Civil Engineering Reference
In-Depth Information
TESTING FRESHLY MIXED CONCRETE
layer, and overfilled for the third layer. Each layer is
rodded 25 times. Following rodding, the last layer is struck
off and the cone is slowly raised vertically 300 mm (12 in.)
in 5 ± 2 seconds. As the concrete subsides or settles to a
new height, the empty slump cone is then inverted and
gently placed next to the settled concrete. The slump is the
vertical distance the concrete settles, measured to the near-
est 5 mm (
1
⁄
4
in.); a ruler is used to measure from the top of
the slump cone (mold) to the displaced original center of
the subsided concrete (see Fig. 16-2).
A higher slump value is indicative of a more fluid
concrete. The entire test through removal of the cone
should be completed in 2
1
⁄
2
minutes, as concrete loses
slump with time. If a falling away or shearing off occurs
from a portion of the concrete, another test should be run
on a different portion of the sample.
Another test method for flow of fresh concrete
involves the use of the K-Slump Tester (ASTM C 1362).
This is a probe-type instrument that is thrust into the
concrete in any location where there is a minimum depth
of 175 mm (7 in.) of concrete a 75-mm (3-in.) radius of
concrete around the tester. The amount of mortar flowing
into openings in the tester is reported as a measure of flow.
Additional consistency tests include: the FHWA
vibrating slope apparatus (
Wong and others 2001
and
Saucier 1966
); British compacting factor test (
BS 1881
);
Powers remolding test (
Powers 1932
); German flow table
test (DIN 1048-1); Vebe consistometer for roller-compacted
concrete (ASTM C 1170); Kelly ball penetration test (ASTM
C 360-92 now discontinued); Thaulow tester; the inverted
slump cone for fiber-reinforced concrete (ASTM C 995);
Powers and Wiler plastometer (
Powers and Wiler 1941
);
Sampling Freshly Mixed Concrete
The importance of obtaining truly representative samples
of freshly mixed concrete for control tests must be empha-
sized. Unless the sample is representative, test results will
be misleading. Samples should be obtained and handled in
accordance with ASTM C 172 (AASHTO T 141). Except for
routine slump and air-content tests performed for process
control, ASTM C 172 (AASHTO T 141) requires that sample
size used for acceptance purposes be at least 28 liter (1 cu ft)
and be obtained within 15 minutes between the first and
final portions of the sample. The composite sample, made
of two or more portions, should not be taken from the very
first or last portion of the batch discharge. The sample
should be protected from sunlight, wind, and other sources
of rapid evaporation during sampling and testing.
Consistency
The slump test, ASTM C 143 (AASHTO T 119), is the most
generally accepted method used to measure the consis-
tency of concrete (Fig. 16-2). The test equipment consists of
a slump cone (a metal conical mold 300 mm [12 in.] high,
with a 200-mm [8-in.] diameter base and 100-mm [4-in.]
diameter top) and a steel rod 16 mm (
5
⁄
8
in.) in diameter and
600 mm (24 in.) long with a hemispherically shaped tip.
The dampened slump cone, placed upright on a flat,
nonabsorbent rigid surface, should be filled in three layers
of approximately equal volume. Therefore, the cone
should be filled to a depth of about 70 mm (2
1
⁄
2
in.) for the
first layer, a depth of about 160 mm (6 in.) for the second
A
B
Fig. 16-2. Slump test for consistency of concrete. Figure A illustrates a lower slump, Figure B a higher slump. (69786, 69787)
