The consistency of concrete is determined by Vee-Bee Consistometer, which determines the time required to transform by vibration a concrete specimen in the shape of a conical frustum into a cylinder.
This is a very good laboratory test, particularly for dry mixes. In case of compacting factor test, some of the concrete may remain sticking with the hopper if the concrete is dry, which may affect the results. Further this test has the advantage that the treatment of concrete during the test is comparatively closely related to the method of placing concrete in practice. The name Vee-Bee has been derived from the initials of V. Bahrner of Sweden who developed this test.
This test is not suitable for concrete having slump 75 mm or more. This test gives satisfactory performance of concrete, of consistencies for which slump cannot be measured.
The apparatus is shown diagrammatically in Fig. 7.10 and a line diagram in Fig. 7.11.
A Vee-Bee consistometer consists of the followings:
(a) A vibrating table resting on elastic supports
(b) A metal pot
(c) A sheet metal cone, open at both ends as slump test apparatus
(d) A standard iron rod 20 mm in diameter and 500 mm long.
The vibrating table (G) Fig. 7.11, is 380 mm long and 260 mm wide. It is supported on rubber shock absorbers at a height of about 305 mm above the floor level. The table is mounted on a base (K) which rests on three rubber feet and is fitted with an electrically operated vibrator mounted under it. A sheet metal cone- (B) open at both ends is placed in a metal pot (A) and the pot is fixed on the vibrating table by means of two wing nuts (H).
The sheet metal cone is 30 cm high, having bottom and top diameters as 20 cm and 10 cm respectively. A swivel arm holder (M) is fixed to the base and into this is telescoped another swivel arm (N) with funnel (D) and guide sleeve (E) The swivel arm can be readily detached from the vibrating table. The graduated rod (J) is fixed on to the swivel arm and at the end of graduated arm a glass disc (C) is screwed. The division of the scale on the rod records the slump of the concrete cone in Centimeters and the volume of concrete after vibration of the concrete in the pot.
First of all slump of the concrete is determined by putting the metal cone B in the sheet metal cylinder – ‘A’ of the consistometer. The glass disc ‘C’ is placed on the top of the slump cone in the cylinder before lifting the cone by moving the swivel arm and the position of concrete cone noted by adjusting the glass disc. Now the cone is lifted up and the slump is read on the graduated rod by lowering the E glass disc on the top of concrete cone. The electric vibrator is c now switched on and the concrete is allowed to spread in the cylinder.
The vibration is continued until the whole concrete surface uniformly adheres to the glass disc and time taken in this operation noted with the help of a stop watch. The time is recorded in seconds. The consistency of concrete is expressed in Vee-Bee seconds or (Vee-Bee degrees) which are equal to the time in seconds as recorded above. The required slump is obtained by the Table 7.5.
The curve of Fig. 7.12 shows a relationship between slump in cms and consistency. Vee-Bee seconds.
Thus the time required for complete re-moulding of concrete in seconds is nearest of workability and expressed in number as Vee-Bee second.
From the above table it will be seen that for dry mixes where slump test is less sensitive, Vee-Bee test gives reliable results. All concretes intended to be vibrated lie between the range of 40 to 25 Vee-Bee degrees. For this range this test is very sensitive and accurate. The variation of ½ Vee-Bee degrees corresponds to a change of 0.004 in water/cement ratio or a change in strength of about 0.8%. This is a great sensitivity for concrete. A concrete having V-B degree equal to 4, corresponds to a slump of 3 cm, and is plastic enough to be shaped into a ball between the palms of hands.
As all these tests measure only a particular aspect of workability, hence there is no rigid correlation between the work-abilities of concrete measured by different test methods. Hence it is recommended that for a given concrete, the appropriate test method be decided before hand to decide the workability.
The major draw backs of the above methods are summarised below:
1. The tests are quite arbitrary and empirical as far as the measurement of workability is concerned. Each of these tests is a single point test measuring a single quantity, which at times may classify two concretes as identical that may behave quite differently on the job or site.
2. Even minor variations in techniques of conducting the test, may vary the results of tests.
3. None of these tests is capable of covering the whole range of work abilities of concrete. The slump test is incapable of differentiating between concretes of very low (zero slumps) workability or two concretes of very high workability (collapse slump). More over the test results may be used as simple statement of qualitative behaviour of concrete under specific conditions.