混凝土攪拌機(jī)傳動(dòng)卸料系統(tǒng)設(shè)計(jì)

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新拌混凝土的性能 作者：H.-J. Wierig 新拌混凝土為水、水泥、集料和外加劑（如果有的話）的混合物。攪拌后，新拌混凝土的操作如輸送、澆注、密實(shí)和終飾也會(huì)顯著影響硬化混凝土的性能。組成材料在施工的不同時(shí)期保持在混凝土中的均勻分布及完全密實(shí)是很重要的。若這些條件不理想，成品硬化混凝土的性能如強(qiáng)度和耐久性就有不利影響。 新拌混凝土影響完全密實(shí)的特性是其稠度、流動(dòng)性和密實(shí)性。在混凝土實(shí)踐中這些一起被稱為和易性?；炷辆S持其均勻性的能力由其穩(wěn)定性控制，穩(wěn)定性又取決于稠度和粘聚性。由于對(duì)混凝土拌和物運(yùn)輸、澆注和搗固采用的方法與澆注構(gòu)件的性質(zhì)一樣隨工程不同而異，因此相應(yīng)的和易性和穩(wěn)定性要求也會(huì)改變。對(duì)特定工作新拌混凝土的適應(yīng)性的評(píng)定在某種程度上總存在人為判斷的問題。 盡管很重要，但塑性混凝土的行為通常被忽視。建議學(xué)生應(yīng)學(xué)會(huì)鑒定塑性狀態(tài)混凝土的不同特性的重要性，了解在包括澆注混凝土結(jié)構(gòu)的施工操作時(shí)如何去改變它們。 和易性 混凝土的和易性從未被準(zhǔn)確定義。實(shí)踐時(shí)一般認(rèn)為是指混凝土拌和物從攪拌機(jī)施工到其最終密實(shí)形狀的容易程度。和易性的三個(gè)主要特性是稠度、流動(dòng)性和密實(shí)性。稠度指濕潤度或流度的度量。流動(dòng)性指拌和物流進(jìn)并完全充滿模板或模具的容易程度。密實(shí)性指給定拌和物完全密實(shí)，排除所有截留空氣的容易程度。本章要求的拌和物和易性不僅取決于組成材料的特性和相應(yīng)比例，而且取決于（1）運(yùn)輸和密實(shí)采用的方法，（2）模板或模具的尺寸、形狀和表面粗糙度，（3）鋼筋的數(shù)量和間距（布筋）。 另一個(gè)普遍接受和易性的定義指產(chǎn)生完全密實(shí)所必須的有用內(nèi)功的數(shù)量。應(yīng)認(rèn)識(shí)到必需功又取決于被澆注構(gòu)件的性質(zhì)。內(nèi)功的確定存在許多困難，為此已發(fā)展了幾種方法，但沒有一種能給出和易性的絕對(duì)確定。 通常用于確定和易性的實(shí)驗(yàn)不能確定和易性的單一特性（稠度、流動(dòng)性和密實(shí)性）。然而它們的確給出了拌和物和易性的一個(gè)有用、實(shí)際的指導(dǎo)。和易性影響混凝土的質(zhì)量，并直接影響成本，如和易性不好的混凝土拌和物完全密實(shí)要求更多時(shí)間和勞力。最重要的是在對(duì)適宜的混凝土配比下任何結(jié)論之前要求對(duì)給定現(xiàn)場(chǎng)條件的和易性作出現(xiàn)實(shí)評(píng)定。 和易性的確定 三個(gè)廣泛應(yīng)用確定和易性的實(shí)驗(yàn)是坍落度、密實(shí)系數(shù)和V-B稠度計(jì)實(shí)驗(yàn)（圖13.1），是英國的標(biāo)準(zhǔn)實(shí)驗(yàn)，詳細(xì)描述在英標(biāo)1881第2部分。在實(shí)施法規(guī)110第1部分也推薦使用。重要的是注意到不同混凝土的坍落度、密實(shí)系數(shù)和V-B值間沒有單一關(guān)系。下列章節(jié)討論了這些實(shí)驗(yàn)的突出特點(diǎn)及其優(yōu)點(diǎn)和局限性。 坍落度實(shí)驗(yàn) 此實(shí)驗(yàn)由美國Chapman于1913年發(fā)展的。標(biāo)準(zhǔn)條件（英標(biāo)1881第2部分）下準(zhǔn)備的300mm高混凝土圓錐下沉，錐體下沉或高度的降低被確定為和易性的度量。儀器便宜、輕便、結(jié)實(shí)，是所有確定和易性方法中最簡(jiǎn)單的。盡管存在一些局限性，坍落度實(shí)驗(yàn)的普及是不足為奇的。 實(shí)驗(yàn)主要確定塑性混凝土的稠度，盡管很難看出坍落度與和易性有象先前定義的任何顯著聯(lián)系，但它適用于檢測(cè)和易性的改變。如，用水量增加或細(xì)集料比例不足會(huì)引起坍落度增加。實(shí)驗(yàn)適用于質(zhì)量控制目的，但應(yīng)記住一般認(rèn)為不適用于配比設(shè)計(jì)，因密實(shí)需不同工作量的混凝土可能有相似的坍落度數(shù)值。實(shí)驗(yàn)檢測(cè)不同拌和物和易性改變的靈敏性和可靠性主要取決于其對(duì)稠度的靈敏性。實(shí)驗(yàn)不適用于很干或濕的拌和物。坍落度為0或接近0的很干拌和物，和易性的一般改變不會(huì)引起坍落度有可測(cè)量的變化。對(duì)濕拌和物，混凝土的完全崩坍會(huì)產(chǎn)生不可信的坍落度值。 圖13.1儀器對(duì)工作性測(cè)量 (a) 坍落度, (b) 壓縮因子and (c) V-B .濃度測(cè)試器 通常觀察的三種坍落度為真實(shí)坍落度、剪切坍落度和崩坍坍落度，見插圖13.2。粘性富拌和物可看到真實(shí)坍落度，一般對(duì)和易性改變較敏感。剪切坍落度通常有很濕拌和物相關(guān)，一般表現(xiàn)為差質(zhì)量的混凝土，最常是由組成材料的離析引起。崩坍坍落度在貧拌和物中比富拌和物更常發(fā)生，指缺少粘性，一般與干硬性拌和物（砂漿含量少）相關(guān)。只要出現(xiàn)剪切坍落度就應(yīng)重復(fù)實(shí)驗(yàn)，若一再重復(fù)，就應(yīng)記載此實(shí)驗(yàn)現(xiàn)象和結(jié)果，因?yàn)楂@得相差大的不同坍落度值取決于坍落度是真實(shí)或是剪切形式。 標(biāo)準(zhǔn)坍落度儀器僅適用于集料最大粒徑不超過37.5mm的混凝土。應(yīng)注意坍落度值隨攪拌后時(shí)間而改變，因?yàn)檎５乃鸵恍┯坞x水的蒸發(fā)，因此在一固定時(shí)間內(nèi)完成實(shí)驗(yàn)是比較理想的。 圖13.2三種坍落度 密實(shí)系數(shù)實(shí)驗(yàn) 由英國Glanville（1947）等發(fā)展的這個(gè)實(shí)驗(yàn)確定對(duì)于標(biāo)準(zhǔn)工作量下的密實(shí)程度，因此給出了如前定義的混凝土和易性的直接而合理可信的評(píng)價(jià)。儀器是相對(duì)簡(jiǎn)單的機(jī)械裝置（圖13.1），描述在英標(biāo)1881第2部分中。實(shí)驗(yàn)要求確定部分和完全密實(shí)混凝土的重量，部分對(duì)完全密實(shí)重量的比值總小于1，即是密實(shí)系數(shù)。對(duì)于普通范圍的混凝土，密實(shí)系數(shù)為0.80~0.92。實(shí)驗(yàn)尤其適用于坍落度實(shí)驗(yàn)不理想的較干拌和物。在普通范圍的和易性之外時(shí)密實(shí)系數(shù)靈敏性減小，通常密實(shí)系數(shù)超過0.92時(shí)就是不理想的。 也應(yīng)認(rèn)識(shí)到，嚴(yán)格地說，實(shí)驗(yàn)的一些基本假設(shè)是不正確的。用于克服檢測(cè)圓柱體的表面摩擦的工作可能隨拌和物的特性而異。Cusens（1956）指出對(duì)很低和易性的混凝土，當(dāng)密實(shí)系數(shù)保持明顯不變時(shí)獲得完全密實(shí)要求的實(shí)際工作取決于拌和物的富度。因此通常認(rèn)為有相同密實(shí)系數(shù)的混凝土完全密實(shí)要求的工作量相同的觀念不總是正確的。應(yīng)注意的另一點(diǎn)是澆注混凝土到檢測(cè)圓柱體的程序與現(xiàn)場(chǎng)通常采用的方法并不相同。與坍落度實(shí)驗(yàn)一樣，密實(shí)系數(shù)的確定必須在某一特定時(shí)間內(nèi)。標(biāo)準(zhǔn)儀器適用于集料最大粒徑達(dá)37.5mm的混凝土。 V-B稠度計(jì)實(shí)驗(yàn) 實(shí)驗(yàn)由瑞典Bhrner（1940）發(fā)展（看圖13.1）。盡管一般將其作為主要用于研究的實(shí)驗(yàn)，但其潛力現(xiàn)在正在工業(yè)中被更廣泛公認(rèn)，實(shí)驗(yàn)逐漸被接受。實(shí)驗(yàn)中（英標(biāo)1881第2部分）記錄了通過振動(dòng)把一個(gè)標(biāo)準(zhǔn)混凝土圓錐變成密實(shí)的平圓柱體所用的時(shí)間，即V-B時(shí)間，用s做單位，規(guī)定精確到0.5s。與前兩個(gè)實(shí)驗(yàn)不同，此實(shí)驗(yàn)處理混凝土與實(shí)際密實(shí)混凝土方法類似。而且，此實(shí)驗(yàn)對(duì)稠度、流動(dòng)性和密實(shí)性改變敏感，因此認(rèn)為在實(shí)驗(yàn)結(jié)果與現(xiàn)場(chǎng)和易性評(píng)定之間存在合理的相關(guān)關(guān)系。 實(shí)驗(yàn)適用于大范圍拌和物，與坍落度和密實(shí)系數(shù)實(shí)驗(yàn)不同，它對(duì)很干和引氣混凝土和易性變化很敏感，對(duì)集料特性如形狀和表面紋理的變化也更敏感。實(shí)驗(yàn)結(jié)果的復(fù)驗(yàn)性好。如其它實(shí)驗(yàn)一樣，其準(zhǔn)確性趨于隨集料最大粒徑增加而降低，大于19.0mm實(shí)驗(yàn)結(jié)果有點(diǎn)不可信。對(duì)于密實(shí)要求很少振動(dòng)的混凝土V-B時(shí)間僅約3s。這樣的結(jié)果可能可信度比大V-B時(shí)間要低，因?yàn)楣烙?jì)時(shí)間終點(diǎn)（混凝土接觸塑料盤的整個(gè)下面）比較困難。在和易性范圍的另一面，如很干拌和物，記錄的V-B時(shí)間可能超過真實(shí)和易性，因?yàn)橄该鞅P下截留的氣泡要求延長振動(dòng)。為克服這個(gè)困難，可在儀器上附上一個(gè)記錄相對(duì)于時(shí)間的盤垂直下沉量的自動(dòng)裝置。這個(gè)記錄裝置也能消除判斷終點(diǎn)的人為誤差。V-B實(shí)驗(yàn)儀器比坍落度和密實(shí)系數(shù)實(shí)驗(yàn)更貴，要求有一電源，操作要更有經(jīng)驗(yàn)，所有這些使其比普通現(xiàn)場(chǎng)使用，更適于預(yù)制混凝土工業(yè)和預(yù)拌混凝土工廠。 影響和易性的因素 已知影響新拌混凝土和易性的各種因素見圖13.3。從下述討論看與組成材料相關(guān)和易性的改變主要受用水量和水泥與集料的比表面積的影響。 水泥和水 圖13.3對(duì)新拌混凝土的影響因素 不同和易性的灰水比（體積計(jì)）和水泥體積分?jǐn)?shù)的典型關(guān)系見圖15.5。對(duì)給定變化的灰水比，若改變用水量其和易性的變化比僅改變水泥用量要大些。一般水泥用量的影響對(duì)較富拌和物更大些。Hughes（1971）指出存在與組成材料的性能無關(guān)的類似線性的關(guān)系。 對(duì)給定拌和物，混凝土和易性由于比表面積增加而隨水泥細(xì)度增加而降低，這種影響在富混合物中更顯著。也應(yīng)注意更細(xì)的水泥會(huì)改善拌和物的粘聚性。除石膏外，水泥的成分對(duì)和易性沒有顯著影響。不穩(wěn)定的石膏會(huì)產(chǎn)生假凝而削弱和易性，除非對(duì)新拌混凝土延長攪拌或重新攪拌。適于配制混凝土的水質(zhì)量的變化對(duì)和易性沒有重大影響。 外加劑 有助于混凝土和易性改善的主要外加劑是減水劑和加氣劑。和易性改善的程度取決于所用外加劑的種類和用量及新拌混凝土的常規(guī)特性。 和易性外加劑當(dāng)配比保持恒定時(shí)用于增加和易性，或當(dāng)和易性保持恒定時(shí)減少用水量。前者會(huì)引起混凝土強(qiáng)度的輕微降低。 加氣劑是到目前為止最普遍應(yīng)用的和易性外加劑，因?yàn)樗鼈円哺纳扑苄曰炷恋恼尘坌院统善坊炷恋目箖鲂浴ｊP(guān)于加氣混凝土的兩點(diǎn)實(shí)踐要點(diǎn)是對(duì)于給定加氣量時(shí)圓形集料或小灰水比（體積計(jì)）混凝土的和易性增加趨于更小，并且，一般和易性增加的速度趨于隨含氣量的增加而降低。然而，原則上可假定含氣量每增加1%就會(huì)使密實(shí)系數(shù)增加0.01，使V-B時(shí)間降低10%。 集料 對(duì)于給定水泥、水和集料用量，混凝土和易性主要受集料的總表面積影響。集料表面積受最大粒徑、級(jí)配和形狀影響。比表面積增加，和易性降低，因?yàn)檫@要求有更大比例的水泥漿潤濕集料顆粒，因此潤滑所用漿體數(shù)量更少。因此，其它條件相同時(shí)當(dāng)集料最大粒徑增加，集料顆粒變圓或綜合級(jí)配更粗時(shí)和易性將增加。然而，和易性這種變化的大小取決于配比，對(duì)很富拌和物（集灰比接近2），集料的影響可忽略不計(jì)。實(shí)際意義指對(duì)給定和易性和灰水比，能用于拌和物的集料數(shù)量的變化取決于集料的形狀、最大粒徑和級(jí)配，見圖13.4和表13.1、表13.2。加氣（4.5%）對(duì)和易性的影響也見圖13.4。 Maximum aggregate size (mm) Aggregate-cement ratio (by weight) Low workability Medium workability High workability Irregular gravel Crushed rock Irregular gravel Crushed rock Irregular gravel Crushed rock 9.5 19.0 37.5 5.3 6.2 7.6 4.8 5.5 6.4 4.7 5.4 6.5 4.2 4.7 5.5 4.4 4.9 5.9 3.7 4.4 5.2 TABLE 13.1集料的形狀、最大粒徑和級(jí)配 Type of aggregate Aggregate-cement ratio Coarse grading Fine grading Rounded gravel Irregular gravel Crushed rock 7.3 5.5 4.7 6.3 5.1 4.3 TABLE 13.2集料的形狀、最大粒徑和級(jí)配 圖13.4對(duì)和易性的影響 已發(fā)展了幾種方法評(píng)價(jià)集料形狀，在第12章已討論了。棱角系數(shù)與級(jí)配模量和當(dāng)量平均粒徑一起提供了考慮集料形狀、粒徑和級(jí)配的相應(yīng)影響的方法（看第15章）。因?qū)o定材料和灰水比的完全密實(shí)混凝土的強(qiáng)度并不取決于粗集料對(duì)細(xì)集料的比值，因此對(duì)給定水泥用量采用粗集料用量配制最大和易性能獲得最大經(jīng)濟(jì)效益（Hughes，1960）（看圖13.5）。第15章記述了混凝土配比設(shè)計(jì)中最佳粗集料用量。應(yīng)注意的是集料的體積分?jǐn)?shù)而不是重量是重要的。 圖13.5一個(gè)典型的關(guān)系的工作性和粗集料混凝土 表面紋理對(duì)和易性的影響見圖13.6。能看出具有光滑紋理的集料比粗糙紋理的集料產(chǎn)生的和易性更高。當(dāng)采用干或部分干燥集料時(shí)集料的吸水性也影響和易性。在這種情況下，和易性降低，降低程度取決于集料用量和其吸水能力。 環(huán)境條件 可能導(dǎo)致和易性降低的環(huán)境因素為溫度、濕度和風(fēng)速。對(duì)給定混凝土，和易性變化受水泥水化速度和水的蒸發(fā)速度的支配。因此從攪拌開始到密實(shí)的時(shí)間間隔和裸露情況都影響和易性的降低。溫度升高加快了水用于水化的速度，也加快了它蒸發(fā)損失的速度。同樣，風(fēng)速和濕度由于影響蒸發(fā)速度而影響和易性。值得記住的是實(shí)際上這些因素取決于天氣條件，并不受控制。 圖13.6光滑紋理的集料比粗糙紋理的集料產(chǎn)生的和易性更高 時(shí)間 混凝土攪拌到最終密實(shí)經(jīng)歷的時(shí)間取決于常規(guī)施工條件，如攪拌機(jī)和澆注點(diǎn)的距離、現(xiàn)場(chǎng)程序和常規(guī)管理。相應(yīng)和易性的降低是游離水隨時(shí)間蒸發(fā)、集料吸收和水泥初始水化而損失造成的直接結(jié)果。和易性損失的速度受組成材料的某些特性的影響，如水泥的水化和放熱發(fā)展特性、集料的初始含水量和孔隙率，還有環(huán)境條件。 對(duì)于給定混凝土和一組環(huán)境條件，和易性隨時(shí)間損失的速度取決于施工條件?；炷翑嚢韬蟊３朱o止直至澆注的地方，最初一個(gè)小時(shí)內(nèi)和易性損失較明顯，和易性損失速度隨時(shí)間降低見圖13.7曲線A。相反，若持續(xù)攪拌，如預(yù)拌混凝土，和易性損失減小，尤其是最初1h左右（看圖13.7曲線B）。然而，在運(yùn)輸中延長攪拌可能由于摩擦?xí)黾庸腆w顆粒的細(xì)度，使和易性更加降低。對(duì)運(yùn)輸中持續(xù)攪拌和靜止混凝土，從攪拌開始到輸送到現(xiàn)場(chǎng)的允許（英標(biāo)1926）時(shí)間間隔分別為2h和1h。 對(duì)實(shí)際應(yīng)用，當(dāng)混凝土和易性差，不能有效密實(shí)，從而對(duì)其強(qiáng)度和其它性能產(chǎn)生不利影響時(shí)和易性損失為主要因素。經(jīng)常采用的確保混凝土在澆注時(shí)有理想和易性的矯正措施是或者增加初始用水量，或者臨在混凝土卸出前增加用水量繼續(xù)攪拌。當(dāng)這些導(dǎo)致用水量比初始確定的大，將會(huì)出現(xiàn)硬化混凝土強(qiáng)度和耐久性的降低，除非相應(yīng)地增加水泥用量。這個(gè)重要事實(shí)在現(xiàn)場(chǎng)經(jīng)常被忽略。應(yīng)回想和易性損失隨拌和物、環(huán)境條件、施工條件和交付時(shí)間而變化。英國實(shí)施法規(guī)110第1部分沒有限制交付時(shí)間，但混凝土必須能在不再加水條件下被澆注和有效密實(shí)。對(duì)預(yù)拌混凝土使用的細(xì)節(jié)，建議讀者參考Dewar（1973）的工作。 圖13.7混凝土的和易性損失時(shí)間 穩(wěn)定性 除了充分的可操作性，新拌混凝土應(yīng)具有組分使其組成材料攪拌和密實(shí)期間及密實(shí)后到混凝土變硬前的期間能在混凝土中保持均勻分布。由于組成材料顆粒尺寸和比重存在差異，因此存在使它們分離的自然趨勢(shì)。能保持要求的均勻性的混凝土被認(rèn)為是穩(wěn)定的，大多數(shù)粘性拌和物屬于這個(gè)范疇。對(duì)不穩(wěn)定拌和物，組成材料分離的程度取決于運(yùn)輸、澆注和密實(shí)的方法。不穩(wěn)定混凝土的兩種最普遍特征是離析和泌水。 離析 若拌和物中粗顆粒和細(xì)顆粒有分離的顯著趨勢(shì)時(shí)就認(rèn)為發(fā)生了離析。一般拌和物粘性越差，離析發(fā)生的趨勢(shì)越大。離析受包括水泥在內(nèi)的固體顆粒的總表面積和拌和物中砂漿數(shù)量的支配。干硬性、極端濕和干的拌和物與那些缺少砂，尤其是較細(xì)顆粒的拌和物一樣易于離析。應(yīng)盡可能避免有助于離析的條件，如運(yùn)輸中振搗混凝土、澆注時(shí)過高下落及密實(shí)中過度振動(dòng)。 表面缺陷、砂質(zhì)條痕、多孔層和蜂窩麻面是離析的直接結(jié)果。這些特征不僅難看，而且會(huì)對(duì)硬化混凝土的強(qiáng)度、耐久性和其它性能產(chǎn)生不利影響。重要的是看清離析的影響可能不會(huì)被控制試件的常規(guī)強(qiáng)度實(shí)驗(yàn)所預(yù)示，因?yàn)樵嚰沧⒑兔軐?shí)條件與實(shí)際結(jié)構(gòu)不同。沒有特定規(guī)律來猜疑可能的離析，但在經(jīng)過一些攪拌和操作混凝土的經(jīng)驗(yàn)后不難看出可能會(huì)發(fā)生離析的拌和物。如用手抓一把混凝土擠壓后松開，讓其在手掌中，粘性混凝土仍能保持其形狀。此條件下不能保持形狀的混凝土肯定易于離析，尤其是對(duì)濕拌和物。 泌水 在密實(shí)到水泥漿硬化期間，固體顆粒有一向下運(yùn)動(dòng)的自然趨勢(shì)，這取決于粒徑和比重。若拌和物稠度使其不能容住所有水，一些水就逐漸轉(zhuǎn)移并上浮到表面，一些水可能通過模板接縫滲漏出去。水從拌和物以這種方式分離即泌水。部分水到達(dá)上表面，而部分水在較大顆粒下和鋼筋條下被截留?；炷林杏行в盟康淖罱K變化引起其性能的相應(yīng)改變。例如，鋼筋和粗集料顆粒正下方的混凝土強(qiáng)度可能比平均強(qiáng)度小些，這些地方抵抗?jié)B水的能力也降低。通常，混凝土從上表面下其強(qiáng)度隨深度增加而增加。到達(dá)上表面的水產(chǎn)生了最嚴(yán)重的實(shí)際問題。若這些水不除去，則上表面及其附近的混凝土比其它地方的混凝土更脆弱，更不耐久。這尤其困擾著有大表面積的板材。另一方面，除去表面水將不適當(dāng)?shù)匮娱L了現(xiàn)場(chǎng)的終飾施工。 當(dāng)振動(dòng)密實(shí)混凝土?xí)r泌水可能性增加，但采用正確設(shè)計(jì)的拌和物及確?；炷敛贿^度振動(dòng)可使其最小。富拌和物比貧拌和物趨于少泌水。所用水泥品種也很重要，泌水發(fā)生趨勢(shì)隨水泥細(xì)度或堿和C3A含量增加而降低。加氣為控制泌水提供了另一個(gè)很有效的方法，如在離析和泌水經(jīng)常困擾的濕、貧拌和物中。 Properties of Fresh Concrete Edited by H.-J. Wierig Fresh concrete is a mixture of water, cement, aggregate and admixture (if any). After mixing, operations such as transporting, placing, compacting and finishing of fresh concrete can all considerably affect the properties of hardened concrete. It is important that the constituent materials remain uniformly distributed within the concrete mass during the various stages of its handling and that full compaction is achieved. When either of these conditions is not satisfied the properties of the resulting hardened concrete, for example, strength and durability, are adversely affected. The characteristics of fresh concrete which affect full compaction are its consistency, mobility and compactability. In concrete practice these are often collectively known as workability. The ability of concrete to maintain its uniformity is governed by its stability, which depends on its consistency and its cohesiveness. Since the methods employed for conveying, placing and consolidatingd a concrete mix, as well as the nature of the section to be cast, may vary from job to job it follows that the corresponding workability and stability requirements will also vary. The assessment of the suitability of a fresh concrete for a particular job will always to some extent remain a matter of personal judgment. In spite of its importance, the behaviour of plastic concrete often tends to be overlooked. It is recommended that students should learn to appreciate the significance of the various characteristics of concrete in its plastic state and know how these may alter during operations involved in casting a concrete structure. 13.1 Workability Workability of concrete has never been precisely defined. For practical purposes it generally implies the ease with which a concrete mix can be handled from the mixer to its finally compacted shape. The three main characteristics of the property are consistency, mobility and compactability. Consistency is a measure of wetness or fluidity. Mobility defines the ease with which a mix can flow into and completely fill the formwork or mould. Compactability is the ease with which a given mix can be fully compacted, all the trapped air being removed. In this context the required workability of a mix depends not only on the characteristics and relative proportions of the constituent materials but also on (1) the methods employed for conveyance and compaction, (2) the size, shape and surface roughness of formwork or moulds and (3) the quantity and spacing of reinforcement. Another commonly accepted definition of workability is related to the amount of useful internal work necessary to produce full compaction. It should be appreciated that the necessary work again depends on the nature of the section being cast. Measurement of internal work presents many difficulties and several methods have been developed for this purpose but none gives an absolute measure of workability. The tests commonly used for measuring workability do not measure the individual characteristics (consistency, mobility and compactability) of workability. However, they do provide useful and practical guidance on the workability of a mix. Workability affects the quality of concrete and has a direct bearing on cost so that, for example, an unworkable concrete mix requires more time and labour for full compaction. It is most important that a realistic assessment is made of the workability required for given site conditions before any decision is taken regarding suitable concrete mix proportions. 13.2 Measurement of Workability Three tests widely used for measuring workability are the slump, compacting factor and V-B consistometer tests (figure 13.1). These are standard tests in the United Kingdom and are described in detail in BS 1881: Part 2. Their use is also recommended in CP 110: Part 1. It is important to note that there is no single relationship between the slump, compacting factor and V-B results for different concretes. In the following sections the salient features of these tests together with their merits and limitations are discussed. Slump Test This test was developed by Chapman in the United States in 1913. A 300 mm high concrete cone, prepared under standard conditions (BS 1881: Part 2) is allowed to subside and the slump or reduction in height of the cone is taken to be a measure of workability. The apparatus is inexpensive, portable and robustd and is the simplest of all the methods employed for measuring workability. It is not surprising that, in spite of its several limitations, the slump test has retained its popularity. Figure 13.1 Apparatus for workability measurement: (a) slump cone, (b) compacting factor and (c) V-B consistometer The test primarily measures the consistency of plastic concrete and although it is difficult to see any significant relationship between slump and workability as defined previously, it is suitable for detecting changes in workability. For example, an increase in the water content or deficiency in the proportion of fine aggregate results in an increase in slump. Although the test is suitable for quality-control purposes it should be remembered that it is generally considered to be unsuitable for mix design since concretes requiring varying amounts of work for compaction can have similar numerical values of slump. The sensitivity and reliability of the test for detecting variation in mixes of different workabilities is largely dependent on its sensitivity to consistency. The test is not suitable for very dry or wet mixes. For very dry mixes, with zero or near-zero slump, moderate variations in workability do not result in measurable changes in slump. For wet mixes, complete collapse of the concrete produces unreliable values of slump. Figure 13.2 Three main types of slump The three types of slump usually observed are true slump, shear slump and collapse slump, as illustrated in figure 13.2. A true slump is observed with cohesive and rich mixes for which the slump is generally sensitive to variations in workability. A collapse slump is usually associated with very wet mixes and is generally indicative of poor quality concrete and most frequently results from segregation of its constituent materials. Shear slump occurs more often in leaner mixes than in rich ones and indicates a lack of cohesion which is generally associated with harsh mixes (low mortar content). whenever a shear slump is obtained the test should be repeated and, if persistent, this fact should be recorded together with test results, because widely different values of slump can be obtained depending on whether the slump is of true or shear form. The standard slump apparatus is only suitable for concretes in which the maximum aggregate size does not exceed 37.5 mm. It should be noted that the value of slump changes with time after mixing owing to normal hydration processes and evaporation of some of the free water, and it is desirable therefore that tests are performed within a fixed period of time. Compacting Factor Test This test, developed in the United Kingdom by Glanville et al. (1947), measures the degree of compaction for a standard amount of work and thus offers a direct and reasonably reliable assessment of the workability of concrete as previously defined. The apparatus is a relatively simple mechanical contrivance (figure 13.1) and is fully described in BS 1881: Part 2. The test requires measurement of the weights of the partially and fully compacted concrete and the ratio of the partially compacted weight to the fully compacted weight, which is always less than 1, is known as the compacting factor. For the normal range of concretes the compacting factor lies between 0.80 and 0.92. The test is particularly useful for drier mixes for which the slump test is not satisfactory. The sensitivity of the compacting factor is reduced outside the normal range of workability and is generally unsatisfactory for compacting factors greater than 0.92. It should also be appreciated that, strictly speaking, some of the basic assumptions of the test are not correct. The work done to overcome surface friction of the measuring cylinder probably varies with the characteristics of the mix. It has been shown by Cusens (1956) that for concretes with very low workability the actual work required to obtain full compaction depends on the richness of a mix while the compacting factor remains sensibly unaffected. Thus it follows that the generally held belief that concretes with the same compacting factor require the same amount of work for full compaction cannot always be justified. One further point to note is that the procedure for placing concrete in the measuring cylinder bears no resemblance to methods commonly employed on the site. As in the slump test, the measurement of compacting factor must be made within a certain specified period. The standard apparatus is suitable for concrete with a maximum aggregate size of up to 37.5 mm. V-B Consistometer Test This test was developed in Sweden by Bhrner (1940) (see figure 13.1). Although generally regarded as a test primarily used in research its potential is now more widely acknowledged in industry and the test is gradually being accepted. In this test (BS 1881: Part 2) the time taken to transform, by means of vibration, a standard cone of concrete to a compacted flat cylindrical mass is recorded. This is known as the V-B time, in seconds, and is stated to the nearest 0.5 s. Unlike the two previous tests, the treatment of concrete in this test is comparable to the method of compacting concrete in practice. Moreover, the test is sensitive to change in consistency, mobility and compactability, and therefore a reasonable correlation between the test results and site assessment of workability can be expected. The test is suitable for a wide range of mixes and, unlike the slump and compacting factor tests, it is sensitive to variations in workability of very dry and also air-entrained concretes. It is also more sensitive to variation in aggregate characteristics such as shape and surface texture. The reproducibility of results is good. As for other tests its accuracy tends to decrease with increasing maximum size of aggregate; above 19.0 mm the test results become somewhat unreliable. For concretes requiring very little vibration for compaction the V-B time is only about 3 s. Such results are likely to be less reliable than for larger V-B times because of the difficulty in estimating the time of the end point (concrete in contact withd the whole of the underside of the plastic disc). At the other end of the workability range, such as with very dry mixes, the recorded V-B times are likely to be in excess of their true workability since prolonged vibration is required to remove the entrapped air bubbles under the transparent disc. To overcome this difficulty an automatic device which records the vertical settlement of the disc with respect to time can be attached to the apparatus. This recording device can also assist in eliminating human error in judging the end point. The apparatus for the V-B test is more expensive than that for the slump and compacting factor tests, requiring an electric power supply and greater experience in handling; all these factors make it more suitable for the precast concrete industry and ready-mixed concrete plants than for general site use. 13.3 Factors Affecting Workability Various factors known to influence the workability of a freshly mixed concrete are shown in figure 13.3. From the following discussion it will be apparent that a change in workability associated with the constituent materials is mainly affected by water content and specific surface of cement and aggregate. Cement and Water Figure 13.3 Factors affecting workability of fresh conrete Typical relationships between the cement-water ratio (by volume) and the volume fraction of cement for different workabilities are shown in figure 15.5. The change in workability for a given change in cement-water ratio is greater when the water content is changed than when only the cement content is changed. In general the effect of the cement content is greater for richer mixes. Hughes (1971) has shown that similar linear relationships exist irrespective of the properties of the constituent materials. For a given mix, the workability of the concrete decreases as the fineness of the cement increases as a result of the increased specific surface, this effect being more marked in rich mixtures. It should also be noted that the finer cements improve the cohesiveness of a mix. With the exception of gypsum, the composition of cement has no apparent effect on workability. Unstable gypsum is responsible for false set, which can impair workability unless prolonged mixing or remixing of the fresh concrete is carried out. Variations in quality of water suitable for making concrete have no significant effect on workability. Admixtures The principal admixtures affecting improvement in the workability of concrete are water-reducing and air-entraining agents. The extent of the increase in workability is dependent on the type and amount of admixture used and the general characteristics of the fresh concrete. Workability admixtures are used to increase workability while the mix proportions are kept constant or to reduce the water content while maintaining constant workability. The former results in a slight reduction in concrete strength. Air-entraining agents are by far the most commonly used workability admixtures because they also improve both the cohesiveness of the plastic concrete and the frost resistance of the resulting hardened concrete. Two points of practical importance concerning air-entrained concrete are that for a given amount of entrained air, the increase in workability tends to be smaller for concretes containing rounded aggregates or low cement-water ratios (by volume) and, in general, the rate of increase in workability tends to decrease with increasing air content. However, as a guide it may be assumed that every 1 per cent increase in air content will increase the compacting factor by 0.01 and reduce the V-B time by 10 per cent. Aggregate For given cement, water and aggregate contents, the workability of concrete is mainly influenced by the total surface area of the aggregate. The surface area is governed by the maximum size, grading and shape of the aggregate. Workability decreases as the specific surface increases, since this requires a greater proportion of cement paste to wet the aggregate particles, thus leaving a smaller amount of paste for lubrication. It follows that, all other conditions being equal, the workability will be increased when the maximum size of aggregate increases, the aggregate particles become rounded or the overall grading becomes coarser. However, the magnitude of this change in workability depends on the mix proportions, the effect of the aggregate being negligible for very rich mixes (aggregate-cement ratios approaching 2). The practical significance of this is that for a given workability and cement-water ratio the amount of aggregate which can be used in a mix varies depending on the shape, maximum size and grading of the aggregate, as shown in figure 13.4 and tables 13.1 and 13.2. The influence of air-entrainment (4.5 per cent) on workability is shown also in figure 13.4. TABLE 13.1 Effect of maximum size of aggregate of similar grading zone on aggregate-cement ratio of concrete having water-cement ratio of 0.55 by weight, based on McIntosh (1964) Maximum aggregate size (mm) Aggregate-cement ratio (by weight) Low workability Medium workability High workability Irregular gravel Crushed rock Irregular gravel Crushed rock Irregular gravel Crushed rock 9.5 19.0 37.5 5.3 6.2 7.6 4.8 5.5 6.4 4.7 5.4 6.5 4.2 4.7 5.5 4.4 4.9 5.9 3.7 4.4 5.2 TABLE 13.2 Effect of aggregate grading (maximum size 19.0 mm) on aggregate-cement ratio of concrete having medium workability and water-cement ratio of 0.55 by weight, based on McIntosh (1964) Type of aggregate Aggregate-cement ratio Coarse grading Fine grading Rounded gravel Irregular gravel Crushed rock 7.3 5.5 4.7 6.3 5.1 4.3 Figure 13.4 Effect of aggregate shape on aggregate-cement ratio of concretes for different workabilities, based on Cornelius (1970) Several methods have been developed for evaluating the shape of aggregate, a subject discussed in chapter 12. Angularity factors together with grading modulus and equivalent mean diameter provide a means of considering the respective effects of shape, size and grading of aggregate (see chapter 15). Since the strength of a fully compacted concrete, for given materials and cement-water ratio, is not dependent on the ratio of coarse to fine aggregate, maximum economy can be obtained by using the coarse aggregate content producing the maximum workability for a given cement content (Hughes, 1960) (see figure 13.5). The use of optimum coarse aggregate content in concrete mix design is described in chapter 15. It should be noted that it is the volume fraction of an aggregate, rather than its weight, which is important. Figure 13.5 A typical relationship between workability and coarse aggregate content of concrete, based on Hughes (1960) The effect of surface texture on workability is shown in figure 13.6. It can be seen that aggregates with a smooth texture result in higher workabilities than aggregates with a rough texture. Absorption characteristics of aggregate also affect workability where dry or partially dry aggregates are used. In such a case workability drops, the extent of the reduction being dependent on the aggregate content and its absorption capacity. Ambient Conditions Environmental factors that may cause a reduction in workability are temperature, humidity and wind velocityd. For a given concrete, changes in workability are governed by the rate of hydration of the cement and the rate of evaporation of water. Therefore both the time interval from the commencement of mixing to compaction and the conditions of exposure influence the reduction in workability. An increase in the temperature speeds up the rate at which water is used for hydration as well as its loss through evaporation. Likewise wind velocity and humidity influence the workability as they affect the rate of evaporation. It is worth remembering that in practice these factors depend on weather conditions and cannot be controlled. Figure 13.6 Effect of aggregate surface texture on aggregate-cement ratio of concretes for different workabilities, based on Cornelius (1970) Time The time that elapses between mixing of concrete and its final compaction depends on the general conditions of work such as the distance between the mixer and the point of placing, site procedures and general management. The associated reduction in workability is a direct result of loss of free water with time through evaporation, aggregate absorption and initial hydration of the cement. The rate of loss of workability is affected by certain characteristics of the constituent materials, for example, hydration and heat development characteristics of the cement, initial moisture content and porosity of the aggregate, as well as the ambient conditions. For a given concrete and set of ambient conditions, the rate of loss of workability with time depends on the conditions of handling. Where concrete remains undisturbed after mixing until it is placed, the loss of workability during the first hour can be substantial, the rate of loss of workability decreasing with time as illustrated by curve A in figure 13.7. On the other hand, if it is continuously agitated, as in the case of ready-mixed concrete, the loss of workability is re