Civil Engineering Reference
In-Depth Information
with freezing and thawing cycles, cracking of the concrete
starts in the saturated aggregate (Fig. 5-16) 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.
The performance of aggregates under exposure to
freezing and thawing can be evaluated in two ways: (1)
past performance in the field, and (2) laboratory freeze-
thaw tests of concrete specimens. If aggregates from the
same source have previously given satisfactory service
when used in concrete, they might be considered suitable.
Aggregates not having a service record can be considered
acceptable if they perform satisfactorily in air-entrained
concretes subjected to freeze-thaw tests according to
ASTM C 666 (AASHTO T 161). In these tests concrete spe-
cimens made with the aggregate in question are subjected
to alternate cycles of freezing and thawing in water.
Deterioration is measured by (1) the reduction in the
dynamic modulus of elasticity, (2) linear expansion, and
(3) weight loss of the specimens. An expansion failure
criterion of 0.035% in 350 freeze-thaw cycles or less is used
by a number of state highway departments to help indi-
cate whether or not an aggregate is susceptible to D-crack-
ing. Different aggregate types may influence the criteria
levels and empirical correlations between laboratory
freeze-thaw tests. Field service records should be made to
select the proper criterion (
Vogler and Grove 1989
).
Specifications may require that resistance to weather-
ing be demonstrated by a sodium sulfate or magnesium
sulfate test (ASTM C 88 or AASHTO T 104). The test
consists of a number of immersion cycles for a sample of
the aggregate in a sulfate solution; this creates a pressure
through salt-crystal growth in the aggregate pores similar
to that produced by freezing water. The sample is then
ovendried and the percentage of weight loss calculated.
Unfortunately, this test is sometimes misleading.
Aggregates behaving satisfactorily in the test might
produce concrete with low freeze-thaw resistance;
conversely, aggregates performing poorly might produce
concrete with adequate resistance. This is attributed, at
least in part, to the fact that the aggregates in the test are
not confined by cement paste (as they would be in
concrete) and the mechanisms of attack are not the same as
in freezing and thawing. The test is most reliable for strat-
ified rocks with porous layers or weak bedding planes.
An additional test that can be used to evaluate aggre-
gates for potential D-cracking is the rapid pressure release
method. An aggregate is placed in a pressurized chamber
and the pressure is rapidly released causing the aggregate
with a questionable pore system to fracture (
Janssen and
Snyder 1994
). The amount of fracturing relates to the
potential for D-cracking.
Wetting and Drying Properties
Weathering due to wetting and drying can also affect the
durability of aggregates. The expansion and contraction
coefficients of rocks vary with temperature and moisture
content. If alternate wetting and drying occurs, severe
strain can develop in some aggregates, and with certain
types of rock this can cause a permanent increase in
volume of the concrete and eventual breakdown. Clay
lumps and other friable particles can degrade rapidly with
repeated wetting and drying. Popouts can also develop
due to the moisture-swelling characteristics of certain
aggregates, especially clay balls and shales. While no
specific tests are available to determine this tendency, an
experienced petrographer can often be of assistance in
determining this potential for distress.
Abrasion and Skid Resistance
The abrasion resistance of an aggregate is often used as a
general index of its quality. Abrasion resistance is essential
when the aggregate is to be used in concrete subject to
abrasion, as in heavy-duty floors or pavements. Low abra-
sion resistance of an aggregate may increase the quantity
of fines in the concrete during mixing; consequently, this
may increase the water requirement and require an adjust-
ment in the water-cement ratio.
The most common test for abrasion resistance is the
Los Angeles abrasion test (rattler method) performed in
accordance with ASTM C 131 (AASHTO T 96) or ASTM C
535. In this test a specified quantity of aggregate is placed
in a steel drum containing steel balls, the drum is rotated,
and the percentage of material worn away is measured.
Specifications often set an upper limit on this mass loss.
However, a comparison of the results of aggregate abra-
sion tests with the abrasion resistance of concrete made
with the same aggregate do not generally show a clear
Fig. 5-16. Fractured carbonate aggregate particle as a
source of distress in D-cracking (magnification 2.5X) (
Stark
1976
). (30639-A)
