Table 15-6.-Number of Roddings to be Used in Molding
Cylinder Specimens
Diameter of Cylinder,
Number of
in. (mm)
Strokes/Layer
6 (152)
25
8 (200)
50
10 (250)
75
plates and squeezing out excess material to pro-
vide a cap that is as thin as possible. Sulfur caps
may be applied in time to harden for at least 2 hr
before testing. Plaster caps cannot be stored over
4 hr in a moist room. Neat cement caps must be
aged 6 days or more in a moist room (2 days when
Type II cement is used). During capping, protect
moist, cured specimens against drying by cover-
ing them with wet burlap.
Beam Specimens
Tests are performed on concrete beam
specimens to evaluate the flexural strength of the
concrete. The standard beam specimen is 6 in. by
6 in. by 21 in. (152 mm by 152 mm by 532 mm)
for concrete in which the maximum size of the
coarse aggregate is 2 in. (50 mm). When the max-
imum size of the coarse aggregate exceeds 2 in.
(50 mm), the smaller cross-sectional dimension is
to be increased to at least three times the nominal
maximum size of the coarse aggregate. All beam
specimens prepared in the field are to be at least
6 in. wide and 6 in. deep unless required other-
wise by project specifications.
STANDARDS FOR BEAM MOLDS.— The
beam molds are to be smooth on all interior sur-
faces and free from warpage. The molds are to
produce specimens that do not exceed the required
cross-sectional dimensions by 1/8 in. The length
of the specimens is not to be more than 1/16 in.
shorter than the specified length, but it may
exceed that length.
RODDING.— Place the concrete in the mold
in the required number of layers. (See table 15-5.)
Rod the bottom layer throughout, distributing the
strokes uniformly over the cross section of the
mold. When rodding the upper layers, allow the
rod to penetrate the previous layer 1/2 in.,
providing the previous layer is 4 in. or less and
1 in. if the previous layer is greater than 4 in. The
number of strokes’ per layer is one for each 2 in.2
(13 cm2) of the top surface area of the specimen.
After each layer is rodded, spade the concrete with
a trowel along the sides of the mold to help in
the removal of surface voids. Strike off the top
surface with a straightedge, and finish it with a
wooden float.
CURING.— YOU should cure the beam
specimens in the same manner as the cylinder
specimens with the following exceptions: (1) ex-
tend the initial curing period to 48 hr ± 4 hr and
(2) do not allow the surface of the beam specimen
to become dry between the time of removal from
curing and the completion of testing.
When transporting specimens from the field
to the laboratory, be sure they are sufficiently
cushioned to protect them from damage by
jarring. Additional measures are required to
prevent damage by freezing temperatures and
moisture loss. You can prevent moisture loss by
covering the specimens with plastic or surround-
ing them by wet sand or wet sawdust.
15-39
