Which of the following is a primary property of hardened concrete?

The primary property of hardened concrete is compressive strength, which refers to the ability of concrete to withstand axial loads without failing. Compressive strength is a crucial performance metric because it largely defines the structural capacity of concrete in construction applications. It is measured in pounds per square inch (psi) or megapascals (MPa) and is determined through standardized tests such as the cylinder test, where concrete samples are subjected to increasing loads until failure occurs.

Hardened concrete is specifically designed to perform well under compression, making this property essential for various applications, such as beams, slabs, and structural foundations. High compressive strength contributes to the durability and longevity of concrete structures, ensuring they can support the intended loads over their lifespan.

While flexibility, elasticity, and thermal conductivity are important characteristics in some contexts, they are not fundamental properties of hardened concrete that define its performance under typical structural conditions. Flexibility is generally more associated with materials that can deform without breaking, and elasticity pertains to materials' ability to return to their original shape after deformation—both of which are not inherent to the typical behavior of hardened concrete. Thermal conductivity deals with the heat transfer properties of the material and is less critical compared to compressive strength when assessing performance in structural applications.