Durability
Concrete Technology
Home > Durability > Freeze-thaw
Resistance
Freeze-Thaw Resistance
 When
water freezes, it expands about 9%. As the water in moist concrete
freezes, it produces pressure in the pores of the concrete. If the
pressure developed exceeds the tensile strength of the concrete,
the cavity will dilate and rupture. The accumulative effect of successive
freeze-thaw cycles and disruption of paste and aggregate can eventually
cause expansion and cracking, scaling, and crumbling of the concrete.
Deicing chemicals
used for snow and ice removal, such as sodium chloride, can aggravate
freeze-thaw deterioration. Moisture tends to move towards zones
with higher salt concentrations (by osmosis). Therefore, if salts
are present in the pore solution the osmotic pressure is increased.
In addition, the application of deicing salts to pavements increases
the rate of cooling, increasing the potential for freeze-thaw deterioration
at the concrete surface. However, properly designed and placed air-entrained
concrete can withstand deicers for many years.
Click here for the effect of different types of deicers on concrete.
Click here for a case study
on conductive concrete used for bridge deck deicing.
 D-Cracking.
Cracking of concrete pavements caused by the freeze-thaw deterioration
of the aggregate within concrete is called D-cracking. D-cracks
are closely spaced crack formations parallel to transverse and longitudinal
joints that later multiply outward from the joints toward the center
of the pavement panel. D-cracking is a function of the pore properties
of certain types of aggregate particles and the environment in which
the pavement is placed. Due to the natural accumulation of water
under pavements in the base and subbase layers, the aggregate may
eventually become saturated. Then with freezing and thawing cycles,
cracking of the concrete starts in the saturated aggregate at the
bottom of the slab and progresses upward until it reaches the wearing
surface. This problem can be reduced either by selecting aggregates
that perform better in freeze-thaw cycles or, where marginal aggregates
must be used, by reducing the maximum particle size. Also, installation
of effective drainage systems for carrying free water out from under
the pavement may be helpful.
 |
| Cross section of air-entrained (right) and
non-air-entrained concrete. Large size air voids are entrapped
air. Small pinpoint size bubbles (entrained air) uniformly distributed
through the paste are beneficial air voids. Note comparison
with common pin. |
Air entrainment. The severity of freeze-thaw exposure
varies with different areas of the United States. Local weather
records can help determine the severity of exposure. The resistance
of concrete to freezing and thawing in a moist condition is significantly
improved by the use of intentionally entrained air. The tiny entrained
air voids act as empty chambers in the paste for the freezing and
migrating water to enter, thus relieving the pressure in the pores
and preventing damage to the concrete. Concrete with a low permeability
(that is, a low water-cement ratio and adequate curing) is better
able to resist freeze-thaw cycles. In rare cases, air-void clustering
can occur, leading to a loss of compressive strength. More
on air-void clustering.
 |
| Typical example of scaled concrete surface |
Prevention of Concrete Scaling
Scaling is defined as a general loss of surface mortar or mortar surrounding
the coarse aggregate particles on a concrete surface. This problem
is typically caused by the expansion of water due to freezing and
thawing cycles and the use of deicing chemicals; however properly
specified, produced, finished, and cured quality concrete need not
suffer this type of deterioration. There is a distinct chain of responsibility
for the production of scale resistant concrete. More
on preventing scaling.
 |
| Closeup view of ice impressions in paste of
frozen fresh concrete. The ice crystal formations occur as unharden
concrete freezes. |
Freezing temperatures. Concrete gains very little
strength at low temperatures. Accordingly, freshly placed concrete
must be protected against freezing until the degree of saturation
of the concrete has been sufficiently reduced by cement hydration.
The time at which this reduction is accomplished corresponds roughly
to the time required for the concrete to attain a compressive strength
of 3.5 MPa (500 psi). Concrete to be exposed to deicers should attain
a strength of 28 MPa (4000 psi) prior to repeated cycles of freezing
and thawing.
 Optimizing
the Use of Fly Ash in Concrete Cold weather and winter
conditions can be challenging when concrete contains fly ash. Especially
when used at higher levels, fly ash concrete typically has extended
setting times and slow strength gain, leading to low early-age strengths
and delays in rate of construction. In addition, concretes containing
fly ash are often reported to be more susceptible to surface scaling
when exposed to deicing chemicals than portland cement concrete.
It is therefore important to know how to adjust the amount of fly
ash to minimize the drawbacks, while maximizing the benefits.
The architect for the Bayview high-rise apartment
optimized the amount of fly ash on the basis of the requirements
of the concrete specification, the construction schedule and the
temperature. He limited the amount of fly ash in slabs on grade
placed during winter months to 20%. If adequate curing cannot be
provided or if the concrete is exposed to freezing and thawing in
the presence of deicer salts, the amount of fly ash should always
be less than 25%. More on optimizing
the use of fly ash in concrete.
Publications
 Different
concretes require different degrees of durability depending on the
exposure environment and the properties desired.
The Specifer’s Guide to Durable Concrete is intended
to provide sufficient information to allow the practitioner to select
materials and mix design parameters to achieve durable concrete
in a variety of environments.
 Optimizing
the Use of Fly Ash in Concrete (IS548) discusses
issues related to using low to very high levels of fly ash in concrete
and provides guidance for the use of fly ash without compromising
the construction process or the quality of the finished product.
Case studies were selected as examples of some of the more demanding
applications of fly ash concrete for ASR mitigation, chloride resistance,
and green building.
To search PCA’s bookstore for additional related publications
click here.
Images
To search for related images, click
here.
|