土木工程-外文翻譯---混凝土的施工過程中溫度與裂縫控制

1、<p>　　混凝土的施工過程中溫度與裂縫控制</p><p>　　By G. K. Kululanga, W. Kuotcha, R. McCaffer, Member, ASCE, and F. Edum-Fotw美國土木工程師學(xué)會。</p><p>　　摘 要本文通過多年的現(xiàn)場觀察，通過查閱有關(guān)混凝土內(nèi)部應(yīng)力方面的專著，對混凝土溫度裂縫產(chǎn)生的原因、現(xiàn)場混凝土溫度的控制和預(yù)防裂

2、縫的措施進行等進行闡述。</p><p>　　關(guān)鍵詞：混凝土；溫度應(yīng)力；裂縫；控制</p><p>　　1 混凝土在現(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所不在。盡管我們在施工中采取各種措施，小心謹慎，但裂縫仍然時有出現(xiàn)。究其原因，我們對混凝土溫度應(yīng)力的變化注意不夠是其中之一。</p><p>　　在大體積混凝土中，溫度應(yīng)

3、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，在運轉(zhuǎn)過程中，溫度變化對結(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，因此本文僅對施工中混凝土裂縫的成因和處理措施做一探討。</p><p><b>　　1.1 裂縫的原因</b></p><p>　　混凝土中產(chǎn)生裂縫

4、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。</p><p>　　混凝土硬化期間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。后期在降溫過程中，由于受到基礎(chǔ)或老混凝上的約束，又會在混凝土內(nèi)部出現(xiàn)拉應(yīng)力。氣溫的降低也會在混凝土表面引起很大的拉應(yīng)力。當這些拉應(yīng)力超出混凝土的抗裂能力時，即會出現(xiàn)裂縫。許多混凝土的內(nèi)部濕度

5、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)護不周、時干時濕，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。混凝土是一種脆性材料，抗拉強度是抗壓強度的1／10左右，短期加荷時的極限拉伸變形只有（0.6～1.0）×104， 長期加荷時的極限位伸變形也只有（1.2～2.0）×104.由于原材料不均勻，水灰比不穩(wěn)定，及運輸和澆筑過程中的離析現(xiàn)象，在同一塊混凝土中其抗拉強度又是不均勻的，存在著許多抗拉能

6、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，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。一般設(shè)計中均要求不出現(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轉(zhuǎn)時期的穩(wěn)定溫度，往往在混凝土內(nèi)部引起相當大的拉應(yīng)力。有</p><p>　　1.2 溫度應(yīng)力的分析</p><p>　　根據(jù)溫

7、度應(yīng)力的形成過程可分為以下三個階段：</p><p>　?。?）早期：自澆筑混凝土開始至水泥放熱基本結(jié)束，一般約30天。這個階段的兩個特征，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，這一時期在混凝土內(nèi)形成殘余應(yīng)力。</p><p>　?。?）中期：自水泥放熱作用基本結(jié)束時起至混凝土冷卻到穩(wěn)定溫度時止，這個時期中，溫度應(yīng)力主要是由于混凝土的冷卻及外界氣溫變化

8、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。</p><p>　?。?）晚期：混凝土完全冷卻以后的運轉(zhuǎn)時期。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。</p><p>　　根據(jù)溫度應(yīng)力引起的原因可分為兩類：</p><p>　?。?）自生應(yīng)力：邊界上沒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，

9、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)尺寸相對較大，混凝土冷卻時表面溫度低，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。</p><p>　?。?）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欄混凝土。</p><p>　　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。</p><p>

10、　　要想根據(jù)已知的溫度準確分析出溫度應(yīng)力的分布、大小是一項比較復(fù)雜的工作。在大多數(shù)情況下，需要依靠模型試驗或數(shù)值計算?；炷恋男熳兪箿囟葢?yīng)力有相當大的松馳，計算溫度應(yīng)力時，必須考慮徐變的影響，具體計算這里就不再細述。</p><p>　　1.3 溫度的控制和防止裂縫的措施</p><p>　　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方面著手。</p><

11、;p>　　控制溫度的措施如下：</p><p>　?。?）采用改善骨料級配，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；</p><p>　?。?）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；</p><p>　?。?）熱天澆筑混凝土?xí)r減少澆筑厚度，利用澆筑層面散熱；</p><p>　　（4）

12、在混凝土中埋設(shè)水管，通入冷水降溫；</p><p>　　（5）規(guī)定合理的拆模時間，氣溫驟降時進行表面保溫，以免混凝土表面發(fā)生急劇的溫度梯度；</p><p>　　（6）施工中長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；</p><p>　　改善約束條件的措施是：</p><p>　　（1）合理地分縫分塊；</p>

13、<p>　?。?）避免基礎(chǔ)過大起伏；</p><p>　?。?）合理的安排施工工序，避免過大的高差和側(cè)面長期暴露；</p><p>　　此外，改善混凝土的性能，提高抗裂能力，加強養(yǎng)護，防止表面干縮，特別是保證混凝土的質(zhì)量對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。</p>&l

14、t;p>　　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。當混凝土溫度高于氣溫時應(yīng)適當考慮拆模時間，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)，表面引起相當大的拉應(yīng)力，此時表面溫度亦較氣溫為高，此時拆除模板，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)力達到很大的數(shù)

15、值，就有導(dǎo)致裂縫的危險，但如果在拆除模板后及時在表面覆蓋一輕型保溫材料，如泡沫海棉等，對于防止混凝土表面產(chǎn)生過大的拉應(yīng)力，具有顯著的效果。</p><p>　　加筋對大體積混凝土的溫度應(yīng)力影響很小，因為大體積混凝土的含筋率極低。只是對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，鋼的各項性能是穩(wěn)定的，而與應(yīng)力狀態(tài)、時間及溫度無關(guān)。鋼的線脹系數(shù)與混凝土線脹系數(shù)相差很小，在溫度變化時兩者間只發(fā)生很小的

16、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，當內(nèi)混凝土應(yīng)力達到抗拉強度而開裂時，鋼筋的應(yīng)力將不超過100~200kg／cm2..因此，在混凝土中想要利用鋼筋來防止細小裂縫的出現(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而間距密時，對提高混凝土抗裂性的效果較好。混凝土和鋼筋混凝土結(jié)構(gòu)的表面常常會發(fā)生細而淺的裂縫，其中大多數(shù)屬于干縮裂縫。雖然這種裂縫一般都較淺，但它對結(jié)構(gòu)的強度和耐

17、久性仍有一定的影響。</p><p>　　為保證混凝土工程質(zhì)量，防止開裂，提高混凝土的耐久性，正確使用外加劑也是減少開裂的措施之一。例如使用減水防裂劑，筆者在實踐中總結(jié)出其主要作用為：</p><p>　?。?）混凝土中存在大量毛細孔道，水蒸發(fā)后毛細管中產(chǎn)生毛細管張力，使混凝土干縮變形。增大毛細孔徑可降低毛細管表面張力，但會使混凝土強度降低。這個表面張力理論早在六十年代就已被國際上所確認。

18、</p><p>　?。?）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％。</p><p>　?。?）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度的條件下可減少15％的水泥用量，其體積增加骨料用量來補充。</p><p>　?。?）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。</p&

19、gt;<p>　　（5）提高水泥漿與骨料的粘結(jié)力，提高的混凝土抗裂性能。</p><p>　　（6）混凝土在收縮時受到約束產(chǎn)生拉應(yīng)力，當拉應(yīng)力大于混凝土抗拉強度時裂縫就會產(chǎn)生。減水防裂劑可有效的提高的混凝土抗拉強度，大幅提高混凝土的抗裂性能。</p><p>　　（7）摻加外加劑可使混凝土密實性好，可有效地提高混凝土的抗碳化性，減少碳化收縮。</p><p

20、>　?。?）摻減水防裂劑后混凝土緩凝時間適當，在有效防止水泥迅速水化放熱基礎(chǔ)上，避免因水泥長期不凝而帶來的塑性收縮增加。</p><p>　　（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.</p><p>　　許多外加劑都有緩凝、增加和易性、改善塑性的功能，我們在工程實踐中應(yīng)多進行這方面的實驗對比和研究，比單純的靠改善外部條件，可能會更加簡捷、經(jīng)濟。

21、</p><p>　　1.4 混凝土的早期養(yǎng)護</p><p>　　實踐證明，混凝土常見的裂縫，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。因此說混凝土的保溫對防止表面早期裂縫尤其重要。從溫度應(yīng)力觀點出發(fā)，保溫應(yīng)達到下述要求：</p><p>　　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)

22、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）防止老混凝土過冷，以減少新老混凝土間的約束。</p><p>　　混凝土的早期養(yǎng)護，主要目的在于保持適宜的溫濕條件，以達到兩個方面的效果，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行，以期達到設(shè)計的強度和抗裂能力。</p><p>　　適宜的溫濕度條件是相互關(guān)聯(lián)的?；炷系谋?/p>

23、溫措施常常也有保濕的效果。</p><p>　　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)護的關(guān)鍵時期，在施工中應(yīng)切實重視起來。</p><p>　　2.防水混凝土施工質(zhì)量控制</p><p>　　結(jié)合經(jīng)驗，對模

24、板的設(shè)計、制作安裝、鋼筋綁扎、混凝土澆筑及養(yǎng)護等關(guān)鍵工序的操作要點進行了闡述，并介紹了成型后混凝土的質(zhì)量控制標準，以降低工程造價，具有很好的經(jīng)濟和社會效益。</p><p>　　影響防水混凝土的質(zhì)量因素很多。任何一個環(huán)節(jié)不注意均會不同程度地損失防水混凝土阻 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、混凝土的拌制和運輸、混凝土的澆筑和振搗、施工縫作法、混凝土的養(yǎng)護與拆模以及做好回填土等方面

25、嚴把質(zhì)量關(guān) 方法確保防水混凝土自防水，并通過實踐獲得成功。</p><p><b>　　施工方法</b></p><p>　　2.1 模板制作與安裝 </p><p>　　由于防水混凝土特別強調(diào)密實性，所以對模板的制作和安裝也相應(yīng)提高 了要求 ，即要求密實不漏漿，堅固不變形 ，吸水性要小 ，宜優(yōu)先選用竹膠板模板或鋼模板。嚴格控制模板間的縫隙大小

26、 ，超過2 mm的必須用海綿條塞縫 ，對于板面多孔的模板一律不用，同時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，先注入同標號的水泥砂漿 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。</p><p>　　2.2 鋼筋制作與綁扎 </p><p>

27、;　　防水混凝土結(jié)構(gòu)對鋼筋有如下要求 ：</p><p>　　1)鋼筋應(yīng)盡可能采用螺紋鋼筋 ，增加握裹力和止水能力。</p><p>　　2)鋼筋接頭應(yīng)盡量采用焊接接頭 ，盡量少用和不用綁扎接頭。</p><p>　　4)嚴格控制鋼筋保護層厚度。</p><p>　　防水混凝土鋼筋保護層最小厚度不小于25 mm，特別是迎水面的保護層不允許有負

28、誤差，用砂漿墊塊作保護層時，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，也應(yīng)加焊止水鐵板或固定在墊塊上，以增強防水效果,本工程用新型材料尼龍固定片有效 的防止了鋼筋保護層偏差過大的質(zhì)量通病。 </p><p>　　2.3混凝土的拌制和運輸</p><p>　　由于防水混凝土要求較高的密實性，所以拌制也要有較好的均勻性，為此應(yīng)做好如下幾點：</p>&

29、lt;p>　　1)確保攪拌時間，每次攪拌從投料到出料一般不少于 2 min。</p><p>　　2)當使用外加劑，應(yīng)將外加劑制成一定濃度的溶液后加入攪 拌機內(nèi)，不得將外加劑的干粉或高濃度溶液直接加入攪拌機內(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。</p><p>　　3)施工過程中應(yīng)定期抽查測定混凝土坍落度，施工中如遇下 </p><

30、p>　　雨或其他原因，砂石含水量發(fā)生變化時，應(yīng)及時測定其含水率，并 </p><p><b>　　調(diào)整施工配合比。</b></p><p>　　4)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，嚴格控制了混凝土水灰比及坍落度。 </p><p>　　5)混凝土的澆筑采用泵送工藝，有效的避免了混凝土在運輸過程中產(chǎn)生離

31、析泌水及漏漿現(xiàn)象。</p><p>　　2.4 施工中的注意事項 </p><p>　　1)施工作業(yè)面的合理劃分。防水混凝土澆筑應(yīng)嚴格分層進行 ，并一次連續(xù)施工完成前后及上下層之間接頭應(yīng)嚴格控制在 水泥的初凝時間內(nèi)為此 ，可將操作工人分成幾個作業(yè)組同時相 向或相背而行。</p><p>　　2)嚴格做到定點定量下料。根據(jù)分層澆筑高度和運輸工具的車容量，嚴格進行定點定

32、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)。</p><p>　　3)堅持開門子下料或用串桶(溜槽)下料。</p><p>　　為防止水泥漿與 骨料分離，防水混凝土下料時自由高度應(yīng)不超過防水結(jié) 構(gòu)鋼筋密集，當澆筑高度較高時 ，應(yīng)按規(guī)定在模板上開門子下料或用串桶(溜槽)下料，防止水泥漿與骨料分離，同時也避免水泥 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

33、難 ，又影響結(jié)構(gòu)表面光滑，成為日后滲漏隱患。</p><p>　　4)混凝土的振搗是保證混凝土密實、防滲的重要一環(huán)， 點定量均勻下料的基礎(chǔ)上，也應(yīng)采取定點振搗，精心操作注意上 下層的銜接，既要防止漏振、欠振，又要防止過振，特別要禁止用 撬動鋼筋或振動鋼筋來助振的方法，這樣將使前面已澆筑振實部 分的鋼筋與混凝土分離，造成嚴重后果。</p><p>　　5)認真做好接漿措施，及時調(diào)整坍落度。&

34、lt;/p><p>　　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。在澆筑豎壁時應(yīng)在接縫處先澆一層 5cm～10 cm厚的同標號水 泥砂漿銜接層，以保證接縫處振搗密實。同時，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。</p><p>　　2.5 施工縫作法 </p><p>　　施工

35、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，當混凝土澆筑受到設(shè)備或支模條件的限制時，則可在征得設(shè)計單 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200 mm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

36、縫是防水結(jié)構(gòu)的一個薄弱環(huán)節(jié)，施工中應(yīng)專人負責，精心操作，確保施工質(zhì)量。采用止水鋼板時不得有漏焊，采用橡膠止水帶在接長時務(wù)必要將接頭用專用膠水粘貼牢固可靠。 </p><p>　　2.6 混凝土的養(yǎng)護與拆模</p><p>　　防水混凝土澆筑后養(yǎng)護工作的好壞，不僅影響到混凝土的強 度 ，而且影響到混凝土的抗?jié)B性能。試驗證明，防水混凝土早期脫水或養(yǎng)護過程缺水 ，將大幅度降低其抗?jié)B性能。防水混凝

37、土養(yǎng)護應(yīng)不少于 14 d防水混凝土拆模要求也比較高，拆模時間不宜過早 ，拆模時 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。</p><p><b>　　2.7 做好回填土</b></p><p>　　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，這實在是一個很大的失誤。當構(gòu)筑物防水工作結(jié)束后應(yīng)立即進行回填土施工。首先應(yīng)重視回填土本身的質(zhì)量；其次回

38、填前應(yīng)清理 坑底的雜物和積水 ，回填時應(yīng)分層夯實。地下防水結(jié)構(gòu)周圍一層 夯實的回填土 ，本身是一道可靠而又經(jīng)濟的防水帷幕。 在工程地下室工程施工申，由于切切實實嚴把以上七道關(guān)口，組織防水混凝土施工，效果比較理想 ，沒有出現(xiàn)滲漏水現(xiàn)象。</p><p><b>　　結(jié)束語</b></p><p>　　以上對混凝土的施工溫度與裂縫之間的關(guān)系進行了理論和實踐上的初步探討，雖

39、然學(xué)術(shù)界對于混凝土裂縫的成因和計算方法有不同的理論，但對于具體的預(yù)防和改善措施意見還是比較統(tǒng)一，同時在實踐中的應(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。 </p><p>　　Discuss the construction temperature and crack of the concrete lightly &l

40、t;/p><p>　　By G. K. Kululanga, W. Kuotcha, R. McCaffer, Member, ASCE, and F. Edum-Fotwe ,The American Society of Civil Engineers</p><p>　　The summary Through observation live for many years, throug

41、h consulting the monograph about stress within the concrete, explain to concrete temperature reason , on-the-spot concrete control and measure , prevention of crack of temperature that crack produce. </p><p>

42、;　　Keyword: Concrete;Temperature stress;Crack ;Control </p><p>　　1. The concrete occupies the important position in modern engineering construction. But today, the crack of the concrete is comparatively gen

43、eral, the cracks are nearly omnipresent in the science of bridge building. Though we take various kinds of measures in constructing, careful, but the crack still occurs now and then. Tracing it to its cause, it is one of

44、 them incompletely that our change to concrete temperature stress pays attention to. In the large volume concrete, temperature stress and t</p><p>　　Reason of a crack </p><p>　　Have many kinds o

45、f reasons to produce the crack in the concrete, it is mainly the changes of temperature and humidity, fragility and disparity of the concrete, and the structure is unreasonable, the raw materials is not up to standard (i

46、f the alkali aggregate react), the template is out of shape, the foundation does not subside etc. evenly . The cement emits a large amount of heat of hydration when the concrete is hardenned, inside temperature is rising

47、 constantly, cause the stress of drawing on</p><p>　　Can be divided into following three stages according to the forming process of the temperature stress: </p><p>　　(1)It is early: Build concre

48、te is it is it over basically to send out heat to cement to begin , generally one one day by oneself. Two characteristics at this stage, first, the cement emits a large amount of heat of hydration, second , mix and conge

49、al the changing sharply of elastic model quantity. Because of the change of elastic model quantity , form the remaining stress in the concrete in this period. </p><p>　　(2)Middle period: Up till the concrete

50、 is cooled until stability temperature from cement send out heat function basically when expiring, in this period, the temperature stress is mainly because the cooling of the concrete and external temperature change caus

51、e, these stresses and remnants stresses that is formed in early days are superposed , mix and congeal the elastic mould amount that goes to and does not change much during this period. </p><p>　　(3)Later per

52、iod: Operation period after the complete cooling of concrete. Temperature stress whether external temperature change cause mainly, these stresses and first two kinds of remnants stresseses are changed and added . </p&

53、gt;<p>　　Can be divided into two kinds according to the reason why the temperature stress causes: </p><p>　　(1)Spontaneous stress: There are not any restraint or totally static structure at the border

54、, if inside temperature is non-linear distribution, temperature stress appearing because structure restrains from each other. For example, the body of mound of the bridge, the physical dimension is relatively large, surf

55、ace temperature is low when the concrete is cooled, inside temperature is high, present the stress of drawing on the surface, present the stress of pressing in the middle. </p><p>　　(2)Restrain the stress: A

56、ll of the structure ones or it restrain external one some border,can't out of shape and stress not cause not free. Such as case roof beam roof concrete and guardrail concrete. </p><p>　　This two kinds of

57、 temperature stresses draw back stresses caused to act on with the doing of concrete together frequently. It is a more complicated job to want to analyse the distribution , size of the temperature stress accurately accor

58、ding to known temperature. In case of great majority , need to rely on the model test or the number value to calculate. To is it make temperature stress have sizable limp to creep concrete, at the stress accounting tempe

59、rature, must consider the influence that cr</p><p>　　Control and preventing the measure of the crack of 3 temperature </p><p>　　For prevent crack , lighten temperature stress can from control te

60、mperature and is it is it set about to restrain terms from two to improve. </p><p>　　The measure of controlling temperature is as follows: </p><p>　　(1)Is it improve aggregate grade mix , is it

61、do rigid concrete to spend , mix mixture to adopt, is it guide angry pharmaceutical or plastification pharmaceutical ,etc. measure in order to reduce cement consumption of concrete to add; </p><p>　　(2)Add w

62、ater or the water to cool the broken stone in order to reduce the temperature of building of the concrete while mixing and shutting the concrete; </p><p>　　(3)Reduce the thickness of building while building

63、the concrete on hot day, utilize and build the aspect to dispel the heat; </p><p>　　(4)Bury the water pipe underground in the concrete, enter the cold water to lower the temperature openly; </p><p

64、>　　(5)Stipulate rational form removal time, the temperature keeps warm the surface while lowering suddenly, in case that the rapid temperature gradient takes place in the concrete surface; </p><p>　　(6)Th

65、e concrete with medium and long-term and exposed construction builds a piece of surface or thin wall structure, take the measure of keeping warm in cold season; </p><p>　　The measure of improving condition o

66、f restraining is: </p><p>　　(1)Divide and sew and divide one rationally ; </p><p>　　(2)Prevent the foundation from rising and falling too big; </p><p>　　(3)Rational arrangement cons

67、truction process, prevent the too big discrepancy in elevation and side from exposing for a long time; </p><p>　　In addition, improve the performance of the concrete and improve and resist the ability of spl

68、itting, strengthen maintenance , prevent the surface from being done and contracted , especially guarantee the quality of the concrete is very important to preventing the crack, should pay special attention to avoiding p

69、roducing and running through the crack , the globality resumed its structure after appearing is very difficult, so should rely mainly on preventing the emergence of the running through c</p><p>　　In construc

70、tion of concrete , for raise turnover rate of template , demand concrete form removal as soon as possible that build newly often. Should consider form removal time properly when concrete temperature is higher than the te

71、mperature, so as not to cause the superficial early crack of concrete. Building the early form removal newly, cause very large stress of drawing on the surface, the phenomenon that " temperature is assaulted "

72、appears. Build initial stage in concrete, because heat of h</p><p>　　Add muscle influence to large volume temperature stress of concrete very light , because large volume concrete include muscle to be rate v

73、ery much low. Just have influence on the general armored concrete. On terms that temperature is not very high and the stress is less than limit of surrendering, every performance of the steel is steady, and have nothing

74、to do with stress state , time and temperature. Line bloated coefficient of steel and concrete line bloated coefficient difference very light,</p><p>　　In order to guarantee concrete project quality , preven

75、t fracturing , improve the durability of the concrete, use the admixture to reduce one of the measures that fractures correctly. Whether is it reduce water is it split pharmaceutical to defend , I summarize his main func

76、tion in practice to use. </p><p>　　(1)There is pore Dao of a large number of mao in the concrete , produce capillary tension in the capillary after water is evaporated, make concrete is it contract out of sh

77、ape to do. Increasing the thin aperture of hair can reduce the capillary surface tension , but will make the intensity of concrete reduce . This surface tension theory has already been confirmed in the world as far back

78、as the sixties. </p><p>　　(2)Water dust than influence important factor that concrete shrink, is it reduce water is it split pharmaceutical can make concrete water consumption reduce by 25% to defend to use.

79、 </p><p>　　(3)Cement consumption important factor, concrete of person who shrink too, is it add and subtract water is it split concrete reducible 15% of the cement consumption on terms that keep the intensit

80、y of concrete of pharmaceutical to defend to mix, its volume is supplemented by increasing aggregate consumption. </p><p>　　(4)Reduce water is it split pharmaceutical can improve consistency of grout , reduc

81、e concrete secrete ink to defend, reduce and sink and draw back deforming. </p><p>　　(5)Improve glueing the strength of forming of the grout and aggregate, the concrete improved resists the performance of sp

82、litting. </p><p>　　(6)Concrete is it produce stress of drawing to restrain from while shrinking, crack when drawing the stress and is greater than concrete tensile strength can produce. Reduce water is it sp

83、lit pharmaceutical effective concrete tensile strength of improvement very to defend , improve resisting the performance of splitting of concrete by a wide margin. </p><p>　　(7)It can make the concrete densi

84、ty good to add the admixture to mix , can improve resisting carbonization of concrete effectively , reduce carbonization to shrink. </p><p>　　(8)Is it reduce water is it split slow coagulation time proper co

85、ncrete under pharmaceutical to defend , on the basis of preventing the fast water of cement from sending out heat effectively to mix, prevent the plasticity shrink that brings because the cement is not congealed for a lo

86、ng time from increasing. </p><p>　　(9)Mix admixture concrete and getting easy and kind , surface easy to feel flat , form little membrane, reduce the moisture to evaporate, reduce drily and shrink. A lot of

87、admixture all have the functions of slow coagulation , increasing and apt , improvement plasticity, the experiment that we should carry on in this respect more in the project practice is compared with and studied, than l

88、ean against not improving terms more simple,may getting simple and more direct, economy. </p><p>　　Early maintenance of 4 concrete </p><p>　　Practice has proved , the common crack of concrete ,

89、most is the surface crack of different depth, main reason its whether temperature gradient cause cold temperature of area lower too easy to form crack suddenly. So say the warm - keeping of the concrete is especially imp

90、ortant to preventing the early crack of surface. </p><p>　　From the viewpoint of temperature stress, should reach and require to keep warm followingly: </p><p>　　1)Prevent concrete internal and

91、external temperature poor and concrete surface gradient from , prevent the surface crack. </p><p>　　2)Prevent concrete from to be ultra and cold , should is it is it make the minimum temperature is not lower

92、 than the steady temperature of concrete service time construction time in concrete to try to try one's best. </p><p>　　3)Prevent the old concrete subcooling , in order to reduce the restraint among the

93、old and new concrete. </p><p>　　The early maintenance of the concrete, the main purpose lies in keeping the suitable warm and humid condition , in order to get the result of two respects, on the other hand m

94、ake the concrete avoid the invasion and attack of the unfavorable and warm , humidity out of shape, the ones that prevent from harmfully are cold to contract and do to contract. On one hand make cement water function go

95、on smoothly , is in the hope of reaching the intensity designed and resisting the ability of splitting. </p><p>　　The suitable warm damp condition is interrelated. Mix warm - keeping measure paid to congeal

96、often protect wet results too. Analyse , water concrete include moisture can meet demand , cement of water have enough and to spare newly theoretically. But because the reason of evaporating etc. often causes losses of t

97、he moisture, thus postpone or hinder water of the cement from, the surface concrete receives this kind of adverse effect easiestly and directly. Key period when maintained in initial a fe</p><p>　　Conclusion

98、s</p><p>　　Construction temperature and relation of crack in concrete the above carry on preliminary discussion of theory and practice, though the academia has different theories to origin cause of formation

99、 and computing technology of the concrete crack, but to concrete prevention and improving the measure suggestion to relatively unify , application in practice result fine too at the same time, concrete to is it observe ,

100、 compare more more by us to want in constructing, analyse more , summarize more afte</p><p>　　2.Quality control of waterproof concrete construction</p><p>　　Combined with experience, from formwo

101、rk design, fabrication and installation, assembing reinforoement, pouring and curing of concrete and other aspects construction technology of fair-faced concrete is introduced as well as quality control measures and stan