Slump of Hydraulic Cement Concrete
Mold and filling
The slump test uses a standard mold 4 in. (100 mm) across the top, 8 in. (200 mm) across the bottom, and 12 in. (300 mm) high, with a base plate. Dampen the mold and plate, place the mold on a level, rigid surface, and fill in three layers, each about one-third of the volume. Rod each layer 25 times with a 5/8 in. (16 mm) rounded rod. Rod the bottom layer through its full depth; for the upper layers, penetrate about 1 in. (25 mm) into the layer below.
Strike-off and removal
Strike off the top with a trowel, clean the base plate, and remove the mold vertically within 5 ± 2 seconds. Measure the slump immediately as the difference between the mold height and the height of the slumped concrete at the center, to the nearest 1/4 in. (5 mm). Complete filling and rodding within 2.5 minutes. If the concrete shears or collapses, the test is not valid; obtain a new sample.
Traps and worked example
Typical traps are too few strokes, rocking the mold while lifting, lifting at an angle, measuring from the side rather than the center, and testing stale concrete. A very stiff or nonplastic mix may not be suitable for slump.
Rule of thumb: 3 layers, 25 strokes, 5 seconds to lift, 1/4 in. precision. Worked example: mold height is 12 in.; slumped height at the center is 7 3/4 in.; slump = 12 − 7 3/4 = 4 1/4 in.