The concrete slump test measures how workable a batch of fresh concrete is by filling a metal cone, lifting it straight off, and measuring how far the concrete drops. It takes under three minutes, requires only a few pieces of equipment, and is the most common field test for concrete consistency. Here’s exactly how to do it right.
Equipment You Need
The slump cone (also called an Abrams cone) is a truncated metal or plastic cone that measures 12 inches tall, 8 inches across the base, and 4 inches across the top, with a tolerance of plus or minus 1/8 inch on each dimension. Metal cones need a minimum wall thickness of 0.060 inches, while plastic cones require at least 0.100 inches. The cone has two foot holds welded to the base and handles near the top.
The tamping rod is a straight, smooth steel rod, 5/8 inch in diameter, with one or both ends rounded into a hemispherical tip. It needs to be at least 4 inches longer than the depth of the cone but no longer than 24 inches overall. You’ll also need a tape measure or ruler and a flat, level, non-absorbent surface that’s free from vibration. A dampened steel plate or concrete slab works well. Dampen the cone and the base surface before you start.
Step-by-Step Procedure
The entire test, from the moment you begin filling the cone to the moment you lift it off, must be completed within 2 1/2 minutes. If you’re sampling from a truck, you need to begin the slump test within 5 minutes of taking the sample. Work steadily and don’t pause between steps.
Fill the Cone in Three Layers
Place the dampened cone on your base surface and stand on the foot holds to keep it from shifting. Fill the cone in three layers, each roughly one-third the volume of the cone. After adding each layer, rod it exactly 25 times with the tamping rod, distributing strokes evenly across the cross section.
The bottom layer requires a slightly different technique. Angle the rod slightly and make about half your strokes around the perimeter, then spiral inward toward the center with vertical strokes. Push the rod through the full depth of this first layer. For the second and third layers, rod through the full depth of that layer so each stroke penetrates about 1 inch into the layer below it.
Finish the Top
When filling the third layer, heap concrete above the rim of the cone before you start rodding. If rodding causes the concrete to sink below the top edge, add more so there’s always excess above the rim. After all 25 strokes, strike off the surface flush with the top of the cone using a screening and rolling motion of the tamping rod. Clear away any concrete that has fallen around the base of the cone so it won’t interfere when the concrete slumps outward.
Lift the Cone
Grip both handles and raise the cone straight up in one steady motion. Lift it 12 inches in 5 seconds (give or take 2 seconds). Do not twist, tilt, or jerk the cone. Any lateral movement will distort the result.
How to Measure the Slump
Once the cone is off, flip it upside down and set it next to the slumped concrete. Lay the tamping rod horizontally across the top of the inverted cone so it extends out over the pile of concrete. Using your tape measure, measure the vertical distance from the bottom of the rod down to the displaced original center of the top surface of the specimen. That distance, in inches, is your slump.
Finding the “displaced original center” can be tricky. A useful trick: before you lift the cone, place a coin on the center of the struck-off surface. After the cone comes off, measure down to wherever the coin ended up. That gives you a reliable reference point.
Types of Slump and What They Mean
Not every slump result is usable. There are three patterns to watch for:
- True slump: The concrete subsides evenly, roughly keeping the cone shape. This is the result you want and the only type that gives a valid measurement.
- Shear slump: One half of the cone slides down at an angle while the other half stays mostly in place. This indicates the mix lacks cohesion, often a sign of a harsh mix with poor aggregate proportions. If you see a shear slump, discard the result and run a new test on a fresh portion of the sample.
- Collapse slump: The concrete flattens out completely. This typically means the mix is too wet. A collapse can still give you a measurable number, but it signals a workability problem worth investigating.
Typical Slump Ranges by Application
The right slump depends on what you’re building. ACI 211.1 provides recommended ranges that account for differences in aggregate characteristics and placing conditions. As a general guide:
- Pavements and slabs on grade: 1 to 3 inches
- Beams and reinforced walls: 3 to 4 inches
- Building columns: 3 to 4 inches
- Mass concrete (footings, caissons): 1 to 3 inches
Air-entrained concrete generally allows you to use a lower slump while still achieving the same workability. The proper slump for a given job can range from zero (very stiff mixes used in roller-compacted applications) all the way to flowing concrete used with high-range water reducers. Your project specifications will state the target slump and the acceptable tolerance, usually plus or minus 1 to 1.5 inches from the target.
Common Mistakes That Skew Results
Most slump test errors come from rushing or sloppy technique. Failing to dampen the cone and base plate lets the surface absorb water from the concrete, artificially stiffening the mix and producing a lower reading. Not standing on the foot holds allows the cone to shift during filling, which introduces voids and inconsistency.
Uneven rod distribution is another frequent problem. If you cluster your 25 strokes in the center and neglect the perimeter, the concrete won’t be uniformly compacted. On the bottom layer especially, use that spiral pattern from the outside in. Lifting the cone too fast, too slow, or with any sideways wobble will also distort the result. Aim for that steady 5-second lift straight up.
Finally, timing matters more than people realize. If you take too long filling and rodding, the concrete begins to stiffen, and your measurement won’t reflect the true workability of the batch. Keep everything within that 2 1/2 minute window, and start the test within 5 minutes of collecting your sample from the mixer or truck.