Compressive Strength Testing: Sulfur Capping or Neoprene Pads?

Compressive Strength Testing: Sulfur Capping or Neoprene Pads

Importance of Capping

Prior to performing compressive strength tests on cylindrical concrete specimens, it’s important to understand both of the most popular options for preparing the cylinder ends. Proper end preparation ensures that the ends of concrete test cylinders or cores have smooth, parallel bearing surfaces perpendicular to the applied axial load to ensure uniform distribution of forces during testing. ASTM C617 is a method using molten sulfur mortar to form bonded caps. ASTM C1231 specifies the procedure to use unbonded neoprene pads in metal retaining rings. No matter which method you choose, specific criteria should comply with ASTM C39 specifications for the planeness and perpendicularity of specimens. Be sure to scroll down below the infographic for a brief overview of each method as well as the necessary equipment.

Sulfur Capping vs Neoprene Pads

Sulfur Mortar Capping


Capping of concrete cylinders and drilled concrete cores with sulfur mortar or other capping compounds prepares specimens for compressive strength testing. Specifically, this procedure provides plane surfaces perpendicular to the specimen axis to evenly distribute loading forces.  As noted in ASTM C617, sulfur mortar may be used to test specimens with greater compressive strengths than its own with user verification. When using this method, it’s important to periodically check for voids by tapping hardened caps with the edge of a coin and listening. Capped cylinders must cure for at least two hours prior to testing or for 16 hours if the design strength is 5,000psi (35MPa) or greater.


  • Sulfur Mortar Capping Compound:
    • Flake Style, supplied in 50lb bags and often higher in strength
    • Ingots sometimes preferred for ease of handling and storage
  • Melting pot to prepare capping compound. Capacities from 4 to 28qt (3.8 to 26.5L) are available
  • Ladle for transferring molten capping compound
  • Cylinder Capper positions cylinders properly for insertion into the compound and during hardening
  • The Thermometer ensures the temperature of the compound is between 265° and 290°F (130° to 145°C)
  • Gloves and Eye Protection promote safety during capping

Neoprene Pads and Metal Retaining Rings


Neoprene Pads or Pad Caps are ideal for tests between 1,500 and 7,000psi, but can also be used for concrete strengths up to 12,000psi with proper verification by the user as noted in ASTM C1231. These pads spread the compressive force evenly across specimen surfaces. The durometer hardness of neoprene pads must be matched to the design strength range of the concrete.

Gilson Neoprene Pads Chart


  • Neoprene pads evenly distribute compressive force
  • Retainer rings are placed on both ends of concrete cylinders and confine pads, so they do not extend beyond the specimen diameter
  • Polysaccharide powder serves as a lubricant for easy pad-change out
  • Gloves promote safety during testing

Whether you use the sulfur mortar capping method or neoprene pads with metal retainer rings, proper end preparation guarantees that concrete test cylinder ends or cores have smooth, parallel bearing surfaces for uniform distribution of forces during testing.

We hope this article has helped you. Please contact our testing experts for questions or help with your application.

 About the Author Ben Backus