Mortar and Grout: Proportioning, Mixing, and Field Testing
Mortar
ASTM C270 covers proportion and property specifications. Common volume proportions are Type M 1 cement : 1/4 lime : 3 sand; Type S 1 : 1/2 : 4-1/2; Type N 1 : 1 : 6; and Type O 1 : 2 : 9. Property-spec minimum 28-day strengths are M 2,500 psi; S 1,800 psi; N 750 psi; and O 350 psi. Mortar must be mechanically mixed, used within the TMS 602 time limit (commonly 1-1/2 hours), and retempered only if permitted.
Grout
ASTM C476 grout uses fine grout at 1 part cement : 2-1/4 sand, or coarse grout at 1 : 2-1/4 : 2 coarse aggregate. Minimum 28-day strength is 2,000 psi. Grout slump is typically 8 to 11 in. and must remain workable.
Field testing
ASTM C780 covers field mortar sampling and testing; C1019 covers grout sampling; C1314 covers prism testing. Frequencies come from TMS 602 and the project specifications; common practice is at least one set per 5,000 sq ft and not less than one set per day.
Worked example
A mortar submittal lists 1 part masonry cement : 3 sand. That may be a proprietary masonry-cement mortar, not the same as Type M. The inspector must verify the mortar type by the C270 property or proportion table and the approved mix design, not by the mason's shorthand.
Traps
- Confusing masonry cement with portland cement.
- Adding water after initial set to loosen mortar.
- Testing grout with a mortar test method instead of C1019.