Mixing time
ASTM C270-19a §7.3 requires mechanical batch mixing for 3 to 5 minutes. Hand mixing is allowed only with the specifier's written approval. One to two minutes is too short; five to ten minutes or fifteen minutes and more runs past the 5 minute maximum.
Retempering is allowed
ASTM C270-19a §7.4 lets mortar that has stiffened from evaporation be retempered with water as often as needed. Two traps sit here:
- Retempering is not prohibited. Discarding stiffened mortar on that belief is a common error.
- There is no "once only" limit. §7.4 says as often as needed.
Retempering means water. §7.4 does not allow adding cement to restore strength, and adding anything never removes the time limit.
The normal time limit
§7.4 bars use of any mortar more than 2 1/2 h after mixing. Add 2 hours 30 minutes to the mixing time.
Mixed at 7:45 a.m. on a mild day: 7:45 + 2:30 = 10:15 a.m. Stiffened mortar at 40 minutes can be retempered and used, as long as it is placed within that 2 1/2 h window.
The hot-weather limit
BIA TN 8 (2020) limits mortar to 2 h after initial mixing in hot weather, and says mortar should not be used once it has begun to set.
Mixed at 1:15 p.m. in hot weather: 1:15 + 2:00 = 3:15 p.m.
The trap runs both ways. Applying the normal 2 1/2 h C270 limit in hot weather (which would give 3:45 p.m. here) is the common error. Applying the 2 h hot-weather figure on a mild day (which would give 9:45 a.m. in the morning example) is the reverse mistake. Check the weather in the question, pick the limit, then do the clock arithmetic carefully.