Are you preparing to construct your dream home? Or are you about to renovate your personal space? Concrete will become the major element of your construction project. Hence, you have to ensure the quality of concrete and must ensure its workability to get better results. The slump test is one of the methods used to determine the workability of concrete. In today’s blog of Brick & Bolt, you are going to learn more about concrete slump testing, its importance, why it matters, and how it’s performed.
But first, we need to understand the workability of concrete and why it is important.
What is the Workability of Concrete and Its Importance?
Workability of concrete is a term that signifies how easily the freshly prepared concrete can be placed or poured, tamped, and cured without segregation or unwanted bleeding. If workability is not ideal, the structures are likely to exhibit air pockets and voids that affect the quality, strength, and durability of the completed building. Correct workability means that the concrete can be filled perfectly in the forms, spread around the reinforcement and compact with no air pockets or voids to be found. It also correlates with the surface finish and the overall construction or installation process.
Workability is significant for the performance of the final concrete by meeting quality, strength, and durability while reducing construction time and labour costs. Proper workability guarantees that structures are built of high quality and last long as well. Workability can be measured by slump test, compaction factor test, vee bee consistometer, key ball test, and flow test. Among all the workability tests, the slump test is widely used at the construction site due easiness of its performance.
What is the Slump Value of Concrete?
The slump value of concrete indicates its consistency and workability before it gets set. It signifies how easily the concrete can be placed, consolidated, and finished without segregation. The slump test is generally used to assess the freshly prepared concrete’s fluidity and workability.
Importance of the Slump Test
The concrete slump test is very important for the following reasons:
- Workability assessment of concrete: By conducting the slump test, you can determine whether the concrete is suitable for the intended application or not.
- Water content indication: The test allows for adjusting the water content to be suitable for the specific project, reducing any excess water.
- The moulding capacity of concrete: The slump value of concrete shows how easily the concrete can be placed and consolidated.
- Verification of mix design: Reaffirms that the prepared concrete meets specified requirements.
Slump Cone Test Equipment
To conduct the slump cone test, the following are the essential apparatus:
- Slump cone: height 300 mm, bottom diameter 200 mm, and top diameter 100 mm.
- Measuring Tape
- Standard tamping rod
- Base plate
Slump Test Procedure
The following are the general steps taken while conducting the slump test:
- Clean the inner surface of the mould and apply oil.
- Set the mould onto a non-porous, horizontal base plate.
- Pour three equal layers of freshly prepared concrete into the mould.
- With the usual tamping rod, tap each layer 25 times to make it more compact. After compacting, level the top surface of the concrete.
- Level the upper surface of the concrete after compacting.
- Gently lift the mould in an upward direction, leaving the concrete cone in place.
- Gauge the difference in height between the mould and the concrete sample. This difference is referred to as the slump, measured in millimetres
Different Types of Concrete Slump
The following are the major types of concrete slump based on the concrete slump test values:
- True Slump
- Shear Slump
- Collapse Slump
- Zero Slump
1. True Slump
When someone lifts the cone in an upward direction, the concrete sample keeps its shape and slumps roughly the same symmetrical shape as the cone, it is called a true slump. Although each project and particular type of concrete may have different genuine slump specifications, general slump values typically range from 1 to 8 inches (25 to 200 mm). This kind of slump signifies that the mixture is quite workable. True slumps are usually found in concrete mixtures that have the right amount of water and a proportionate amount of cement, additives, and particles.
2. Shear Slump
A shear slump occurs when one-half of the cone-shaped concrete mass slides down or shears off an angle of 45 degrees or more from the other half. This type of slump often indicates issues with the mix’s cohesion and internal friction. It might be caused by improper proportions of fine and coarse aggregates or an improper grading of aggregates. Shear slumps indicate potential challenges in achieving uniform compaction and might result in uneven concrete surfaces.
3. Collapse Slump
When someone lifts the cone, the concrete slump collapses or flattens out. It’s taken to be a collapse slump. This is attributed to the very high amounts of water and superplasticisers in concrete. Collapse slumps serve as an indication to modify the water-cement ratio of the concrete mix and reconsider the proportion of the other ingredients.
4. Zero Slump
Zero slump occurs to the dry concrete mixes with less water content. If the mixture keeps its shape the same as the mould without any settling or deformation after the removal of the cone, it’s considered Zero slump. It implies very low workability, making it difficult to place properly and compact the concrete.
Concrete Slump Ranges for Different Types of Requirements
Depending on the type of construction, these various techniques and specifications for project requirements give acceptable ranges for concrete slumps. General considerations include:
- Foundations and footings: 25-75 mm
- Slabs and pavements: 50-100 mm
- Reinforced concrete walls and columns: 50-125 mm
- Mass concrete: 25-50 mm
- Pumped concrete: 75-150 mm
Factors Affecting Slump Value of Concrete
The Slump value of concrete can vary based on the following key factors:
- Amount of Water: The water-cement ratio or quantity of water significantly influences the slump value.
- Cement content and type: The type of cement that is being used in the concrete determines its workability.
- Aggregate size and shape: Aggregates of various sizes have unique characteristics that influence slump value.
- Admixtures: Chemical admixtures can modify slump without changing water content.
- Temperature: Higher temperatures may reduce slump over time.
- Time: The slump generally decreases as the concrete is set.
Limitations of the Slump Test
While the slump test is helpful, it also has some limitations:
- It cannot determine durability or strength directly.
- Results depend upon the skill of the person performing the test.
- It may not be suitable for all types of concrete mixes, especially those with very low or very high workability.
- While the slump test is useful for many concrete mixes, some specialised types, like self compacting concrete, may require different workability tests.
Slump testing is a simple but powerful technique to determine the workability of concrete. Understanding slump values provides the assurance that concrete mixes will meet the required specifications for workability and consistency, which in turn augments the overall quality and durability of the concrete structure. Although it has its own limitations, the test is widely used and considered necessary in current concrete quality control. The integration of slump testing with other quality control measures will ensure that competent construction