混凝土的澆筑與養(yǎng)護(hù)—外文資料翻譯

1、<p><b>　　外文資料翻譯譯文</b></p><p><b>　　混凝土的澆筑與養(yǎng)護(hù)</b></p><p><b>　　摘 要</b></p><p>　　根據(jù)一般承認(rèn)的慣例看,巴基斯坦的混凝土結(jié)構(gòu)建筑物在結(jié)構(gòu)上的質(zhì)量，效用和安全需要上都留下了很多值得關(guān)注的問題。 當(dāng)2005年1

2、0月8日發(fā)生在巴基斯坦北部的一個(gè)7.6級(jí)地震的時(shí)候，這些施工技術(shù)上的缺點(diǎn)袒露無遺了，破壞了數(shù)以千計(jì)建筑物、橋梁，而且造成估計(jì)79000人的死亡。在巴基斯坦這種十分低質(zhì)量的混凝土施工技術(shù)是造成這次極廣破壞的主要原因。</p><p>　　關(guān)鍵字：混凝土；澆筑；養(yǎng)護(hù) </p><p><b>　　1混凝土澆筑</b></p><p>　　若混凝土在

3、地面上澆筑，地面應(yīng)該具有足夠的含水量以避免它吸收混凝土中的水分。如果新混凝土將被澆筑在已經(jīng)凝固的混凝土上或其旁邊，已澆混凝土表面應(yīng)該徹底清除干凈，最好使用高壓空氣、高壓水流或鋼絲刷。其表應(yīng)潮濕，但不應(yīng)有集水，應(yīng)當(dāng)在整個(gè)范圍內(nèi)刷上少量的水泥漿，然后立即鋪上一層厚度為1/2in的砂漿。新混凝土應(yīng)鋪筑在砂漿上或其旁邊。</p><p>　　為減少混凝土在澆筑后由于運(yùn)輸導(dǎo)致的離析，混凝土的制備應(yīng)該盡可能靠近最后澆筑的地方

4、。它應(yīng)該分層澆筑以使每層可被均壓實(shí)。每層澆筑間隔的時(shí)間應(yīng)該限制在能夠保證新澆筑的混凝土與前面澆筑的有良好的結(jié)合。</p><p>　　當(dāng)向較深的模板中澆筑混凝土?xí)r，就應(yīng)該使用一根導(dǎo)管來限制混凝土的自由降落高度不超過3或4ft，以防止混凝土離析。這種導(dǎo)管是由輕金屬制成的管子，具有可調(diào)節(jié)的長度而且被連接到儲(chǔ)存的混凝土的罐的底部。當(dāng)模板被澆筑滿后，就可以抽出管節(jié)。</p><p>　　混凝土一澆

5、筑完成，應(yīng)立即用手提振搗器或機(jī)器振搗器進(jìn)行搗密后就應(yīng)該移走，否則就會(huì)發(fā)生骨料離析。一般來說，振搗器不應(yīng)該集中在先前提起的混凝土上。</p><p>　　振搗主要優(yōu)點(diǎn)是它允許使用較干的混凝土，由于減少了混凝土的含水量，這種混凝土有較高的強(qiáng)度。振搗混凝土的優(yōu)點(diǎn)如下：</p><p>　　1、水分的減少允許水泥和細(xì)集料的用量，因?yàn)檫@時(shí)需要較少的水泥漿。</p><p>　

6、　2、較低的含水量可減少收縮和空隙。</p><p>　　3、較干的混凝土減少混凝土表面處理的費(fèi)用。</p><p>　　4、機(jī)械振搗可代替3—8個(gè)手持振搗棒。</p><p>　　5、較低的含水量可提高混凝土的強(qiáng)度。</p><p>　　6、較干的混合料允許較早地拆除一些模板，這樣可降低模板的造價(jià)。</p><p>

7、<b>　　2混凝土的養(yǎng)護(hù)</b></p><p>　　若混凝土想要獲得最高強(qiáng)度或其它必要特性，則應(yīng)該對(duì)其用足夠的水分和適當(dāng)?shù)臏囟冗M(jìn)行養(yǎng)護(hù)。如果沒能提供這些條件，則會(huì)導(dǎo)致劣質(zhì)的混凝土。</p><p>　　混凝土中最初的水分足夠所有水泥的水化反應(yīng)，只要在使用前沒有被蒸發(fā)。這可以用多種方法來實(shí)現(xiàn)，例如把模板留在混凝土中，保持表面潮濕，或在混凝土表面覆蓋一層液體養(yǎng)生化合物

8、，它可以形成一種不透水的隔膜來阻止混凝土中的水分遺失。養(yǎng)生化合物可以用刷子或壓力噴嘴加到混凝土表面。一加侖可以覆蓋200~300ft2。</p><p>　　混凝土應(yīng)該在40°F以上或80°F以下進(jìn)行澆筑。低溫會(huì)降低凝結(jié)速度，而高溫會(huì)降低混凝土的極限強(qiáng)度。</p><p>　　3寒冷天氣下的混凝土澆筑</p><p>　　當(dāng)混凝土在寒冷天氣下澆筑

9、時(shí)，通常有必要預(yù)先加熱水、骨料或兩者同時(shí)預(yù)熱，以便使其初始溫度能夠保證混凝土的初凝并獲得強(qiáng)度。對(duì)水進(jìn)行預(yù)熱是提供必要的溫度最有效的方法。出于這一目的，蓄水池應(yīng)該安裝一個(gè)通有蒸氣的蛇形管或?qū)⒄魵饪梢灾苯油ǖ剿?，這是可以使用多個(gè)排氣口以提供良好的熱量分布。</p><p>　　當(dāng)混合料的溫度已知，一些專用圖可以用來計(jì)算混凝土的溫度。一條直線穿過所有三種比例，通過任意兩個(gè)已知溫度，就可以確定出第三個(gè)溫度。如果砂的表面

10、干燥，則應(yīng)使用實(shí)線計(jì)算混凝土的溫度。然而，如果砂含有3%的水分，則應(yīng)使用虛線。</p><p>　　規(guī)范通常要求新澆筑的混凝土在澆筑后應(yīng)保持3天溫度不低于70°F或5天不低于50°F。當(dāng)預(yù)計(jì)有寒冷天氣時(shí)必須提供一些適當(dāng)?shù)姆椒▉肀３忠蟮臏囟取?lt;/p><p>　　4用于混凝土中的鋼筋</p><p>　　與混凝土相比，鋼是一種高強(qiáng)度材料。普通鋼筋

11、在抗拉和抗壓時(shí)可以利用的強(qiáng)度，即屈服強(qiáng)度，約為普通的結(jié)構(gòu)混凝土抗壓強(qiáng)度的1.5倍，而且超過抗拉強(qiáng)度的100倍。另一方面，與混凝土相比，鋼材的成本要高得多。所以，兩種材料最好的結(jié)合使用是混凝土用于抵抗壓應(yīng)力。因此，在鋼筋混凝土梁內(nèi)，混凝土抵抗壓應(yīng)力，縱向鋼筋配置在靠近受拉面處以抵抗拉應(yīng)力，通常還附加配有一些鋼筋，抵抗梁內(nèi)的剪應(yīng)力所引起的斜向拉應(yīng)力。然而，鋼材也可以用于抵抗應(yīng)力，主要是為了減小受壓構(gòu)件的截面尺寸，例如用于多層建筑的下部樓層柱

12、。即使不存在這種必要性，所有受壓構(gòu)件也要配置最少數(shù)量的鋼筋，以保證這些構(gòu)件在偶然出現(xiàn)的小彎矩作用下的安全性，在這情況下，不加鋼筋的混凝土構(gòu)件可能會(huì)開裂，甚至破壞。</p><p>　　使配筋最有效的發(fā)揮作用的基本條件是鋼筋和混凝土的變形要一致，即這兩種材料間要有足夠強(qiáng)的黏結(jié)力，以確保鋼筋和其周圍混凝土間不發(fā)生相對(duì)移動(dòng)。這種黏結(jié)力是由鋼筋-混凝土結(jié)合面上較強(qiáng)的化學(xué)粘合作用，熱軋鋼筋表面層的固有粗糙度，以及間距較小的

13、肋形表面變形等所構(gòu)成的。鋼筋的邊面并行為兩種材料間提供了很高的咬合作用。</p><p>　　鋼材以兩種不同方式應(yīng)用于混凝土中：普通鋼筋和預(yù)應(yīng)力鋼筋。普通鋼筋在澆筑混凝土之前先置于模板內(nèi)。鋼筋中的應(yīng)力，與硬化混凝土中的應(yīng)力一樣，除了由收縮或類似原因造成的附加應(yīng)力外，僅僅是由結(jié)構(gòu)上作用的荷載引起的，比較起來，在預(yù)應(yīng)力混凝土結(jié)構(gòu)中，在鋼筋與混凝土共同工作承受外部荷載之前，對(duì)鋼筋已施加了很大的拉力。</p>

14、<p>　　最常見的鋼筋的形式為圓棒狀?，F(xiàn)在可以使用的鋼筋的直徑范圍很大，在一般的應(yīng)用中從10到35毫米，兩種大型鋼筋的尺寸為44和57毫米。對(duì)這些鋼筋表面進(jìn)行了變形處理，其目的是增加鋼筋與混凝土之間的抗滑能力。對(duì)這些變形的最低要求已經(jīng)通過實(shí)驗(yàn)研究予以確定。不同的鋼筋制造廠家采用不同的變形花紋，他們?nèi)慷寄軌驖M足這些要求。</p><p>　　為了對(duì)鋼筋進(jìn)行拼接，或者便于制作置于模板內(nèi)的鋼筋骨架所進(jìn)

15、行的焊接，可能會(huì)引起金相的變化而降低材料的強(qiáng)度和延性，因此，必須對(duì)所有鋼材的類型和焊接規(guī)程加以特殊的限制。ASTM中的A706的條款是專門使用于焊接的。</p><p>　　長期以來，在鋼筋混凝土領(lǐng)域明顯的趨向于高強(qiáng)度材料，包括鋼筋和混凝土。屈服強(qiáng)度為40KSI的鋼筋，在20年前幾乎是標(biāo)準(zhǔn)的盲目前大部分已由屈服強(qiáng)度為60鋼筋所取代。因?yàn)楹笳吒鼮榻?jīng)濟(jì)，而且使用他們可以減少模板內(nèi)鋼筋的擁擠狀況。</p>

16、<p>　　ACI規(guī)范允許使用強(qiáng)度FY=80KSI的鋼筋。這類高強(qiáng)度鋼筋通常是逐漸屈服的沒有屈服平臺(tái)。在這種情況下，ACI規(guī)范要求在規(guī)定的最小屈服強(qiáng)度時(shí)的總應(yīng)變不應(yīng)超過0.0035。這是將現(xiàn)行的設(shè)計(jì)方法應(yīng)用于這類高強(qiáng)度鋼筋時(shí)必須遵守的?，F(xiàn)行的設(shè)計(jì)方法是按鋼材突然屈服，而且有屈服平臺(tái)的情況而制定的。ASTM規(guī)范中沒有關(guān)于屈服強(qiáng)度高于60KSI的變形鋼筋的條款，但是在實(shí)際中可能使用這種鋼筋，根據(jù)ACI規(guī)范，他們可以在根據(jù)上述的要

17、求的情況下使用。在特殊情況下，例如高層建筑的下部樓層的柱子，使用這一高強(qiáng)度范圍內(nèi)的鋼筋就非常適合。</p><p>　　在惡劣的環(huán)境田間下，例如受除冰化學(xué)劑侵蝕的蕎麥內(nèi)，要求使用鍍鋅或環(huán)氧樹脂涂層的鋼筋，以便使鋼筋的腐蝕和隨之發(fā)生的混凝土的剝落減至最小。</p><p><b>　　5混凝土結(jié)構(gòu)的修復(fù)</b></p><p>　　鋼筋混凝土是非

18、常耐用的結(jié)構(gòu)材料，它需要的修復(fù)工作很少。然而，它的耐用性受很多因素的影響。這包括那些設(shè)計(jì)和施工的失誤，使用劣質(zhì)材料和在侵蝕性環(huán)境下暴露。修復(fù)的必要性主要取決于對(duì)破壞程度的診斷。如果某項(xiàng)工程不只需要作裝飾性處理，好的工作技巧很重要。</p><p>　　5.1修復(fù)系統(tǒng)的功能要求</p><p>　　通過對(duì)事故的仔細(xì)診斷并找到損壞的原因之后，下一步是考慮修復(fù)方法的要求，這樣會(huì)我到一個(gè)解決問題的

19、有效方法。</p><p><b>　　5.1.1耐久性</b></p><p>　　選擇有相當(dāng)耐久性的修復(fù)材料非常重要。修復(fù)工作所用的材料最起碼也要像基層混凝土那樣耐用。</p><p>　　5.1.2鋼筋的保護(hù)</p><p>　　對(duì)加勁鋼筋的保護(hù)機(jī)理取決于所使用的修復(fù)材料的類型。例如，粘性材料能促使混凝土堿性增加，

20、其抑制作用能保護(hù)鋼筋免受更深的侵蝕，而環(huán)氧樹脂砂漿可防止氧氣、水分和其它有害物的侵人。</p><p><b>　　5.1.3基片粘結(jié)</b></p><p>　　使用基片粘結(jié)可進(jìn)行完整的修復(fù)，它可防止水分和大氣從界面進(jìn)人。大多數(shù)的修復(fù)材料，通過使用合適的粘合手段，其粘合性能已大大地改善了，如環(huán)氧樹脂系統(tǒng)使用未填充的環(huán)氧底漆，普通水泥修復(fù)系統(tǒng)使用普通水泥稀漿加上任何一

21、種乳膠添加劑。所要采用的預(yù)防措施是：對(duì)即將被粘結(jié)的表面，必須先清理其上面的疏松和易碎的物質(zhì)。</p><p>　　5.1.4尺寸穩(wěn)定性</p><p>　　在養(yǎng)護(hù)期間，材料的收縮要保持到最小，為了防止修復(fù)失敗、材料的尺寸變化應(yīng)與基層材料相當(dāng)接近。</p><p>　　5.1.5對(duì)環(huán)境引起的損傷的早期防護(hù)</p><p>　　一些開始就暴露在外

22、的情況會(huì)引起修復(fù)的過早損壞。例如，熱天會(huì)阻礙水泥的水合作用從而使局部修復(fù)的普通水泥發(fā)生變質(zhì)，為了防止發(fā)生這種情況，養(yǎng)護(hù)期間應(yīng)對(duì)其作特別的保護(hù)。</p><p><b>　　5.1.6操作方便</b></p><p>　　材料應(yīng)當(dāng)易混合和方便使用，這樣可以隨時(shí)填到裂縫和孔隙里去，材料最好是不粘工具，用泥刀修平時(shí)不剝落，填筑后不坍塌。</p><p&g

23、t;<b>　　5.1.7外觀</b></p><p>　　修復(fù)材料與現(xiàn)有混凝土的匹配程度取決于所修補(bǔ)的結(jié)構(gòu)和業(yè)主的要求。當(dāng)外觀很重要或鋼筋的保護(hù)層很薄時(shí)，要求采用鑲面處理。</p><p>　　5.2修復(fù)方法的選擇</p><p>　　適當(dāng)?shù)男迯?fù)就是消除與結(jié)構(gòu)使用有關(guān)的所有缺陷。認(rèn)真考慮選擇正確的方法和某一特定操作所用的材料，弄清到底是要滿足

24、強(qiáng)度、耐用性的特定要求還是滿足其它的長期或短期的性能要求。這些因素包括：</p><p><b>　　1.損傷的性質(zhì)</b></p><p>　　如果活動(dòng)裂縫里填的是剛性材料，那么不但填補(bǔ)材料會(huì)斷裂，而且舊裂縫周圍也會(huì)產(chǎn)生新的裂縫，修復(fù)活動(dòng)裂縫的方法是要么采用柔性材料以適應(yīng)結(jié)構(gòu)位移，要么在修復(fù)之前采取一定措施消除結(jié)構(gòu)位移。</p><p>&l

25、t;b>　　2.斷裂的部位</b></p><p>　　依靠材料重力填充裂縫技術(shù)在水平面上裂縫修補(bǔ)中的成功率較高，但在豎直面上裂縫修補(bǔ)中效果很差。</p><p><b>　　3.環(huán)境</b></p><p>　　如果裂縫里有水氣、水和雜物時(shí)，要先補(bǔ)好漏洞。如果結(jié)構(gòu)還處于工作狀態(tài)并且環(huán)境潮濕時(shí)，會(huì)使得堵住漏洞這一工作更加困難。

26、</p><p><b>　　4.工藝</b></p><p>　　工人所采用的修復(fù)工藝是另一個(gè)與修復(fù)有關(guān)的因素，有時(shí)它意味著水久性修復(fù)或者修復(fù)材料過早損壞。</p><p><b>　　5.成本</b></p><p>　　與為修復(fù)工作提供通路、做準(zhǔn)備工作和實(shí)際勞力的成本相比，所用的修復(fù)材料的成本

27、是相當(dāng)小的。</p><p><b>　　6.外觀</b></p><p>　　修復(fù)部位的表面可能很難看，特別是當(dāng)它處于建筑物顯眼的地方時(shí)。在這種情況下，修復(fù)工作系統(tǒng)就要包括對(duì)整個(gè)表面進(jìn)行某種處理。</p><p><b>　　外文原文</b></p><p>　　Concrete Placing

28、Curing</p><p><b>　　Abstract</b></p><p>　　Viewed in terms of accepted practices, concrete construction operations leave much to be desired with respect to the quality, serviceability,

29、 and safety of completed structures. The shortcomings of these operations became abundantly clear when a magnitude 7.6 earthquake struck northern Paki-stan on October 8, 2005, destroying thousands of buildings, damaging

30、bridges, and killing an esti-mated 79,000 people. The unusually low quality of construction operations prevalent was a major cause of the i</p><p>　　Keywords: Concrete; Placing; Curing</p><p>　　

31、1Placing Concrete</p><p>　　If concrete is placed in the surface, the surface should be filled with water sufficiently to prevent it from absorbing the concrete of its water. If fresh concrete is to be placed

32、 on or nearby to concrete that has solidified, the surface of the placed concrete should be cleaned absolutely, preferably with a high-pressure air or water jetor steel-wire brushes. The surface should be wet, but there

33、should be no much water. A little quantity of cement grout should be brushed over the whole area, an</p><p>　　In order to decrease the disintegration resulting from carriage after it is placed. The concrete

34、should be placed as nearly as probably in its final point. It should be placed in layers to permit uniform compaction. The time interval between the placing of layers should be limited to assure perfect bond between the

35、fresh and previously placed concrete.</p><p>　　In placing concrete in deeper patters, a vessel should be used limit the free fall to not over 3 or 4 ft, in order to prevent concrete disintegration. The vesse

36、l is a pipe made of lightweight metal, having adjustable lengths and attached to the bottom of a hopper into which the concrete is deposited. As the patters are filled, sections of the pipe may be removed.</p><

37、;p>　　Immediately after the concrete is placed, it should be compacted by hand pudding or a mechanical vibrator to eliminate voids. The vibrator should be left in one position only long enough to reduce the concrete ar

38、ound it to a plastic mass; then the vibrator should be moved, or disintegration of the aggregate will occur. In general, the vibrator should not be permitted to penetrate concrete in the prior lift.</p><p>　

39、　The mainly advantage of vibrating is that it permits the use of a drier concrete, which has a higher strength because of the reduced water content. Among the advantages of vibrating concrete are the following:</p>

40、<p>　　1. The decreased water permits a reduction in the cement and fine aggregate because less cement paste is needed.</p><p>　　2. The lower water content decreases shrinkage and voids.</p><

41、;p>　　3. The drier concrete decreases the cost of finishing the surface.</p><p>　　4. Mechanical vibration may replace three to eight hand puddles.</p><p>　　5. The lower water content increases

42、 the strength of the concrete.</p><p>　　6. The drier mixture permits the removal of some patters more quickly, which may reduce the cost of patters.</p><p>　　2Curing Concrete</p><p>

43、;　　If concrete is to gain its maximum strength and other desirable properties, it should be cured with adequate moisture and at a favorable temperature. Failure to provide these conditions may result in an inferior concr

44、ete.</p><p>　　The initial moisture in concrete is adequate to hydrate all the cement, provided it is not should replace the moisture that does evaporate. This may be accomplished by many methods, such as lea

45、ving the patters in place, keeping the surface wet, or covering the surface with a liquid. curing compound, which comes being to a water-tight membrane that prevents the escape of the initial water. Curing compounds may

46、be applied by brushes or pressure sprayers. A gallon will cover 200 to 300 sq ft.</p><p>　　Concrete should be placed at a temperature not less than 40 or more than 80° F.A lower temperature will decreas

47、e the rate of setting, while a higher temperature will decrease the ultimate strength.</p><p>　　3Placing Concrete in Cold Weather</p><p>　　When the concrete is placed during cold weather, it is

48、usually necessary to preheat the water, the aggregate, or both in order that the initial temperature will assure an initial set and gain in strength. Preheating the water is the most effective method of providing the nec

49、essary temperature. For this purpose a water reservoir should be equipped with pipe coils through which steam can be passed, or steam may be discharged directly into the water, several outlets being used to given better

50、distr</p><p>　　When the temperatures of the mixtures are known, some specific charts may be used to calculate the temperature of concrete. A straight line pass all three scales, passing through every two kno

51、wn temperatures, will assure the determination of the third temperature. If the surface of sand is dry, the fact lines of the scales giving the temperature of concrete should be used. However, if the sand contains about

52、3 percent moisture, the dotted lines should be used.</p><p>　　Specifications usually demand that freshly placed concrete shall be kept at a temperature of not less than 70°F for 3 days or 50°F for

53、5 days after it is placed. Some proper method must be provided to keep the demanded temperature when the cold weather is estimated.</p><p>　　4Reinforcing steels for concrete</p><p>　　Compared wi

54、th concrete, steel is a high strength material. The useful strength of ordinary reinforcing steels in tension as well as compression, the yield strength, is about 15 times the compressive strength of common structural co

55、ncrete, and well over 100 times its tensile strength. On the other hand, steel is a high-cost material compared with concrete. It follow that the two materials are the best used in combination if the.</p><p>

56、;　　Concrete is made to resist the compressive stresses and the compressive force, longitudinal steel reinforcing bars are located close to the tension face to resist the tension force. and usually additional steel bars a

57、re so disposed that they resist the inclined tension stresses that are caused by the shear force in the beams. However, reinforcement is also used for resisting compressive forces primarily where it is desired to reduce

58、the cross-sectional dimensions of compression members, as in th</p><p>　　For most effective reinforcing action, it is essential that steel and concrete deform together, that there be a sufficiently strong bo

59、nd between the two materials to ensure that no relative movements of the steel bars and the surrounding concrete occur. This bond is provided by the relatively large chemical adhesion which develops at the steel-concrete

60、 interface, by the natural roughness of the mill scale of hot-rolled reinforcing bars, and by the closely spaced rib-shaped surface deformations w</p><p>　　Steel is used in two different ways in concrete str

61、uctures: as reinforcing steel and as prestressing steel .reinforcing steel is placed in the forms prior to casting of the concrete. Stresses in the steel, as in the hardened concrete, are caused only by the loads on the

62、structure, except for possible parasitic stresses from shrinkage or similar causes. In contrast, in priestesses concrete structures large tension forces are applied to the reinforcement prior to letting it act jointly wi

63、th the co</p><p>　　The most common type of reinforcing steel is in the form of round bars, sometimes called rebars, available in a large range of diameters, from 10 to 35 mm for ordinary applications and in

64、two heavy bar sizes off 44 and 57 mm these bars are furnished with surface deformations for the purpose of increasing resistance to slip between steel and concrete minimum requirements for these deformations have been de

65、veloped in experimental research. Different bar producers use different patterns, all of whi</p><p>　　Welding of rebars in making splices, or for convenience in fabricating reinforcing cages for placement in

66、 the forms, may result in metallurgical changes that reduce both strength and ductility, and special restrictions must be placed both strength and ductility, and special restrictions must be placed both on the type of st

67、eel used and the welding procedures the provisions of ASTM A706 relate specifically to welding.</p><p>　　In reinforced concrete a long-time trend is evident toward the use of higher strength materials, both

68、 steel and concrete. Reinforcing bars with 40ksi yield stress, almost standard 20 years ago, have largely been replaced by bars with 60ksi yield stress, both because they are more economical and because their use tends t

69、o reduce congestion of steel in the forms.</p><p>　　The ACI Code permits reinforcing steels up to FY=80KIS. Such high strength steels usually yield gradually but have no yield plateau in this situation the A

70、CI Code requires that at the specified minimum yield strength the total strain shall not exceed 0.0035 this is necessary to make current design methods, which were developed for sharp-yielding steels with a yield plateau

71、, applicable to such higher strength steels. there is no STM specification for deformed bars may be used , according to the A</p><p>　　In order to minimize corrosion of reinforcement and consequent spelling

72、of concrete under sever exposure conditions such as in bridge decks subjected to deicing chemicals, galvanized or epoxy-coated rebars may be specified.</p><p>　　5Repair of Concrete Structures</p><

73、p>　　Reinforced concrete is generally a very durable structural material and very little repair work is usually needed. However, its durability can be affected by a variety of causes, including those of design and cons

74、truction faults, use of inferior materials and exposure to aggressive environment. The need for a repair is primarily dictated by the severity of the deterioration as determined from the diagnosis. Good workmanship is es

75、sential if anything more than just a cosmetic treatment to the crea</p><p>　　5.1Performance requirements of repair system</p><p>　　Having established the causes of the defect by carefully diagno

76、sing the distress, the next step should be to consider the requirements of the repair method that will offer an effective solution to the problem (see fig.).</p><p>　　5.1.1Durability</p><p>　　It

77、 is important to select repair materials that provide adequate durability. Materials used for the repair job should be at least as durable as the substrate concrete to which it is applied.</p><p>　　5.1.2Prot

78、ection of steel</p><p>　　The mechanism of protection provided to the reinforcing depends on the type of repair materials used. For example, cementations materials can protect the steel from further corrosion

79、 by their inhibitive effect of increasing the alkalinity of the concrete, whereas epoxy resin mortars can give protection against the ingress of oxygen, moisture and other harmful agents. </p><p>　　5.1.3Bond

80、 with substrate</p><p>　　The bond with the substrate must produce an integral repair to prevent entry of moisture and atmospheric gases at the interface. With most repair materials, the bond is greatly enhan

81、ced with the use of a suitable bonding aid such as an unfilled epoxy resin systems and slurry of Portland cement, plus any latex additives for a Portland cement-based repair system. Precautions should also be taken to re

82、move all loose and friable materials from the surfaces to be bonded.</p><p>　　5.1.4Dimensional Stability</p><p>　　Shrinkage of materials during curing should be kept to a minimum. Subsequent dim

83、ensional change should be very close in the substrate inorder to prevent failure.</p><p>　　5.1.5Initial Resistance to Environmentally Induced Damage</p><p>　　Some initial exposure conditions may

84、 lead to premature damage lo repairs. For example, partially cured Portland cement repairs can deteriorate from hot weather preventing full hydration of the cement. To prevent this from happening extra protection during

85、curing time may be necessary.</p><p>　　5.1.6Ease of Application</p><p>　　Materials should be easily mixed and applied so that they can be worked readily into small crevices and voids. Ideally, t

86、he material should not stick to tools, and should not shear while being trowel led nor slump after placement.</p><p>　　5.1.7Appearance</p><p>　　The degree to which the repair material should mat

87、ch the existing concrete will depend on the use of the structure and the client' s requirements. A surface coating may be required when appearance is important or when cover to reinforcement is small.</p><

88、p>　　5.2Selection of Repair Methods</p><p>　　A suitable repair counteracts all the deficiencies which are relevant to the use of the structure. The selection of tile correct method and material for a parti

89、cular, application requires careful consideration, whether to meet special requirements for placing strength, durability or other short-or long-term properties. These considerations include:</p><p>　　1. Natu

90、re of the Distress</p><p>　　If alive crack is filled with a rigid material, then either the repair material will eventually fail or some new cracking will occur adjacent to the original crack. Repairs to liv

91、e cracks must either use flexible materials to accommodate movements or else steps must be taken prior to the repair to eliminate the movement.</p><p>　　2. Position of the Crack</p><p>　　Techniq

92、ues which rely on gravity to introduce the material into the crack are more successfully carried out on horizontal surfaces but are rarely effective on vertical ones.</p><p>　　3. Environment</p><p

93、>　　If moisture, water or contaminants are found in the crack, then it is necessary to rectify the leaks Repair to slop leaks may be further complicated by the need to make the repairs while the structure is in service

94、 and the environment is damp.</p><p>　　4. Workmanship</p><p>　　The skill the operatives available to carry put the repairs is another relevant factors. Sometimes this can mean the difference bet

95、ween a permanent repair and premature failure of the repair material.</p><p><b>　　5. Cost</b></p><p>　　The cost of repair materials is usually small compared with the costs of provid

96、ing access, preparation and actual labor.</p><p>　　6. Appearance</p><p>　　The repair surface may be unsightly, particularly when it appears on a prominent part of the building. In this case, the