Civil Engineering Reference
In-Depth Information
and air contents than high vibration frequencies (14,000
vpm). High frequencies can significantly increase spacing
factors and decrease air contents after 20 seconds of vibra-
tion (
Brewster 1949
and
Stark 1986
).
For pavements, specified air contents and uniform air
void distributions can be achieved by operating within
paving machine speeds of 1.22 to 1.88 meters per minute
(4 to 6 feet per minute) and with vibrator frequencies of
5,000 to 8,000 vibrations per minute. The most uniform
distribution of air voids throughout the depth of concrete,
in and out of the vibrator trails, is obtained with the com-
bination of a vibrator frequency of approximately 5,000 vi-
brations per minute and a slipform paving machine
forward track speeds of 1.22 meters per minute (4 feet per
minute). Higher frequencies of speeds singularly or in
combination can result in discontinuities and lack of re-
quired air content in the upper portion of the concrete
pavement. This in turn provides a greater opportunity for
water and salt to enter the pavement and reduce the dura-
bility and life of the pavement (
Cable, McDaniel,
Schlorholtz, Redmond, and Rabe 2000
).
concrete increases, particularly as slump is increased. This
effect is especially important during hot-weather con-
creting when the concrete might be quite warm. A
decrease in air content can be offset when necessary by
increasing the quantity of air-entraining admixture.
In cold-weather concreting, the air-entraining admix-
ture may lose some of its effectiveness if hot mix water is
used during batching. To offset this loss, such admixtures
should be added to the batch after the temperature of the
concrete ingredients have equalized.
Although increased concrete temperature during
mixing generally reduces air volume, the spacing factor
and specific surface are only slightly affected.
Supplementary Cementitious Materials
The effect of fly ash on the required dosage of air-entraining
admixtures can range from no effect to an increase in dosage
of up to five times the normal amount (
Gebler and Klieger
1986
). Large quantities of slag and silica fume can double the
dosage of air-entraining admixtures (
Whiting and Nagi 1998
).
Concrete Temperature
Admixtures and Coloring Agents
Temperature of the concrete affects air content, as shown
in Fig. 8-19. Less air is entrained as the temperature of the
Coloring agents such as carbon black usually decrease the
amount of air entrained for a given amount of admixture.
This is especially true for coloring materials with
increasing percentages of carbon (
Taylor 1948
).
Water-reducing and set-retarding admixtures gener-
ally increase the efficiency of air-entraining admixtures by
50% to 100%; therefore, when these are used, less air-
entraining admixture will usually give the desired air con-
tent. Also, the time of addition of these admixtures into
the mix affects the amount of entrained air; delayed addi-
tions generally increasing air content.
Set retarders may increase the air-void spacing in con-
crete. Some water-reducing or set-retarding admixtures are
not compatible with some air-entraining admixtures. If
they are added together to the mixing water before being
dispensed into the mixer, a precipitate may form. This will
settle out and result in large reductions in entrained air.
The fact that some individual admixtures interact in this
manner does not mean that they will not be fully effective
if dispensed separately into a batch of concrete.
Superplasticizers (high-range water reducers) may
increase or decrease the air content of a concrete mixture
based on the admixture's chemical formulation and the
slump of the concrete. Naphthalene-based superplasti-
cizers tend to increase the air content while melamine-
based materials may decrease or have little effect on air
content. The normal air loss in flowing concrete during
mixing and transport is about 2 to 4 percentage points
(
Whiting and Dziedzic 1992
).
Superplasticizers also affect the air-void system of
hardened concrete by increasing the general size of the
entrained air voids. This results in a higher-than-normal
Concrete temperature,
°
F
50
60
70
80
90
7
175-mm (7-in.) slump
6
125-mm (5-in.) slump
5
75-mm (3-in.) slump
25-mm (1-in.) slump
4
3
2
1
Cement: 335 kg/m
3
(565 lb/yd
3
)
Aggregate: 37.5-mm (1
1
/
2
-in.) max. size
0
5
10
15
20
25
30
35
Concrete temperature,
C
°
Fig. 8-19. Relationship between temperature, slump, and air
content of concrete. PCA Major Series 336 and
Lerch 1960
.
