What Is Concrete Slump and How Do I Test for It?

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In this article, we’ll be sharing with you everything you need to know about concrete slump and how to test for it.

Concrete slump is essentially a measure of how workable a batch of concrete is. A concrete slump test is performed by filling a conical mould with wet concrete and then observing how the concrete flows when the mould is removed.

Why Is Concrete Slump Testing Important?

If you’re going to be pouring concrete for a job, it’s important to make sure that the concrete you’re using is of the right consistency. In addition, if you’re pouring multiple batches of concrete, you need a way to ensure that each batch is of the same quality. That’s where a slump test comes in.

In short, a slump test measures how easily a batch of concrete flows, which is determined by its water content. A batch of concrete with very little water is going to have a low slump, whereas concrete with a lot of water will have a high slump.

Concrete with a lot of water is easier to work with, but isn’t as strong as dry concrete. Therefore, it’s important to have just the right amount of water content in your concrete to find the balance between strength and workability.

What Equipment Do I Need for a Concrete Slump Test?

Fortunately, you don’t need any kind of expensive specialty equipment to perform a slump test. There are one or two things you will probably need to get if you haven’t performed a slump test before, but these shouldn’t be too hard to acquire. 

Here’s what you’ll need for a slump test:

  • Slump cone: This is the mould that you’ll use for the test. Look for a 12″ tall mould for the best results.
  • Tamping rod: A metal rod used to tamp the concrete down inside the slump cone.
  • Baseplate: This is what the slump cone sits on during the test. The baseplate should be made of some kind of non-absorbent material. Ideally, it should also come with clamps to hold the slump cone securely during the test.
  • Scoop: For placing the concrete sample into the slump cone.
  • Measuring tape: For measuring the slump of the concrete.

How to Perform a Concrete Slump Test

Assuming you have your batch of concrete prepared and you have the requisite testing materials, you should be all ready to perform a concrete slump test. Here’s what you need to do to perform the test successfully:

  1. Secure your slump cone to the baseplate, if possible. It’s important that the slump cone doesn’t move at all for the duration of the test, so if your baseplate doesn’t come with clamps, you might want to find another way to hold the slump cone steady.
  2. Use the scoop to fill up about 1/3 of the slump cone with concrete.
  3. Take your tamping rod and tamp down the concrete inside the slump cone. Make sure you distribute your tamping evenly across the concrete so that it evens out. A good rule of thumb is to tamp down the concrete no less than 25 times.
  4. Fill up the next third of the slump cone with a second layer of concrete.
  5. Tamp the second layer down exactly as you did the first layer.
  6. For the final concrete layer, fill the slump cone up until it’s slightly overflowing, and tamp it down as before. 
  7. Level the excess concrete from the top of the slump cone using the side of the tamping rod.
  8. Unlock the clamps keeping the slump cone in place, if there are any. Very slowly and carefully, lift the slump cone vertically off of the concrete. It’s crucial that you remove the slump cone as smoothly and vertically as possible, otherwise, it can throw off the results of the test. When you do this, the concrete will slump and decrease in height.
  9. Turn the slump cone upside down and place it on the baseplate just beside (but not touching) the concrete.
  10. Place a straight edge on the base of the slump cone, extending over the concrete. The tamping rod is a good tool to use for this. 
  11. Measure the distance from the bottom of the straight edge to the displaced centre of the concrete to the nearest 1/4 of an inch. This distance is the slump of the concrete.

Slump Test Results

The distance the concrete travels during the slump isn’t the only thing that matters; how the concrete slumps can also tell you a lot about the quality of the concrete. Here’s how to interpret the test results based on how the concrete slumps:

  • If the height of the concrete cone decreases but the sides of the cone remain intact, this is a “true slump”. This is the result you want because it indicates that the concrete is mixed well and has the right water content.
  • If one side of the concrete cone shears away from the main portion, this is a “shear slump”. This indicates that the concrete is not mixed well.
  • If the slump completely collapses, this is a “collapse slump”. This indicates that the water content of the concrete is too high.
  • If the slump doesn’t drop at all and maintains the shape of the slump cone, this is a “zero slump”. This indicates that the concrete doesn’t have enough water and thus has no workability. 
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