Objective and Theory of Vee-Bee Test on Concrete
The main objective of Vee-Bee test is to determine the workability of the freshly mixed concrete. The Vee-Bee test gives an indication about the mobility and the compactibility aspect of the freshly mixed concrete.
Vee-bee test carries out the relative effort measurement to change the mass of the concrete from a definite shape to the other. That is, as per the test, from the conical shape to the cylindrical shape by undergoing vibration process.
The measurement of the effort is done by time measurement in seconds. The amount of work measured in seconds is called as the remolding effort. The time required for the complete remolding is a measure of the workability and is expressed in the Vee-Bee seconds.
The experiment is named after the developer V Bahrmer of Sweden. The method can be also applied for dry concrete. For concrete that have slump value more than 50mm, the remolding activity will be so fast that the measurement of time is not possible.
Apparatus for Vee-Bee test
The Vee-Bee test apparatus consist of a Vee-Bee consistometer as per IS: 119 – 1959, as shown in the figure-1. The apparatus consists of a vibrating table which is supported and mounted on elastic supports. It also consists of a sheet metal slump cone, a weighing balance, cylindrical container, a standard iron tamping rod and trowels.
Fig.1: Consistometer Used in Vee-Bee Test of Concrete
The vibrating table as shown in figure-1 has a dimension of 380mm length and a width of 260mm. At a height of 305mm, it is supported on a rubber shock absorber above the level of floor.
A vibrator is provided under the table. This vibrator is operated electrically. The whole mentioned assembly is mounted on a base as shown above which is in turn resting on rubber supports three in number.
The sheet metal slump cone mold has opening at both ends and is placed in a cylindrical container as shown in figure-1. The cylinder container is mounted over the vibration table with the help of wing nuts.
The cone used in the arrangement have height equal to 300mm, the top and the bottom diameters as 200 and 100mm respectively. Base consist of a swivel arm holder. There is another swivel arm that is fixed into it that consist of a funnel and a guide sleeve.
The detachment from the vibrating table is possible for the swivel arm. A graduated rod is fixed to the swivel arm through the guide sleeve. The graduated rod has the provision for screwing the transparent disc.
The slump of the concrete cone is measured through the divisions on the scale marked on the rod. A 20mm diameter standard iron tamping rod is used that have a length of 500mm.
Procedure of Vee-Bee Test on Concrete
The procedure for conducting the Vee-Bee test are as follows:
Initially the sheet metal slump cone is placed inside the cylinder container that is placed in the consistometer. The cone is filled with four layers of concrete. Each concrete layer is one fourth the height of the cone. Each layer after pouring is subjected to twenty-five tamping with the standard tamping rod. The tamping is done with the rounded end of the rod.
The strokes are distributed in uniform manner. This must be done in such a way theta the strokes conducted for the second and the subsequent layers of concrete must penetrate the bottom layers.
Once the final layer has been placed and compacted, the concrete is struck off to make it in level with the help of a trowel. This makes the cone to be exactly filled.
After the preparation of the concrete cone, the glass disc attached to the swivel arm is moved and is placed on the top of the slump cone placed inside the cylindrical container. The glass disc has to be placed such that it touches the top of the concrete level and the reading is measured from the graduated rod.
Now the cylindrical cone is removed immediately by raising the cone slowly in the vertical direction. The transparent disc on the top of the concrete is placed down to the new position and the reading is determined.
The difference in the values measured from step 3 and step 4 will give the slump.
Now the electrical vibrator is switched on and at the same time we have to start the stop watch. The concrete is allowed to spread out in the cylindrical container. Until the concrete is remolded the vibration is continued. This stage is when the surface of the concrete becomes horizontal and the concrete surface completely adheres uniformly to the transparent disc.
The time required for complete remolding in seconds is recorded. This time in seconds gives us the measure of workability of the fresh concrete. This time is expressed in Vee-Bee seconds.
Observation and Calculations in Vee-Bee Test
- Initial reading from the graduated rod, before unmolding (a) in mm
- The final reading on the graduated rod after removing the mold (b) in mm
- Slump = a – b in mm
- The time required for complete remolding in seconds
Hence the consistency of the concrete is measured in ———- vee-bee seconds.
Precautions Necessary in Vee-Bee Test
- The mold should be cleaned and free from moisture internally before adding the concrete mix.
- While the strokes are applied over the layers, care must be taken to apply it uniformly all throughout the layers. This helps in having the impact of the strokes in full depth.
- The removal of the slump cone should be lifted upward in such a way that the concrete cone is not disturbed in any means.
- The vee-bee tests must be conducted at a distance away from any other source of vibration than the vibration procedure provided in the test.
- When a state attains at which the transparent disc rider completely covers the concrete and all the voids and cavities present in the concrete surface get disappeared, the remolding of concrete is attained completely.
Vee-Bee Test Compared with Other Workability Tests
The test procedure of Vee-Bee test carries out the similar procedure that a freshly mixed concrete is subjected to in its actual state. This is an added advantage of Vee bee test when compared with other tests, the slump test and the compaction factor test.
The remolding completion is visually ascertained that can cause difficulty in measuring the end-point and hence have the chances to have errors. This chance of error is more pronounced in the concrete mixes that have higher workability. This mix thus has a lower value for Vee-Bee time.
In the cases of concrete mixes that have slump value greater than 125mm, the phenomenon of remolding is found to be very quick and the time cannot be measured. This means that the Vee bee test is not suitable for measuring the mobility of concrete of higher workability. This higher workability comes in the range of slump value greater than 75mm.
In some situations, this issue is overcome by making use of an automatic operating device that records the movement time. Generally, the Vee-bee test is best suited for concrete mixes that have low or very low value of workability.
Among the three-workability test recommended by IS: 119 -1959 i.e. the slump test, the compaction factor test, and the Vee-bee test, the slump test method is most popular test to measure concrete workability.
Table below shows the Vee-bee time in seconds for various workability as per American Concrete Institute 211 (ACI Committee 211).
Table.1: Consistency Measurement for Different Workability Mix in Slump Test, Compaction Factor Test and Vee -Bee Test