Civil Engineering Reference
In-Depth Information
Table 8-5. Effect of Production Procedures, Construction Practices, and Environment on Control of Air
Content in Concrete (Continued)
Procedure/Variable
Effects
Guidance
Belt conveyors
Reduces air content by an average
Avoid long conveyed distance if possible.
of 1%.
Reduce the free-falling effect at the end of conveyor.
Pumping
Reduction in air content ranges from
Use of proper mix design provides a stable air-void
2% to 3%.
system.
Does not significantly affect air-void
Avoid high slump, high air content concrete.
system.
Keep pumping pressure as low as possible.
Minimum effect on freeze-thaw
Use loop in descending pump line.
resistance.
Shotcrete
Generally reduces air content in wet-
Air content of mix should be at high end of target
process shotcrete.
zone.
Internal vibration
Air content decreases under prolonged
Do not overvibrate. Avoid high-frequency vibrators
vibration or at high frequencies.
(greater than 10,000 vpm). Avoid multiple passes of
vibratory screeds.
Proper vibration does not influence the
Closely spaced vibrator insertion is recommended
air-void system.
for better consolidation.
Finishing
Air content reduced in surface layer by
Avoid finishing with bleed water still on surface.
excessive finishing.
Avoid overfinishing. Do not sprinkle water on surface
prior to finishing. Do not steel trowel exterior slabs.
Temperature
Air content decreases with increase in
Increase air-entraining admixture dosage as temper-
temperature.
ature increases.
Changes in temperature do not signifi-
cantly affect spacing factors.
9.0
25-mm (1-in.) immersion-type vibrator.
All mixes contained same amount
of air-entraining admixture.
tents. The effect of water hardness in most municipal
water supplies is generally insignificant; however, very
hard water from wells, as used in rural communities, may
decrease the air content in concrete.
8.0
7.0
Slump and Vibration
The effect of slump and vibration on the air content of con-
crete is shown in Fig. 8-18. For a constant amount of air-
entraining admixture, air content increases as slump
increases up to about 150 or 175 mm (6 or 7 inches); then it
begins to decrease with further increases in slump. At all
slumps, however, even 15 seconds of vibration (the
ACI 309
limit) will cause a considerable reduction in air content.
Prolonged vibration of concrete should be avoided.
The greater the slump, air content, and vibration time,
the larger the percentage of reduction in air content
during vibration (Fig. 8-18). However, if vibration is prop-
erly applied, little of the intentionally entrained air is lost.
The air lost during handling and moderate vibration con-
sists mostly of the larger bubbles that are usually undesir-
able from the standpoint of strength. While the average
size of the air voids is reduced, the air-void spacing factor
remains relatively constant.
Internal vibrators reduce air content more than
external vibrators. The air loss due to vibration increases
as the volume of concrete is reduced or the vibration fre-
quency is significantly increased. Lower vibration fre-
quencies (8000 vpm) have less effect on spacing factors
6.0
137-mm (5.4-in.) slump
5.0
96-mm (3.8-in.) slump
4.0
3.0
46-mm (1.8-in.) slump
2.0
1.0
0
10
20
30
40
50
Vibration time, seconds
Fig. 8-18. Relationship between slump, duration of vibra-
tion, and air content of concrete (
Brewster 1949
).
