sidered to be a measure of the beneficial voids in
that the entire aggregate is pressurized, as
the aggregate. A second reading, taken after 15
opposed to only freezing sites, suggests that this
minutes, indicates the relative portion of voids
test could be quite different from natural freez-
ing. Generally, aggregates that can withstand
that are frost susceptible. Large amounts of sec-
more than 100 pressure cycles are quite durable,
ondary water indicate that the aggregate is sus-
and those that produce fracture in 5% of the
ceptible to frost damage. This test seems to give
aggregate particles in less than 50 pressure cycles
good correlation to ASTM C 666 (1990) results,
tend to exhibit poor frost resistance. Though this
though there is not always a good correlation
test has shown reasonable correlation to field per-
with field performance.
formance in limited testing, more experience is
Assuming that the first minute represents ben-
needed to assess it.
eficial voids appears to be a shortcoming of this
test. In nature, the first pores to fill with water are
those that are small enough to exhibit capillarity.
Cryogenic test
The cryogenic test (Korhonen and Charest
The larger "beneficial" voids, provided they are
1995) examines the efficacy of cycling aggregate
not connected to the surface, will not fill with
between hot water and liquid nitrogen. Modified
water unless they are under hydrostatic pressure
from AASHTO T 103 (1990), the test consists of
(or gravity drainage). Thus, a 24-hr soak followed
immersing water-saturated aggregate in liquid
by additional wetting under pressure should be
nitrogen for about 1 minute followed by a
investigated as a modification to this test.
2-minute immersion in hot water. Within 10
In summary, the experience of transportation
freezethaw cycles, aggregates susceptible to
officials, pavement engineers, researchers, and
frost damage are readily identified. The test,
others is that, whether aggregates are confined
within Portland cement concrete (PCC) or within
however, is not capable of ranking aggregates of
asphalt cement concrete (ACC) pavements, they
moderate to good performance.
are not always immune to damage caused by cold
weather. D-cracking, a common distress in PCC
Slow freeze test (ASTM C 671)
It has been documented that cooling rates in
pavements, is the disintegration of coarse aggre-
gates by frost action, and stripping, a common
the field rarely exceed a few degrees per hour, but
distress in HMA pavements, is the disbonding of
the ASTM C 666 (confined rapid freezethaw) test
subjects concrete to much higher cooling rates.
coarse aggregates from the asphalt matrix during
ASTM C 671 (1981), on the other hand, consists of
cold weather. Both deterioration mechanisms
cooling concrete at a rate of 2.8C per hour. The
lead to pavement damage whose root cause
specimens are held in a 2C bath for two weeks
appears to be excessive volume changes caused
before being frozen. Though this test has shown
by the freezing of saturated aggregate. When water
good correlation with field performance, it is very
freezes inside particles of rock, the tendency is for
time consuming.
the rock to expand, which can be great enough to
crack the pavement matrix or to rupture the
aggregate or both.
VPI single-cycle freeze test
In the VPI single-cycle freeze test, a concrete
Currently, the best way to measure volume
beam is subjected to 18C air while length
expansion of aggregate is when it is confined with-
in concrete. In this manner, stresses from hydraulic
change and temperature are measured over a 4-hr
period. The test is rapid compared with ASTM C
forces within discrete particles of aggregate and
671 (1981) and has been successful in identifying
those from water escaping from the aggregate
very durable and nondurable aggregate. Other
into the surrounding matrix are accounted for.
testing is needed to rank aggregates of moderate
However, this test is time-consuming--making,
durability.
curing, and testing specimens requires consider-
able effort--and does not always correlate to field
performance.
Iowa Pore Index test
The Iowa Pore Index test acknowledges the
A simpler method is to measure the volume
importance of pore size in aggregate durability. It
expansion of unconfined aggregate. Though this
is different from being confined in concrete, it
consists of measuring the amount of water that
enters aggregate submerged in water when pres-
would identify those aggregates that expand
surized to 241 kPa. The amount of water
unusually or that expel a large amount of water. It
absorbed during the first minute of the test is con-
would also avoid the variabilities associated with
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