Table of Contents
Introduction
The quality of concrete satisfying the above requirements is termed as workable concrete. The word “workability” or workable concrete signifies much wider and deeper meaning than the other terminology “consistency” often used loosely for workability. Consistency is a general term to indicate the degree of fluidity or the degree of mobility. A block of concrete which has high consistency and which is more mobile, need not be of right workability for a particular job. Every job requires particular workability.
A theoretical water/cement ratio aim not going to give an ideal situation for maximum strength. Hundred per cent compaction concrete is an important parameter for contributing to the maximum strength. Lack of compaction will result in air voids whose damaging effect on strength and durability is equally or more predominant than the presence of capillary cavities.
Many research workers tried to define the word workability. But as it signifies much wider properties and qualities of concrete, and does not project any one particular meaning, it eludes all precise definitions. Road Research laboratory, UK, who have extensively studied the field of compaction and workability, defined workability as “the property of concrete which determines the amount of useful internal work necessary to produce full compaction.”
Another definition which envelopes a wider meaning is that it is defined as the “ease with which concrete can be compacted hundred per cent having regard to the mode of compaction and place of deposition.” Without dwelling much on the merits and demerits of various definitions of workability, having explained the importance and full meaning of the term workability, we shall see the factors affecting workability.
Measurement of Workability
It is discussed earlier that workability of concrete is a complex property. Just as it eludes all precise definition, it also eludes precise measurements. Numerous attempts have been made many research workers to quantitatively measure this important and vital property of concrete. But none of these methods is satisfactory for precisely measuring or expressing this Property to bring out its full meaning. Some of the tests, measure the parameters very close to workability and provide useful information.
The following tests are commonly employed to measure workability-
- Slump Test
- With Slump Cone
- With K-Slump Tester
- Compacting Factor Test
- Flow Test & Flow Table Test
- Kelly Ball Test
- Vee Bee Consistometer Test
1. Slump Test
1.1 Slump Test With Slump Cone
The slump test is the most commonly used method of measuring the consistency of concrete which can be employed either in the laboratory or at the site of work. It is not a suitable method for very wet or very dry concrete. It does not measure all factors contributing to workability, nor is it always representative of the placability of the concrete. However, it is used conveniently as a control test and gives an indication of the uniformity of concrete from batch to batch. Figure 1.a shows the apparatus for the test.
To read more about this test, Please read our this article.
Additional information on workability and quality of concrete can be obtained by observing the manner in which concrete slumps. Quality of concrete can also be further assessed by giving a few tappings or blows by tamping rod to the base plate. The deformation (Figure- 1.b) shows the characteristics of concrete with respect to the tendency for segregation.
Despite many limitations, the slump test is very useful on-site to check day-to-day or hour-to-hour variation in the quality of the mix. An increase in a slump may mean for instance that the moisture content of the aggregate has suddenly increased or there has been a sudden change in the grading of aggregate. The slump test gives the warning to correct the causes for change of slump value. The simplicity of this test is yet another reason, why this test is still popular in spite of the fact that many other workability tests are in vogue.
1.2 Slump Test With K-Slump Tester
Very recently a new apparatus called “K-Slump Tester” has been devised. It can be used to measure the slump directly in one minute after the tester is inserted in the fresh concrete to the level of the floater disc. This tester can also be used to measure the relative workability.
In the concrete industry, the slump test is still the most widely used test to control the consistency of concrete mixtures, even though there are some questions about its Significance and its effectiveness. Many agree that the test is awkward and is not in keeping with the strides that the industry has made since 1913 when the slump cone was first introduced. Several apparatus have been proposed to replace or supplement the slump cone, but in general, they have proved to be rich in theory and poor in practice. Their use is still limited mainly to research work in laboratories.
The K-Slump apparatus is very simple, practical, and economical to use, both in the field and the laboratory. It has proven, with over 450 tests, that it has a good correlation with the slump cone.
The K-slump tester can be used to measure slump in one minute in cylinders, pails, buckets, wheel-barrows, slabs or any other desired location where the fresh concrete is placed. A workability index can be determined by the tester. Read this article to know more about the apparatus and the procedure of testing.
2. Compacting Factor Test
The compacting factor test is designed primarily for use in the laboratory, but it can also be used in the field. It is more precise and sensitive than the slump test and is particularly useful for concrete mixes of very low workability as are normally used when concrete is to be compacted by vibration. Such dry concrete is insensitive to slump test. The diagram of the apparatus is shown in Figure-3. The essential dimensions of the hoppers and mould and the distance between them and other detailed description are discussed in this article.
The compacting factor test has been developed at the Road Research Laboratory, UK, and it is claimed that it is one of the most efficient tests for measuring the workability of concrete. This test works on the principle of determining the degree of compaction achieved by a standard amount of work done by allowing the concrete to fall through a standard height. The degree of compaction called the compacting factor is measured by the density ratio i.e., the ratio of the density actually achieved in the test to the density of the same concrete fully compacted.
The weight of fully compacted concrete can also be calculated by knowing the proportion of materials, their respective specific gravities, and the volume of the cylinder. It is seen from experience, that it makes very little difference in compacting factor value, whether the weight of fully compacted concrete is calculated theoretically or found out actually after IOO per cent compaction.
It can be realized that the compacting factor test measures the inherent characteristics of the concrete which relates very close to the workability requirements of concrete and as such it is one of the good tests to depict the workability of concrete.
3. Flow Test
This is a laboratory test, which gives an indication of the quality of concrete with respect to consistency, cohesiveness and the proneness to segregation. In this test, a standard mass of concrete is subjected to Jolting. The spread or the flow of the concrete is measured and this flow is related to workability.
Fig. 6.8 shows the details of the apparatus used. It can be seen that the apparatus consists flow table, about 76 cm. in diameter over which concentric circles are marked. A mould Made from smooth metal casting in the form of a frustum of a cone is used with the following internal dimensions. The base is 25 cm. in diameter, upper surface 17 cm. in diameter, and height of the cone is 12 cm.
A close look at the pattern of spread of concrete can also give a good indication of the characteristics of concrete such as a tendency for segregation. Presently this test is not commonly used. In its place Flow Table Test is used as given in IS 9103 of 1999.
3.1 Flow Table Test
The BIS has recently introduced another new equipment for measuring flow value of concrete. This new flow table test is in line with BS 1881 part 105 of 1984 and DIN 1048 part 1. The apparatus and method of testing is described in this article.
The flow table apparatus is to be constructed in accordance with Fig. 4.a and 4.b. The top of the Flow table is constructed from a flat metal of minimum thickness of 1.5 mm. The top is in plan 700 mm x 700 mm. The centre of the table is marked with a cross, the lines which run parallel to and out to the edges of the plate, and with a central circle 200 mm in diameter. The front of the flow table top is provided with a lifting handle as shown in Fig. 4.b. The total mass of the flow table top is about 16 ± 1 kg.
The flow table top is hinged to a base frame using externally mounted hinges in such a way that no aggregate can become trapped easily between the hinges or hinged surfaces. The front of the base frame shall extend a minimum 120 mm beyond the flow table top in order to provide a top board. An upper stop similar to that shown in Figure- 4.a is provided on each side of the table so that the lower front edge of the table can only be lifted 40 ± 1 mm.
The lower front edge of the flow table top is provided with two hard rigid stops which transfer the load to the base frame. The base frame is so constructed that this load is then transferred directly to the surface on which the flow table is placed so that there is minimal tendency for the flow table top to bounce when allowed to fail.
4. Kelly Ball Test
This is a simple field test consisting of the measurement of the indentation made by 15 cm diameter metal hemisphere weighing 13.6 kg when freely placed on fresh concrete. The test has been devised by Kelly and hence known as Kelly Ball Test. This has not been covered by Indian Standards Specification. The advantages of this test is that it can be performed on the concrete placed in site and it is claimed that this test can be performed faster with a greater precision than slump test. The disadvantages are that it requires a large sample of concrete and it cannot be used when the concrete is placed in thin section. The minimum depth of concrete must be at least 20 cm and the minimum distance item the centre of the bail to nearest edge of the concrete 23 cm.
The surface of the concrete is struck off level, avoiding excess working, the ball is lowered gradually on the surface of the concrete. The depth of penetration is read immediately on the stem to the nearest 6 mm. The test can be performed in about 15 seconds and it gives much more consistent results than the Slump Test. Figure-5 shows the Kelly Bail apparatus.
5. Vee Bee Consistometer Test
This is a good laboratory test to measure indirectly the workability of concrete. This test consists of a vibrating table, a metal pot, a sheet metal cone, a standard iron rod. The apparatus is shown in Figure-6.
Slump test as described earlier is performed, placing the slump cone inside the sheet metal cylindrical pot of the consistometer. The glass disc attached to the swivel arm is turned and placed on the top of the concrete in the pot. The electrical vibrator is then switched on and simultaneously a stopwatch started. The vibration is continued till such time as the conical shape of the concrete disappears and the concrete assumes a cylindrical shape.
This can be judged by observing the glass disc from the top for the disappearance of transparency. Immediately when the concrete fully assumes a cylindrical shape, the stopwatch is switched off. The time required for the shape of concrete to change from slump cone shape to cylindrical shape in seconds is known as Vee Bee Degree. This method is very suitable for very dry concrete whose slump value cannot be measured by the Slump Test, but the vibration is too vigorous for concrete with a slump greater than about 50 mm.
Read this article for the detailed procedure.
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