The slump test measures the consistency and workability of fresh concrete. This is to see how well the freshly mixed concrete will flow, settle and hold its shape after placement so the material will perform for the project. Slumps can indicate many things in the concrete sample, whether the batch is too wet, too dry or just right. These are important for pavements, hard standing slabs, trench filled foundations and other critical parts of modern infrastructure.
In simple terms the concrete slump test is used to measure the workability or consistency of hydraulic cement concrete. A concrete slump is a measure of how much a pile of concrete mixture settles after a concrete slump cone is filled and then removed. This simple test on hydraulic cement concrete gives an indication of the concrete’s ease of handling, placement and ability to consolidate around reinforcement like steel.
The concrete slump test is a standard procedure for testing the workability and consistency of fresh concrete mixes before they are poured. It is done using a concrete slump cone, sometimes called an Abrams cone, which is a hollow metal mold that is open at both ends and has a flat base. To fill the cone, fresh concrete is added in three layers, each layer is compacted with a steel rod. Once the cone is filled and levelled at the top, it is lifted carefully and the concrete is allowed to “slump” under its own weight. The concrete slump test measures the concrete slump value, which is the distance the concrete has dropped from its original shape.
There are several types of concrete slump in a slump test which indicates the workability and consistency of the concrete:
True Slump: In a true slump the concrete settles evenly, maintains its original shape but with a decrease in height. This means the mix has a good balance of workability and cohesion which is ideal for normal reinforced concrete structures like foundations.
Shear Slump: This type of concrete slump occurs when part of the batch shears off and slips sideways. A shear slump means the mix is not well balanced, either due to poorly freshly mixed concrete batch or too much water which can lead to poor performance in the final structure.
Collapse Slump: A collapse concrete slump occurs when the concrete collapses completely when the cone is removed. This is usually due to excess water in the mix which can reduce the strength and durability of the slabs or foundations being built. A collapse slump is not desirable for most projects but may be acceptable for specialist applications that require very fluid concrete.
Zero: If the concrete does not collapse or settle at all when the cone is removed this is called a concrete slump at zero. This means the mix is very dry and lacks workability and is not suitable for most projects unless used in specific conditions like strip footings or pipework bedding. Zero slump concrete is too stiff to be workable under field conditions without extensive vibration or water reducing admixtures.
How to Measure Slump with Plastic Slump Cones
Measuring slump is a simple process. Once the cone is removed, measure the height of the slumped concrete using a ruler or straightedge placed horizontally across the top of the mold. The difference between the height of the original cone and the slumped concrete is the slump value which is usually measured in inches or millimeters. The average slump value will give an indication of the workability of concrete and consistency of the mix and if the mix is suitable for the job.
The slump value can vary due to many factors such as water cement ratio, admixtures, presence of reinforcement, shape and size of aggregate used in the concrete. For example low values means dry concrete and high concrete slump values means wet concrete mix. In most cases the right consistency for a batch is between true slump and moderate slump value depending on the project requirement. All improperly freshly mixed batch should be discarded. Ensuring multiple batches of the same concrete mix meets the required specification is important for uniformity in strength and workability. An improperly mixed batch can cause confusion in the results.
Slump Test Equipment and Preparation
To do a slump test you need specific equipment to get accurate and reliable results. The required tools are:
Slump cone mold: A non-porous, frustum shaped cone with a height of 30 cm, a bottom diameter of 20 cm and a top diameter of 10 cm.
Steel tamping rod: A 16 mm diameter, 60 cm long steel tamping rod with a rounded end to compact the concrete in the slump cone mold.
Measuring tape or scale: To measure the concrete slump value with the steel tamping rod
Level: Optional but recommended to ensure the base plate is level.
Tamping tool: Optional but can be useful.
Before you start the slump test make sure all equipment is clean and free of any previous test residue. The inside surface of the concrete slump cone mold and the base plate should be damp to prevent the concrete from sticking. Place the concrete slump cone mold on the base plate, make sure it’s on a flat surface. This is very important to get accurate and consistent results.
Slump Test Results Interpretation
Interpreting the slump test results is important to know the workability and consistency of the concrete mix. Here’s what different concrete slump types mean:
True Slump: The concrete mass drops evenly without any disintegration, good workability and balanced mix.
Shear Slump: One side of the concrete shears or falls away from the main portion, means mix issue, poor mixing or imbalance in water cement ratio.
Collapse Slump: The concrete collapses completely, means high water cement ratio and mix strength and durability issue.
Zero Slump: The concrete maintains the shape of the mold, means very low workability not suitable for most applications.
Excess Concrete: If there’s excess concrete at the top of the slump cone mold it can affect the test and give misleading results.
Follow the standard safety procedure and retest if the results are not clear or show mix issue. Accurate interpretation of slump test results help you to make necessary adjustment to the concrete mix to get the optimal performance.
Factors Affecting Concrete Slump
Several factors can affect the concrete slump, the test results and the mix workability:
Water cement ratio: Higher water cement ratio means higher concrete slump value, means more fluid mix. Lower water cement ratio means lower slump value, means stiffer mix.
Aggregate size and type: Larger aggregate means higher slump value, smaller aggregate means lower slump value. Shape and texture of aggregate also matters.
Cement type and content: Different type and amount of cement can affect the slump value, the mix consistency and workability.
Admixtures: Chemical admixtures can alter the workability of the concrete, the slump value. For example water reducing admixtures can increase workability without increasing the water content.
Temperature and humidity: Extreme temperature and humidity can affect the slump value, higher temperature generally reduce workability and higher humidity can increase it.
Knowing these factors is important to interpret the slump test results and make necessary adjustment to the concrete mix to get the desired workability and performance.
Concrete Slump Tests on Site
Concrete slump tests are a procedure to check the workability and consistency of fresh concrete at construction site. It’s a quick and easy way to check if a concrete mix ready for placement will meet the project specification. On site especially for big project with multiple loads of concrete, it’s important to ensure each batch meet the required consistency. This is more important for concrete designed for hard standing slabs and strip footings where the mix should flow easily and set with minimal defects.
Concrete technologist or site supervisor can use slump test to check if any adjustment need to be made to the mix, like add more water or cement to improve workability or consistency. They can also detect the problem in the batch if the slump value is too low or high. Higher slump means too much water and will segregate during placement. Lower slump means the mix is too dry for easy handling and will crack or poor adhesion in the final structure.
Slump in Different Applications
The slump value required for a project will depend on the application and condition. The slump of the concrete between 2-4 inches (50-100 mm) is typical for light reinforcement structure like walls, strip footings or pipework bedding. Higher slump value is required for project that need more fluidity like placing concrete in tight spacing or complex reinforcement grid. Dry mix with zero slump is rarely used except for trench filled foundation or where vibration is used to settle the concrete.
In some specialized application like concrete pavement where high precision is required the slump test becomes more important. Low slump concrete is used in strip footings, slabs and pavements because it has more strength and water resistance while higher slump is used in areas that requires more flexibility in placement.
Problems and Variability in Concrete Slump Testing
While the concrete slump test is a widely used method, it has its limitation. Concrete slump testing may not give accurate picture of the workability for concretes with water reducing admixtures or for mixes designed to have high flowability without high water content. In such cases concrete slump testing may give misleading result, the concrete is too wet when in fact it’s been engineered for specific flow and consistency.
Another problem is field variability. Differences in how the test is performed like how the cone is filled or how the concrete is compacted can result to inconsistent or unreliable slump value. It’s important to ensure that the test is conducted by trained personnel using standardized procedure as outlined in guidelines like ASTM C143 or BS EN 12350-2 to minimize error and get consistent result.
Limitations and Others
While the slump test is widely used, it has some:
Aggregate size: Not suitable for concrete with aggregate size larger than 40 mm as larger aggregate can affect the result.
Slump range: Not suitable for concrete with very high or very low slump value (outside 10-220 mm) as it may not give reliable information about the mix workability.
For concrete mixes that fall outside these parameters, other tests can be used:
Flow table or slump-flow test: This test is used for concrete that is too fluid to be measured using the standard slump test, gives information about the mix flowability.
Vebe test: This test measures the time for a certain amount of concrete to flow through a narrow opening, gives information about the mix workability.
Compaction factor test: This test measures the density of the concrete after compaction, gives additional information about the mix consistency and workability.
These other tests can supplement the slump test, gives a more complete understanding of the concrete mix properties and ensures it meets the project requirement.
Advantages of Slump Testing
Despite the limitations the slump test has some advantages in ensuring a concrete mix is workable and suitable for the project. It’s a quick and easy way to check the workability of fresh concrete to avoid problem in placement and finishing stage. This is more important in big project where multiple load of concrete is delivered and consistency must be maintained across the entire project.
By doing slump testing, site supervisors and engineers can ensure the concrete has the right consistency for easy handling, placement and consolidation to minimize the risk of segregation, cracks or other defects in the finished structure. Also the test will avoid excess concrete being wasted or wrongly mixed, time and money saved in the project.
The concrete slump test is a key tool in determining the workability and consistency of fresh concrete in various construction applications. Whether building reinforced concrete structure, constructing pavement or placing concrete in foundation and walls, the slump test gives you an idea how the concrete mix will perform. Simple as it may be, this test is crucial in ensuring the quality and durability of modern infrastructure, concrete technologists and engineers must have control of the concrete batch and get the desired result.
By knowing the different types of slumps like true slump by an experienced concrete technologist, collapse slump, shear slump and zero slump, the project team can evaluate the concrete mix and make necessary adjustment before placement. The slump value is an indicator of potential problem like water or wrongly mixed batch, so corrections can be made on the fly. In the end the concrete slump test is a simple but effective way to ensure the concrete is right for the job to avoid costly mistake and get strong structure.
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