外文翻譯--新拌混凝土的性能

1、<p>　　Properties of Fresh Concrete</p><p>　　Edited by H.-J. Wierig</p><p>　　Fresh concrete is a mixture of water, cement, aggregate and admixture (if any). After mixing, operations such as

2、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 co

3、ncrete 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 resu</p><p>　　The characteristics of fresh c

4、oncrete 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 governe

5、d 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 fr

6、om job to job it follows that </p><p>　　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

7、 of the various characteristics of concrete in its plastic state and know how these may alter during operations involved in casting a concrete structure.</p><p>　　13.1 Workability</p><p>　　Worka

8、bility 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 characteris

9、tics 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. Comp

10、actability is the ease with which a given mix can be fu</p><p>　　Another commonly accepted definition of workability is related to the amount of useful internal work necessary to produce full compaction. It

11、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 no

12、ne gives an absolute measure of workability.</p><p>　　The tests commonly used for measuring workability do not measure the individual characteristics (consistency, mobility and compactability) of workability

13、. 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

14、 more time and labour for full compaction. It is most important that a realistic assessment is made of the workability required</p><p>　　13.2 Measurement of Workability</p><p>　　Three tests wide

15、ly 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 al

16、so 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

17、of these tests together with their merits an</p><p>　　Slump Test</p><p>　　This test was developed by Chapman in the United States in 1913. A 300 mm high concrete cone, prepared under standard co

18、nditions (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

19、 methods employed for measuring workability. It is not surprising that, in spite of its several limitations, the slump test has retained its popularity.</p><p>　　Figure 13.1 Apparatus for workability measure

20、ment: (a) slump cone, (b) compacting factor and (c) V-B consistometer</p><p>　　The test primarily measures the consistency of plastic concrete and although it is difficult to see any significant relationship

21、 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 inc

22、rease in slump. Although the test is suitable for quality-control purposes it should be remembered that it is generally considered to be unsuitable for </p><p>　　Figure 13.2 Three main types of slump</p&g

23、t;<p>　　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 general

24、ly 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 m

25、aterials. Shear slump occurs more often in leaner mixes than in rich ones a</p><p>　　The standard slump apparatus is only suitable for concretes in which the maximum aggregate size does not exceed 37.5 mm. I

26、t 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 fixe

27、d period of time.</p><p>　　Compacting Factor Test</p><p>　　This test, developed in the United Kingdom by Glanville et al. (1947), measures the degree of compaction for a standard amount of work

28、 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 18

29、81: Part 2. The test requires measurement of the weights of the partially and fully compacted concrete and the ratio of the partially compacted we</p><p>　　It should also be appreciated that, strictly speaki

30、ng, 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)

31、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

32、 held bel</p><p>　　V-B Consistometer Test</p><p>　　This test was developed in Sweden by Bhrner (1940) (see figure 13.1). Although generally regarded as a test primarily used in research its pote

33、ntial 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 fl

34、at 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 t</p><p>　　The test is suitable for a wide range of mixes and, unlike t

35、he 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 surfa

36、ce 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 re

37、</p><p>　　13.3 Factors Affecting Workability</p><p>　　Various factors known to influence the workability of a freshly mixed concrete are shown in figure 13.3. From the following discussion it wi

38、ll 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.</p><p>　　Cement and Water</p>&

39、lt;p>　　Figure 13.3 Factors affecting workability of fresh conrete</p><p>　　Typical relationships between the cement-water ratio (by volume) and the volume fraction of cement for different workabilities ar

40、e 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 co

41、ntent is greater for richer mixes. Hughes (1971) has shown that similar linear relationships exist irrespective of the properties of the constituent ma</p><p>　　For a given mix, the workability of the concre

42、te 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

43、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 t

44、he fresh concr</p><p>　　Admixtures</p><p>　　The principal admixtures affecting improvement in the workability of concrete are water-reducing and air-entraining agents. The extent of the increase

45、 in workability is dependent on the type and amount of admixture used and the general characteristics of the fresh concrete.</p><p>　　Workability admixtures are used to increase workability while the mix pro

46、portions are kept constant or to reduce the water content while maintaining constant workability. The former results in a slight reduction in concrete strength.</p><p>　　Air-entraining agents are by far the

47、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

48、 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, th

49、e rate of increase in w</p><p><b>　　Aggregate</b></p><p>　　For given cement, water and aggregate contents, the workability of concrete is mainly influenced by the total surface area

50、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

51、 wet the aggregate particles, thus leaving a smaller amount of paste for lubrication. It follows that, all other conditions being equal, the workability will</p><p>　　TABLE 13.1</p><p>　　Effect

52、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)</p><p>　　TABLE 13.2</p><p>　　Effe

53、ct 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)</p><p>　　Figure 13.4 Effect o

54、f aggregate shape on aggregate-cement ratio of concretes for different workabilities, based on Cornelius (1970)</p><p>　　Several methods have been developed for evaluating the shape of aggregate, a subject d

55、iscussed 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

56、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 co</p><p>　　Fig

57、ure 13.5 A typical relationship between workability and coarse aggregate content of concrete, based on Hughes (1960)</p><p>　　The effect of surface texture on workability is shown in figure 13.6. It can be s

58、een 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 use

59、d. In such a case workability drops, the extent of the reduction being dependent on the aggregate content and its absorption capacity.</p><p>　　Ambient Conditions</p><p>　　Environmental factors

60、 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 wat

61、er. 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 i

62、s used for hydration as well as its los</p><p>　　Figure 13.6 Effect of aggregate surface texture on aggregate-cement ratio of concretes for different workabilities, based on Cornelius (1970)</p><p

63、><b>　　Time</b></p><p>　　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

64、 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 cemen

65、t. The rate of loss of workability is affected by certain characteristics of the constituent materials, for exam</p><p>　　For a given concrete and set of ambient conditions, the rate of loss of workability w

66、ith 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 decrea

67、sing 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 reduced, particularl</p><p>　　Fo

68、r practical purposes, loss of workability assumes importance when concrete becomes so unworkable that it cannot be effectively compacted, with the result that its strength and other properties become adversely affected.

69、Corrective measures frequently taken to ensure that concrete at the time of placing has the desired workability are either an initial increase in the water content or an increase in the water content with further mixing

70、shortly before the concrete is discharged. When this resul</p><p>　　Figure 13.7 Loss of workability of concrete with time: (A) no agitation and (B) continuously agitated after mixing</p><p>　　13

71、.4 Stability</p><p>　　Apart from being sufficiently workable, fresh concrete should have a composition such that its constituent materials remain uniformly distributed in the concrete during both the period

72、between mixing and compaction and the period following compaction before the concrete stiffens. Because of differences in the particle size and specific gravities of the constituent materials there exists a natural tende

73、ncy for them to separate. Concrete capable of maintaining the required uniformity is said to be</p><p>　　Segregation</p><p>　　When there is a significant tendency for the large and fine particle

74、s in a mix to become separated, segregation is said to have occurred. In general, the less cohesive the mix the greater the tendency for segregation to occur. Segregation is governed by the total specific surface of the

75、solid particles including cement and the quantity of mortar in the mix. Harsh, extremely wet and dry mixes as well as those deficient in sand, particularly the finer particles, are prone to segregation. As far as</p&g

76、t;<p>　　Blemishes, sand streaks, porous layers and honeycombing are a direct result of segregation. These features are not only unsightly but also adversely affect strength, durability and other properties of the

77、hardened concrete. It is important to realize that the effects of segregation may not be indicated by the routine strength tests on control specimens since the conditions of placing and compaction of the specimens differ

78、 from those in the actual structure. There are no specific rules for suspec</p><p><b>　　Bleeding</b></p><p>　　During compaction and until the cement paste has hardened there is a na

79、tural tendency for the solid particles, depending on size and specific gravity, to exhibit a downward movement. Where the consistency of a mix is such that it is unable to hold all its water some of it is gradually displ

80、aced and rises to the surface, and some may also leak through the joints of the formwork. Separation of water from a mix in this manner is known as bleeding. While some of the water reaches the top surface som</p>

81、<p>　　The risk of bleeding increases when concrete is compacted by vibration although this may be minimized by using a correctly designed mix and ensuring that the concrete is not over-vibrated. Rich mixes tend to

82、bleed less than lean mixes. The type of cement employed is also important, the tendency for bleeding to occur decreasing as the fineness of the cement or its alkaline and tricalcium aluminate (C3A) content increases. Air

83、-entrainment provides another very effective means of controlling bleedi</p><p><b>　　新拌混凝土的性能</b></p><p>　　作者：H.-J. Wierig</p><p>　　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

84、土的操作如輸送、澆注、密實和終飾也會顯著影響硬化混凝土的性能。組成材料在施工的不同時期保持在混凝土中的均勻分布及完全密實是很重要的。若這些條件不理想，成品硬化混凝土的性能如強度和耐久性就有不利影響。</p><p>　　新拌混凝土影響完全密實的特性是其稠度、流動性和密實性。在混凝土實踐中這些一起被稱為和易性。混凝土維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。由于對混凝土拌和物運輸、澆注和搗固采用

85、的方法與澆注構件的性質一樣隨工程不同而異，因此相應的和易性和穩(wěn)定性要求也會改變。對特定工作新拌混凝土的適應性的評定在某種程度上總存在人為判斷的問題。</p><p>　　盡管很重要，但塑性混凝土的行為通常被忽視。建議學生應學會鑒定塑性狀態(tài)混凝土的不同特性的重要性，了解在包括澆注混凝土結構的施工操作時如何去改變它們。</p><p><b>　　和易性</b></

86、p><p>　　混凝土的和易性從未被準確定義。實踐時一般認為是指混凝土拌和物從攪拌機施工到其最終密實形狀的容易程度。和易性的三個主要特性是稠度、流動性和密實性。稠度指濕潤度或流度的度量。流動性指拌和物流進并完全充滿模板或模具的容易程度。密實性指給定拌和物完全密實，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比例，而且取決于（1）運輸和密實采用的方法，（2）模板或模具的尺寸、形狀和表

87、面粗糙度，（3）鋼筋的數量和間距（布筋）。</p><p>　　另一個普遍接受和易性的定義指產生完全密實所必須的有用內功的數量。應認識到必需功又取決于被澆注構件的性質。內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確定。</p><p>　　通常用于確定和易性的實驗不能確定和易性的單一特性（稠度、流動性和密實性）。然而它們的確給出了拌和物和易性的一個有用、實際的

88、指導。和易性影響混凝土的質量，并直接影響成本，如和易性不好的混凝土拌和物完全密實要求更多時間和勞力。最重要的是在對適宜的混凝土配比下任何結論之前要求對給定現場條件的和易性作出現實評定。</p><p><b>　　和易性的確定</b></p><p>　　三個廣泛應用確定和易性的實驗是坍落度、密實系數和V-B稠度計實驗（圖13.1），是英國的標準實驗，詳細描述在英標1

89、881第2部分。在實施法規(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、密實系數和V-B值間沒有單一關系。下列章節(jié)討論了這些實驗的突出特點及其優(yōu)點和局限性。</p><p><b>　　坍落度實驗</b></p><p>　　此實驗由美國Chapman于1913年發(fā)展的。標準條件（英標1881第2部分）下準備的300mm高混凝土圓錐下沉，錐體下沉或高度的

90、降低被確定為和易性的度量。儀器便宜、輕便、結實，是所有確定和易性方法中最簡單的。盡管存在一些局限性，坍落度實驗的普及是不足為奇的。</p><p>　　實驗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系，但它適用于檢測和易性的改變。如，用水量增加或細集料比例不足會引起坍落度增加。實驗適用于質量控制目的，但應記住一般認為不適用于配比設計，因密實需不同工作量的混凝土可能有相似的坍落度數

91、值。實驗檢測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稠度的靈敏性。實驗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引起坍落度有可測量的變化。對濕拌和物，混凝土的完全崩坍會產生不可信的坍落度值。</p><p>　　圖13.1儀器對工作性測量 (a) 坍落度, (b) 壓縮因子and (c) V-B .濃度測試器</p><p>　　通常觀察的三種

92、坍落度為真實坍落度、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坍落度，一般對和易性改變較敏感。剪切坍落度通常有很濕拌和物相關，一般表現為差質量的混凝土，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。只要出現剪切坍落度就應重復實驗，若一再重復，就應記載此實驗現象和結果，因為獲得相差大的不同坍落度值取決于坍落度是真實或是剪切形式。</p>

93、;<p>　　標準坍落度儀器僅適用于集料最大粒徑不超過37.5mm的混凝土。應注意坍落度值隨攪拌后時間而改變，因為正常的水化和一些游離水的蒸發(fā)，因此在一固定時間內完成實驗是比較理想的。</p><p>　　圖13.2三種坍落度</p><p><b>　　密實系數實驗</b></p><p>　　由英國Glanville（1947

94、）等發(fā)展的這個實驗確定對于標準工作量下的密實程度，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價。儀器是相對簡單的機械裝置（圖13.1），描述在英標1881第2部分中。實驗要求確定部分和完全密實混凝土的重量，部分對完全密實重量的比值總小于1，即是密實系數。對于普通范圍的混凝土，密實系數為0.80~0.92。實驗尤其適用于坍落度實驗不理想的較干拌和物。在普通范圍的和易性之外時密實系數靈敏性減小，通常密實系數超過0.92時就是不理想

95、的。</p><p>　　也應認識到，嚴格地說，實驗的一些基本假設是不正確的。用于克服檢測圓柱體的表面摩擦的工作可能隨拌和物的特性而異。Cusens（1956）指出對很低和易性的混凝土，當密實系數保持明顯不變時獲得完全密實要求的實際工作取決于拌和物的富度。因此通常認為有相同密實系數的混凝土完全密實要求的工作量相同的觀念不總是正確的。應注意的另一點是澆注混凝土到檢測圓柱體的程序與現場通常采用的方法并不相同。與坍落度

96、實驗一樣，密實系數的確定必須在某一特定時間內。標準儀器適用于集料最大粒徑達37.5mm的混凝土。</p><p><b>　　V-B稠度計實驗</b></p><p>　　實驗由瑞典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驗，但其潛力現在正在工業(yè)中被更廣泛公認，實驗逐漸被接受。實驗中（英標1881第2部分）記錄了通過振動把一個標準

97、混凝土圓錐變成密實的平圓柱體所用的時間，即V-B時間，用s做單位，規(guī)定精確到0.5s。與前兩個實驗不同，此實驗處理混凝土與實際密實混凝土方法類似。而且，此實驗對稠度、流動性和密實性改變敏感，因此認為在實驗結果與現場和易性評定之間存在合理的相關關系。</p><p>　　實驗適用于大范圍拌和物，與坍落度和密實系數實驗不同，它對很干和引氣混凝土和易性變化很敏感，對集料特性如形狀和表面紋理的變化也更敏感。實驗結果的復驗

98、性好。如其它實驗一樣，其準確性趨于隨集料最大粒徑增加而降低，大于19.0mm實驗結果有點不可信。對于密實要求很少振動的混凝土V-B時間僅約3s。這樣的結果可能可信度比大V-B時間要低，因為估計時間終點（混凝土接觸塑料盤的整個下面）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時間可能超過真實和易性，因為消除透明盤下截留的氣泡要求延長振動。為克服這個困難，可在儀器上附上一個記錄相對于時間的盤垂直下沉量的自動裝置。這個記錄裝置也