4.1 Capping Equipment & Material Specifications

Capping cylindrical concrete specimens is a critical preparation step prior to performing compressive strength tests. The ultimate goal of compressive testing is to apply a pure, axial compressive force to the concrete specimen to determine its true load-bearing capacity. However, concrete cylinders cast in plastic molds rarely have perfectly flat, smooth, or perpendicular ends. Surface irregularities, such as bumps, depressions, ridges, or sloped ends, can concentrate stresses, leading to localized premature failures. To eliminate these stress concentrations, ASTM C617 outlines the procedure for capping these specimens with sulfur mortar to create smooth, perpendicular, and planar bearing surfaces. Capping technicians must understand the stringent specifications of the equipment and materials utilized in this process to avoid introducing testing biases that could artificially reduce measured compressive strengths.

Capping Plate Specifications and Materials

The foundation of the sulfur capping process is the capping plate. The flat cap is formed by pouring molten sulfur mortar onto a capping plate and pressing the concrete cylinder into it. Therefore, any deviation in the planeness of the plate is directly transferred to the cap itself.

ASTM C617 requires capping plates to be made of materials that are rigid, wear-resistant, and stable. The permitted materials include:

• Metal Plates: Must be at least 1 in. (25 mm) thick to prevent warping or bending under repeated heating and mechanical cleaning.
• Glass Plates: Must be at least 1/4 in. (6 mm) thick. Glass is highly resistant to chemical attack and provides a very flat surface, but it is fragile and prone to chipping.
• Granite or Diabase (Stone) Plates: Must be at least 3 in. (75 mm) thick. Stone plates provide excellent stability but are less common due to their weight and cost.

Regardless of the material, capping plates must meet strict planeness requirements. The surface of the plate must not depart from a plane by more than 0.002 in. (0.05 mm) in any 6 in. (150 mm), or 0.003 in. (0.07 mm) overall. The plate must be large enough to accommodate the specimen diameter plus at least 1 in. (25 mm) of clearance around the perimeter. This extra space allows the molten capping material to overflow slightly, ensuring full coverage of the cylinder end.

Furthermore, plates used with sulfur mortar must have a surface hardness that resists scratching and wear. The concrete aggregates and the steel scrapers used to clean the plates can quickly gouge soft metals. Consequently, ASTM C617 mandates that capping plates for sulfur mortar must have a Rockwell hardness of at least 45 HRC.

Technicians must perform daily visual inspections of the plates. Any scratch, gouge, or depression must be checked. Under ASTM C617, individual depressions must not exceed 0.010 in. (0.25 mm) in depth or 0.05 square inches (30 square millimeters) in total area. If a capping plate fails to meet these planeness or depression tolerances, it must be replaced or refinished.

Melting Pots and Temperature Control

Sulfur mortar must be melted in an electrically heated melting pot. This pot must be equipped with automatic temperature control capable of maintaining the molten material within a range of 265 to 290°F (130 to 145°C).

Melting pots must be constructed of metal or lined with a material that does not react with sulfur. The heating elements should distribute heat uniformly across the bottom and sides of the pot to prevent localized hot spots. Hot spots can cause the sulfur to burn or polymerize, degrading the material properties.

Additionally, melting pots must be operated under a ventilated exhaust hood that vents to the outside. This is a crucial safety requirement, as heating sulfur releases vapors, and overheating can generate dangerous sulfur dioxide gas (SO2), which is highly irritating and toxic to the respiratory system.

Sulfur Mortar Material Composition and Strength Requirements

Sulfur mortar is a mixture containing elemental sulfur and an inert mineral filler, such as silica, carbon, fly ash, or clay. The sulfur acts as the binder that melts and solidifies, while the filler increases the strength of the mortar, controls shrinkage during cooling, and ensures the thermal expansion coefficient of the cap is compatible with the concrete. Typically, the composition consists of:

• Elemental Sulfur: 40% to 60% by weight.
• Mineral Filler: 40% to 60% by weight.

The mineral filler must be finely ground and thoroughly mixed with the sulfur. Because the filler has a higher density than sulfur, it tends to settle to the bottom of the melting pot during heating. Technicians must stir the mixture thoroughly and regularly before pouring to ensure a homogeneous mix. If the filler settles, the caps will contain a disproportionately high amount of sulfur at the top and filler at the bottom, resulting in weak, brittle caps that fail prematurely.

The strength of the capping material must be compatible with the concrete being tested. To verify this, technicians must cast and test 2-inch (50-mm) compression test cubes of the sulfur mortar. The strength requirements are based on the design strength of the concrete:

• Concrete strength less than 5,000 psi (35 MPa): The sulfur mortar must achieve a minimum compressive strength of 5,000 psi (35 MPa) at 2 hours.
• Concrete strength of 5,000 psi (35 MPa) or greater: The sulfur mortar must achieve a strength equal to or greater than the design strength of the concrete at 2 hours or 16 hours.

Sulfur mortar strength must be tested at least once per week during active capping operations, or whenever a new lot of material is used. The cubes are cast in a 2-inch metal mold, allowed to cool, and tested in compression. The average strength of the cubes determines compliance.