4.5 Filling, Rodding, Lifting, and Measuring Slump

Filling by Volume, Not by Height

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 one-third by height. The widely taught fill heights are: first layer to about 2-5/8 in. (67 mm), second layer to about 6-1/8 in. (155 mm), and the third layer heaped above the 12 in. top before rodding and strike-off. Scooping concrete to a flat one-third-height mark would under-fill the bottom and over-fill the top, biasing consolidation.

Move the scoop around the perimeter as each layer is added so the concrete is distributed evenly rather than dumped on one side.

The reason for filling by volume is geometric. The cone is wide at the bottom (8 in.) and narrow at the top (4 in.), so equal volumes correspond to shrinking heights as you move up. If a technician instead split the 12 in. height into three equal 4 in. layers, the bottom layer would hold far less than one-third of the volume and the top far more, so the 25 strokes per layer would over-consolidate the bottom and under-consolidate the top. Hitting the ~2-5/8 in. and ~6-1/8 in. marks keeps the energy per unit volume roughly constant.

Rodding Each Layer 25 Times

Each layer is rodded exactly 25 strokes with the 5/8 in. hemispherical-tip rod, distributed uniformly over the cross section of the layer. Rod the bottom layer throughout its depth, holding the rod slightly inclined and making about half the strokes near the perimeter in a spiral toward the center, without forcing the rod against the base. Rod the middle and top layers throughout their depth so the rod just penetrates about 1 in. into the layer below, knitting the layers together.

During the top layer, keep a surplus of concrete above the mold by adding more before rodding is complete if the level drops below the rim, so concrete remains over the top throughout the 25 strokes.

All strokes use the rounded end. Stabbing the center repeatedly, hammering the base, or rodding only the wall produces uneven consolidation and a distorted slump.

A useful mental model for distributing strokes: imagine the cross section divided into rings. On each layer, place several strokes near the perimeter, then spiral inward toward the center, so all 25 strokes are spread over the area rather than concentrated. For the bottom layer the rod is inclined slightly near the wall to follow the taper; for the upper two layers the rod is driven straight through the current layer and about 1 in. into the layer beneath so the boundary between layers is consolidated and not left as a weak plane.

Maintaining surplus concrete on the top layer is its own checkpoint. As the rod consolidates the heaped third layer, the surface drops. If it falls below the rim before the 25th stroke, the technician adds concrete to keep a surplus above the mold so the layer stays full while rodding finishes. Only after all 25 strokes are complete is the surface struck off flush. Striking off before the layer is fully rodded, or rodding an under-filled top layer, both bias the slump.

Strike-Off, Lift, and Measure

After the top layer is rodded, strike off the surface flush with the top of the mold using a screeding and rolling motion of the rod, then clean spilled concrete from the base around the mold. Immediately remove the mold by raising it vertically 12 in. (300 mm) in 5 plus or minus 2 seconds, in one smooth motion with no lateral or torsional movement. The whole filling-through-removal operation is performed without interruption and completed within 2.5 minutes.

Measure slump immediately after removal. The slump is the vertical difference between the top of the mold (12 in.) and the displaced original center of the top surface of the specimen. A common technique inverts the empty mold beside the slumped concrete and lays the rod across the top so the ruler can read down to the displaced original center. Record to the nearest 1/4 in. (5 mm).

Worked example: the rod laid across the inverted mold sits at the 12 in. datum; the ruler reads 4-3/16 in. down to the original-center high point of the concrete. Rounded to the nearest 1/4 in., the reported slump is 4-1/4 in.

Sequence memory list:

• Fill three layers by volume (~2-5/8 in., ~6-1/8 in., heaped).

• Rod each layer exactly 25 strokes; upper layers ~1 in. into the layer below.

• Strike off flush and clean the base area.

• Lift 12 in. in 5 plus or minus 2 seconds, straight up, no twist.

• Measure to the displaced original center, nearest 1/4 in., within 2.5 minutes total.