Concrete is widely used in construction projects. However, improperly mixed concrete or a lack of uniformity in the different batches compromises the strength and durability of construction work. How do you ensure the quality of a concrete mix and uniformity for the different batches? This is where the concrete slump test comes in!
The slump test measures the workability of a concrete mix and the consistency between batches. It can help you identify defects in your mix so you can make amendments and ensure you use quality concrete that delivers strong and durable construction works.
This article will tell you everything you need to know about the Slump test. We’ll examine what the slump test is, the equipment and procedures for doing it, how to interpret the result, and the precautions to take when doing the test.
Understanding the Concrete Test for Slump
Concrete should meet a certain quality standard before it is poured and used in a construction project. The slump test is just one of the ways to ensure the quality of freshly mixed concrete.
The concrete slump test measures the consistency of workability of the concrete mix. It provides immediate results and is low cost.
Why is a Slump Concrete Test Important?
The slump test can do the following:
Identify Defects in a Concrete Mix
The slump concrete test measures the workability of a concrete mix or how easily the fresh concrete flows.
Workability tests can help you identify defects in your concrete mix. For example, it can reveal whether your water-cement ratio is okay, too low, or too high.
Needless to say, a low or high water-cement ratio affects the strength and durability of the concrete work.
Determine the Consistency of Different Concrete Batches
The slump test also measures the uniformity of concrete between different batches used as part of a construction.
Know that the workability of concrete often varies between concrete batches. However, working with inconsistent concrete batches compromises the quality of the construction work.
Thus, using the slump test to ensure concrete batches are consistent helps you deliver quality concrete works with high strength and durability.
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Common Factors that Influence the Slump Concrete Test
The factors that affect the concrete slump are as follows:
Water Content
The water-cement ratio is critical in determining the slump of concrete. Typically, the higher the water content, the more fluid the mix, and the greater the slump in the test.
Chemical Admixtures
Admixtures are agents used to improve the behavior of concrete. They come in different forms and perform different functions. For example, water-reducing admixtures or plasticizers can reduce concrete water demand to reduce the slump in the test.
That said, the admixture type, dosage, sequence of addition, and effectiveness can affect the slump test.
Aggregates Properties
The size, shape, and grading of the aggregates in the concrete affect workability (hence effects slump).
- Grading: Because well-graded aggregates fill up voids better than their angular counterparts, they get workability easily. As a result, well-graded aggregates produce a higher slump than poorly-graded aggregates.
- Shape: Because more-rounded aggregates have less surface area and friction resistance than angular aggregates, they get workability easily. As a result, more-rounded aggregates show higher slump than angular aggregates.
- Size: Because bigger aggregates demand less water for wetting, they easily get workability than finer aggregates. Thus, increases in coarse aggregate size usually correspond to an increase in slump value.
- Moisture content: Aggregates with higher moisture content attain workability easily. As a result, they produce a higher slump.
Cement Properties
The chemical characteristics of the cement used in the concrete can also affect slump. For example, the higher the alkali content, the lower the workability of concrete. Hence, the lower the slump.
Mix Proportion
The proportion of cement, sand, and aggregates in the concrete affects slump. So, different mix designs will produce different slumps. For example, a rich concrete mix (with high cement content) is more workable. Hence, it will produce a higher slump than concrete with lower cement content.
Air Content of Concrete
Air bubbles within concrete act as a lubricant to increase the fluidity of the concrete mix. Hence, air-entrained concrete produces a higher slump than non-air-entrained concrete.
Temperature
The ambient temperature and the temperature of the concrete mix can affect slump. Higher temperatures accelerate setting times and reduce slump, while colder temperatures will produce higher slump because of delayed setting times.
Mixing Time and Method
Mixing time and method can also affect slump. For example, continuing to mix concrete after blending is achieved causes fluidity loss, reducing the workability of the mix. Hence, a long mixing time produces a lower slump.
Time
As concrete begins to set, its slump decreases. So, allowing a long time between mixing the concrete and testing will produce a lower slump.
Testing Method and Equipment
Not using standardized equipment and following standardized procedures can affect the result. For example, you may read off a lower or higher slump if you do not use calibrated equipment.
Transportation and Handling
How you transport and place the concrete can affect slump. For example, vibration during transportation can remove entrapped air, making the concrete less fluid. Hence, causing a reduction in slump.
International Standards for Concrete Slump Test
The slump test is carried out according to the procedures outlined in the following standards:
- ASTM C143 in the United States
- IS: 1199 in India
- EN 12350-2 in Europe
Equipment and Materials Required for Concrete Test
The materials needed for the concrete slump are:
- Slump cone: A mold for creating a concrete cone to measure its slump. It’s a frustum of a cone, measuring 30cm in height, 20cm in bottom diameter, and 10cm in top diameter.
- Non-porous base plate: To place the mold to make the concrete cone.
- Slump cone filling funnel: To guide the concrete into the slump cone to avoid spillage.
- Sample scoop: To scoop concrete into the slump cone.
- Tamping rod: A rod for tamping the concrete in the slump cone. It’s usually a steel rod rounded at one end, measuring 60mm in length and 16mm in diameter.
- Trowel: To make the concrete in the slump cone level with the top of the cone.
- Steel rod: To gauge the vertical displacement of the concrete cone after it slumps.
- Measuring tape: To measure the slump of the concrete cone.
- Scrub brush: To clean concrete spills on the base plate (around the concrete cone).
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Procedures for a Concrete Slump
The procedures for conducting slump are as follows:
- Place the slump cone on the flat, smooth, non-porous plate.
- Fill one-third of the slump cone with the prepared concrete mix and tamp 25 times with the rod.
- Fill another one-third of the cone and tamp 25 times.
- Fill the last one-third of the cone and tamp 25 times.
- Remove excess concrete from the top of the cone, ensuring that the concrete is level with the cone’s top.
- Lift the cone off the concrete in a vertical direction. The concrete will slump (subside) when the cone comes off.
- Place the cone upside down next to the concrete.
- Place the steel rod across the top of the cone so it overhangs the concrete.
- Measure the slump by checking the vertical distance between the rod and the top of the slumped concrete. The slump is measured to the nearest 5mm.
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Results of a Concrete Slump Test
The level of slump is usually one of the following:
Zero Slump
In a zero slump, the concrete does not subside at all but retains its shape completely.
The zero slump level indicates a very low water-cement ratio, making the mix too dry. These kinds of concrete are fine for road construction.
True Slump
In a true slump, the concrete subsides a little but more or less retains its shape. The true slump is the most desirable, as it shows that the mix is cohesive and its workability is not too high.
Shear Slump
In a shear slump, the concrete subsides dramatically as one half falls more than the other. The concrete leans to one side as the top portion shears off and slips sideways.
The shear level of slump indicates that the test is inconclusive, meaning the concrete has to be retested.
Collapsed Slump
In a collapsed slump, the concrete does not retain its shape at all but collapses completely.
The collapsed slump indicates a high workability mix. It shows a very high water-cement ratio, making the mix too wet. These concrete need to be corrected before they can be used.
Precautions to Take During Concrete Slump Test
Some precautions to take to ensure reliable results when conducting the slump test include:
- Do the test in a place free of vibrations and shocks. This is because vibrations will make the concrete cone subside more than it otherwise would.
- The cone and base plate should be non-porous. This ensures they do not absorb liquid and impact the concrete’s behavior.
- Clean the slump cone before using it so it is clear of contaminants that may affect the behavior of the concrete.
- Moisten the inside of the mold and base plate before using them for the test. This reduces surface friction.
- Use freshly prepared concrete. Concrete begins to set if you wait too long to test after mixing.
- Use a trowel to ensure the concrete flushes with the mold after the third fill so that excess concrete will not affect the slump.
- Tamp each layer of concrete 25 times in a uniform manner. When tamping a layer, ensure the tamping rod penetrates the layers beneath it.
- Before lifting the mold, clean off any concrete and water that spill around the base of the cone. This ensures nothing interferes with the movement of the concrete as it slumps.
- Lift the mold off the concrete vertically so you do not disturb the concrete cone.
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Takeaway: Conduct a Slump Test and Ensure the Quality of your Fresh Concrete
The concrete slump test measures the workability of a concrete mix and the consistency between batches. It can help you identify defects in your concrete mix so you can make amendments and ensure you use quality concrete that delivers strong and durable construction works.
Following the procedures and precautions above, you’ll have no problem conducting a concrete slump test.
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