The workability of concrete refers to how easily or difficultly the concrete can be handled, transported, and placed between forms while maintaining its homogeneity. It is crucial to determine the workability of concrete before starting a construction project to ensure that it is suitable for the intended purpose.
If the concrete mixture is too wet, the coarse aggregates may settle at the bottom of the concrete mass, causing the composition to become non-uniform. On the other hand, if the concrete mixture is too dry, it can be challenging to handle and place it in position. Therefore, determining the workability of concrete is critical to ensure that the final product is of the desired quality.
At construction sites, various tests are performed to determine the workability of concrete. Some commonly used tests include the slump test, Vee-bee test, and Compaction factor test.
The slump test involves filling a mold with fresh concrete and then removing it to measure the slump or the difference in height between the top of the mold and the highest point of the concrete. The Vee-bee test involves measuring the time taken for a vibrating table to compact the concrete and reach a predetermined level of workability. The Compaction factor test measures the workability of concrete by determining the ratio of the weight of the concrete before and after compaction.
The recommended values for these tests may vary depending on the intended use of the concrete. For instance, a higher slump value may be suitable for a concrete mix used in a foundation, while a lower value may be preferred for a mix used in a column. Therefore, it is essential to select the appropriate test and values for the specific project to ensure the desired quality of the final product.
Tests for Workability of Concrete
Concrete mixture workability can be evaluated through several tests. These tests include laboratory tests such as the Vee-bee consistometer test and the compaction factor test, as well as field tests like the slump test.
The Vee-bee consistometer test and the compaction factor test are both laboratory tests that measure the workability of a concrete mixture. These tests are conducted in a controlled environment and require specialized equipment to measure the consistency and flow of the concrete.
On the other hand, the slump test is a field test that can be conducted on-site to assess the workability of a concrete mixture. This test involves filling a cone-shaped mold with concrete and then removing the mold to observe the slump or deformation of the concrete.
Overall, these tests are important in determining the workability of a concrete mixture, which is essential for ensuring the successful placement and finishing of concrete structures. While laboratory tests provide more precise measurements, field tests like the slump test provide a quick and convenient way to evaluate concrete workability in real-world settings.
Concrete Slump Test
The test is conducted using a mould called a slump cone, which has a top diameter of 10 cm, a bottom diameter of 20 cm, and a height of 30 cm. The procedure for the test can be summarized in the following steps: First, the slump mould is placed on a smooth, flat, and non-absorbent surface. Then, the dry ingredients of the concrete are mixed thoroughly until a uniform color is achieved, and the required amount of water is added. Next, the mixed concrete is placed in the mould, filling it to approximately one-fourth of its height.
The process of compacting concrete involves several steps. Firstly, the concrete is compacted 25 times using a tamping rod, ensuring uniform coverage over the entire area. Next, the concrete is placed in the mould, filling it to about half of its height, and then compacted again using the tamping rod. The same process is repeated as the concrete is filled up to three fourths of its height and finally to the top of the mould. Each layer is compacted 25 times with the tamping rod, and for subsequent layers, the rod should penetrate into the underlying layers for better consolidation.
Once the mould is filled to the top with the concrete, the excess is struck off using a trowel or tamping rod, ensuring that the mould is completely filled. The mould is then immediately removed, taking care to lift it in a vertical direction to avoid disturbing the concrete. The settlement of the concrete is allowed to stop, and the subsidence of the concrete is measured in millimeters, which gives the required slump of the concrete.
Figure: Measurement of slump
Suitability of Slump Test:
The slump test is a method used to measure the consistency or workability of fresh concrete. It involves filling a slump cone with freshly mixed concrete and then removing the cone to observe the slump, or the amount of settling or subsidence of the concrete.
However, it is important to note that the slump test is not suitable for all types of concrete. Specifically, it is generally recommended for use with concrete of high or medium workability. This is because if the concrete has low workability, it may not be able to be properly tested using the slump test method. In such cases, other methods may need to be used to determine the consistency of the concrete.
Therefore, it is important to consider the specific properties of the concrete being tested and to use appropriate testing methods based on those properties. While the slump test can be a useful tool for measuring the consistency of fresh concrete, it may not be appropriate in all situations, particularly when working with concrete of low workability.
Recommended Values of Concrete Slump Tests for Various Purposes:
| No. | Types of concrete | Slump |
| 1 | Concrete for road constructionhttps://82d83799462603984fa1d11a4579b4f8.safeframe.googlesyndication.com/safeframe/1-0-40/html/container.htmlReport this ad | 20 to 40 mm |
| 2 | Concrete for tops of curbs, parapets, piers, slabs and wall | 40 to 50 mm |
| 3 | Concrete for canal lining | 70 to 80 mm |
| 4 | Normal RCC work | 80 to 150 mm |
| 5 | Mass concrete | 20 to 50 mm |
| 6 | Concrete to be vibrated | 10 to 25 mm |