Step 5: Concrete Slump Test

A slump test is used to determine the correct hydration of a batch of concrete. The slump is the distance the wet concrete settles after the slump cone is lifted off. The ideal mix will not be too stiff or too soft, but will have a slump of about 4". If you're working in very hot or dry conditions, or if the mold has intricate shapes, you may want a slightly wetter mix. Keep in mind that the more water in the mix, the higher the risk of cracks. A mix that is too dry is hard to pour and vibrate.

Performing a slump test will help give you an idea of what the right mix should feel like. Over time you'll develop a feel for it, but in the beginning it helps to keep track of the amount of water you add so you can reproduce the same results throughout projects.

Professional Slump Test Supplies:
  • 12" Slump Cone
  • Mixed Concrete
  • Straight Edge
  • Plywood Base Board

1. Moisten the inside of the cone and put it on a smooth, level surface such as a moist piece of plywood or a concrete slab.
2. Fill the cone one-third full with a sample of concrete. Push a rod down into the concrete (called tamping) to help the concrete fully settle and allow any air to escape.
3. Fill the cone until it's two-thirds full, and tamp this layer with the rod.
4. Add more concrete until the cone is slightly over-filled and tamp this layer.
5. Scrape off excess concrete
6. Slowly lift the cone away from the concrete.
7. Put the cone beside the concrete, but not touching it. Rest a straightedge across the cone and measure the distance down to the top of the sample as seen in the photos.

Homebrew Slump Test Supplies:
  • 15+ oz. (450+ mL) Plastic Cup
  • Pen to poke hole
  • Mixed Concrete
  • Ruler
1. Poke a 1/4" hole in the bottom of the plastic cup with the pen to allow air to escape.
2. Fill the cup with freshly mixed concrete and pack it well.
3. Place the filled cup upside-down on a flat, rigid surface.
4. With your hands, carefully vibrate the cup in a steady lifting motion without stopping.
5. Measure the distance of the slump with a ruler.

The ideal slump should be about half the height of the plastic cup. If the mix is too wet, add more bagged concrete mix by the cup.

Hopefully this Instructable has given you some tips that will help when it's time to mix the concrete for your next project. For some easy concrete project ideas, check out our other Instructables.

Feel free to ask questions or offer any tips that haven't been mentioned. Thanks for following!
<p>When mixing larger jobs, how do you dry mix once the concrete mixer already mixed wet concrete?</p>
<p>when mixing large jobs, the moisture from last job will blend with newer when mixing</p>
Very detailed and easy to understand about the batch of concrete. Thank you!
Nice, thanks for this.
Really helpful. Thanks to share.
I've read your countertop book. It's not a project I'll probably ever tackle, but it was extremely interesting, as was this instructable. Thanks for sharing more of your knowledge.
wow thats some great instructable right there, thanks for the share!
thats a really helpful share a banch of thanx

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