Civil Engineering Reference
In-Depth Information
Therefore, further hydration and chemical shrinkage is
compensated by the formation of voids in the microstruc-
ture (Fig. 15-1). Most of this volume change is internal
and does not significantly change the visible external
dimensions of a concrete element.
The amount of volume change due to chemical
shrinkage can be estimated from the hydrated cement
phases and their crystal densities or it can be determined
by physical test as illustrated in Fig. 15-2. The Japan
Concrete Institute has a test method for chemical
shrinkage of cement paste (
Tazawa 1999
). An example of
long-term chemical shrinkage for portland cement paste is
illustrated in Fig. 15-3. Early researchers sometimes
referred to chemical shrinkage as the absorption of water
during hydration (
Powers 1935
).
Le Chatelier (1900)
was
the first to study chemical shrinkage of cement pastes.
Autogenous Shrinkage
Autogenous shrinkage is the macroscopic volume reduc-
tion (visible dimensional change) of cement paste, mortar,
or concrete caused by cement hydration. The macroscopic
volume reduction of autogenous shrinkage is much less
than the absolute volume reduction of chemical shrinkage
because of the rigidity of the hardened paste structure.
Chemical shrinkage is the driving force behind autogenous
shrinkage. The relationship between autogenous shrink-
age and chemical shrinkage is illustrated in Figs. 15-1, 15-4,
and 15-5. Some researchers and organizations consider that
Water level
at time
zero
1.2
Autogenous shrinkage
Chemical shrinkage
Chemical
shrinkage
volume at
time n
1
0.8
Lime
saturated
water
0.6
Water
level at
time n
0.4
Start of
setting
0.2
Cement
paste
0
0
2
4
6
8
10
Time after mixing, hours
Fig. 15-4. Relationship between autogenous shrinkage and
chemical shrinkage of cement paste at early ages (
Hammer
1999
).
Fig. 15-2. Test for chemical shrinkage of cement paste
showing flask for cement paste and pipet for absorbed
water measurement.
Chemical shrinkage
Subsidence
Chemical
shrinkage
Bleed water
Autogenous shrinkage
8
Cumulative
hydration voids
Water
w/c = 0.50
Water
Water
6
Cement
4
Cement
Ordinary portland cement
Cement
Hydrated
cement
2
Hydrated
cement
At casting
At initial setting
After hardening
0
0
1
10
100
1000
Fig. 15-5. Volumetric relationship between subsidence,
bleed water, chemical shrinkage, and autogenous shrink-
age. Only autogenous shrinkage after initial set is shown.
Not to scale.
Age, hours
Fig. 15-3. Chemical shrinkage of cement paste (
Tazawa
1999
).
