Civil Engineering Reference
In-Depth Information
Table 8-4. Effect of Mixture Design and Concrete Constituents on Control of Air Content in Concrete
(Continued)
Characteristic/Material
Effects
Guidance
Water chemistry
Very hard water reduces air content.
Increase air entrainer dosage.
Batching of admixture into concrete
Avoid batching into wash water.
wash water decreases air.
Algae growth may increase air.
Water-to-cement ratio
Air content increases with increased
Decrease air-entraining admixture dosage as water
water to cement ratio.
to cement ratio increases.
Slump
Air increases with slumps up to about
Adjust air-entraining admixture dosages for slump.
150 mm (6 in.).
Air decreases with very high slumps.
Avoid addition of water to achieve high-slump con-
crete.
Difficult to entrain air in low-slump
Use additional air-entraining admixture; up to ten
concretes.
times normal dosage.
Table 8-5. Effect of Production Procedures, Construction Practices, and Environment on Control of Air
Content in Concrete
Procedure/Variable
Effects
Guidance
Batching sequence
Simultaneous batching lowers air
Add air-entraining admixture with initial water or
content.
on sand.
Cement-first raises air content.
Mixer capacity
Air increases as capacity is approached.
Run mixer close to full capacity. Avoid overloading.
Mixing time
Central mixers: air content increases
Establish optimum mixing time for particular mixer.
up to 90 sec. of mixing.
Truck mixers: air content increases
Avoid overmixing.
with mixing.
Short mixing periods (30 seconds)
Establish optimum mixing time (about 60 seconds).
reduce air content and adversely affect
air-void system.
Mixing speed
Air content gradually increases up to
Follow truck mixer manufacturer recommendations.
approx. 20 rpm.
Air may decrease at higher mixing
Maintain blades and clean truck mixer.
speeds.
Admixture metering
Accuracy and reliability of metering
Avoid manual-dispensing or gravity-feed systems
system will affect uniformity of air
and timers. Positive-displacement pumps interlocked
content.
with batching system are preferred.
Transport and delivery
Some air (1% to 2%) normally lost
Normal retempering with water to restore slump will
during transport.
restore air.
Loss of air in nonagitating equipment
If necessary, retemper with air-entraining admixture
is slightly higher.
to restore air.
Dramatic loss in air may be due to factors other than
transport.
Haul time and agitation
Long hauls, even without agitation,
Optimize delivery schedules. Maintain concrete
reduce air, especially in hot weather.
temperature in recommended range.
Retempering
Regains some of the lost air.
Retemper only enough to restore workability. Avoid
addition of excess water.
Does not usually affect the air-void
system.
Higher admixture dosage is needed for jobsite
admixture additions.
Retempering with air-entraining ad-
mixtures restores the air-void system.
