Mass Concrete
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Mass Concrete—How Do You Handle the
Heat?
By John Gajda, PE
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| Mass concrete columns and footings for the
James River Bridge. (Courtesy of Fred Parkinson, PB.) |
Mass concrete is a hot topic. Owners desire long service lives so
engineers design concrete mixes for low permeability. These mixes
typically have high cementitious material contents, which results
in high temperatures within the concrete. To avoid cracking and other
temperature related damage to the concrete, contractors must control
the maximum temperature and temperature difference between the interior
and the surface of the concrete. This can pit the schedule against
the service life. When all involved parties work together, appropriate
changes can be made to achieve the desired service life with minimal
impacts to the schedule. The key is an understanding of mass concrete.
First of all, what is mass concrete? Mass concrete is defined
by the American Concrete Institute (ACI) as: Any volume of concrete
with dimensions large enough to require that measures be taken to
cope with generation of heat from hydration of the cement and attendant
volume change to minimize cracking. While this is a perfect
definition, the question is often asked, “so, is this placement
considered mass concrete?” As a general rule of thumb, any
placement of structural concrete with a minimum dimension equal
to or greater than 1 meter (36 in.) should be considered mass concrete.
Similar considerations should be given to other concrete placements
that do not meet this minimum dimension, but contain ASTM C150 Type
III or ASTM C1157 HE cement, accelerating admixtures, or cementitious
materials in excess of 355 kg/m3 (600 lb/yd3)
of concrete.
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| A severe case of thermal cracking in a concrete
footing. |
Now that we know what placements are considered mass concrete,
what makes a mass concrete placement any different than a typical
placement? The answer is that temperatures in a mass concrete
placement can get high enough to damage the concrete. All concretes
generate heat. Heat is a byproduct of the hydration reactions which
gives concrete its strength and durability. In most placements, the
heat escapes almost as rapidly as it is generated. In a mass concrete
placement, the heat escapes more slowly than it is generated. The
result is that temperatures within the concrete can get quite hot.
If the internal temperature exceeds 70°C (158°F), the long
term durability of some concretes can be affected by delayed ettringite
formation (DEF). DEF is rare and only certain concretes can be
affected. When DEF occurs, the concrete paste expands and cracks the
concrete with detrimental results, which may not be evident for many
years. Additionally, while the interior can be quite hot, the surface
can be relatively cool. The resulting large temperature difference
results in large thermal stresses which can cause cracking of the
surface. Historically, limiting the temperature difference between
the interior and surface so that it is less than 20°C (35°F)
has been found to prevent or minimize thermal cracking. Certain concretes
are more tolerant of thermal cracking than others, and these concretes
can withstand a higher temperature difference without thermally cracking.
How do I prevent high internal temperatures and large temperature
differences? The first step is to select an appropriate
mix design. This will reduce other efforts to control temperatures
and temperature differences after placement. The temperature rise
of concrete is directly related to the types and quantities of cementitious
materials in the concrete. An appropriate mix design contains the
least amount of cementitious materials needed for strength and durability.
Placeability of concrete must also factor into the concrete mix
design. This sometimes increases the cementitious content. To reduce
heat of hydration, Class F fly ash or slag cement is typically used
to replace a portion of the cement. The percentage depends on several
factors including environmental exposure and durability requirements.
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| Concrete insulating blankets on a column. |
Once I have a reasonable concrete mix design, do I need
to do anything else?
In most cases, the answer is yes; two items must be considered. First,
you must ensure that the maximum temperature in the concrete will
not exceed 70°C (158°F). In placements over about 1.80 m (6
ft) thick, the maximum temperature is the sum of the installed concrete
temperature plus the temperature rise of the concrete. The temperature
rise can be measured or estimated. If the maximum temperature of the
concrete is predicted to exceed 70°C (158°F), the concrete
can be precooled by using chilled batch water, substituting ice for
a portion of the batch water, or by liquid nitrogen injection into
the fresh concrete. If significant precooling is required, internal
cooling pipes can be used to reduce the amount of precooling. Second,
the concrete surface will likely also need to be insulated. Insulation
is needed to limit the temperature difference between the center and
surface so that thermal cracking is prevented or minimized. One or
two layers of concrete insulating blankets are often used. Thermal
modeling is sometimes done to optimize the amount of insulation and
precooling, so that the most cost-effective measures are used.
Do I need to do anything else? To document the
means and methods that are required and will be used, a thermal
control plan should be developed. A thermal control plan is similar
to a quality control plan, and will allow all involved parties to
agree on the measures that will be used, and the expected results.
Such measures may include precooling of the concrete, cooling pipe
installation and operation, insulation, temperature monitoring equipment
and locations.
Additional and more detailed information on mass concrete can be
found in PCA’s publication Mass
Concrete for Buildings and Bridges (EB547).
John Gajda, PE,
Principal Engineer
CTLGroup, Skokie, Ill.
JGajda@ctlgroup.com
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