Civil Engineering Reference
In-Depth Information
spacing factor, occasionally higher than what may be con-
sidered desirable for freeze-thaw durability. However,
tests on superplasticized concrete with slightly higher
spacing factors have indicated that superplasticized con-
cretes have good freeze-thaw durability. This may be due
to the reduced water-cement ratio often associated with
superplasticized concretes.
A small quantity of calcium chloride is sometimes
used in cold weather to accelerate the hardening of con-
crete. It can be used successfully with air-entraining
admixtures if it is added separately in solution form to the
mix water. Calcium chloride will slightly increase air con-
tent. However, if calcium chloride comes in direct contact
with some air-entraining admixtures, a chemical reaction
can take place that makes the admixture less effective.
Nonchloride accelerators may increase or decrease air
content, depending upon the individual chemistry of the
admixture, but they generally have little effect on
air content.
5
11 rpm
4
4 rpm
3
2
1
Cement: 305 kg/m
3
(510 lb/yd
3
)
Mixer: 4 m
3
(6 yd
3
) transit mixer
Mixing time: starts after charging completed
0
0
10
20
30
40
50
60
Mixing time, minutes
Fig. 8-20. Relationship between mixing time and air content
of concrete. PCA Major Series 336.
Mixing Action
Mixing action is one of the most important factors in the
production of entrained air in concrete. Uniform distribu-
tion of entrained air voids is essential to produce scale-
resistant concrete; nonuniformity might result from
inadequate dispersion of the entrained air during short
mixing periods. In production of ready mixed concrete, it
is especially important that adequate and consistent
mixing be maintained at all times.
The amount of entrained air varies with the type and
condition of the mixer, the amount of concrete being
mixed, and the rate and duration of mixing. The amount
of air entrained in a given mixture will decrease appre-
ciably as the mixer blades become worn, or if hardened
concrete is allowed to accumulate in the drum or on the
blades. Because of differences in mixing action and time,
concretes made in a stationary mixer and those made in a
transit mixer may differ significantly in amounts of air
entrained. The air content may increase or decrease when
the size of the batch departs significantly from the rated
capacity of the mixer. Little air is entrained in very small
batches in a large mixer; however, the air content increases
as the mixer capacity is approached.
Fig. 8-20 shows the effect of mixing speed and dura-
tion of mixing on the air content of freshly mixed con-
cretes made in a transit mixer. Generally, more air is
entrained as the speed of mixing is increased up to about
20 rpm, beyond which air entrainment decreases. In the
tests from which the data in Fig. 8-20 were derived, the air
content reached an upper limit during mixing and a
gradual decrease in air content occurred with prolonged
mixing. Mixing time and speed will have different effects
on the air content of different mixes. Significant amounts
of air can be lost during mixing with certain mixtures and
types of mixing equipment.
Fig. 8-21 shows the effect of continued mixer agita-
tion on air content. The changes in air content with pro-
longed agitation can be explained by the relationship
between slump and air content. For high-slump con-
cretes, the air content increases with continued agitation
as the slump decreases to about 150 or 175 mm (6 or 7 in.).
Prolonged agitation will decrease slump further and
decrease air content. For initial slumps lower than
150 mm (6 in.), both the air content and slump decrease
with continued agitation. When concrete is retempered
(the addition of water and remixing to restore original
slump), the air content is increased; however, after
4 hours, retempering is ineffective in increasing air con-
tent. Prolonged mixing or agitation of concrete is accom-
panied by a progressive reduction in slump.
8
Agitating speeds: 2 or 4 rpm
Transit mixer: 4.5 and 6.1 m
3
(6 and 8 yd
3
)
Initial mixing: 70 rev. at 10 rpm
7
225-mm (9-in.) initial slump
6
5
100-mm (4-in.) initial slump
4
10
20
30
40
50
60
70
80
90
Agitating time, minutes (after initial mixing)
Fig. 8-21. Relationship between agitating time, air content,
and slump of concrete. PCA Major Series 336.
