Air-Entrained Concrete - Design and Control of Concrete Mixtures - page 136

percentage point for concretes with a low-to-moderate
slump and constant air-entraining admixture dosage.
However, this approximation is greatly affected by con-
crete temperature, slump, and the type and amount of
cement and admixtures present in the concrete. A low-
slump concrete with a high dosage of water-reducing and
air-entraining admixtures can undergo large increases in

slump and air content with a small addition of water. On
the other hand, a very fluid concrete mixture with a 200 to
250-mm (8 to 10-in.) slump may lose air with the addition
of water. Refer to Tables 8-4 and 8-5 for more information.
The mixing water used may also affect air content.
Algae-contaminated water increases air content. Highly
alkaline wash water from truck mixers can affect air con-

Table 8-4. Effect of Mixture Design and Concrete Constituents on Control of Air Content in Concrete

Characteristic/Material Effects Guidance

Alkali content Air content increases with increase Changes in alkali content or cement source require

in cement alkali level. that air-entraining agent dosage be adjusted.

Less air-entraining agent dosage Decrease dosage as much as 40% for high-alkali

needed for high-alkali cements. cements.

Air-void system may be more unstable
with some combinations of alkali level
and air-entraining agent used.

Fineness Decrease in air content with increased Use up to 100% more air-entraining admixture for

fineness of cement. very fine (Type III) cements. Adjust admixture if

cement source or fineness changes.

Cement content in Decrease in air content with increase Increase air-entraining admixture dosage rate as

mixture in cement content. cement content increases.

Smaller and greater number of voids
with increased cement content.

Contaminants Air content may be altered by contam- Verify that cement meets ASTM C 150 (AASHTO M 85)

ination of cement with finish mill oil. requirements on air content of test mortar.

Fly ash Air content decreases with increase in Changes in LOI or fly ash source require that air-

loss on ignition (carbon content). entraining admixture dosage be adjusted.

Perform “foam index” test to estimate increase in
dosage.

Air-void system may be more unstable Prepare trial mixes and evaluate air-void systems.

with some combinations of fly ash/
cement/air-entraining agents.

Ground granulated Decrease in air content with increased Use up to 100% more air-entraining admixture for

blast-furnace slag fineness of GGBFS. finely ground slags.

Silica fume Decrease in air content with increase Increase air-entraining admixture dosage up to 100%

in silica fume content. for fume contents up to 10%.

Metakaolin No apparent effect. Adjust air-entraining admixture dosage if needed.

Water reducers Air content increases with increases in Reduce dosage of air-entraining admixture.

dosage of lignin-based materials.

Select formulations containing air-detraining agents.

Spacing factors may increase when Prepare trial mixes and evaluate air-void systems.

water-reducers used.

Retarders Effects similar to water-reducers. Adjust air-entraining admixture dosage.

Accelerators Minor effects on air content. No adjustments normally needed.

High-range water Moderate increase in air content when Only slight adjustments needed.

reducers (Plasticizers) formulated with lignosulfonate.

Spacing factors increase. No significant effect on durability.

Maximum size Air content requirement decreases Decrease air content.

with increase in maximum size.
Little increase over 37.5 mm (1 1 ⁄ 2 in.)
maximum size aggregate.

Sand-to-total aggregate Air content increases with increased Decrease air-entraining admixture dosage for

ratio sand content. mixtures having higher sand contents.

Sand grading Middle fractions of sand promote air- Monitor gradation and adjust air-entraining admixture

entrainment. dosage accordingly.