ABSTRACT
For any particular job it is necessary to know what workability is required so that a specification may be written, so, obviously, unless the decision is to be based on personal practical experience, information relating workability levels to various particular types of job is needed. Road Note 41, which for many years was the best-known U K guide to concrete-mix design, gave such information in tabular form and listed four categories of workability in purely descriptive terms ranging from Very low7 to 'high7. With each category there was associated a corresponding compacting factor, a range of slump values, and examples of jobs for which that grade of workability was said to be suitable. A caveat was entered in a footnote which said: T h e slump is not definitely related to the workability or the compacting factor. The figures given must, therefore, be regarded as a rough indication of the order of the slump and nothing more7. The value of attempting to give general rules at all has more recently been questioned, and in Design of Normal Concrete Mixes2, which was produced as an intended replacement for Road Note 4, it is stated, as was pointed out earlier, that 'it is not considered practical for this Note to define the workability required for various types of construction or placing conditions since this is affected by many factors'.*
In practice therefore, workability specifications are devised on the basis of experience, and are stated in terms of results from one of the standard tests. In this connection, by far the most commonly quoted test is the slump test; the flow table is occasionally quoted, compacting factor rarely, and the Vebe hardly ever. Of course, as with any specification, it is necessary to quote tolerances on the desired value and this is done in British Standard BS 5328:1981 Methods for specifying concrete, where it is stated that workability shall be within the limits given in Table 13.1.
