How is mortar for masonry construction specified?

ANSWER: 

The “Standard Specification for Mortar for Unit Masonry” is designated as ASTM C270, and is produced by the American Society for Testing and Materials (ASTM) (www.astm.org). ASTM C270 defines two approaches for specifying mortar. One is called the proportion method and the other is known as the property method.

The proportion method is the simpler approach and is used on most projects. The proportion method specifies mortar by giving a recipe and requires no mortar testing (Refer to FAQ, “When should mortar cube testing be performed?”). The proportion recipe spells out relative quantities of sand and cementitious materials, each measured by volume, to mix together to create the appropriate mortar Type (refer to FAQ How do I determine what mortar Type to specify?”).

The property method requires mortar testing by a qualified laboratory.  A laboratory-prepared mix is tested to confirm that three physical properties, compressive strength, air content, and water-retentivity, meet the requirements of ASTM C270.  The laboratory then defines the recipe for mixing mortar in the field.

Note that regardless of whether the proportion method or property method of specifying mortar is used, the quantity of water in field-mixed mortar is not limited by ASTM C270.  ASTM C270 identifies the proportion method as the default, so when a specifier is silent on the method of mortar specification, the proportion method governs.

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