The concrete slump test, also known as the slump cone test, is an essential procedure to evaluate the workability or consistency of concrete mix. It is performed both at the laboratory and on construction sites to ensure that the quality of concrete remains uniform throughout the construction process.
The test is simple to carry out, inexpensive, and provides immediate results, making it a popular choice for workability testing since its introduction in 1922. The procedures for conducting the slump test are defined in ASTM C143 in the United States, IS: 1199 – 1959 in India, and EN 12350-2 in Europe.
The slump value of concrete is primarily used to determine workability, which is an indicator of the water-cement ratio. However, other factors such as the properties of the materials used, mixing methods, dosage, and admixtures can also impact the concrete slump value. Therefore, it is crucial to perform the slump test regularly to maintain uniformity and ensure the concrete mix meets the required specifications.
Factors which influence the concrete slump test:
The properties of cementitious materials, such as their chemistry, fineness, particle size distribution, moisture content, and temperature, can have a significant impact on the quality of concrete. Similarly, the size, texture, combined grading, cleanliness, and moisture content of the aggregates used in concrete production can also play a crucial role in determining the final properties of the concrete.
The use of chemical admixtures in concrete, including their dosage, type, combination, interaction, sequence of addition, and effectiveness, must also be carefully considered. The air content of concrete is another important factor that can affect its performance.
Concrete batching, mixing, and transporting methods and equipment must also be carefully selected and monitored to ensure that the final product is of high quality. Additionally, the temperature of the concrete during production and placement can have a significant impact on its final properties.
The sampling of concrete, the slump-testing technique used, and the condition of the testing equipment are also critical factors that can affect the accuracy and reliability of test results. The amount of free water in the concrete and the time since mixing at the time of testing are also important variables that must be taken into account. Overall, attention to these factors is essential for ensuring that concrete meets the desired performance specifications.
Equipments Required for Concrete Slump Test
The context provided describes the mold used for a slump test, which is an important test conducted in the field of civil engineering. The mold used for the test is in the form of a frustum of a cone, with a height of 30 cm, bottom diameter of 20 cm, and top diameter of 10 cm. The other equipment used in the test includes a non-porous base plate, a measuring scale, and a tamping rod made of steel. The tamping rod is 16 mm in diameter and 60 cm long, with one end rounded. The purpose of the slump test is to determine the workability of concrete, which is an essential property that affects the ease of handling, placing, and compacting of the material. The test involves filling the mold with freshly mixed concrete, compacting it with the tamping rod, and then removing the mold to measure the slump or the degree of deformation of the concrete. The results of the slump test can be used to adjust the water-cement ratio of the mix, which is critical in achieving the desired strength and durability of the concrete.
Sampling of Materials for Slump Test
In the laboratory, a concrete mix is prepared by weighing the necessary components according to the desired ratio, which may be M15 or another specified mix. The appropriate water-to-cement ratio is also taken into consideration during the preparation process. This procedure is similar to what was described in section 5.9.
Once the mix is prepared, a Slump test is conducted to determine the consistency of the concrete. Based on the results of the test, adjustments may be made to the water-to-cement ratio if necessary to achieve the desired consistency.
After the mix has been properly adjusted and meets the required consistency, it is ready for casting. In this case, the objective is to cast six cubes using the prepared mix. This process requires careful attention to detail to ensure that the cubes are properly formed and cured for accurate results.
Figure-1: Measuring Slump of Concrete
Procedure for Concrete Slump Cone Test
To ensure proper concrete casting, it is important to clean the internal surface of the mould and apply oil. Afterward, the mould should be placed on a smooth, horizontal, non-porous base plate. The prepared concrete mix should then be poured into the mould in approximately equal layers, with four layers being the norm.
For each layer, 25 strokes of the rounded end of the tamping rod should be used to tamp the concrete in a uniform manner over the cross-section of the mould. When tamping the subsequent layers, the rod should penetrate into the underlying layer. Once each layer is complete, excess concrete should be removed, and the surface leveled with a trowel.
It is crucial to clean away any mortar or water that may have leaked out between the mould and the base plate. After completing these steps, the mould should be raised from the concrete immediately and slowly in a vertical direction. This allows the concrete to set properly without any disturbances.
Finally, the slump should be measured by finding the difference between the height of the mould and the height of the point on the specimen being tested. Proper attention to detail during these steps will ensure successful and accurate concrete casting.
Figure-2: Concrete Slump Test Procedure
NOTE:
It is important to carry out the above operation, which refers to a particular procedure, in a location that is free from any form of vibrations or shock. This is crucial because any sort of disturbance can have an impact on the accuracy of the results obtained from the procedure. Therefore, the location where this operation is performed should be carefully selected to ensure that it meets the necessary requirements.
Additionally, it is recommended that the operation be completed within a period of two minutes after sampling has been done. This time frame is essential because it ensures that the samples are processed promptly, and there is less chance of any external factors affecting the results. Time management is a critical aspect of this procedure, and it should be given due consideration to obtain the desired outcomes.
Slump Value Observation:
During the test, it is necessary to record the slump or vertical settlement of the specimen in terms of millimeters of subsidence. This measurement is important in determining the behavior of the specimen under load and can provide insight into its structural properties.
The slump refers to the degree of deformation or settling that occurs in the specimen when a load is applied. This can be caused by a variety of factors, including the composition of the material, its moisture content, and the amount of stress it is subjected to.
To accurately measure the slump, it is important to record the amount of subsidence in terms of millimeters. This provides a precise and standardized measurement that can be compared across different specimens and tests.
Overall, recording the slump in terms of millimeters of subsidence is a crucial step in conducting tests on materials and understanding their structural properties. By accurately measuring this parameter, researchers and engineers can gain valuable insights into the behavior of materials under load and make informed decisions about their use in various applications.
Results of Slump Test on Concrete
When conducting a slump test for concrete, the observed shape of the slump can vary. The result of the test is usually expressed in millimeters as the “slump” of the concrete sample. The slump refers to the amount by which the concrete slumps or settles after the test is conducted. The specific value of the slump would depend on the particular sample being tested and can vary from sample to sample. The slump test is an important procedure in determining the workability and consistency of concrete, and the observed slump can provide valuable information about the quality of the concrete mixture being tested.
Figure-3: Types of Concrete Slump Test Results
The True slump is the only type of slump that can be measured in the test. It is determined by measuring the distance between the top of the cone and the top of the concrete after the cone has been removed, as shown in figure-1.
Zero slump, on the other hand, indicates a very low water-cement ratio, resulting in dry mixes. This type of concrete is typically used for road construction.
On the contrary, a Collapsed slump is an indication that the water-cement ratio is too high, resulting in a mix that is too wet or has high workability. In such cases, a slump test may not be appropriate as the result would be unreliable.
Lastly, Shear slump is an indication that the test result is incomplete and the concrete needs to be retested to obtain accurate measurements.