What is Mass Concrete?
According to ACI 207, "mass concrete is
any large volume of concrete with dimensions
large enough to require that measures
be taken to cope with the generation of heat
and attendant volume change to minimize
cracking." Cement hydration generates
heat. Heat dissipates from concrete slowly;
the thicker the section, the longer it will
take the interior to cool. This can result in
large temperature differentials between the
concrete surface and its interior. The
concrete is then subject to high thermal
stresses, which can result in cracking and
loss of structural integrity.
Reducing Thermal Stress
There are three generally accepted strategies for reducing thermal stress in concrete:
- Reduce the total cementitious content
- Reduce the portland cement content
- Slow down the hydration process through the use of various admixtures or
cooling the concrete.
How Does Slag Cement Help?
When slag cement is incorporated in a concrete mixture, less heat is generated and
thermal stress is reduced:
- Due to increased strength with slag cement, the total cementitious content
can be reduced
- Portland cement content is reduced by the percentage of slag cement used
- Hydration characteristics of slag cement are such that the early rate of heat
generation and peak temperature of the concrete are reduced.
Slag Replacement Levels
Generally 65 to 80 percent is considered
an optimum replacement range for mass
concrete applications. These levels typically
provide significant heat reduction
while achieving desired strengths.
Levels from 50 to 65 percent have been
used successfully in smaller mass concrete
placements. Mixtures should be
tested with job materials to ensure required
thermal and strength characteristics.
Figure 2 shows the specific heat of
hydration of an ordinary portland
cement and combinations from 25 to 75
percent slag cement substitution. Note
that each increasing level of slag cement
reduces peak heat of hydration of the
binder. More importantly, the area under
each curve represents total heat generated.
This area reduces dramatically when
the substitution rate increases from 50 to
75 percent, indicating that significantly
less total heat is generated for 75 percent
slag cement, even though the peak heat
of hydration is not much reduced from
50 percent substitution.
Figure 3 shows the effect of slag cement
on temperature rise in mass concrete.
Note that peak temperature dropped by
19 degrees F with 65 percent slag
cement substitution, and by 42 degrees F
for 80% substitution.
- ACI 207.1R-97, Mass Concrete; Concrete Institute, Farmington Hills, Michigan, 1992.