matrix to absorb the escaping water into empty
pore, the lower the freezing temperature. Since
pores. In addition, even if the aggregate can dilate
aggregate includes a range of pore sizes, not all
without cracking, pavement damage can only be
pores freeze at one temperature, whereas in the
avoided if the pavement matrix can also expand
sulfate test, all pores, regardless of size, experi-
without cracking. Thus, aggregate that can elasti-
ence expansive forces caused by crystal growth.
cally accommodate water frozen within its pores
Consequently, the mechanism of internal destruc-
could show no effect from unconfined testing but
tion caused by the sulfate soundness test does not
could show considerable damage when tested in
duplicate that of natural freezing. The literature
concrete if the concrete matrix was relatively
contains examples of aggregate that failed the
impermeable or if it was unable to accommodate
sulfate test but performed well in service and vice
the dilation of the aggregate. Aggregate that wets
versa.
by capillarity may take on more water when
exposed to a free water surface than when con-
Unconfined freezethaw test (AASHTO T 103)
fined in a pavement, particularly if the matrix has
Modern refrigeration equipment provides an
smaller pores than those in the aggregate, in
alternative to the sulfate soundness test for mea-
which case water will be preferentially absorbed
suring aggregate durability. This equipment was
by the matrix. In this case, the aggregate would
first used to test the aggregate in an unconfined
likely suffer more damage when tested uncon-
condition; that is, not confined within concrete
fined than when tested confined.
mixtures as it is normally found. Traditionally,
unconfined freezethaw tests have either evalu-
ated the damage caused by a certain number of
Confined rapid freezethaw test
freezethaw cycles or they have determined the
(ASTM C 666 A and B)
A popular test procedure for evaluating the
number of freezethaw cycles needed to cause a
frost durability of concrete and aggregate is
certain amount of damage. The basic procedure,
AASHTO T 103 (1990), is to subject aggregate to
ASTM C 666 (1990) and its many variants around
repeated cycles of freezing and thawing. Varia-
the world. It consists of two methods: method A,
tions on this procedure have included vacuum
which freezes and thaws concrete in water, and
saturation of aggregate, as opposed to merely sub-
method B, which freezes concrete in air and
merging the aggregate in water; alcoholwater, as
thaws it in water. One freezethaw cycle lasts
opposed to water, as the wetting medium to aid
from 2 to 5 hr, and the test can last up to 300 or
penetration of pores; soaking the aggregate in salt
more cycles. The correlation between this test and
solutions, as opposed to just water, to increase
field performance has not always been good. The
frost damage; and varying the freezing and thaw-
major weakness with this test is a very rapid cool-
ing rates. Despite these and other variations on
ing rate compared with that in nature. The
the freezethaw test procedure, the results
hydraulic pressures that might be caused by a rela-
obtained by unconfined freezing and thawing of
tively slowly advancing freeze front in nature are
aggregate have not always provided good corre-
probably much lower than the unrealistically
lation to service life.
fast-moving freeze fronts caused by this test.
The problem is that freezing unconfined
Another objection to this test is that up to 5 months
aggregate is not the same as freezing aggregate
can pass before results are available. Though this
confined in concrete. Though the freezing process
test has several shortcomings, it will continue to
is the same in either case, the interaction between
be popular until a better method is available.
the aggregate and its immediate surroundings
creates the difference between the two testing
Hydraulic fracture test
conditions. As ice forms in a pore of an aggregate,
Recently, a new test was developed to simulate
the approximate 9% volume change that accom-
the hydraulic forces generated by freezing water
panies this process squeezes unfrozen water
inside aggregate without having to freeze the
against the walls of the pore. This hydraulic pres-
aggregate. In this test, aggregate submerged in
sure, in the case of unconfined aggregate, can be
water is subjected to high pressures. Upon sudden
handled if the aggregate can elastically deform,
release of the pressure, air compressed within the
or if the water can escape into a nearby empty
pores forces water to flow away from the pore in
void or to the outside boundary of the aggregate.
much the same way as freezing causes unfrozen
For confined aggregates, the outside pressure
water to flow away from a freeze front. The fact
relief depends on the ability of the concrete
that this creates very rapid pressure changes and
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