外文翻譯---淺談混凝土的施工過(guò)程中溫度與裂縫控制

1、<p>　　淺談混凝土的施工過(guò)程中溫度與裂縫控制</p><p>　　By G. K. Kululanga, W. Kuotcha, R. McCaffer, Member, ASCE, and F. Edum-Fotwe</p><p><b>　　美國(guó)土木工程師學(xué)會(huì)</b></p><p>　　摘 要 為了防止業(yè)主對(duì)混凝土工程的索

2、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(zhuān)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。</p><p>　　關(guān)鍵詞：混凝土；溫度應(yīng)力；裂縫；控制</p><p>　　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(wú)所不在。盡管我們?cè)谑┕ぶ胁扇「鞣N措施，小

3、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。</p><p>　　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。</

4、p><p><b>　　1、裂縫的原因</b></p><p>　　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。</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(guò)程中，由于受到基礎(chǔ)或老混凝上

5、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，短期加荷時(shí)的極限拉伸變形只有（0.6～1.0）×104， 長(zhǎng)期加荷時(shí)的極限位伸變形也只有（1.

6、2～2.0）×104.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

7、凝土內(nèi)部引起相當(dāng)大的拉應(yīng)力。有</p><p><b>　　2、溫度應(yīng)力的分析</b></p><p>　　根據(jù)溫度應(yīng)力的形成過(guò)程可分為以下三個(gè)階段：</p><p>　?。?）早期：自澆筑混凝土開(kāi)始至水泥放熱基本結(jié)束，一般約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

8、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(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。</p><p>　　（3）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。</p>

9、<p>　　根據(jù)溫度應(yīng)力引起的原因可分為兩類(lèi)：</p><p>　?。?）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(xiàn)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。</p><p>　　（2）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

10、引起的應(yīng)力。如箱梁頂板混凝土和護(hù)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>　　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。混凝土的徐變使溫度應(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。</p><p>

11、;　　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(gè)方面著手。</p><p>　　控制溫度的措施如下：</p><p>　?。?）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；</p><p>　?。?）拌合混凝土?xí)r加水或用水將碎石

12、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>　?。?）在混凝土中埋設(shè)水管，通入冷水降溫；</p><p>　?。?）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；</p><p>　?。?）施工中長(zhǎng)期暴露的混凝土澆筑塊表面或薄壁結(jié)構(gòu)，

13、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>　?。?）合理地分縫分塊；</p><p>　?。?）避免基礎(chǔ)過(guò)大起伏；</p><p>　?。?）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；</p><p>　　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，

14、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。</p><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(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)，

15、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。</p><p>　　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

16、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(xiàn)脹系數(shù)與混凝土線(xiàn)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／cm2..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

17、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。</p><p>　　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：</p><p>　?。?）

18、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)。</p><p>　　（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％。</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(qiáng)度的條件

19、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。</p><p>　　（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。</p><p>　?。?）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>　?。?）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

20、提高混凝土的抗裂性能。</p><p>　?。?）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。</p><p>　　（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。</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.<

21、;/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?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)。</p><p>　　4、混凝土的早期養(yǎng)護(hù)</p><p>　　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

22、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：</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)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。</p><p>　　混凝土的早期養(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

23、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。</p><p>　　適宜的溫濕度條件是相互關(guān)聯(lián)的?；炷系谋卮胧┏３Ｒ灿斜竦男Ч?。</p><p>　　從理論上分析，新澆混凝土中所含水分完全可以滿(mǎn)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

24、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)。</p><p>　　2.防水混凝土施工質(zhì)量控制</p><p>　　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。</p><p>　　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

25、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) 方法確保防水混凝土自防水，并通過(guò)實(shí)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>

26、　　由于防水混凝土特別強(qiáng)調(diào)密實(shí)性，所以對(duì)模板的制作和安裝也相應(yīng)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) 2 mm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

27、，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>　　防水混凝土結(jié)構(gòu)對(duì)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&g

28、t;<p>　　4)嚴(yán)格控制鋼筋保護(hù)層厚度。</p><p>　　防水混凝土鋼筋保護(hù)層最小厚度不小于25 mm，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。 </p><p>

29、;　　2.3混凝土的拌制和運(yùn)輸</p><p>　　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)：</p><p>　　1)確保攪拌時(shí)間，每次攪拌從投料到出料一般不少于 2 min。</p><p>　　2)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

30、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(guò)程中應(yīng)定期抽查測(cè)定混凝土坍落度，施工中如遇下 </p><p>　　雨或其他原因，砂石含水量發(fā)生變化時(shí)，應(yīng)及時(shí)測(cè)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

31、了混凝土攪拌質(zhì)量及 各類(lèi)外加劑的摻量，嚴(yán)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(yùn)輸過(guò)程中產(chǎn)生離析泌水及漏漿現(xiàn)象。</p><p>　　2．4 施工中的注意事項(xiàng) </p><p>　　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

32、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。</p><p>　　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(chē)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)。</p><p>　　3)堅(jiān)持開(kāi)門(mén)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(shí)自由高度應(yīng)不超過(guò)防水結(jié) 構(gòu)鋼筋

33、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(mén)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。</p><p>　　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

34、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。</p><p>　　5)認(rèn)真做好接漿措施，及時(shí)調(diào)整坍落度。</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(shí)應(yīng)在接縫處先澆一層 5cm～10 cm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

35、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>　　Discuss the construction temperature and crack of the concrete lightly </p><p>　　By G. K. Kululanga, W. Kuotcha, R. McCaffer, Member, ASCE, and F. Ed

36、um-Fotwe ,The American Society of Civil Engineers</p><p>　　The summary , In order to prevent the owners of the concrete work of claims, we must do a good job in the construction process in the temperature an

37、d crack control,through observation live for many years, through consulting the monograph about stress within the concrete, explain to concrete temperature reason , on-the-spot concrete control and measure , prevention o

38、f crack of temperature that crack produce. </p><p>　　Keyword Concrete Temperature stress Crack Control </p><p>　　1. The concrete occupies the important position in modern engineering con

39、struction. But today, the crack of the concrete is comparatively general, the cracks are nearly omnipresent in the science of bridge building. Though we take various kinds of measures in constructing, careful, but the cr

40、ack still occurs now and then. Tracing it to its cause, it is one of them incompletely that our change to concrete temperature stress pays attention to. In the large volume concrete, temperature stress and t</p>&

41、lt;p>　　Reason of a crack </p><p>　　Have many kinds of 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 s

42、tructure is unreasonable, the raw materials is not up to standard (if 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 hy

43、dration when the concrete is hardenned, inside temperature is rising constantly, cause the stress of drawing on</p><p>　　Can be divided into following three stages according to the forming process of the tem

44、perature stress: </p><p>　　(1)It is early: Build concrete 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 ceme

45、nt emits a large amount of heat of hydration, second , mix and congeal the changing sharply of elastic model quantity. Because of the change of elastic model quantity , form the remaining stress in the concrete in this p

46、eriod. </p><p>　　(2)Middle period: Up till the concrete is cooled until stability temperature from cement send out heat function basically when expiring, in this period, the temperature stress is mainly beca

47、use the cooling of the concrete and external temperature change cause, 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

48、 change much during this period. </p><p>　　(3)Later period: Operation period after the complete cooling of concrete. Temperature stress whether external temperature change cause mainly, these stresses and fi

49、rst two kinds of remnants stresseses are changed and added . </p><p>　　Can be divided into two kinds according to the reason why the temperature stress causes: </p><p>　　(1)Spontaneous stress: T

50、here are not any restraint or totally static structure at the border, if inside temperature is non-linear distribution, temperature stress appearing because structure restrains from each other. For example, the body of m

51、ound of the bridge, the physical dimension is relatively large, surface temperature is low when the concrete is cooled, inside temperature is high, present the stress of drawing on the surface, present the stress of pres

52、sing in the middle. </p><p>　　(2)Restrain the stress: All 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 concr

53、ete and guardrail concrete. </p><p>　　This two kinds of 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 analys

54、e the distribution , size of the temperature stress accurately according 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

55、 have sizable limp to creep concrete, at the stress accounting temperature, must consider the influence that cr</p><p>　　Control and preventing the measure of the crack of 3 temperature </p><p>

56、　　For prevent crack , lighten temperature stress can from control temperature and is it is it set about to restrain terms from two to improve. </p><p>　　The measure of controlling temperature is as follows:

57、</p><p>　　(1)Is it improve aggregate grade mix , is it do rigid concrete to spend , mix mixture to adopt, is it guide angry pharmaceutical or plastification pharmaceutical ,etc. measure in order to reduce ce

58、ment consumption of concrete to add; </p><p>　　(2)Add water 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

59、><p>　　(3)Reduce the thickness of building while building 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, ent

60、er the cold water to lower the temperature openly; </p><p>　　(5)Stipulate rational form removal time, the temperature keeps warm the surface while lowering suddenly, in case that the rapid temperature gradie

61、nt takes place in the concrete surface; </p><p>　　(6)The 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

62、 season; </p><p>　　The measure of improving condition of restraining is: </p><p>　　(1)Divide and sew and divide one rationally ; </p><p>　　(2)Prevent the foundation from rising and

63、falling too big; </p><p>　　(3)Rational arrangement construction process, prevent the too big discrepancy in elevation and side from exposing for a long time; </p><p>　　In addition, improve the p

64、erformance of the concrete and improve and resist the ability of splitting, strengthen maintenance , prevent the surface from being done and contracted , especially guarantee the quality of the concrete is very important

65、 to preventing the crack, should pay special attention to avoiding producing and running through the crack , the globality resumed its structure after appearing is very difficult, so should rely mainly on preventing the

66、emergence of the running through c</p><p>　　In construction of concrete , for raise turnover rate of template , demand concrete form removal as soon as possible that build newly often. Should consider form r

67、emoval time properly when concrete temperature is higher than the temperature, 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

68、surface, the phenomenon that " temperature is assaulted " appears. Build initial stage in concrete, because heat of h</p><p>　　Add muscle influence to large volume temperature stress of concrete ve

69、ry light , because large volume concrete include muscle to be rate very 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 surre

70、ndering, every performance of the steel is steady, and have nothing to do with stress state , time and temperature. Line bloated coefficient of steel and concrete line bloated coefficient difference very light,</p>

71、<p>　　In order to guarantee concrete project quality , prevent fracturing , improve the durability of the concrete, use the admixture to reduce one of the measures that fractures correctly. Whether is it reduce wa

72、ter is it split pharmaceutical to defend , I summarize his main function 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 capilla

73、ry after water is evaporated, make concrete is it contract out of shape to do. Increasing the thin aperture of hair can reduce the capillary surface tension , but will make the intensity of concrete reduce . This surface

74、 tension theory has already been confirmed in the world as far back as the sixties. </p><p>　　(2)Water dust than influence important factor that concrete shrink, is it reduce water is it split pharmaceutical

75、 can make concrete water consumption reduce by 25% to defend to use. </p><p>　　(3)Cement consumption important factor, concrete of person who shrink too, is it add and subtract water is it split concrete red

76、ucible 15% of the cement consumption on terms that keep the intensity of concrete of pharmaceutical to defend to mix, its volume is supplemented by increasing aggregate consumption. </p><p>　　(4)Reduce water

77、 is it split pharmaceutical can improve consistency of grout , reduce concrete secrete ink to defend, reduce and sink and draw back deforming. </p><p>　　(5)Improve glueing the strength of forming of the grou

78、t and aggregate, the concrete improved resists the performance of splitting. </p><p>　　(6)Concrete is it produce stress of drawing to restrain from while shrinking, crack when drawing the stress and is great

79、er than concrete tensile strength can produce. Reduce water is it split pharmaceutical effective concrete tensile strength of improvement very to defend , improve resisting the performance of splitting of concrete by a w

80、ide margin. </p><p>　　(7)It can make the concrete density good to add the admixture to mix , can improve resisting carbonization of concrete effectively , reduce carbonization to shrink. </p><p>

81、;　　(8)Is it reduce water is it split slow coagulation time proper concrete under pharmaceutical to defend , on the basis of preventing the fast water of cement from sending out heat effectively to mix, prevent the plasti

82、city shrink that brings because the cement is not congealed for a long time from increasing. </p><p>　　(9)Mix admixture concrete and getting easy and kind , surface easy to feel flat , form little membrane,

83、reduce the moisture to evaporate, reduce drily and shrink. A lot of admixture all have the functions of slow coagulation , increasing and apt , improvement plasticity, the experiment that we should carry on in this respe

84、ct more in the project practice is compared with and studied, than lean against not improving terms more simple,may getting simple and more direct, economy. </p><p>　　Early maintenance of 4 concrete </p&g

85、t;<p>　　Practice has proved , the common crack of concrete , 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 s

86、uddenly. So say the warm - keeping of the concrete is especially important to preventing the early crack of surface. </p><p>　　From the viewpoint of temperature stress, should reach and require to keep warm

87、followingly: </p><p>　　1)Prevent concrete internal and external temperature poor and concrete surface gradient from , prevent the surface crack. </p><p>　　2)Prevent concrete from to be ultra and

88、 cold , should is it is it make the minimum temperature is not lower than the steady temperature of concrete service time construction time in concrete to try to try one's best. </p><p>　　3)Prevent the o

89、ld concrete subcooling , in order to reduce the restraint among the old and new concrete. </p><p>　　The early maintenance of the concrete, the main purpose lies in keeping the suitable warm and humid conditi

90、on , in order to get the result of two respects, on the other hand make 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 con

91、tract and do to contract. On one hand make cement water function go on smoothly , is in the hope of reaching the intensity designed and resisting the ability of splitting. </p><p>　　The suitable warm damp co

92、ndition is interrelated. Mix warm - keeping measure paid to congeal often protect wet results too. Analyse , water concrete include moisture can meet demand , cement of water have enough and to spare newly theoretically.

93、 But because the reason of evaporating etc. often causes losses of the moisture, thus postpone or hinder water of the cement from, the surface concrete receives this kind of adverse effect easiestly and directly. Key per

94、iod when maintained in initial a fe</p><p>　　Conclusions</p><p>　　Construction temperature and relation of crack in concrete the above carry on preliminary discussion of theory and practice, tho

95、ugh the academia has different theories to origin cause of formation and computing technology of the concrete crack, but to concrete prevention and improving the measure suggestion to relatively unify , application in pr

96、actice result fine too at the same time, concrete to is it observe , compare more more by us to want in constructing, analyse more , summarize more afte</p><p>　　2.Quality control of waterproof concrete cons

97、truction</p><p>　　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 standards in o

98、rde Combined with experience, from formwork design, fabrication and installation, r to reduce engineering cost to acquire satisfied economic and social benefits.</p><p>　　The factors of influening waterproo

99、f- concrete quality are very many </p><p>　　Any links does not pay attention to the water-proof concrete of field loss hinders the water function without exception jointly with degree.Engineering constructio

100、n in the basement adopts secondary form board fabrication and installation, reinforced bar fabrication and bind, concrete stirring and mixing system and transport, concrete concrete covibration beat with a stick, constru

101、ction joint practice, concrete curing and dismantle model and beingready for backfill and so on aspects.These are ver</p><p>　　Method being under construction</p><p>　　2.1 Fabrication and Instal