Translating Tech Data Sheets – Compressive Strength

Compressive Strength

The next stop on our grand tour of physical properties is compressive strength, the ability of a material to resist forces that would cause it to, well, compress. It is a feature of structures and of substances. For the purposes of this article, we are most interested in the compressive strength of chemical-resistant mortars, grouts, monolithic surfacings and polymer concretes, the types of materials we use at PennCoat.

Don’t Be Tense

Compression is the opposite of tension (which we’ll discuss next week). Compression is considered a negative strain, and compressive strength arises from the reluctance of molecular bonds to shorten. As negative strain increases, the pressure tends to cause buckling, which is sideways deflection. The material’s ultimate compressive strength is the point at which it fails.

Standards

The industry standard applicable to our focus of materials is ASTM International, (formerly known as the American Society for Testing and Material) C579, last updated in 2012. The test applies to materials based on resin, silicate, silica or sulfur bindings. In simplified form, the test involves an apparatus that applies compressive force and detectors that measure and plot pressure.

Three different test methods exist, one each for materials containing aggregate sizes:

  • Less than 1.6 mm
  • 6 to 10 mm
  • Greater than 10 mm

The tests can use two different load rates:

  • Load Rate I: 6 psi per minute
  • Load Rate II: 0.20 to 0.25 inches per minute crosshead speed

For example, some epoxy grouts involves using brass or steel molds for creating two-inch cubes of the material, which cures for 24 hours at a temperature no lower than 65 degrees. It tests the cubes using Load Rate II.

The two rates might produce different numbers for compressive strength. Five-Star takes tests after 1, 7 and 28 days. Care is taken not to apply compression load to the original top or bottom cube surfaces.

The compressive strength of the material in derived by dividing the load into the cross sectional area, and is expressed in psi.

Foot Wear

The reason compression strength is important is because many of the tested materials must stand up to years of heavy foot traffic and/or vehicle use. The constant pounding can lead to failure over time — stress cracks and so forth. We refer to a product’s data sheet to compare the compressive strength of various competing materials. For example, Poly-Crete MD, a cementitious urethane from Dur-A-Flex, Inc., has a compressive strength of 9,000 psi when tested under ASTM C579 methodology. This means the material can absorb a load of up to 9,000 psi without failing. We think that’s pretty good. The compressive strength of Five-Star DP Epoxy Grout ranges from 11,000 to 15,500 psi, depending on curing time and conditions.