4.2 Capping Procedures & Planeness Verification

To ensure a concrete cylinder distributes the compressive load uniformly during testing, the capping process must be executed with high precision. Even the highest-quality capping plate and sulfur mortar will fail to produce reliable test results if the cylinder is not aligned correctly or if the caps contain hidden voids. Capping technicians must master the step-by-step procedures required to prepare, align, pour, and verify sulfur mortar caps in strict accordance with ASTM C617.

Specimen End Preparation and Moisture Control

The capping procedure begins with the inspection and preparation of the concrete cylinder ends. Before applying sulfur mortar, the ends of the cylinder must be completely clean. Technicians must remove any dust, loose concrete particles, oil, grease, paint, or curing compounds. These contaminants prevent the sulfur mortar from bonding to the concrete surface, which can cause the cap to delaminate or slip under load.

A critical aspect of preparation is moisture control. While concrete cylinders must be kept in a moist condition during curing, the ends to be capped must not have free, standing water at the time of capping. If molten sulfur (which is heated to over 265°F) comes into contact with free water, the water instantly flashes into steam. This steam becomes trapped under the cylinder, causing the sulfur mortar to bubble, foam, and splatter. The resulting cap will contain large air pockets, reducing the effective bearing area and weakening the cap structure. Conversely, the cylinders must not be allowed to dry out completely, as drying affects the concrete's hydration and compressive strength. The correct practice is to wipe the cylinder ends with a damp cloth to remove excess water immediately before capping, ensuring the surface is damp but free of pooling water.

Furthermore, technicians must check the flatness of the cylinder ends. Projections or bumps on the concrete ends must not exceed 3/8 in. (10 mm) before capping. If a projection exceeds this limit, it must be removed using a hammer, chisel, or grinding wheel before the cylinder is capped. If not removed, the projection will create a localized thin spot in the cap or prevent the cylinder from aligning vertically.

Capping Plate Preparation and Lubrication

Before pouring the sulfur mortar, the capping plate must be prepared. The plate should be clean and free of hardened sulfur from previous operations. Technicians must apply a very thin layer of mineral oil or water-soluble lubricant to the plate surface. This lubricant acts as a release agent, preventing the sulfur mortar from bonding to the metal or glass plate.

However, technicians must avoid over-oiling. Excess oil will float on the surface of the molten sulfur and get trapped between the sulfur and the concrete, creating surface voids, reducing the bond strength, and weakening the capping material. A light wipe with an oily rag is sufficient.

Alignment and Capping Stand Execution

To prevent eccentric loading during the compression test, the caps must be perpendicular to the longitudinal axis of the cylinder. This requires the use of a capping stand or alignment device equipped with a guide bar. The capping process proceeds as follows:

1. Melt and Stir: Ensure the sulfur mortar is within the required temperature range (265 to 290°F) and stir it thoroughly to distribute the mineral filler.
2. Pour: Pour a sufficient amount of molten sulfur mortar onto the greased capping plate to form a pool slightly larger than the cylinder diameter.
3. Lower the Specimen: Using the alignment guide, immediately lower the cylinder end into the pool of sulfur mortar. The cylinder must be held firmly against the guide bar to maintain perpendicularity. The axis of the cylinder must be within 0.5 degrees of perpendicular to the capping plate. This corresponds to a deviation of approximately 1/8 in. in 12 in. (10 mm in 1 m) or 1/16 in. in 6 in.
4. Hold and Cool: Hold the cylinder steady for a few seconds until the sulfur begins to solidify. Do not disturb the specimen until the cap has fully hardened, as any movement during crystallization will fracture the sulfur and weaken the cap.
5. Strip: Carefully remove the capped cylinder from the plate. A scraper can be used to clean any overflow sulfur from the sides of the cylinder.

Planeness Verification and Feeler Gauge Testing

Once the cap has hardened, its planeness must be verified. The finished cap must be flat within 0.002 in. (0.05 mm).

Technicians must verify the planeness of the caps on a regular basis. ASTM C617 requires checking at least 1 of every 10 specimens of a given strength, or daily when starting operations. The verification procedure is:

1. Place a straightedge across the diameter of the capped cylinder end.
2. Attempt to slide a 0.002 in. (0.05 mm) feeler gauge between the straightedge and the cap surface.
3. Perform this check along at least three diameters across the surface.
4. If the feeler gauge passes under the straightedge at any point, the cap is out of tolerance and must be replaced.

Acoustic Sounding for Hollow Caps

In addition to planeness, the bond between the cap and the concrete must be sound. Technicians must perform an acoustic check, commonly known as tapping or sounding.

Gently tap the cap surface at several locations using a coin, key, or a small metal tool.

• A solid, high-pitched ringing sound indicates a good bond and a solid cap.
• A dull, hollow thudding sound indicates the presence of air voids, steam pockets, or delamination between the sulfur and the concrete.

If any cap is found to be hollow or out of planeness tolerance, it is rejected. The technician must completely remove the cap, clean the concrete end, and repeat the capping process.