Table 15-5.-Numbers of Layers Required for Specimens
Specimen Type and Size,
Mode of
Number of
Approximate Depth of
as Depth, in. (mm)
Compaction
Layers
Layer, in (mm)
Cylinders:
12 (305)
rodding
3 equal
4 (100)
Over 12 (305)
rodding
as required
4 (100)
Beams:
6 (125) to 8 (200)
rodding
2 equal
half depth of specimen
Over 8 (200)
rodding
3 or more
4 (100)
(table 15-5). As you fill a mold, rotate each scoop-
ful of the concrete around the top edge of the
mold as the concrete slides from it. This ensures a
symmetrical concrete distribution within the mold.
Tamp each layer with the tamping rod,
distributing the strokes uniformly over the cross
section of the mold and penetrating the underlying
layer. Tamp the bottom layer throughout its
depth. The number of roddings is determined by
the diameter of the cylinder. (See table 15-6.)
After tamping the top layer, strike off the sur-
face with a trowel or rod so that the concrete fills
the mold exactly. Do not add unrepresentative
concrete to an underfilled mold. If voids are left
by the tamping rod, tap the sides of the mold
lightly with your open hand to close the voids.
If desired, cap the top surface of freshly made
cylinders with a thin layer of stiff portland cement
paste, which you should then permit to harden
and cure with the specimens. When finished, move
the specimens to the storage place and leave them
undisturbed for the initial curing period.
CURING AND STORING CYLINDERS.–
During the initial curing period of test specimens,
be sure to take precautions to prevent the evapora-
tion and loss of water in the specimens. Cover the
specimens with a sheet of plastic. You may place
wet burlap on top of the plastic to help retard
evaporation, but be sure that it does not come in
contact with the concrete surface. The exterior of
cardboard molds must be protected against the
absorption of water or molds may expand, allow-
ing the specimens to be damaged. The test
specimens now begin the initial curing period of
24 hr ± 8 hr. Test specimens maybe transported
after the initial curing period, providing they
remain in the mold. Upon completion of the
initial curing period, remove the specimens
from the molds and place them immediately in
a moist environment with water maintained on
their surface at a temperature of 73.4°F ± 3°
(23°C ± 1.7°). You can also obtain the required
condition by immersing the specimens in saturated
limewater or by storing them in a moist room or
cabinet. Do not expose test specimens to a flow
of running or dripping water.
CAPPING CYLINDERS.— The ends of com-
pression-test specimens must be planed within
0.002 in. and within 0.5 degrees of being
perpendicular to the axis of the cylinder.
Specimens formed in strong metal molds
having accurately flat baseplates can be capped
with neat cement at 2 to 4 hr after molding. A
stiff paste of portland cement and water is made
at the time the cylinder is molded so that the
capping mixture will have shrunk before applica-
tion. Any free water or laitance (layer of fine
particles on the surface) is removed from the end
of the specimen. The paste is applied to the top
of the concrete and worked with a flat plate until
it is smooth and level with the top of the mold.
Hardened concrete specimens may be ground
to place ends or capped with a material having
greater compressive strength than the concrete.
Prepared mixtures of sulfur and granular mate-
rials, special high-strength gypsum plasters, and
neat high-early strength cement are satisfactory
capping materials (ordinary low-strength plaster
of paris, compressible rubber, or fibrous materials
are not suitable for caps). You should apply these
materials in a plastic state and finish them to the
desired plane surface by applying glass or metal
15-38
