The document discusses air entraining admixtures, which generate tiny stable bubbles in concrete to protect against freezing and thawing cycles by providing spaces for expanding water to enter and exit without damaging the concrete; it describes how air entrainment improves durability in freeze-thaw and deicer exposed environments and explains the mechanism by which air voids in concrete provide this protection against frost damage and scaling.

1. CON 122
Concrete Admixtures
Session 3
Air Entraining Admixtures

2. Air-Entraining Admixtures
ASTM C 260 or AASHTO M 154
 Improve durability in concrete exposed to
 Freeze-thaw
 Deicers
 Sulfates
 Alkali-reactive environments
 Improve workability

3. Why do we need Air-Entrainment?
 DEFINITION: Air-
Entraining Admixtures
are primarily used to
stabilize tiny bubbles
generated in concrete
to protect against
freezing and thawing
cycles.

4. History
 Air-entrainment was
discovered accidentally
in the 1930s
 Several pavements in
New York had survived
severe freeze-thaw
exposure
 Cement manufactured
with grinding aids Beef Tallow

5. Freeze-Thaw Distress
 Frost Damage
 Hydraulic Pressure
 Scaling Distress
 Hydraulic & Osmotic
Pressures
Photos courtesy of M. Thomas

6. Freeze-Thaw Distress
Photos courtesy of M. Thomas

7. Frost Damage
Photos courtesy of M. Thomas

8. Frost Damage
Photos courtesy of M. Thomas

9. Air Entraining Admixtures
 Air entrained admixtures produce voids
 Anionic
 Hydrophobic (repel water)
 Electric charged
 Mechanical mixing dispersed
 Size: 10-1000 microns
 Bubbles not interconnected

10. Concrete Air Voids
 Non-air entrained concrete
 1 in (25mm) max size
 Entrapped air voids: 11/2%
 Air-entrained concrete
 1 in (25mm) max size
 Entrained air voids: 6%
 Total air consists of entrapped and entrained

11. Freezing and Thawing
 Concrete water freezes
 Produces Osmotic and hydraulic pressures
 As pressure exceeds tensile strength of paste
cavity will dilate and rupture
 Effect of successive F-T cycles
 Disruption of paste and aggregate
 Expansion of concrete and deterioration
 Water travels to nearest air void for relief

12. Mechanism of Protection by Air Voids
 As the temperature of saturated
concrete(.91.7%) in service is lowered, the
water held in the capillary pores in the harden
concrete paste freezes.

13. Mechanism of
Protection by Air Voids Air-entrained
32oF
23oF
Saturation > 91.7%

14. Mechanism of Protection by Air Voids
The microscopic air bubbles are filled with
freezing of water which increases in volume by
9%, so the excess water in the cavity is expelled
by dilating pressure.

15. Mechanism of
Protection by Air Voids Air-entrained
32oF
23oF
Saturation > 91.7%
15

16. Mechanism of Protection by Air Voids
 Diffusion of water leading to a growth of a
relatively small number of bodies of ice.
 When salts are used for de-icing, they are
absorbed by the upper part of the concrete.
 This produces an osmotic pressure with a
consequence movement of water towards the
coldest zone where freezing takes place.

17. Mechanism of
Protection by Air Voids Air-entrained
32oF
23oF
Saturation > 91.7%
17

18. Mechanism of Protection by Air Voids
The microscopic air bubbles are filled with
freezing of water which increases in volume by
9%, so the excess water in the cavity is expelled
by dilating pressure.

19. Mechanism of
Protection by Air Voids Air-entrained
32oF
23oF
Saturation > 91.7%
19

20. Entrained Air and Resistance to F/T
 Effect of entrained air
on the resistance of
concrete to freezing
and thawing in
laboratory tests.

21. Hydraulic Pressures
 Causes 9% expansion of water
 Osmotic pressures developed differential
concentration of alkali solutions
 As water freezes , alkali concentration
increases in adjacent unfrozen water
 A high alkali solution draws water from lower
alkali solutions into pores

22. Effects of Weathering
Effect of weathering on boxes and slabs on ground: Top are air-entrained

23. Effects of Weathering
Effect of weathering on boxes and slabs on ground: Top are exhibiting severe
crumbling and scaling

24. Air-Void System
Spacing factor ( L ):
 An index related to the
maximum distance of any
point in the cement paste
from the periphery of an
air void.
 ASTM 457: less than 0.2
mm (0.008 in.)

25. Air-Void System
Specific surface ( ):
 The surface area of a
quantity of air voids that
have a total volume of
one cubic inch.
 ASTM 457: 24
mm2/mm3 (600 in.2/in.3)
or more

26. Acts at Air-Water Interface

27. Acts at Air-Water Interface

28. Mechanism of
Protection by Air Voids Air-entrained
32oF
23oF
courtesy of M. Thomas
Saturation > 91.7%
29

29. Mechanism of
Protection by Air Voids Air-entrained
32oF
23oF
courtesy of M. Thomas
Saturation > 91.7%
30

30. Mechanism of
Protection by Air Voids Air-entrained
32oF
23oF
courtesy of M. Thomas
Saturation > 91.7%
31

31. Mechanism of
Protection by Air Voids Air-entrained
32oF
23oF
courtesy of M. Thomas
Saturation > 91.7%
32

32. Spacing Factor and Air Content
 Spacing factor as a
function of total air
content in concrete.

33. Improved Freeze Thaw Resistance
 Good Quality Aggregates
 Low Water/Cement ratio (max 0.45)
 Minimum cementitious content (564 lbs/yd3;
335 kg/m3)
 Proper curing practices
 Compressive strength at 28 days of 4000 psi
or 28 MPa

34. Improved Deicer-Scaling Resistance
 Low Water/Cement ratio (max 0.45)
 Slump: less 4 in (100 mm); use plasticizer
 Minimum cementitious content
 Proper finishing( bleed water evaporation)
 Adequate drainage
 Minimum compressive strength
 Minimum 30 day drying period

35. Effect of Air and Cement Content on
Performance of Concrete in Sulfate Soil
Cement
Without air content With air
222 kg/m3
(375 lb/yd3)
306 kg/m3
(515 lb/yd3)
392 kg/m3
(660 lb/yd3)
Type II-Cement
5 years exposure to sulfate soil

36. Effect of Air Content on Reducing
Expansion Due to ASR
 Effect of air content on
the reduction of
expansion due to alkali-
silica reaction.

37. Strength and W/C-Ratio
 Typical relationship
between 28-day
compressive strength
and water-cement ratio
for a wide variety of air-
entrained concretes
using Type I cement.

38. Effect of Air Entrainment on
Concrete
 Increased air necessitates increase of cement
content
 When cement content and slump are constant, air
entrainment reduces sand and water requirements
 Air entrained concrete have lower W/C ratio
 Each percentile increase of air reduces compressive
strength from 2%-9%

39. Water Reduction vs. Air Content
 Reduction of water
content obtained at
various levels of air and
cement contents.

40. Sand Reduction vs. Air Content
 Reduction of sand
content obtained at
various levels of air and
cement contents.

41. Relationship Between 28-Day
Strength and Air Content
 Relationship between
air content and 28-day
compressive strength
for concrete at three
cement contents.

42. Workability
 Entrained air improves workability
 Very effective in lean cement content
 Mixes with angular and poorly graded aggregates
workability improved
 Freshly mixed air-entrained concrete is
cohesive, easier to handle
 Entrained air also reduces segregation and bleeding
 High air mixes are sticky and difficult to finish

43. Measuring Air Content
ASTM C 231 ASTM44C 173

44. Tests for Air Content
Hardened Concrete
(ASTM 457):
 Linear-Traverse Method
 Point-Count Method

45. Measuring Air in Hardened Concrete

46. Fresh Concrete Air Void Analyzer

47. RECOMMENDED AVERAGE AIR CONTENT
PERCENTAGE FOR LEVEL OF EXPOSURE
Nominal maximum sizes of aggregates
Exposure 3/8 in. 1/2 in. 3/4 in. 1 in. 1-1/2 in. 2 in.
(9.5mm) (12.5mm) (19mm) (25mm) (37.5mm) (50mm)
Mild 4.5 4.0 3.5 3.0 2.5 2.0
Moderate 6.0 5.5 5.0 4.5 4.5 4.0
Severe 7.5 7.0 6.0 6.0 5.5 5.0

48. Air Entraining
Please return to Blackboard and watch the
following video:
 Video 1: Air Entrainment