Characteristics of hardened mortar include compressive strength and tensile bond strength. Only the first is controlled by ASTM C270. If mortar is specified by the property specification of ASTM C270, compressive strength is controlled directly. It is measured using 2 in. mortar cubes, made with laboratory flow mortar, cured for 28 days at 100 percent relative humidity and 70°F. It typically ranges from 500 to 3000 psi. It does not significantly affect the compressive strength of masonry assemblages. ASTM C270 requires minimum compressive strengths of 2500, 1800i, 750, and 350 psi for Types M, S, N, and O mortar, respectively.
If mortar is specified by the proportion specification of ASTM C270, compressive strength is controlled indirectly. Masonry-cement mortar meeting the proportion specification usually has a compressive strength slightly greater than the minimum value specified in the property specification. Cement-lime mortar meeting the proportion specification usually has a compressive strength considerably greater than the minimum value specified in the property specification.
Other Characteristics of Hardened Mortar
One other characteristic of hardened mortar is tensile bond strength (the tensile strength of the bond between mortar and units). Cement-lime mortar has traditionally satisfied practical requirements for tensile bond strength. Tensile bond strength is not specified directly for cement-lime mortar, or for masonry-cement mortar. It is addressed directly in the specification for mortar-cement mortar. Strictly speaking, it can be measured only in conjunction with units, and is therefore not a property of the hardened mortar alone. Nevertheless, certain characteristics of the mortar itself contribute to good tensile bond strength regardless of the type of unit used. High tensile bond strength can be obtained using cement-lime mortar or mortar-cement mortar. It is also enhanced by the use of mortars with air content below 12 percent.