Civil Engineering Reference
In-Depth Information
Fig. 8-3. Effect of weathering on boxes and slabs on ground at the Long-Time Study outdoor test plot, Project 10, PCA,
Skokie, Illinois. Specimens at top are air-entrained, specimens at bottom exhibiting severe crumbling and scaling are non-
air-entrained. All concretes were made with 335 kg (564 lb) of Type I portland cement per cubic meter (cubic yard).
Periodically, calcium chloride deicer was applied to the slabs. Specimens were 40 years old when photographed (see
Klieger 1963
for concrete mixture information). (69977, 69853, 69978, 69854)
small for ice crystals to form, water attempts to migrate to
locations where it can freeze.
Entrained air voids act as empty chambers in the
paste where freezing and migrating water can enter, thus
relieving the pressures described above and preventing
damage to the concrete. Upon thawing, most of the water
returns to the capillaries due to capillary action and pres-
sure from air compressed in the bubbles. Thus the bubbles
are ready to protect the concrete from the next cycle of
freezing and thawing (
Powers 1955
,
Lerch 1960
, and
Powers 1965
).
The pressure developed by water as it expands
during freezing depends largely upon the distance the
water must travel to the nearest air void for relief.
Therefore, the voids must be spaced close enough to
reduce the pressure below that which would exceed the
tensile strength of the concrete. The amount of hydraulic
pressure is also related to the rate of freezing and the per-
meability of the paste.
The spacing and size of air voids are important factors
contributing to the effectiveness of air entrainment in con-
crete. ASTM C 457 describes a method of evaluating the
air-void system in hardened concrete. Most authorities
consider the following air-void characteristics as represen-
tative of a system with adequate freeze-thaw resistance
(
Powers 1949
,
Klieger 1952
,
Klieger 1956
,
Mielenz and
others 1958
,
Powers 1965
,
Klieger 1966
,
Whiting and Nagi
1998
, and
Pinto and Hover 2001
):
1. Calculated spacing factor, ¿, (an index related to the
distance between bubbles but not the actual average
spacing in the system)—less than 0.200 mm (0.008 in.)
(
Powers 1954 and 1965
)
2. Specific surface,
, (surface area of the air voids)—24
square millimeters per cubic millimeter (600 sq in. per
cubic inch) of air-void volume, or greater.
α
Current U.S. field quality control practice usually in-
volves only the measurement of total air volume in freshly
mixed concrete; this does not distinguish air-void size in
any way. In addition to total air volume, Canadian prac-
tice also requires attainment of specific spacing factors.
Fig. 8-4 illustrates the relationship between spacing factor
and total air content. Measurement of air volume alone
does not permit full evaluation of the important charac-
teristics of the air-void system; however, air-entrainment
is generally considered effective for freeze-thaw resistance
when the volume of air in the mortar fraction of the con-
crete—material passing the 4.75-mm (No. 4) sieve—is
about 9 ± 1% (
Klieger 1952
) or about 18% by paste volume.
For equal admixture dosage rates per unit of cement, the
air content of ASTM C 185 (AASHTO T 137) mortar would
be about 19% due to the standard aggregate's properties.
