8.2 Cylinder Molding, Consolidation, Finishing, and Identification

Cylinder Molding as a Repeatable Measurement Process

Concrete cylinders are used most often for compressive strength. The cylinder must represent the concrete sampled in the field, and it must have a consistent shape, density, and curing history. That is why C31 gives detailed rules for mold size, layers, consolidation, finishing, protection, and identification. The ACI exam does not reward vague memory; it rewards controlled hand skill.

The mold is placed on a level, firm surface where it will not be bumped or tilted during molding. Concrete is placed in layers appropriate to the cylinder size and consolidation method. For common field cylinders, candidates should study the current layer and rodding requirements from the official materials. The concept is that each layer receives enough consolidation to close large voids while avoiding segregation or loss of concrete.

Rodding requires attention to both depth and pattern. The rod is distributed across the cross section, and lower layers are not ignored. When rodding an upper layer, the rod penetrates into the layer below enough to knit the layers together. The mold is tapped as required to close rod holes and release large trapped air pockets. The technician should not pound the mold in a way that damages it or separates the concrete.

Vibration is used only when the method and concrete condition call for it. The candidate should know that consolidation method is not a personal preference. A very low-slump concrete may require vibration under the standard, while normal plastic concrete may be rodded. Over-vibration can segregate concrete and create a nonrepresentative specimen.

The final layer is slightly overfilled as needed so finishing leaves a full specimen. The top is struck off or finished with the approved tool to produce a plane surface even with the mold rim. The technician should not add water to the top, press down aggregate in a way that creates paste-rich caps, or leave ragged edges that can interfere with later capping, grinding, or testing.

Identification must happen without harming the specimen. Mark the mold, tag, lid, or attached label as the project procedure allows. Do not carve into the fresh concrete or place a loose label where it can be lost during curing or transport. Identification should tie the cylinder to the sample, placement, date, time, curing condition, and intended test age.

Cylinder molding reminders:

• Use clean, undamaged molds of the required size.
• Fill in required layers and keep each layer reasonably even.
• Consolidate according to the concrete consistency and current C31 requirements.
• Tap sides and finish the top without losing material.
• Identify each specimen immediately and protect it from disturbance.

A cylinder that looks neat may still be invalid if it was underconsolidated, overworked, mislabeled, or cured poorly. The technician's job is not simply to produce a full mold. The job is to produce a traceable specimen that can later be tested as evidence of concrete quality.