What Is Concrete Slump?

Concrete is traditionally made out of a mixture of cement, water, sand, and gravel. Modern concrete mixes also add other ingredients such as fibers, plastics, and various chemical mixtures. The ingredients and the mixture proportion determine the properties of concrete, including slump, which can affect how well it works for different types of projects. Concrete slump is a way to describe the consistency of the concrete.

Concrete slump is a measurement of the workability or consistency of concrete. In other words, it measures how easy the concrete is to push, mold, and smooth. Accordingly, its slump rating indicates what construction application the concrete is good for.

The higher the slump, the more workable or fluid the concrete. Low concrete slump means it's less fluid and has less workability. If the slump of concrete is too low, it won't shape very easily. If it's too high, you run the risk of having the gravel, sand and cement settle out of the mixture, making it unusable.

The lower the amount of water in a traditional concrete mix, the lower its slump. Low slump values in traditional mixes generally mean higher quality concrete.

The additional ingredients in modern concrete mixes make it impossible to determine concrete quality from slump, however. Concrete with plasticizers and a high slump may actually have less water than a traditional concrete mix with a low slump. Because of these additional ingredients, you can set the slump of a batch of concrete to practically any value while still retaining a high-quality mixture.

Concrete slump is tested with a slump cone. This is a 12-inch tall, truncated cone that's open at the top and bottom. The top is 4 inches wide and the bottom is 8 inches wide. After mixing the concrete, you can use this device to test the slump. The steps to test slump with the cone are:

1. Place the cone with the small side up on a smooth, nonabsorbent surface. Secure the cone so it doesn't move.
2. Fill the slump cone to 1/3 of its height with the prepared concrete.
3. Tamp down with 25 strokes of a 3/4-inch steel rod, moving the rod around across the sample. An easy way to do this is to hold the rod at an angle to match the side of the cone. Start around the edges and move toward the middle.
4. Scoop in more concrete, filling the cone to the 2/3 mark, and tamp it with another 25 strokes. The rod should also go into the first layer about 1 inch.
5. Add more concrete to fill it to the top so it's slightly overflowing with one last layer and tamp it with a final 25 strokes. The rod should go into the second layer but not go through it completely.
6. Scrape off the excess concrete with the rod to create a smooth top.
7. Measure from the top of the cone to the base.
8. Pull the slump cone up off of the concrete. Pull the cone straight up quickly, being careful not to jerk it or turn it at an angle.
9. Measure how far the wet concrete sinks, or slumps down, by measuring from the top of the pile to the ground.
10. Subtract this measurement from the original height of the slump cone. The distance slumped down is called the slump level.

Don't use slump level to compare the quality of one mixture of concrete to a different mixture. Use it only to compare the quality of different batches of the same mixture. This test doesn't indicate anything about the strength of the mixture.