15 Concrete Mix Design and Quality Control.doc

15Concrete Mix Design and Quality Control混凝土配比设计和质量控制 Concrete mix design can be defined as the procedure by which, for any given set of conditions, the proportions of the constituent materials are chosen so as to produce a concrete with all the required properties for the minimum cost. In this context the cost of any concrete includes, in addition to that of the materials themselves, the cost of the mix design, of batchingbatching： bti n.配料, 定量, 计量, mixing and placing the concrete and of site supervisionsite supervision：现场质量管理；supervision： ,sju:pvin n.监督,管理. Quality control refers in the first instancein the first instance：首先, 起初 to the supervision exercised on site to ensure both that the materials used in the production of the concrete are of the required quality and that the stated mix proportions are adhered toadhere dhi to：附着于,坚持,遵守 as closely as is possible with the available site facilitiesfacility： fsiliti n.设备,装备,工具. It also refers to the control testing of the quality of the hardened concrete to ensure that it conforms withconform knfo:m with：符合,遵守,使适应 the design requirements and, where it deviatesdeviate： di:vieit v.偏离 significantly from this, the taking of the necessary corrective action. Mix design, quality control and the over-all cost of the finished concrete product are all interdependent and this will become increasingly apparent as the factors affecting mix design are discussed. In this chapter the basic requirements for any concrete and the way in which these may be incorporated intobe incorporated inko:preitid into：合并,并入 concrete mix design are described. The essentials of quality control associated with the production of concrete on site have been considered earlier. Some of the statistical methodsstatistical method：统计法,平均法；statistical： sttistikl adj.统计的, 统计学的 of quality control used for the interpretationinterpretation： in,t:pritein n.解释,阐明,口译,通译 of control tests on the hardened concrete are described here. 混凝土配比设计即在任何给定的一系列条件下选择组成材料的比例使之在最小成本的基础上获得满足所有要求性能的混凝土的过程。在本章，任何混凝土的成本除材料本身成本外，还包括配比设计、配料、拌和、浇注混凝土及工地管理成本。 首先，质量控制指可能用现场设备时现场监督以确保用于混凝土配比的材料达到要求质量并且规定的配比尽可能接近地遵守。它也指对硬化混凝土进行质量控制检测以确保与设计要求一致，并在严重偏离时采取必要的更正措施。 配比设计、质量控制和最终混凝土产品的成本是互相依赖的，这使得讨论影响配比设计的因素变得日益显著。 本章讲述任何混凝土的基本要求及其纳入混凝土配比的途径。与现场混凝土产品关联的质量控制的要点已被更早地考虑了。这里也叙述了一些用于解释硬化混凝土质量控制检测的统计方法。15.1 Required Concrete Properties要求的混凝土性能 The basic requirements for concrete are conveniently considered at two stages in its life. In its hardened state (in the completed structure) the concrete should have adequate durability, the required strength and also the desired surface finishfinish： fini n.表面光度, 抛光, 涂层. In its plastic state, or the stage during which it is to be handled, placed and compacted in its final form, it should be sufficiently workable for the required properties in its hardened state to be achieved with the facilities available on site. This means that (1) the concrete should be sufficiently fluidfluid： flu(:)id adj.流动的,不固定的,可改变的, for it to be able to flow into and fill all parts of the formworkformwork： fo:mw:k n.模板，模板工程，制模工作, or mouldmould： muld n.模型,模板,形状；vt.模压(铸造,发霉), into which it is placed; (2) it should do so without any segregation, or separation, of the constituent materials while being handled from the mixermixer： miks n.混合者, 搅拌器, 电 混频器 or during placing; (3) it must be possible to fully compact the concrete when placed in position; and (4) it must be possible to obtain the required surface finish. If concrete does not have the required workability in its plastic state, it will not be possible to produce concrete with the required properties in its hardened state. The dependence of both durability and strength on the degree of compaction has been noted earlier. Segregation results in variations in the mix proportions throughout the bulkbulk： blk n.体积，容积；大量，大批 of the concrete and this inevitably means that in some parts the coarser aggregate particles will predominatepredominate： pridomineit vi.统治,成为主流,支配,占优势. This precludespreclude： priklu:d vt.预防,阻碍,排除 the possibility of full compaction since there is insufficient mortarmortar： mo:t n.水泥砂浆，砂浆，胶浆，灰泥 to fill the voids between the coarser particles in these zoneszone： zun n.地域, 地带, 地区, 环带, 圈. This results in what is descriptivelydescriptively： diskriptivli adv.叙述地 known as honeycombing on the surface of the hardened concrete with reduced durability and strength as well as unacceptableunacceptable： nkseptbl adj.无法接受的,不受欢迎的 surface finish. The means by which each of the required concrete properties may be achieved are now considered. 混凝土基本要求从寿命的两阶段考虑较方便。 在混凝土硬化阶段（在结构上）应有足够的耐久性、要求强度和预期表面光度。 在塑性阶段或在可操作、浇注密实进最终形状的阶段，它应有足够的工作性使其用现场设备获得硬化阶段的要求性能。这意味着： （1）混凝土应有足够的流动性使其能流进并充满整个模板或模型，流进它需浇注的地方； （2）它的组成材料应在搅拌机处理或浇注时没有任何离析或分离； （3）当浇注在最终位置时它应可能完全密实； （4）它应可能获得要求表面光度。 若混凝土在塑性阶段没有要求的和易性，就不可能在硬化阶段获得要求性能的混凝土。更早地注意耐久性和强度依赖于密实程度。离析导致混凝土整体配比的改变，不可避免地在某些部分粗集料颗粒占优势，这阻碍了完全密实的可能性，因在这些区域没有足够的砂浆来填充粗颗粒间的空隙。这引起硬化混凝土表面所谓的蜂窝麻面，耐久性和强度降低以及不受欢迎的表面光洁度。 现在就考虑可获得混凝土每种要求性能的途径。Durability耐久性 Adequate durability of exposed concrete can frequently be obtained by ensuring full compaction, an adequate cement content and a low water-cement ratio, all of which contribute to producing a dense, impermeable concrete. The choice of aggregate is also important particularly for concrete wearing surfaces and where improved fire resistancefire resistance：耐火性 is required. Aggregate having high shrinkage properties should be used with cautionwith caution ko:n ：慎重,留心 in exposed concrete. For particularly aggressiveaggressive： gresiv adj.侵蚀性的 environments additional precautionsprecaution： priko:n n.预防措施,预防,保护 may be necessary, for example, the use of sulphate-resisting cement where the concrete will be exposed to sulphate attack. For airfields and roads where salt is likely to be used for de-icing purposes, the use of air-entrained concreteair-entrained concrete：引气混凝土；加气混凝土 is recommended. Durability is not a readily measured property of the hardened concrete. However, for a correctly designed concrete mix any increase in the water-cement ratio on site with the associated reduction in durability, will be accompanied by a reduction in concrete strength. The latter can be determined quite easily using control specimens and for this reason the emphasis in control testing is on the determination of concrete strength. 裸露混凝土的足够耐久性经常可通过确保完全密实、适当的水泥用量、低水灰比及所有可制备致密抗渗混凝土的手段来获得。集料选择也很重要，尤其对混凝土磨损表面和要求改善耐火性的地方。在裸露混凝土中使用高收缩性的集料应慎重。在特别具侵蚀性的环境中必须采取额外预防，如在混凝土裸露在硫酸盐侵蚀中采用抗硫酸盐水泥。对飞机场和公路，可能用盐来除冰，推荐使用加气混凝土。 硬化混凝土的耐久性不易确定。然而，对正确设计的混凝土拌和物，现场水灰比增加导致耐久性降低，也伴随混凝土强度的降低。强度很容易用控制试件来确定，因此控制检测的重点是确定混凝土的强度。Strength强度 Although durability is sometimes the overridingoverriding：: ,ouvraidi adj.压倒的,占优势的 criterioncriterion： kraitirin n.标准，规范；准则, the strength of the concrete is frequently an important design consideration particularly in structural application where the load-carrying capacityload-carrying capacity：承载能力,载重量；capacity： kpsiti n.容量, 生产量, 能量，电容 of a structural member may be closely related to the concrete strength. This will usually be the compressive strength although occasionally the flexural or indirect tensile strength may be more relevant. The strength requirement is generally specified in terms ofin terms of：根据,按照,在.方面 a characteristic strengthcharacteristic ,kriktristik strength：特性强度 (CPCP：（the Code kud of Practice）实施法规 110: Part 1) coupled withcoupled with：加上,外加；coupled： kpld adj. 耦合的, 连结的, 联系的 a requirement that the probability of the strength falling belowfall below：降到水平之下 this shall not exceed a certain value. Typically this may be 5 per cent or a 1 in 20 chance of a strength falling below the specified characteristic strength, this generally being the 28-day strength. An understanding of the factors affecting concrete strength on site, and of the probable variations in strength, is essential if such specifications are to have any real meaning at the mix design stage. A histogramhistogram： histugrm 页：3n.柱状图,直方图,矩形图 showing the frequency of cube compressive strengths for a road contract is shown in figure 15.1. What might appear to be large variations in strength can be seen, with actual cube strengths ranging from 8.5 to 34.0 N mm-2. Some of the differences between individual cube strengths may be attributed to testing errorserror： er 页：3n.误差,错误 including samplingsampling： sa:mpli 页：3n.取样,抽样, preparation and curing of the test cubes, the testing machine itself and to the actual test procedure. Differences in strength can also occur owing to variations in the quality of the cement but the principal factor affecting the strength is the quantity of water, or more specificallymore specifically spisifikli ：页：3更准确地说 the water-cement ratio, in the concrete mix. If the proportions of aggregate and cement and also the quality of the aggregate are maintained constant, the water-cement ratio can be controlled very effectively at the mixer by adding just sufficient water to give the required workability. Once a suitable mix has been obtained the workability can be assessed quite satisfactorily by an experienced mixer operator, with periodicperiodic： piriodik 页：3adj.周期的,定期的 control tests of the workability. However, human errorhuman error：页：3人为误差 will inevitably result in some variation in the water-cement ratio either side of the desired value. Any variation in mix proportions or significant changes in the aggregate grading will affect the quantity of water needed to maintain the required workability and this too will result in variations in the water-cement ratio and hence in concrete strength. All these factors tend to give water-cement ratios which are as likely to be greater as they are to be less than the target valuetarget value：页：4目标值；target： ta:git n.目标, 对象, 靶子. The actual water-cement ratios tend therefore to have a normal or gaussian distributionnormal distribution（gaussian gausin distribution）：页：4正态分布,高斯分布 about the mean, or target, value页：4mean value：平均值. The relationship between water-cement ratio and concrete strength is nonlinear. Nevertheless over a limited range the relationship will be approximately linear and it might be expected that concrete strengths will also tend to have a normal distribution. This can be seen in figure 15.1 in which a typical bell-shapedbell-shaped：页：4钟形的 normal distribution curve is shown. 尽管有时耐久性是基本的评判标准，但混凝土强度经常作为重要的设计考虑事项，尤其是在结构承载能力接近于混凝土强度的结构应用中。这通常是抗压强度，尽管偶尔与抗弯强度或间接抗拉强度有关。 强度要求通常用特性强度（实施法规110第1部分）外加强度不应低于某一值的概率要求来做规定。典型地是可能有5%或1/20强度低于规定的特性强度，通常为28d强度。 了解影响现场混凝土强度的影响因素和强度的可能变化是很重要的，若这样的描述在配比设计时有任何实际意义。对一道路契约显示在图15.1的直方图显示了立方体抗压强度的周期。可以看出强度变化很大，实际立方体强度变化为8.534.0 N mm-2。单一立方体强度的差异是由于试验误差，包括试验立方体的抽样、制备和养护、试验仪器本身及实际试验程序。强度的差异也会由水泥质量的变化引起，但主要影响因素是混凝土混合物中的水量，更准确地说是水灰比。 若集灰比和集料品质保持不变，水灰比可通过在搅拌机中加入适当水达到要求和易性来很有效地控制住。一旦获得适当的拌和物，其和易性可由富有经验的搅拌机工人通过和易性的定期控制试验很令人满意地评定。然而人为误差不可避免地造成水灰比有大于或小于预期值的变化。配比的任何改变或集料级配的重大变化都会影响需保持需求和易性的水的用量，这也将引起水灰比的变化，进而影响混凝土的强度。 所有这些因素趋于使水灰比可能比目标值偏大或偏小。因此实际水灰比趋于在平均值或目标值周围呈正态或高斯分布。水灰比和混凝土强度之间是非线性的，然而超过一有限范围就近似线性，可能是因为混凝土强度也趋于呈正态分布的原因。这也可从图15.1看到，显示了一个典型的钟形的正态分布曲线。Design Mean Strength设计平均强度 The assumption of a normal distribution of concrete strengths forms the basis of mix design and statistical quality control procedures for satisfying the strength requirement. For a normal distribution, the probability of a strength lying outside specified limits either side of the mean strength can be determined. These limits (figure 15.2) are usually expressed in terms of the standard deviationstandard deviation：页：5标准偏差；deviation： ,di:viein n.背离, 偏差 s, defined by N mm-2where fc is an observed strength, fcm is the best estimate of the mean strength, equal to (fc)/n, and n is the number of observations. The probabilities of a strength lying outside the range (fcms) for different values of are given in table 15.1, in which the probabilities of strengths falling below the lower limit (fcms) are also given. 混凝土强度呈正态分布的假设成为配比设计和确保强度要求的质量控制统计程序的基础。 正态分布确定了强度落于平均值限定范围外的概率。这些限制（图15.2）通常用标准偏差s表示，定义为： N mm-2式中fc为观测强度，fcm为最佳估算平均值，等于(fc)/n，n为观测次数。对不同的值，强度落在(fcms)范围外的概率如表15.1，也列出了强度低于最低限制(fcms)的概率。TABLE 15.1Probability valuesProbability of an observed strength lying outside the range (fcms)Probability of an observed strength being less than (fcms)1 in 501 in 201 in 102.331.961.641 in 1001 in 401 in 20 If the specified characteristic strength fcu is the strength below which not more than 1 in 20 of the population of strengths shall fall, it follows thatfcufcm1.64sorfcufcm1.64sHence if the standard deviation likely to be obtained on site can be assessed, the mean strength for which the concrete must be designed can be determined. The free water-cement ratio required to give this mean concrete strength can then be estimated using curves such as those in figure 15.3. The effect of site control on the mean strength fcm required to give a characteristic strength fcu is shown in figure 15.4. The poorer the control the higher the mean strength required and this will mean a lower water-cement ratio. This in turn will require a smaller aggregate-cement ratio, or larger cement content per unit volume, for a given workability and a consequent increase in the total cost of materials. 若规定的特性强度是指低于它的强度数量不超过1/20的强度，那么fcufcm1.64s或fcufcm1.64s因此若可能在现场获得的标准偏差能被评定，混凝土必须设计的平均强度就能确定。要求给出这一混凝土平均强度的自由水灰比也可用如图15.3中的曲线算出。 现场控制对要求给出特性强度fcu的平均强度fcm的影响见图15.4。控制越差，要求的平均强度越高，意味着更低的水灰比。这对给定和易性也要求集灰比更小，或每单位体积水泥用量更大，随之产生材料总成本的增加。Workability和易性 Suitable workabilities for different types of work are given in table 15.2, workability in this context referring to those qualities measured by the slump页：8slump： slmp n.坍落度（稠度）, compacting factor页：8compacting factor：捣实因数，密实系数 and V-B tests. In this table the bracketedbracketed： brkitid 页：8adj.框入;加括号的 values are the least reliable although the associated tests can generally be used for control purposes once the actual value corresponding to a suitable workability has been determined. 不同工程的适宜和易性见表15.2，本文的和易性是指由坍落度、捣实因数和维勃实验确定的性质。表中加括号的最不可信，尽管一旦确定了对应适宜和易性的实际值时相应的实验一般可用于控制的目的。TABLE 15.2Workability requirementsType of workV-B (s)Compacting factorSlump (mm)WorkabilityHeavily reinforcedheavily reinforced：页：8加重的,超配筋的 sections with vibrationvibration： vaibrein n.振动, 颤动, 摇动, 摆动. Simply reinforced sections without vibration密筋构件振动，稀筋构件非振动(3)0.92(25 - 100)MediumSimply reinforced sections with vibration. Mass concrete without vibration6(0.86)(10 - 50)LowMass concrete and large sections with vibration. Road slabs vibrated using power-operatedpower-operated：页：8机动的,自动的 machines12(0.80)Very low For a given water-cement ratio, the principal factors affecting workability are the shape and grading of the coarse and fine aggregates and the aggregate-cement ratio. The choice of suitable concrete mix proportions must take all these factors into account. A number of different approaches to mix design have been proposed, one of the earliest and most widely known being that described in Road Note No.4 (Road Research Laboratory, 1950). A more general approach proposed by Hughes (1971) considers the optimum页：9optimum： optimm adj.最佳，最适宜的 coarse aggregate content, this being the volume fraction页：9volume fraction：体积分数, 体积分率, 容积率；fraction： frkn n. 分数, 小数, 百分率 of coarse aggregate which enables a given workability and strength to be achieved with the minimum cement. The simplified mix design procedure described here is based on Hughes approach. 对给定水灰比，影响和易性的主要因素是粗细集料的形状和级配以及集灰比。选择适当的混凝土配合比必须考虑所有这些因素。已提出许多不同的配给设计方法，最早应用最广的是Road Note No.4（公路研究实验室，1950）。Hughes（1971）提出的更普通的方法考虑最佳粗集料用量，即在最小水泥用量下达到给定和易性或强度的粗集料的体积分数。 这里叙述基于Hughes法的简化配比设计程序。15.2 Concrete Mix Design混凝土配比设计 The basic steps in designing a concrete mix are outlined below. These are followed by an illustrative example in which a concrete mix is designed to meet specific requirements. (1) Durability. For the degree of exposure determine the type of cement, the minimum cement content and maximum water-cement ratio, being guided by relevant specifications. (2) Strength. Where a maximum water-cement ratio has been obtained in step 1, determine the associated concrete strength using relationships such as those shown in figure 15.3. Compare this with the specified characteristic strength and accept the larger value as the required characteristic strength fcu. (3) Determine the required mean concrete strength fromfcm = fcu +s using an appropriate value of . The standard deviation s should be based on recent test results for conditions similar to those which will occur on site. Where no such data are available, conservativeconservative： kns:vtiv 页：9adj.保守的,守旧的 values should be adopted. For example, when = 1.64, values of s = 0.67fcu for fcu22.5 N mm-2 and s = 15.0 N mm-2 for fcu22.5 N mm-2 might be used. (4) Determine the cement-water ratio c/w (by volume) associated with the mean strength fcm using, for example, figure 15.3. (5) Workability. Determine the basic volume fraction of cement required to give the desired workability (see table 15.2), for the cement-water ratio c/w obtained in step 4, using figure 15.5. (6) Using table 15.3 and the gradings of th