Civil Engineering Reference
In-Depth Information
dated beforehand. Fig. 5-13 illustrates how the amount of
bulking of fine aggregate varies with moisture content and
grading; fine gradings bulk more than coarse gradings for
a given amount of moisture. Fig. 5-14 shows similar infor-
mation in terms of weight for a particular fine aggregate.
Since most fine aggregates are delivered in a damp condi-
tion, wide variations can occur in batch quantities if batch-
ing is done by volume. For this reason, good practice has
long favored weighing the aggregate and adjusting for
moisture content when proportioning concrete.
with higher porosity values and medium-sized pores (0.1
to 5 µm) that are easily saturated and cause concrete dete-
rioration and popouts. Larger pores do not usually be-
come saturated or cause concrete distress, and water in
very fine pores may not freeze readily.
At any freezing rate, there may be a critical particle size
above which a particle will fail if frozen when critically
saturated. This critical size is dependent upon the rate of
freezing and the porosity, permeability, and tensile strength
of the particle. For fine-grained aggregates with low perme-
ability (cherts for example), the critical particle size may be
within the range of normal aggregate sizes. It is higher for
coarse-grained materials or those with capillary systems
interrupted by numerous macropores (voids too large to
hold moisture by capillary action). For these aggregates the
critical particle size may be sufficiently large to be of no
consequence, even though the absorption may be high. If
potentially vulnerable aggregates are used in concrete
subjected to periodic drying while in service, they may
never become sufficiently saturated to cause failure.
Cracking of concrete pavements caused by the freeze-
thaw deterioration of the aggregate within concrete is
called D-cracking. This type of cracking has been ob-
served in some pavements after three or more years of
service. D-cracked concrete resembles frost-damaged
concrete caused by paste deterioration. 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 (Fig. 5-15).
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
Resistance to Freezing and Thawing
The frost resistance of an aggregate, an important charac-
teristic for exterior concrete, is related to its porosity,
absorption, permeability, and pore structure. An aggre-
gate particle may absorb so much water (to critical satura-
tion) that it cannot accommodate the expansion and
hydraulic pressure that occurs during the freezing of
water. If enough of the offending particles are present, the
result can be expansion of the aggregate and possible
disintegration of the concrete. If a single problem particle
is near the surface of the concrete, it can cause a popout.
Popouts generally appear as conical fragments that break
out of the concrete surface. The offending aggregate parti-
cle or a part of it is usually found at the bottom of the void.
Generally it is coarse rather than fine aggregate particles
30
20
10
0
2480
150
2280
140
2080
130
Mass of fine aggregate and water in one
unit of volume measured loose in air
120
1880
110
1680
100
1480
90
1280
80
024 68 0 2 4 6 8 0
Moisture in fine aggregate, percent by mass
Fig. 5-15. D-cracking along a transverse joint caused by
failure of carbonate coarse aggregate (
Stark 1976
). (30639)
Fig. 5-14. Bulk density is compared with the moisture con-
tent for a particular sand (PCA Major Series 172).
