Civil Engineering Reference
In-Depth Information
tion reactions. Sulfate
(from gypsum or other
sources) in the cement
regulates setting time,
but setting time is also
affected by cement fine-
ness, water-cement ratio,
and any admixtures that
may be used. Setting
times of concretes do
not correlate directly
with setting times of
pastes because of water
loss to the air or sub-
strate, presence of ag-
gregate, and because of
temperature differences
in the field (as con-
trasted with the con-
trolled temperature in a
testing lab). Fig. 2-40 il-
lustrates mean set times
for portland cements.
Type V*
Type IV**
Type III
Type II
Initial set
Final set
Type I
0 0100
150
200
250
030
350
400
450
Time of set, minutes (Vicat Method)
*Average of two values for final set
**Average of two values for initial set; one value for final set
Fig. 2-40. Time of set for portland cements (
Gebhardt 1995
and
PCA 1996
).
Fig. 2-38. Time of set test for
paste using the Vicat needle.
(23890)
dehydrates in the cement mill to form plaster. Stiffening is
caused by the rapid crystallization of interlocking needle-
like secondary gypsum. Additional mixing without added
water breaks up these crystals to restore workability.
Ettringite precipitation can also contribute to false set.
Flash set or quick set is evidenced by a rapid and early
loss of workability in paste, mortar, or concrete. It is usually
accompanied by the evolution of considerable heat result-
ing primarily from the rapid reaction of aluminates. If the
proper amount or form of calcium sulfate is not available to
control the calcium aluminate hydration, stiffening be-
comes apparent. Flash set cannot be dispelled, nor can the
plasticity be regained by further mixing without the addi-
tion of water.
Proper stiffening results from the careful balance of the
sulfate and aluminate compounds, as well as the tempera-
ture and fineness of the materials (which control the hydra-
tion and dissolution rates). The amount of sulfate formed
into plaster has a significant effect. For example, with one
particular cement, 2% plaster allowed a 5 hour set time,
while 1% plaster caused flash set to occur, and 3% allowed
false set to occur (
Helmuth and others 1995
).
Cements are tested for early stiffening using ASTM C
451 (paste method) (AASHTO T 186), and ASTM C 359
(mortar method) (AASHTO T 185), which use the penetra-
tion techniques of the Vicat apparatus. However, these tests
do not address all the mixing, placing, temperature, and
field conditions that can cause early stiffening. They also
do not address early stiffening caused by interactions with
other concrete ingredients. For example, concretes mixed
for very short periods, less than a minute, tend to be more
susceptible to early stiffening (
ACI 225
).
Fig. 2-39. Time of set as determined by the Gillmore
needle. (23892)
Early Stiffening (False Set and Flash Set)
Early stiffening is the early development of stiffness in the
working characteristics or plasticity of cement paste, mortar,
or concrete. This includes both false set and flash set.
False set is evidenced by a significant loss of plasticity
without the evolution of much heat shortly after mixing.
From a placing and handling standpoint, false-set ten-
dencies in cement will cause no difficulty if the concrete is
mixed for a longer time than usual or if it is remixed with-
out additional water before being transported or placed.
False set occurs when a significant amount of gypsum
