSocietyofCivilEngineersThesummary,Inordertopreventtheownersoftheconcreteworkofclaims,wemustdoagoodjobintheconstructionprocessinthetemperatureandcrackcontrol,throughobservationliveformanyyears,throughconsultingthemonographaboutstresswithintheconcrete,explaintoconcretetemperaturereason,on-the-spotconcretecontrolandmeasure,preventionofcrackoftemperaturethatcrackproduce.KeywordConcreteTemperaturestressCrackControl1.Theconcreteoccupiestheimportantpositioninmodernengineeringconstruction.Buttoday,thecrackoftheconcreteiscomparativelygeneral,thecracksarenearlyomnipresentinthescienceofbridgebuilding.Thoughwetakevariouskindsofmeasuresinconstructing,careful,butthecrackstilloccursnowandthen.Tracingittoitscause,itisoneofthemincompletelythatourchangetoconcretetemperaturestresspaysattentionto.Inthelargevolumeconcrete,temperaturestressandtemperaturecontrolaresignificant.Thisismainlybecauseofthereasonoftworespects.Firstofall,concreteoftenappearthetemperaturecrackinnotconstructing,influencetheglobalityanddurabilityofthestructure.Secondly,inthecourseofoperating,thetemperaturechangehasremarkableinfluencethatcan'tbeignoredonthestressstateofthestructure.Wemeettoconstructtemperaturecrackinmainly,soonlytoorigincauseofformationandtreatmentmeasure,concreteofcrackmakeadiscussioninconstructingthistext.ReasonofacrackHavemanykindsofreasonstoproducethecrackintheconcrete,itismainlythechangesoftemperatureandhumidity,fragilityanddisparityoftheconcrete,andthestructureisunreasonable,therawmaterialsisnotuptostandard(ifthealkaliaggregatereact),thetemplateisoutofshape,thefoundationdoesnotsubsideetc.evenly.Thecementemitsalargeamountofheatofhydrationwhentheconcreteishardenned,insidetemperatureisrisingconstantly,causethestressofdrawingonthesurface.Inthecourseofloweringthetemperature,isitcongealfoundationpayrestraintomixalwayslaterstage,willpresentthestressofdrawingwithintheconcrete.Reducingoftemperaturecansurfacecauseheavystressofdrawingveryinconcretetoo.Whenthesedrawthestressandgobeyondresistingtheabilityofsplittingofconcrete,namelywillpresentthecrack.Alotofinsidehumidityofconcretechangeverylightorchangerelativelyslow,surfacehumiditymightchangeheavytheviolentchangetakesplacing.Suchasmaintainingthoroughly,whengettingwetterwhennotdoing,contractsurfacetherearen'tdeformationdoing,oftencausethecracktoo.Theconcreteisakindoffragilitymaterial,tensilestrengthisabout1/10ofthecompressionstrength,isitcarryonone'sshoulderorbacklimitwhendrawoutofshapetohave*104only,isitcarryonone'sshoulderorbacklimitlocationwhenstretchoutofshapetothereis*104toaddforalongtimetoaddashorttime.Becauserawmaterialseven,waterdustthanunstable,transportandbuildphenomenonofemanatingofcourse,itstensilestrengthisnoteveninthesameconcrete,alotofresisttheabilityofdrawingverylow,itisapttopresenttheweakpositionofthecrack.Amongarmoredconcrete,drawstresstoundertakebyreinforcingbarmainly,concretebearstressofkeepingjust.Orreinforcingbarmixifedgepositiongonetotocongealpresentthestressofdrawinginthestructureinplainconcrete,mustrelyontheconcreteoneselftobear.Requiretoavoidthestressofdrawingoronlyverysmallstressofdrawingappearsofthethegeneraldesign.Buttheconcreteiscooledfrommaximumtemperaturetothesteadytemperatureofoperatingperiodinconstructing,oftencausesizabletodrawthestresswithintheconcrete.Thetemperaturestresscanexceedotheroutsidesandloadthestressescausedsometimes,knowchangelaw,temperatureofstressforcarryonreasonablestructuraldesignandconstructextremelyimportant.Analysisof2temperaturestressesCanbedividedintofollowingthreestagesaccordingtotheformingprocessofthetemperaturestress:(1)Itisearly:Buildconcreteisitisitoverbasicallytosendoutheattocementtobegin,generallyoneonedaybyoneself.Twocharacteristicsatthisstage,first,thecementemitsalargeamountofheatofhydration,second,mixandcongealthechangingsharplyofelasticmodelquantity.Becauseofthechangeofelasticmodelquantity,formtheremainingstressintheconcreteinthisperiod.(2)Middleperiod:Uptilltheconcreteiscooleduntilstabilitytemperaturefromcementsendoutheatfunctionbasicallywhenexpiring,inthisperiod,thetemperaturestressismainlybecausethecoolingoftheconcreteandexternaltemperaturechangecause,thesestressesandremnantsstressesthatisformedinearlydaysaresuperposed,mixandcongealtheelasticmouldamountthatgoestoanddoesnotchangemuchduringthisperiod.(3)Laterperiod:Operationperiodafterthecompletecoolingofconcrete.Temperaturestresswhetherexternaltemperaturechangecausemainly,thesestressesandfirsttwokindsofremnantsstressesesarechangedandadded.Canbedividedintotwokindsaccordingtothereasonwhythetemperaturestresscauses:(1)Spontaneousstress:Therearenotanyrestraintortotallystaticstructureattheborder,ifinsidetemperatureisnon-lineardistribution,temperaturestressappearingbecausestructurerestrainsfromeachother.Forexample,thebodyofmoundofthebridge,thephysicaldimensionisrelativelylarge,surfacetemperatureislowwhentheconcreteiscooled,insidetemperatureishigh,presentthestressofdrawingonthesurface,presentthestressofpressinginthemiddle.(2)Restrainthestress:Allofthestructureonesoritrestrainexternalonesomeborder,can'toutofshapeandstressnotcausenotfree.Suchascaseroofbeamroofconcreteandguardrailconcrete.Thistwokindsoftemperaturestressesdrawbackstressescausedtoactonwiththedoingofconcretetogetherfrequently.Itisamorecomplicatedjobtowanttoanalysethedistribution,sizeofthetemperaturestressaccuratelyaccordingtoknowntemperature.Incaseofgreatmajority,needtorelyonthemodeltestorthenumbervaluetocalculate.Toisitmaketemperaturestresshavesizablelimptocreepconcrete,atthestressaccountingtemperature,mustconsidertheinfluencethatcreep,calculateconcretlythatnolongerstatesthinlyhere.Controlandpreventingthemeasureofthecrackof3temperatureForpreventcrack,lightentemperaturestresscanfromcontroltemperatureandisitisitsetabouttorestraintermsfromtwotoimprove.Themeasureofcontrollingtemperatureisasfollows:(1)Isitimproveaggregategrademix,isitdorigidconcretetospend,mixmixturetoadopt,isitguideangrypharmaceuticalorplastificationpharmaceutical,etc.measureinordertoreducecementconsumptionofconcretetoadd;(2)Addwaterorthewatertocoolthebrokenstoneinordertoreducethetemperatureofbuildingoftheconcretewhilemixingandshuttingtheconcrete;(3)Reducethethicknessofbuildingwhilebuildingtheconcreteonhotday,utilizeandbuildtheaspecttodispeltheheat;(4)Burythewaterpipeundergroundintheconcrete,enterthecoldwatertolowerthetemperatureopenly;(5)Stipulaterationalformremovaltime,thetemperaturekeepswarmthesurfacewhileloweringsuddenly,incasethattherapidtemperaturegradienttakesplaceintheconcretesurface;(6)Theconcretewithmediumandlong-termandexposedconstructionbuildsapieceofsurfaceorthinwallstructure,takethemeasureofkeepingwarmincoldseason;Themeasureofimprovingconditionofrestrainingis:(1)Divideandsewanddivideonerationally;(2)Preventthefoundationfromrisingandfallingtoobig;(3)Rationalarrangementconstructionprocess,preventthetoobigdiscrepancyinelevationandsidefromexposingforalongtime;Inaddition,improvetheperformanceoftheconcreteandimproveandresisttheabilityofsplitting,strengthenmaintenance,preventthesurfacefrombeingdoneandcontracted,especiallyguaranteethequalityoftheconcreteisveryimportanttopreventingthecrack,shouldpayspecialattentiontoavoidingproducingandrunningthroughthecrack,theglobalityresumeditsstructureafterappearingisverydifficult,soshouldrelymainlyonpreventingtheemergenceoftherunningthroughcrackwhileconstructing.Inconstructionofconcrete,forraiseturnoverrateoftemplate,demandconcreteformremovalassoonaspossiblethatbuildnewlyoften.Shouldconsiderformremovaltimeproperlywhenconcretetemperatureishigherthanthetemperature,soasnottocausethesuperficialearlycrackofconcrete.Buildingtheearlyformremovalnewly,causeverylargestressofdrawingonthesurface,thephenomenonthat"temperatureisassaulted"appears.Buildinitialstageinconcrete,becauseheatofhydrationissentout,thesurfacecausessizabletodrawthestress,surfacetemperatureisalsohigherthantemperatureatthismoment,removethetemplateatthismoment,surfacetemperatureisloweredsuddenly,mustcausetemperaturegradient,thusaddanddrawthestressonthesurface,changeandaddwiththeheatofhydrationstress,inaddition,theconcretedriesandcontracts,thesuperficialstressofdrawingreachesverygreatnumbervalue,havedangerofcausingthecrack,butcoveralight-dutyheatinsulatorwithonthesurfaceintimeafterremovingthetemplate,forinstancefoamsponge,etc.,forpreventconcretesurfacefromproducethetoobigstressofdrawing,haveremarkableresults.Addmuscleinfluencetolargevolumetemperaturestressofconcreteverylight,becauselargevolumeconcreteincludemuscletoberateverymuchlow.Justhaveinfluenceonthegeneralarmoredconcrete.Ontermsthattemperatureisnotveryhighandthestressislessthanlimitofsurrendering,everyperformanceofthesteelissteady,andhavenothingtodowithstressstate,timeandtemperature.Linebloatedcoefficientofsteelandconcretelinebloatedcoefficientdifferenceverylight,takeplacelittleinternalstressveryonlybetweenthetwowhilechangingintemperature.Becauseelasticmouldamountofsteelconcreteelasticmould7~15ofquantity,reachasinteriorconcretestresstensilestrengthandwhenfracturing,thestressofthereinforcingbarwillnotexceed10000kg/cm2..Soisitisitpreventtinyappearancedifficultyverymuchofcrackfromtomakeuseofreinforcingbartowantamongconcrete.Butthecrackinthestructuregenerallybecomesnumerous,theintervalislittle,thewidthanddeptharesmallerafteraddingthemuscle.Andifdiameterofreinforcingbardetailedandwhenintervaldense,toimproveconcreteresistresultofpersonwhosplitbetter.Concreteandsurface,armoredconcreteofstructurecantakeplacedetailedandshallowcrackoften,amongthemthegreatmajoritybelongtoanddoanddrawbackthecrack.Thoughthiskindofcrackisgenerallyallrelativelylight,itstillshavecertaininfluenceontheintensityanddurabilityofthestructure.Inordeertooguaarantteecconcrreteprojjectquallity,prrevenntfrractuuringg,iimproovettheddurabbilittyofftheeconncrette,uusettheaadmixxtureetoreduuceooneoofthhemeeasurrestthatfracctureescoorrecctly..Wheetherrisitrreduccewaaterisiitspplitpharrmaceeuticcalttodeefendd,IIsummmariizehhismmainfuncctionninpraccticeetouse..(1)ThereeisporeeDaooofalaargenumbberoofmaaoinntheeconncrette,prodducecapiillarryteensiooninntheecappillaaryaafterrwatteriisevvaporratedd,maakecconcrreteisiitcoontraactooutoofshhapetoddo.IIncreeasinngthhethhinaaperttureofhhaircanreduucettheccapilllaryysurrfaceetennsionn,bbutwwillmakeetheeinttensiityoofcooncreeterreducce.Thisssurrfaceetennsionntheeoryhasalreeadybeennconnfirmmediinthhewoorldasffarbbackastthessixtiies.(2)Waterrdusstthhaniinfluuenceeimpportaantffactoorthhatcconcrreteshriink,isiitreeduceewatteriisittspllitppharmmaceuuticaalcaanmaakecconcrretewateercoonsummptioonreeduceeby25%toddefenndtoousee.(3)Cemenntcoonsummptioonimmporttantfacttor,conccreteeofperssonwwhosshrinnktooo,iisittadddanddsubbtracctwaaterisiitspplitconccreteeredducibble115%oofthheceementtconnsumpptionnontermmsthhatkkeeptheinteensittyoffconncretteoffphaarmacceutiicaltoddefenndtoomixx,ittsvoolumeeissuppplemeenteddbyincrreasiingaaggreegateeconnsumpptionn.(4)Reduccewaaterisiitspplitpharrmaceeuticcalccaniimproovecconsiistenncyoofgrrout,reeduceeconncretteseecretteinnktoodeffend,,redduceandsinkkandddraawbaackddeforrmingg.(5)Improovegglueiingtthesstrenngthoffformiingoofthhegrroutandaggrregatte,tthecconcrreteimprroveddressistsstheeperrformmanceeofspliittinng.(6)Concrreteisiitprroduccesttresssofdrawwingtorrestrrainfrommwhiilesshrinnkingg,crrackwhenndraawinggtheestrressandisggreattertthanconccreteetennsileestrrengtthcaanprroducce.RReduccewaaterisiitspplitpharrmaceeuticcaleeffecctiveeconncretteteensillesttrenggthoofimmprovvemenntveeryttodeefendd,iimprooverresisstinggtheeperrformmanceeofspliittinngoffconncrettebyyawwidemarggin.(7)Itcaanmaaketthecconcrretedenssitygooddtoaddtheadmiixturretoomixx,ccaniimprooverresisstinggcarrboniizatiionoofcooncreeteeeffecctiveely,,redducecarbbonizzatioontooshrrink..(8)Isittredducewateerissitspliitsllowccoaguulatiionttimeproppercconcrreteundeerphharmaaceutticalltodefeend,,onthebasiisoffpreeventtingthefasttwatteroofceementtfroomseendinngouutheeateeffecctiveelyttomiix,ppreveenttthepplastticittyshhrinkkthaatbrringssbeccauseetheecemmentisnnotccongeealeddforrallongtimeefroominncreaasingg.(9)Mixaadmixxtureeconncretteanndgeettinngeaasyaandkkind,suurfacceeaasyttofeeelfflat,foormllittllemeembraane,reduucetthemmoistturetoeevapooratee,reeduceedriilyaandsshrinnk.AAlottofadmiixturrealllhaavettheffuncttionssofslowwcoaagulaationn,iincreeasinnganndappt,imprrovemmentplassticiity,theexpeerimeenttthatwesshoulldcaarryoniinthhisrrespeectmmoreintthepprojeectppractticeisccompaaredwithhanddstuudiedd,thhanlleanagaiinstnotimprrovinngteermsmoreesimmple,,maygetttingsimppleaandmmoredireect,econnomy..Earlymmainttenannceoof4conccreteePracticcehaasprrovedd,ttheccommooncrrackofcconcrrete,moostiisthhesuurfaccecrrackofddiffeerenttdeppth,mainnreaasonitswhetthertempperatturegraddienttcauuseccoldtempperattureofaarealoweertoooeaasyttofoormccrackksudddenlly.SSosaaythhewaarm--keeepinggoftheconccreteeisespeecialllyiimporrtantttoprevventiingttheeearlyycraackoofsuurfacce.Fromthheviiewpoointofttempeeratuuresstresss,sshoulldreeachandrequuiretokkeepwarmmfolllowiinglyy:1)Preveentcconcrreteinteernallanddextternaalteemperraturrepoooraandcconcrretesurffacegraddienttfroom,prevventthesurffacecracck.2)Preveentcconcrretefrommtobeuultraaanddcolld,shouuldiisittisitmmaketheminiimumtempperattureisnnotllowerrthaanthhestteadyytemmperaatureeofconccreteeserrviceetimmecoonstrructiionttimeincconcrretetottryttotrryonne'sbestt.3)Preveentttheooldcconcrretesubccooliing,,inordeertooredducetheresttrainntammongtheoldandnewconccretee.Theearrlymmainttenannceoofthhecooncreete,themainnpurrposeelieesinnkeeepinggtheesuiitabllewaarmaandhhumiddconnditiion,,inordeertoogetttheeressultofttworrespeects,,ontheotheerhaandmmaketheconccreteeavooidttheiinvassionandattaackoofthheunnfavoorablleanndwaarm,,hummidittyouutoffshaape,theonessthaatprrevenntfrromhharmffullyyareecolldtooconntracctannddootoconttractt.Onnoneehanndmaakeccemenntwaaterfuncctionngoonssmootthly,issinthehopeeofreacchinggtheeinttensiityddesiggnedandresiistinngthheabbilittyoffspllittiing.Thesuiitabllewaarmddampcondditioonissintterreelateed.MMixwwarm-keeepinngmeeasurrepaaidttocoongeaalofftenprottectwetresuultstoo..Anaalysee,wwaterrconncretteinncluddemooistuureccanmmeetdemaand,,cemmentofwwaterrhavveennoughhanddtosparreneewlytheooretiicallly.BButbbecauusettherreasoonoffevaaporaatinggetcc.offtencaussesllosseesofftheemoiisturre,tthusposttponeeorhindderwwaterrofthecemeentffrom,,theesurrfaceeconncrettereeceivvestthiskinddofadveerseeffeecteeasieestlyyandddirrectlly.KKeypperioodwhhenmmainttaineedinniniitiallaffewddaysafteersootheeconncretteissbuiilt,shouuldppayaattenntionntoconsscienntiouuslyincconsttructting..ConclussionssConstrructiionttempeeratuureaandrrelattionofccrackkinconccreteetheeabooveccarryyonprelliminnarydisccussiionoofthheoryyanddpraacticce,tthougghthheaccademmiahhasddiffeerentttheeorieestoooriigincausseoffforrmatiionaandccompuutinggtecchnollogyoftthecconcrretecracck,bbutttocooncreeteppreveentioonanndimmprovvingthemeassuresugggestiionttoreelatiivelyyuniify,,appplicaationninpraccticeeressultfineetoooatthesameetimme,cconcrretetoiisittobsservee,ccompaaremmoremoreebyusttowaantiincoonstrructiing,anallysemoree,ssummaarizeemorreafftergoinngwrrong,coombinnemaanykkindssofprevventiionaandddealwithhtheemeaasuree,thhecrrackoftthecconcrretecanbeaavoidded.2.QualiityccontrroloofwaaterpprooffconncrettecoonstrructiionCombineedwiitheexperriencce,ffromformmworkkdessign,,fabbricaationnanddinsstalllatioon,aassemmbinggreiinforroemeent,pourringandcuriingoofcooncreeteaandootherrasppectssconnstruuctioonteechnoologyyoffairr-faccedcconcrreteisiintrooduceedasswelllassquaalityyconntrollmeaasureesanndsttandaardsinoorderrtoreduuceeenginneeriingccosttoaacquiiressatissfieddecoonomiicanndsoociallbennefitts.Theffactoorsoofinnflueeninggwaaterpprooff-cooncreeteqqualiityaarevverymanyyAnylinnksddoesnotpayatteentioontootheewatter-pprooffconncretteofffieeldllosshinddersthewateerfuunctiionwwithoouteexcepptionnjoiintlyywitthdeegreee.Enggineeeringgconnstruuctiooninntheebassemenntaddoptssseccondaaryfformboarrdfaabriccatioonanndinnstalllatiion,reinnforccedbbarffabriicatiionaandbbind,,conncrettesttirriingaandmmixinngsyystemmanddtraanspoort,conccreteeconncrettecoovibrratioonbeeatwwithasttick,,connstruuctioonjoointpraccticee,cooncreeteccurinngannddiismanntlemodeelanndbeeingrreadyyforrbacckfilllanndsooonaspeects..Thessearreveeryccritiicaltoqqualiitymmethoodtooenssurethattwatter-pprooffconncretteseelfwwaterr-prooof,andthewayofppractticehaswanout..Methoddbeiinguunderrconnstruuctioon2.1FabbricaationnanddInsstalllatioonAccordiingttothhecooncreettoofcllosellyknnit,,demmandofreassonwwhyttofoormbboarddsinncetthewwaterr-prooofaalsoconccreteehavvemaadeaandhhaveasseembleedcoorressponddingriseeisspeccial,beetorequuirethattnottleaavinggouttthiickffluidd,ffirmclosselyknittbloockoofwooodddeforrmatiion,,watteraabsorrptioonChharacctershoouldbessmalllanddougghtttogiiveppriorritytosselecctanndussingbambboosslabrubbberfformboarrdorrtheesteeelfform...Sttricttconntrollforrmbooardroommgappsizze,nnecesssaryyexcceediing22mmssuseesfooamrrubbeerorrplaasticctosqueeezeacrrackin,,porrousformmboaardnnonuttilittywiithouutexxcepttiontobboarddfacceBeereaadyfforwwallposttatthesameetimmeroottinngthheprrevenntionnanddcurrejoobAddoptthecemeentmmortaarpoourinngsaame,,indiiciainbbeforrethheroootssegmeentsstickkingthefoammrubbberorpplastticsstripp,tthebbottoompuutsoonacemeentmmortaar,conccreteeacconcrrete,,firrst55cm~100cm..Sinncewwaterr-prooof,conccreteestrructuurewwallthiccknesssissmosstlymoreeinffertiile..Bettoennsureethaatcoomponnentgeommetryydimmensiion,,Chaangaadoptttheeinssideandoutssidebollttoopulllthhemeeasurremeeetinnga