Workability Requirements for Different Concrete Placement:
The workability of concrete is an important factor in determining the type of construction that can be undertaken. It is usually determined by design engineers based on the required concrete strength, durability and ease of construction at the site. Highly workable concrete is recommended for constructions that involve a large percentage of reinforcement, where the gap between reinforcement is small and effective compaction cannot be achieved. Self-compacting concrete is a suitable option in such cases. However, increasing the workability of concrete by adding more water can reduce its strength. Therefore, workability-enhancing concrete admixtures such as air-entraining agents are often used to improve workability without compromising on the strength of concrete. It is important to note that the use of admixtures may also affect the strength of concrete. Therefore, for critical components of a building such as slabs, beams, and columns, and for structures where the strength of concrete is of utmost importance, a low degree of workability is recommended.
On the other hand, for mass concrete and lightly reinforced concrete structures, a low degree of workability can be used. It is important to strike a balance between workability and strength of concrete to ensure that the structure can withstand the intended loads and environmental factors. Design engineers need to consider various factors before deciding on the degree of workability required for a particular construction. Ultimately, the choice of workability of concrete depends on the specific requirements of the construction project, and the design engineers must take into account all relevant factors to make an informed decision.
The workability of concrete can be evaluated by conducting a slump test during the mixing process. The test measures the degree of consistency of the concrete mixture. Different degrees of workability are required for various placement conditions of concrete, and the recommended slump values for each degree of workability are listed in a table. By using the appropriate slump value, the concrete can be properly placed and compacted, resulting in a high-quality finished product.
Degree of Workability for Different Placing Conditions
| Placing Condition | Degree of concrete workability | Slump (mm) |
| Binding concrete, Shallow Sections Pavement using pavers | Very low | Use compacting factor test as per IS: 1199 |
| Mass concrete, Lightly reinforced sections in slabs, beams, walls, columns, floors, hand placed pavements, canal linings, strip footings | Low | 25-75 |
| Heavily Reinforced sections in slabs, columns, walls, slipform work, pumped concrete | Medium | 50 – 100, 75 – 100 |
| Trench fill, in-situ piling | High | 100 – 150 |
| Tensile concrete | Very high | Use determination of flow test as per IS: 9103 |