新拌混凝土的性能【中文5890字】【中英文WORD】

新拌混凝土的性能【中文5890字】【中英文WORD】,中文5890字,中英文WORD,混凝土,性能,中文,5890,中英文,WORD 【中文5890字】 新拌混凝土的性能 作者：H.-J. Wierig 新拌混凝土为水、水泥、集料和外加剂（如果有的话）的混合物。搅拌后，新拌混凝土的操作如输送、浇注、密实和终饰也会显著影响硬化混凝土的性能。组成材料在施工的不同时期保持在混凝土中的均匀分布及完全密实是很重要的。若这些条件不理想，成品硬化混凝土的性能如强度和耐久性就有不利影响。 新拌混凝土影响完全密实的特性是其稠度、流动性和密实性。在混凝土实践中这些一起被称为和易性。混凝土维持其均匀性的能力由其稳定性控制，稳定性又取决于稠度和粘聚性。由于对混凝土拌和物运输、浇注和捣固采用的方法与浇注构件的性质一样随工程不同而异，因此相应的和易性和稳定性要求也会改变。对特定工作新拌混凝土的适应性的评定在某种程度上总存在人为判断的问题。 尽管很重要，但塑性混凝土的行为通常被忽视。建议学生应学会鉴定塑性状态混凝土的不同特性的重要性，了解在包括浇注混凝土结构的施工操作时如何去改变它们。 和易性 混凝土的和易性从未被准确定义。实践时一般认为是指混凝土拌和物从搅拌机施工到其最终密实形状的容易程度。和易性的三个主要特性是稠度、流动性和密实性。稠度指湿润度或流度的度量。流动性指拌和物流进并完全充满模板或模具的容易程度。密实性指给定拌和物完全密实，排除所有截留空气的容易程度。本章要求的拌和物和易性不仅取决于组成材料的特性和相应比例，而且取决于（1）运输和密实采用的方法，（2）模板或模具的尺寸、形状和表面粗糙度，（3）钢筋的数量和间距（布筋）。 另一个普遍接受和易性的定义指产生完全密实所必须的有用内功的数量。应认识到必需功又取决于被浇注构件的性质。内功的确定存在许多困难，为此已发展了几种方法，但没有一种能给出和易性的绝对确定。 通常用于确定和易性的实验不能确定和易性的单一特性（稠度、流动性和密实性）。然而它们的确给出了拌和物和易性的一个有用、实际的指导。和易性影响混凝土的质量，并直接影响成本，如和易性不好的混凝土拌和物完全密实要求更多时间和劳力。最重要的是在对适宜的混凝土配比下任何结论之前要求对给定现场条件的和易性作出现实评定。 和易性的确定 三个广泛应用确定和易性的实验是坍落度、密实系数和V-B稠度计实验（图13.1），是英国的标准实验，详细描述在英标1881第2部分。在实施法规110第1部分也推荐使用。重要的是注意到不同混凝土的坍落度、密实系数和V-B值间没有单一关系。下列章节讨论了这些实验的突出特点及其优点和局限性。 坍落度实验 此实验由美国Chapman于1913年发展的。标准条件（英标1881第2部分）下准备的300mm高混凝土圆锥下沉，锥体下沉或高度的降低被确定为和易性的度量。仪器便宜、轻便、结实，是所有确定和易性方法中最简单的。尽管存在一些局限性，坍落度实验的普及是不足为奇的。 实验主要确定塑性混凝土的稠度，尽管很难看出坍落度与和易性有象先前定义的任何显著联系，但它适用于检测和易性的改变。如，用水量增加或细集料比例不足会引起坍落度增加。实验适用于质量控制目的，但应记住一般认为不适用于配比设计，因密实需不同工作量的混凝土可能有相似的坍落度数值。实验检测不同拌和物和易性改变的灵敏性和可靠性主要取决于其对稠度的灵敏性。实验不适用于很干或湿的拌和物。坍落度为0或接近0的很干拌和物，和易性的一般改变不会引起坍落度有可测量的变化。对湿拌和物，混凝土的完全崩坍会产生不可信的坍落度值。 图13.1仪器对工作性测量 (a) 坍落度, (b) 压缩因子and (c) V-B .浓度测试器 通常观察的三种坍落度为真实坍落度、剪切坍落度和崩坍坍落度，见插图13.2。粘性富拌和物可看到真实坍落度，一般对和易性改变较敏感。剪切坍落度通常有很湿拌和物相关，一般表现为差质量的混凝土，最常是由组成材料的离析引起。崩坍坍落度在贫拌和物中比富拌和物更常发生，指缺少粘性，一般与干硬性拌和物（砂浆含量少）相关。只要出现剪切坍落度就应重复实验，若一再重复，就应记载此实验现象和结果，因为获得相差大的不同坍落度值取决于坍落度是真实或是剪切形式。 标准坍落度仪器仅适用于集料最大粒径不超过37.5mm的混凝土。应注意坍落度值随搅拌后时间而改变，因为正常的水化和一些游离水的蒸发，因此在一固定时间内完成实验是比较理想的。 图13.2三种坍落度 密实系数实验 由英国Glanville（1947）等发展的这个实验确定对于标准工作量下的密实程度，因此给出了如前定义的混凝土和易性的直接而合理可信的评价。仪器是相对简单的机械装置（图13.1），描述在英标1881第2部分中。实验要求确定部分和完全密实混凝土的重量，部分对完全密实重量的比值总小于1，即是密实系数。对于普通范围的混凝土，密实系数为0.80~0.92。实验尤其适用于坍落度实验不理想的较干拌和物。在普通范围的和易性之外时密实系数灵敏性减小，通常密实系数超过0.92时就是不理想的。 也应认识到，严格地说，实验的一些基本假设是不正确的。用于克服检测圆柱体的表面摩擦的工作可能随拌和物的特性而异。Cusens（1956）指出对很低和易性的混凝土，当密实系数保持明显不变时获得完全密实要求的实际工作取决于拌和物的富度。因此通常认为有相同密实系数的混凝土完全密实要求的工作量相同的观念不总是正确的。应注意的另一点是浇注混凝土到检测圆柱体的程序与现场通常采用的方法并不相同。与坍落度实验一样，密实系数的确定必须在某一特定时间内。标准仪器适用于集料最大粒径达37.5mm的混凝土。 V-B稠度计实验 实验由瑞典Bhrner（1940）发展（看图13.1）。尽管一般将其作为主要用于研究的实验，但其潜力现在正在工业中被更广泛公认，实验逐渐被接受。实验中（英标1881第2部分）记录了通过振动把一个标准混凝土圆锥变成密实的平圆柱体所用的时间，即V-B时间，用s做单位，规定精确到0.5s。与前两个实验不同，此实验处理混凝土与实际密实混凝土方法类似。而且，此实验对稠度、流动性和密实性改变敏感，因此认为在实验结果与现场和易性评定之间存在合理的相关关系。 实验适用于大范围拌和物，与坍落度和密实系数实验不同，它对很干和引气混凝土和易性变化很敏感，对集料特性如形状和表面纹理的变化也更敏感。实验结果的复验性好。如其它实验一样，其准确性趋于随集料最大粒径增加而降低，大于19.0mm实验结果有点不可信。对于密实要求很少振动的混凝土V-B时间仅约3s。这样的结果可能可信度比大V-B时间要低，因为估计时间终点（混凝土接触塑料盘的整个下面）比较困难。在和易性范围的另一面，如很干拌和物，记录的V-B时间可能超过真实和易性，因为消除透明盘下截留的气泡要求延长振动。为克服这个困难，可在仪器上附上一个记录相对于时间的盘垂直下沉量的自动装置。这个记录装置也能消除判断终点的人为误差。V-B实验仪器比坍落度和密实系数实验更贵，要求有一电源，操作要更有经验，所有这些使其比普通现场使用，更适于预制混凝土工业和预拌混凝土工厂。 影响和易性的因素 已知影响新拌混凝土和易性的各种因素见图13.3。从下述讨论看与组成材料相关和易性的改变主要受用水量和水泥与集料的比表面积的影响。 水泥和水 图13.3对新拌混凝土的影响因素 不同和易性的灰水比（体积计）和水泥体积分数的典型关系见图15.5。对给定变化的灰水比，若改变用水量其和易性的变化比仅改变水泥用量要大些。一般水泥用量的影响对较富拌和物更大些。Hughes（1971）指出存在与组成材料的性能无关的类似线性的关系。 对给定拌和物，混凝土和易性由于比表面积增加而随水泥细度增加而降低，这种影响在富混合物中更显著。也应注意更细的水泥会改善拌和物的粘聚性。除石膏外，水泥的成分对和易性没有显著影响。不稳定的石膏会产生假凝而削弱和易性，除非对新拌混凝土延长搅拌或重新搅拌。适于配制混凝土的水质量的变化对和易性没有重大影响。 外加剂 有助于混凝土和易性改善的主要外加剂是减水剂和加气剂。和易性改善的程度取决于所用外加剂的种类和用量及新拌混凝土的常规特性。 和易性外加剂当配比保持恒定时用于增加和易性，或当和易性保持恒定时减少用水量。前者会引起混凝土强度的轻微降低。 加气剂是到目前为止最普遍应用的和易性外加剂，因为它们也改善塑性混凝土的粘聚性和成品混凝土的抗冻性。关于加气混凝土的两点实践要点是对于给定加气量时圆形集料或小灰水比（体积计）混凝土的和易性增加趋于更小，并且，一般和易性增加的速度趋于随含气量的增加而降低。然而，原则上可假定含气量每增加1%就会使密实系数增加0.01，使V-B时间降低10%。 集料 对于给定水泥、水和集料用量，混凝土和易性主要受集料的总表面积影响。集料表面积受最大粒径、级配和形状影响。比表面积增加，和易性降低，因为这要求有更大比例的水泥浆润湿集料颗粒，因此润滑所用浆体数量更少。因此，其它条件相同时当集料最大粒径增加，集料颗粒变圆或综合级配更粗时和易性将增加。然而，和易性这种变化的大小取决于配比，对很富拌和物（集灰比接近2），集料的影响可忽略不计。实际意义指对给定和易性和灰水比，能用于拌和物的集料数量的变化取决于集料的形状、最大粒径和级配，见图13.4和表13.1、表13.2。加气（4.5%）对和易性的影响也见图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集料的形状、最大粒径和级配 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集料的形状、最大粒径和级配 图13.4对和易性的影响 已发展了几种方法评价集料形状，在第12章已讨论了。棱角系数与级配模量和当量平均粒径一起提供了考虑集料形状、粒径和级配的相应影响的方法（看第15章）。因对给定材料和灰水比的完全密实混凝土的强度并不取决于粗集料对细集料的比值，因此对给定水泥用量采用粗集料用量配制最大和易性能获得最大经济效益（Hughes，1960）（看图13.5）。第15章记述了混凝土配比设计中最佳粗集料用量。应注意的是集料的体积分数而不是重量是重要的。 图13.5一个典型的关系的工作性和粗集料混凝土 表面纹理对和易性的影响见图13.6。能看出具有光滑纹理的集料比粗糙纹理的集料产生的和易性更高。当采用干或部分干燥集料时集料的吸水性也影响和易性。在这种情况下，和易性降低，降低程度取决于集料用量和其吸水能力。 环境条件 可能导致和易性降低的环境因素为温度、湿度和风速。对给定混凝土，和易性变化受水泥水化速度和水的蒸发速度的支配。因此从搅拌开始到密实的时间间隔和裸露情况都影响和易性的降低。温度升高加快了水用于水化的速度，也加快了它蒸发损失的速度。同样，风速和湿度由于影响蒸发速度而影响和易性。值得记住的是实际上这些因素取决于天气条件，并不受控制。 图13.6光滑纹理的集料比粗糙纹理的集料产生的和易性更高 时间 混凝土搅拌到最终密实经历的时间取决于常规施工条件，如搅拌机和浇注点的距离、现场程序和常规管理。相应和易性的降低是游离水随时间蒸发、集料吸收和水泥初始水化而损失造成的直接结果。和易性损失的速度受组成材料的某些特性的影响，如水泥的水化和放热发展特性、集料的初始含水量和孔隙率，还有环境条件。 对于给定混凝土和一组环境条件，和易性随时间损失的速度取决于施工条件。混凝土搅拌后保持静止直至浇注的地方，最初一个小时内和易性损失较明显，和易性损失速度随时间降低见图13.7曲线A。相反，若持续搅拌，如预拌混凝土，和易性损失减小，尤其是最初1h左右（看图13.7曲线B）。然而，在运输中延长搅拌可能由于摩擦会增加固体颗粒的细度，使和易性更加降低。对运输中持续搅拌和静止混凝土，从搅拌开始到输送到现场的允许（英标1926）时间间隔分别为2h和1h。 对实际应用，当混凝土和易性差，不能有效密实，从而对其强度和其它性能产生不利影响时和易性损失为主要因素。经常采用的确保混凝土在浇注时有理想和易性的矫正措施是或者增加初始用水量，或者临在混凝土卸出前增加用水量继续搅拌。当这些导致用水量比初始确定的大，将会出现硬化混凝土强度和耐久性的降低，除非相应地增加水泥用量。这个重要事实在现场经常被忽略。应回想和易性损失随拌和物、环境条件、施工条件和交付时间而变化。英国实施法规110第1部分没有限制交付时间，但混凝土必须能在不再加水条件下被浇注和有效密实。对预拌混凝土使用的细节，建议读者参考Dewar（1973）的工作。 图13.7混凝土的和易性损失时间 稳定性 除了充分的可操作性，新拌混凝土应具有组分使其组成材料搅拌和密实期间及密实后到混凝土变硬前的期间能在混凝土中保持均匀分布。由于组成材料颗粒尺寸和比重存在差异，因此存在使它们分离的自然趋势。能保持要求的均匀性的混凝土被认为是稳定的，大多数粘性拌和物属于这个范畴。对不稳定拌和物，组成材料分离的程度取决于运输、浇注和密实的方法。不稳定混凝土的两种最普遍特征是离析和泌水。 离析 若拌和物中粗颗粒和细颗粒有分离的显著趋势时就认为发生了离析。一般拌和物粘性越差，离析发生的趋势越大。离析受包括水泥在内的固体颗粒的总表面积和拌和物中砂浆数量的支配。干硬性、极端湿和干的拌和物与那些缺少砂，尤其是较细颗粒的拌和物一样易于离析。应尽可能避免有助于离析的条件，如运输中振捣混凝土、浇注时过高下落及密实中过度振动。 表面缺陷、砂质条痕、多孔层和蜂窝麻面是离析的直接结果。这些特征不仅难看，而且会对硬化混凝土的强度、耐久性和其它性能产生不利影响。重要的是看清离析的影响可能不会被控制试件的常规强度实验所预示，因为试件浇注和密实条件与实际结构不同。没有特定规律来猜疑可能的离析，但在经过一些搅拌和操作混凝土的经验后不难看出可能会发生离析的拌和物。如用手抓一把混凝土挤压后松开，让其在手掌中，粘性混凝土仍能保持其形状。此条件下不能保持形状的混凝土肯定易于离析，尤其是对湿拌和物。 泌水 在密实到水泥浆硬化期间，固体颗粒有一向下运动的自然趋势，这取决于粒径和比重。若拌和物稠度使其不能容住所有水，一些水就逐渐转移并上浮到表面，一些水可能通过模板接缝渗漏出去。水从拌和物以这种方式分离即泌水。部分水到达上表面，而部分水在较大颗粒下和钢筋条下被截留。混凝土中有效用水量的最终变化引起其性能的相应改变。例如，钢筋和粗集料颗粒正下方的混凝土强度可能比平均强度小些，这些地方抵抗渗水的能力也降低。通常，混凝土从上表面下其强度随深度增加而增加。到达上表面的水产生了最严重的实际问题。若这些水不除去，则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。这尤其困扰着有大表面积的板材。另一方面，除去表面水将不适当地延长了现场的终饰施工。 当振动密实混凝土时泌水可能性增加，但采用正确设计的拌和物及确保混凝土不过度振动可使其最小。富拌和物比贫拌和物趋于少泌水。所用水泥品种也很重要，泌水发生趋势随水泥细度或碱和C3A含量增加而降低。加气为控制泌水提供了另一个很有效的方法，如在离析和泌水经常困扰的湿、贫拌和物中。 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