8.2 Cylinder Molding, Consolidation, Finishing, and Identification

Filling and Consolidating by Rodding

Molds are filled in equal layers, and the number of layers and strokes is fixed by mold size. The technician scoops representative concrete around the perimeter and toward the center so each layer is uniform, not segregated.

The tamping rod is a round, smooth steel rod with a hemispherical tip. It is 5/8 in (16 mm) in diameter for molds 6 in or larger and 3/8 in (10 mm) for molds smaller than 6 in. Distribute the strokes uniformly over the cross section in a spiral pattern from the outside toward the center. For the bottom layer, do not let the rod strike the mold base forcibly. For each layer above the bottom, the rod must penetrate the full depth of that layer and approximately 1 in (25 mm) into the layer below, knitting the layers together and breaking up any planes of weakness.

After rodding each layer, tap the outside of the mold smartly 10 to 15 times with the mallet (or an open hand for single-use light-gauge molds). Tapping closes the holes left by the rod and releases entrapped air bubbles that the rod pushed against the mold wall. Skipping the tap leaves voids along the surface that act as stress risers and depress the measured strength.

Consolidation by Vibration, Finishing, and Identification

The consolidation method is governed by slump. When the slump is 1 in (25 mm) or greater, rodding and internal vibration are both acceptable; when the slump is less than 1 in, the concrete is too stiff to rod and must be vibrated. An internal vibrator must operate at a frequency of at least 150 Hz (9,000 vibrations per minute) while immersed. A 6×12 in cylinder filled by vibration is placed in two layers with one insertion near the center of each layer (a 4×8 in cylinder uses one layer, one insertion).

Insert the vibrator vertically about 1 in into the underlying layer, hold only long enough to consolidate (the surface becomes smooth and large bubbles stop rising), and avoid over-vibration, which segregates the mix and brings excess paste and water to the top.

Whether rodded or vibrated, the technician then strikes off the top surface flush with the rim using the rod or a strike-off bar, finishing with the minimum manipulation needed to produce a flat, even surface. Over-troweling brings up a weak paste-rich layer.

Every specimen must be identified without damaging it. Mark the mold, lid, or a tag with the specimen ID, date, and location so the laboratory can trace the result to a placement. Do not scratch or stamp the fresh concrete surface, which would distort the test face.

Use this molding checklist:

• Fill in equal layers (3 for 6×12, 2 for 4×8 when rodding).
• Rod 25 strokes per layer in a spiral, full depth plus ~1 in into the layer below.
• Tap the sides 10-15 times after each layer.
• Vibrate instead of rod when slump is below 1 in.
• Strike off flush with minimal finishing and label without marking the concrete.

Proper consolidation is the single biggest field control on measured strength. Too few strokes, a forgotten side tap, or over-vibration all bias the result, and the exam consistently probes whether the technician knows the exact layer and stroke values rather than a general habit.

Molds, Placement Surface, and Rodding Mechanics

Molds must be watertight, nonabsorbent, and dimensionally stable. Single-use plastic molds are common; reusable steel molds must be clean and lightly coated with a release agent that does not react with the concrete. Absorbent molds (such as untreated cardboard) draw water out of the concrete and lower the measured strength, so they are not acceptable for standard specimens. The molds are set on a rigid, level, horizontal surface free of vibration; a sloped or springy support produces a tilted or poorly consolidated specimen and a non-planar top.

The rodding stroke pattern matters as much as the count. Strokes are distributed uniformly over the cross section and applied in a spiral pattern working from the perimeter toward the center, so consolidation is even rather than concentrated. The rod is used vertically, and the operator avoids striking the mold bottom on the first layer, which can damage single-use molds and does nothing to consolidate.

Reaching about 1 in into the layer below on the upper layers is what eliminates the cold joint between lifts; failing to penetrate leaves a horizontal plane of weakness that can show up as an unusual break pattern in C39 testing.

Capping or grinding to produce a plane test surface is a laboratory step under C39, not a field step, but the technician improves the lab's job by striking off cleanly and not overworking the top.

Common cylinder-molding errors the exam targets:

• Using too few strokes or skipping the side tap, leaving voids.
• Over-vibrating, which segregates the mix and weakens the top.
• Molding on a sloped or springy surface, producing a tilted specimen.
• Using absorbent or leaking molds that drain water from the concrete.

These mechanics are why the exam expects exact recall: the difference between a defensible cylinder and a discarded one is a handful of correctly executed strokes, taps, and finishing moves.