Q & A
Question: Does HPC need to be air entrained
for frost resistance?
Answer: The primary purpose of air
entrainment in concrete is to improve concrete’s resistance
to cycles of freezing and thawing when exposed to water or deicing
chemicals. Consequently, concrete in an environment that does not
experience freeze-thaw cycles, and hence the use of deicing chemicals,
does not need to be frost resistant. Air entrainment, therefore,
is not necessary for concrete in these locations. This is true for
both HPC and conventional concretes.
Structures in locations that experience freeze-thaw cycles are also
likely to be exposed to ice and snow storms and the use of deicing
chemicals. In these locations, HPC members that have the potential
for being critically saturated when exposed to freeze-thaw cycles
must be frost resistant.
PCA publication contains the results of tests on high-strength concrete
for frost and scaling resistance.1 All tests were made
on concretes with portland cement as the only cementitious material.
Tests for resistance of concrete to rapid freezing and thawing were
made in accordance with ASTM C 666 Method A after 14 days of drying.
Durability factors close to 100 were only achieved without air entrainment
when the water-cement ratio was equal to or less than 0.35. Tests
for scaling resistance of concrete surfaces exposed to deicing chemicals
were made in accordance with ASTM C 672. Resistance to scaling of
non-air-entrained concrete was only achieved when the water-cement
ratio was reduced to 0.25.
Most bridge decks in locations that experience freezing and thawing
are directly exposed to freezing rain, snow, and deicing chemicals.
It is also likely that the concretes used in these bridge decks
have water-cementitious materials ratios of about 0.40. No well-documented
field experiments have been made to prove that air entrainment is
not needed in HPC. Until such data are available, current practice
for air entrainment should be followed for decks and other bridge
elements exposed directly to deicing chemicals.
In contrast to decks, HPC bridge beams are generally made with high-strength
concrete. This requires the use of a water-cementitious materials
ratio of less than 0.40, which offers the potential for frost resistant
concrete with a lower percentage of air entrainment or even no air
entrainment. At the same time, bridge beams are protected by the
bridge deck from direct exposure to moisture and deicing salts.
High-strength concretes also have a lower permeability than conventional
strength concretes and moisture penetration is likely to be less.
Non-air-entrained girders used on the Illinois Toll Road bridges
built in 1957-58 are still performing well. Since the use of air
entrainment in high-strength concrete reduces the compressive strength
by about 5 percent or about 500 psi (3.5 MPa) for each 1 percent
increase in air content, it is desirable to minimize the air content
in order to achieve the strength.
All of this indicates that the need for air entrainment is less
critical in HPC bridge beams than in decks. As with all HPC applications,
the specified properties must be consistent with the intended application
and environment. The need for air entrainment in HPC bridge beams
should be based on local conditions and practices.
Pinto, R. C. A. and Hover, K. C., Frost
and Scaling Resistance of High-Strength Concrete, PCA Research
and Development Bulletin RD122, Portland Cement Association, Skokie,
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