新拌混凝土的性能-外文翻译大学论文.doc
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1、Properties of Fresh ConcreteEdited 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
2、 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 an
3、d 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 sta
4、bility, 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
5、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
6、 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 WorkabilityWorkability of concrete has never been precisely defined. For practical purposes it generally implies the ease wit
7、h 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 f
8、ill 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
9、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
10、. 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 mea
11、suring 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 ex
12、ample, 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
13、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 t
14、here 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 TestThis test was developed by Chapman in the United States in 191
15、3. 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
16、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 consistometerThe test primarily measures the consistency of plastic
17、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 i
18、n 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
19、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
20、 slump. For wet mixes, complete collapse of the concrete produces unreliable values of slump. Figure 13.2 Three main types of slumpThe 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
21、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 l
22、eaner 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 val
23、ues 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 hydr
24、ation 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 TestThis test, developed in the United Kingdom by Glanville et al. (1947), measures the degree of compaction for a standard amount of work
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