Vee Bee time, also known as Vee Bee Degree, is the time in seconds it takes for a concrete sample to transform from a slump cone shape to a cylindrical shape under vibration. It's a measure of the workability and consistency of concrete.
Understanding Vee Bee Time
The Vee Bee test is used to assess the workability of concrete mixes, particularly those with low workability, such as those used in pavements and precast concrete. A concrete mix with low workability requires a significant amount of vibration to consolidate and achieve proper density. Vee Bee time provides a quantifiable measure of this required effort.
How the Vee Bee Test Works
The Vee Bee test involves a specific procedure:
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Slump Test: A standard slump test is performed on the concrete sample, filling a slump cone in three layers, each compacted.
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Placement in Vee Bee Consistometer: The slump cone is carefully lifted, and the slumped concrete is placed inside a Vee Bee consistometer. This apparatus includes a vibrating table, a slump cone, and a transparent disc.
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Vibration and Timing: The vibrator is switched on, and a transparent disc is gently lowered onto the top of the concrete.
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Measuring Vee Bee Time: The time it takes for the concrete to be fully compacted and to take on a cylindrical shape, as determined visually through the transparent disc, is recorded as the Vee Bee time (in seconds). The end point is reached when the entire surface of the concrete around the transparent disc has become smooth and cylindrical.
Interpreting Vee Bee Time Results
- Low Vee Bee Time (e.g., 0-5 seconds): Indicates high workability. The concrete is easily compacted.
- Medium Vee Bee Time (e.g., 5-12 seconds): Indicates medium workability. The concrete requires some vibration to compact.
- High Vee Bee Time (e.g., above 12 seconds): Indicates low workability. The concrete requires significant vibration to achieve full compaction. This type of mix is often used in precast applications where high density and strength are required.
Factors Affecting Vee Bee Time
Several factors influence the Vee Bee time of concrete:
- Water Content: Higher water content generally leads to lower Vee Bee time (increased workability).
- Aggregate Properties: The size, shape, and surface texture of aggregates affect workability. Rounded aggregates generally improve workability compared to angular aggregates.
- Cement Content: Higher cement content can improve workability to a certain extent.
- Admixtures: Water-reducing admixtures (plasticizers and superplasticizers) can significantly reduce Vee Bee time and improve workability without increasing the water content.
- Fine Aggregate Content: The proportion of fine aggregates to coarse aggregates also impacts the workability.
Advantages of the Vee Bee Test
- Provides a quantitative measure of workability, which is subjective to assess otherwise.
- Suitable for low workability mixes where slump test is not as sensitive.
Disadvantages of the Vee Bee Test
- Not suitable for highly workable concrete mixes.
- Requires specialized equipment (Vee Bee consistometer).
- Results can be influenced by operator judgment in determining the endpoint.
