The workability of concrete, which refers to the ease with which it can be mixed, placed, consolidated, and finished, is measured using several tests, each evaluating different aspects of the material's behavior. Here are the common methods:
Methods for Measuring Concrete Workability
1. Slump Test
The slump test is a simple and widely used method for assessing the consistency (wetness) of concrete. It primarily measures the plasticity and cohesiveness of the mix.
Procedure:
- A standard slump cone is filled with concrete in three layers, each compacted with a rod.
- The cone is then carefully lifted, and the amount the concrete slumps or drops down is measured.
- The slump value indicates the workability. A higher slump generally suggests higher workability (more flowable), but can also indicate a less cohesive mix prone to segregation.
Limitations:
- Primarily suitable for concrete with medium to high workability.
- Less sensitive to changes in mixes with very low or very high workability.
- Doesn't provide information on cohesiveness or finishability.
2. Compacting Factor Test
The compacting factor test is considered more accurate than the slump test, particularly for concrete mixes with low workability. It measures the compactibility of the concrete under standard conditions.
Procedure:
- Concrete is placed in the upper hopper of the compacting factor apparatus and allowed to fall into a lower hopper, and then into a cylinder.
- The weight of the partially compacted concrete in the cylinder is measured.
- The cylinder is then fully compacted, and the weight of the fully compacted concrete is measured.
- The compacting factor is the ratio of the weight of partially compacted concrete to the weight of fully compacted concrete. A higher compacting factor indicates greater workability.
Advantages:
- More reliable for low workability mixes.
- Less subjective than the slump test.
3. Vee-Bee Consistometer Test
The Vee-Bee consistometer test is used to determine the workability of concrete by measuring the time required for a given mass of concrete to be remolded into a cylinder shape through vibration. It reflects the mobility and finishability of the concrete.
Procedure:
- A slump cone is placed inside the Vee-Bee apparatus.
- The cone is filled with concrete, and the slump is measured.
- A transparent disc is placed on top of the slumped concrete, and the vibrator is switched on.
- The time taken for the concrete to be remolded into a cylindrical shape, as observed through the transparent disc, is recorded. This time is the Vee-Bee degree, and a lower Vee-Bee degree indicates higher workability.
Advantages:
- Suitable for a wide range of workabilities, including dry mixes.
4. Flow Table Test
The flow table test is primarily used for measuring the workability of self-compacting concrete (SCC) or highly flowable concrete. It assesses the concrete's flowability and segregation resistance.
Procedure:
- A slump cone is placed on a flow table and filled with concrete.
- The cone is lifted, and the table is dropped a specified number of times.
- The diameter of the concrete spread is measured in two perpendicular directions.
- The average diameter is the flow value, which indicates the flowability of the concrete. A higher flow value signifies greater workability.
Advantages:
- Specifically designed for highly flowable concrete.
- Provides a good indication of segregation resistance.
5. Kelly Ball Test
The Kelly ball test is an in-situ test that measures the penetration depth of a hemispherical steel ball into the fresh concrete. It provides a quick assessment of workability on the construction site.
Procedure:
- The Kelly ball apparatus is placed on the surface of freshly placed concrete.
- The ball is allowed to penetrate the concrete under its own weight.
- The depth of penetration is measured using a graduated scale on the handle of the apparatus. A greater penetration depth indicates higher workability.
Advantages:
- Quick and easy to perform on-site.
- Provides immediate feedback on concrete consistency.
Summary Table
| Test | Measures | Suitable for | Advantages | Limitations |
|---|---|---|---|---|
| Slump Test | Consistency (wetness), plasticity | Medium to high workability concrete | Simple, widely used | Less sensitive to low/high workability, doesn't measure cohesiveness |
| Compacting Factor Test | Compactibility | Low workability concrete | More accurate than slump test for low workability | |
| Vee-Bee Consistometer | Mobility, finishability | Wide range of workabilities | Suitable for dry mixes | |
| Flow Table Test | Flowability, segregation resistance | Self-compacting concrete (SCC) | Designed for highly flowable concrete, indicates segregation resistance | |
| Kelly Ball Test | Consistency (penetration resistance) | In-situ testing of fresh concrete | Quick, easy to perform on-site |
The selection of the appropriate workability test depends on the type of concrete mix, the desired workability level, and the specific application. Understanding the principles and limitations of each test is crucial for accurate assessment and quality control of concrete.
