4.3 Cap Thickness Limitations

The thickness of a sulfur mortar cap has a profound effect on the measured compressive strength of a concrete cylinder. During a compression test, the testing machine applies load through the steel platens directly onto the capped ends of the cylinder. Because the capping material has different elastic properties than the concrete, the cap undergoes lateral deformation under load. Controlling the thickness of the cap is essential to minimize this deformation and ensure that the test results accurately reflect the strength of the concrete rather than the properties of the capping material.

The Mechanics of Cap Thickness and Stress Distribution

When a cylinder is compressed, both the concrete and the capping material expand laterally due to Poisson's effect. Under heavy loads, sulfur mortar has a higher lateral expansion rate than high-strength concrete. This creates triaxial stresses at the interface.

If the cap is too thick, the sulfur mortar will deform laterally more than the concrete, pushing outward. This lateral expansion induces high tensile stresses at the ends of the concrete specimen, causing it to split vertically at a load significantly lower than its actual capacity. This phenomenon, known as "cap yielding," can artificially reduce the measured compressive strength of the cylinder by 10% or more.

To prevent cap yielding and ensure a uniform stress transfer, the caps must be as thin as practical while still filling all surface irregularities. This is why ASTM C617 establishes strict thickness limits.

ASTM C617 Thickness Limits and Strength Categories

The allowable thickness of sulfur mortar caps depends on the design strength of the concrete. As concrete strength increases, the stresses at failure become much higher, making the thickness of the cap even more critical.

For low to moderate strength concrete (less than 5,000 psi), a slightly thicker cap is permitted. This allows for capping of rougher cylinders without requiring excessive end preparation. The maximum average thickness is 1/4 in. (6 mm), and the thickness at any single point must not exceed 5/16 in. (8 mm).

For high-strength concrete (5,000 psi or greater), the tolerances are much tighter. The maximum average thickness is halved to 1/8 in. (3 mm), and the maximum local thickness at any point must not exceed 3/16 in. (5 mm). Technicians must be extremely precise when capping high-strength concrete, as even a minor deviation can lead to premature failure and low-strength test results.

Controlling Cap Thickness in the Lab

Achieving the required cap thickness starts with proper end preparation. If the end of a concrete cylinder is sloped or has large depressions, pouring sulfur mortar directly onto it will result in a cap that is too thick at one side or in the center.

To control thickness, technicians must inspect the cylinder ends before capping. If the end has a slope exceeding 0.5 degrees or projections greater than 3/8 in. (10 mm), the specimen must be processed before capping:

• Grinding: The ends can be ground using a mechanical grinder to achieve a flatter surface.
• Saw-cutting: The end of the cylinder can be cut off using a concrete saw to create a flat, perpendicular surface.
• Chipping: High spots can be carefully chipped away using a hammer and chisel.

By minimizing the irregularities on the raw concrete end, the technician ensures that the resulting sulfur mortar cap is thin and uniform.

During the capping process, the technician must monitor the depth of the molten sulfur pool in the mold. Pouring too much sulfur will create a thick cap, while pouring too little may result in incomplete coverage. The cylinder must be lowered into the sulfur until it is close to the plate surface, allowing only a thin layer of mortar to remain between the concrete and the plate. The alignment device must hold the cylinder firmly in place to prevent it from tilting, which would cause an uneven thickness.

Inspecting and Measuring Cap Thickness

Technicians must visually inspect the perimeter of the capped cylinder to check the thickness of the caps.

• Look for a uniform layer of sulfur around the entire circumference.
• Any visible tilt or variation in thickness indicates an alignment problem, and the cap must be removed and replaced.

To measure the average and local thickness of the caps, technicians can use calipers or a micrometer:

1. Measure the total length of the cylinder before capping.
2. Measure the total length of the cylinder after capping.
3. The difference between these measurements represents the sum of the two caps' thicknesses.
4. To check the thickness at a specific point (local thickness), use calipers to measure the depth of the sulfur layer at the edges of the cylinder.

If the cap thickness exceeds the limits specified in ASTM C617 for the concrete strength class, the cap must be stripped, the end prepared further (by grinding or saw-cutting if necessary), and the specimen must be recapped.