4.5 Filling, Rodding, Lifting, and Measuring Slump
Key Takeaways
- The slump cone is filled in three layers, each approximately one-third of the mold volume.
- Each layer is rodded 25 times, with upper layers penetrated about 1 in. into the layer below.
- The mold is raised vertically without twisting or lateral motion, commonly 12 in. in 5 plus or minus 2 seconds.
- Slump is measured immediately as the vertical difference from the mold height to the displaced original center of the specimen.
The C143 Performance Sequence
Once the slump test begins, it must proceed without interruption. The mold is filled in three layers, each approximately one-third of the mold volume. Because the cone is tapered, one-third by volume is not the same as one-third by height. The bottom layer reaches a lower height than a simple one-third mark would suggest, the middle layer reaches the next volume level, and the top layer is heaped above the mold before rodding and strike-off.
Each layer is rodded 25 times. Rod the bottom layer throughout its depth without hammering the base. Rod the middle and top layers throughout their depth while penetrating about 1 in. into the layer below. The strokes should be distributed uniformly over the cross section. Rodding all strokes in the center or stabbing randomly at the wall produces inconsistent consolidation and can distort the slump.
After the top layer is rodded, maintain excess concrete above the mold, strike off the top, and clean overflow from around the base. Then remove the mold by raising it vertically, commonly 12 in. in 5 plus or minus 2 seconds, with no lateral or torsional motion. The lift should be smooth. Twisting the mold, bumping the concrete, or dragging the cone sideways can create an artificial slump shape.
| Step | Required control | Performance cue |
|---|---|---|
| Fill layer 1 | About one-third mold volume | Rod 25 strokes through depth |
| Fill layer 2 | About two-thirds mold volume | Rod 25 strokes into layer below |
| Fill layer 3 | Overfill before rodding | Rod 25 strokes into layer below |
| Strike off | Leave top flush with mold | Keep cone held down |
| Lift and measure | Lift vertically, then measure immediately | No twist, no delay |
Slump is measured immediately after the mold is removed. The measurement is the vertical difference between the top of the mold position and the displaced original center of the top surface of the concrete. In practice, the inverted mold and rod are often used to establish a reference, and the ruler is read to the required increment, commonly the nearest 1/4 in. or 5 mm.
The entire operation from beginning filling through mold removal must be completed within the C143 time limit, commonly 2-1/2 minutes. That is why setup matters. If the rod, scoop, ruler, or base is not ready, the candidate will either exceed the time or rush into a procedural error.
Sequence memory list:
-
Dampen and secure the mold on the proper base.
-
Fill three layers by volume, not by simple equal height.
-
Rod each layer 25 times with proper penetration.
-
Strike off cleanly and remove spilled concrete from the base area.
-
Lift straight up, measure immediately, and report to the correct increment.
How many layers and rod strokes per layer are used in ASTM C143?
How should the middle and top layers be rodded?
How is the slump mold removed after strike-off?