A concrete slump test measures how workable a batch of fresh concrete is by filling a metal cone, lifting it straight up, and measuring how much the concrete drops. The entire test takes about five minutes with minimal equipment, but each step follows a precise standard (ASTM C143) that you need to get right for the result to mean anything. Here’s how to do it correctly.
Equipment You Need
The test requires three items: a slump cone, a tamping rod, and a ruler or tape measure. The slump cone is a truncated metal mold that’s 12 inches tall, 8 inches across at the base, and 4 inches across at the top, with a tolerance of plus or minus 1/8 inch on each dimension. The tamping rod is a straight steel rod, 5/8 inch in diameter, with one or both ends rounded into a hemispherical tip. That rounded tip matters because it consolidates the concrete uniformly. A piece of rebar or a rod with a flat end will push aggregate particles down instead of compacting the mix evenly, which throws off your reading.
You also need a flat, non-absorbent surface to work on, like a dampened metal base plate. The surface has to be stable and level. If it flexes or vibrates during the test, your result won’t be accurate.
Step-by-Step Procedure
Start by dampening the inside of the cone and the base plate so they don’t absorb water from the mix. Place the cone on the base plate with the wide end down, then stand on the foot holds or have someone hold it firmly in place so it can’t shift while you’re filling it.
Fill the cone in three roughly equal layers using a scoop. Move the scoop around the perimeter of the opening so concrete distributes evenly rather than piling up on one side. After each layer, rod the concrete 25 times with the rounded end of the tamping rod, distributing strokes evenly across the cross-section. For the first layer, push the rod through the full depth of the concrete. For the second and third layers, push the rod about an inch into the layer below so the layers bond together. Be firm but controlled. You want to penetrate the bottom layer on the first pass without forcefully striking the base plate.
After rodding the third layer, strike off excess concrete flush with the top of the cone. Clear any spilled concrete away from the base so it doesn’t interfere with the measurement.
Now comes the critical moment: lift the cone straight up in a steady motion, raising it 12 inches in 5 seconds (give or take 2 seconds). Don’t twist it, tilt it, or jerk it. Set the empty cone upside down next to the concrete specimen, lay the tamping rod across the top of the inverted cone, and measure the distance from the rod down to the highest point of the slumped concrete. That distance, in inches, is your slump value.
Reading the Three Types of Slump
Not every result gives you a usable number. What the concrete does after you lift the cone tells you as much as the measurement itself.
A true slump is what you want. The concrete subsides evenly and roughly maintains the shape of the cone, just shorter. You can measure this and record it with confidence.
A shear slump means one half of the cone-shaped specimen slides down at an angle while the other half stays mostly upright. This indicates a lack of cohesion in the mix, often from a harsh proportion of aggregate to paste. If you get a shear slump, discard the result and run the test again with a fresh sample. If it shears a second time, the mix itself has a problem.
A collapse slump means the concrete falls apart completely. This points to either an extremely wet mix or one that simply can’t hold together. A collapse doesn’t give you a meaningful measurement.
Target Slump Ranges by Application
The right slump depends on where the concrete is going and how it will be placed. Drier mixes with lower slump numbers are stiffer and harder to work by hand but hold their shape well. Wetter mixes flow more easily into tight spaces but can lose strength if the water content is too high.
- 0 to 1 inch: Very dry mixes used in slip forms or machine-paved surfaces where high-powered vibration does the consolidation work.
- 1 to 2 inches: Low-workability mixes for foundations with light reinforcement or pavements consolidated with hand-operated vibrators.
- 2 to 4 inches: The most common range. Suitable for flat slabs placed by hand, normal reinforced concrete, and heavily reinforced sections consolidated with mechanical vibration.
- 4 to 7 inches: High-workability concrete for sections with congested reinforcement where the mix needs to flow around tight bar spacing. Mixes in this range may not respond well to vibration.
Most ready-mix trucks deliver concrete with a target slump specified on the ticket. If your test result falls outside the specified range, the batch may need adjustment before you place it.
Mistakes That Ruin the Test
Small errors during the test can shift your reading by an inch or more, which is enough to make a good batch look out of spec or let a bad one pass.
The most common mistake is tapping or bumping the cone during filling. Unlike when you mold strength test cylinders, you never tap the slump cone. If the cone gets tapped, moved, or bumped at any point during the test, you have to throw the sample out and start over.
Lifting the cone too fast or too slow also changes the outcome. Yanking it off in under 3 seconds can pull the concrete upward, while lifting slowly over 7 or more seconds lets the concrete settle under its own weight longer than it should. Keep it in that 3-to-7-second window.
Using the wrong rod is another frequent issue. A rod without a hemispherical tip, or one that’s too short to comfortably reach the bottom of the cone, won’t consolidate the layers properly. And skipping the overlap between layers, where you push the rod about an inch into the previous layer, leaves weak planes inside the specimen that affect how it slumps.
Finally, make sure your base plate is on solid ground. Running the test on a surface that bounces or flexes when someone walks by introduces vibration that effectively loosens the concrete and inflates your slump reading.