ASTM C31 does not suggest “curing to be done in temp 60 -80 F” but requires the following curing during the first 48 hour period:
10.1.2 Initial Curing— Immediately after molding and
finishing, the specimens shall be stored for a period up to 48 h
in a temperature range from 60 and 80°F [16 and 27°C] and in
an environment preventing moisture loss from the specimens.
For concrete mixtures with a specified strength of 6000 psi [40
MPa] or greater, the initial curing temperature shall be between
68 and 78°F [20 and 26°C].
After the Initial Curing period of 48 hours or less the specimens are required to be placed in Final Curing:
10.1.3 Final Curing:
10.1.3.1 Cylinders—Upon completion of initial curing and
within 30 min after removing the molds, cure specimens with
free water maintained on their surfaces at all times at a
temperature of 73 ± 3°F [23 ± 2°C] using water storage tanks
or moist rooms complying with the requirements of Specification
C 511, except when capping with sulfur mortar capping
compound and immediately prior to testing. When capping
with sulfur mortar capping compound, the ends of the cylinder
shall be dry enough to preclude the formation of steam or foam
pockets under or in cap larger than 1?4 in. [6 mm] as described
in Practice C 617. For a period not to exceed 3 h immediately
prior to test, standard curing temperature is not required
provided free moisture is maintained on the cylinders and
ambient temperature is between 68 and 86°F [20 and 30°C].
10.1.3.2 Beams—Beams are to be cured the same as cylinders
(see 10.1.3.1) except that they shall be stored in water
saturated with calcium hydroxide at 73 ± 3°F [23 ± 2°C] at
least 20 h prior to testing. Drying of the surfaces of the beam
shall be prevented between removal from water storage and
completion of testing.
Note the combination of Initial Curing and Final Curing is known as Standard Curing and must be used if the results are to be used to determine the following:
4.2 If the specimens are made and standard cured, as
stipulated herein, the resulting strength test data when the
specimens are tested are able to be used for the following
purposes:
4.2.1 Acceptance testing for specified strength,
4.2.2 Checking adequacy of mixture proportions for
strength, and
The Field Curing that you mentioned is only an option if the results are going to be used for the following:
4.3 If the specimens are made and field cured, as stipulated
herein, the resulting strength test data when the specimens are
tested are able to be used for the following purposes:
4.3.1 Determination of whether a structure is capable of
being put in service,
4.3.2 Comparison with test results of standard cured specimens
or with test results from various in-place test methods,
4.3.3 Adequacy of curing and protection of concrete in the
structure, or
4.3.4 Form or shoring removal time requirements.
4.2.3 Quality control.
Why the testing lab told you this 3 day thing I have no idea. You must have misunderstood what they wanted because ASTM C31 requires cylinders to be demolded and placed in Final Curing within 48 hours of molding.
SCET - Techmaximus
