Civil Engineering Reference
In-Depth Information
Consistency
Consistency refers to
the relative mobility of
a freshly mixed cement
paste or mortar or to its
ability to flow. During
cement testing, pastes
are mixed to normal
consistency as defined
by a penetration of 10 ±
1 mm of the Vicat
plunger (see ASTM C
187 or AASHTO T 129
and Fig. 2-36). Mortars
are mixed to obtain
either a fixed water-
cement ratio or to yield a
flow within a prescribed
range. The flow is deter-
mined on a flow table as
described in ASTM C
230 (AASHTO M 152)
and ASTM C 1437 (Fig.
2-37). Both the normal
consistency method and
the flow test are used to
regulate water contents
of pastes and mortars,
respectively, to be used
in subsequent tests; both
allow comparing dis-
similar ingredients with
the same penetrability
or flow.
Fig. 2-33. Quick tests, such as washing cement over this 45-
micrometer sieve, help monitor cement fineness during
production. Shown is a view of the sieve holder with an inset
top view of a cement sample on the sieve before washing
with water. (68818, 68819)
Fig. 2-36. Normal consistency
test for paste using the Vicat
plunger. (68820)
Fig. 2-34. A laser particle analyzer uses laser
diffraction to determine the particle size
distribution of fine powders. Fig. 2-31 (right)
illustrates typical results. (69390)
Fig. 2-37. Consistency test for
mortar using the flow table. The
mortar is placed in a small brass
mold centered on the table (in-
set). For skin safety the tech-
nician wears protective gloves
while handling the mortar. After
the mold is removed and the
table undergoes a succession of
drops, the diameter of the pat is
measured to determine consis-
tency. (68821, 68822)
Setting Time
The object of the setting
time test is to determine
(1) the time that elapses
from the moment water
is added until the paste
ceases to be fluid and
plastic (called initial set)
and (2) the time required
for the paste to acquire a
certain degree of hardness (called final set).
To determine if a cement sets according to the time
limits specified in cement specifications, tests are
performed using either the Vicat apparatus (ASTM C 191
or AASHTO T 131—Fig. 2-38) or the Gillmore needle
(ASTM C 266 or AASHTO T 154—Fig. 2-39).
The Vicat test governs if no test method is specified by
the purchaser. Initial set of cement paste must not occurtoo
early and final set must not occur too late. The settingtimes
indicate that a paste is or is not undergoing normal hydra-
Fig. 2-35. In the soundness test, 25-
mm square bars are exposed to high
temperature and pressure in the
autoclave to determine the volume
stability of the cement paste. (23894)
