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CON 122 Session 3 - Air-Entraining Admixtures

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Air Entrained Concrete
Air Entrained Concrete
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CON 122 Session 3 - Air-Entraining Admixtures

  1. 1. CON 122 Concrete Admixtures Session 3 Air Entraining Admixtures
  2. 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. 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. 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. 5. Freeze-Thaw Distress  Frost Damage  Hydraulic Pressure  Scaling Distress  Hydraulic & Osmotic Pressures Photos courtesy of M. Thomas
  6. 6. Freeze-Thaw Distress Photos courtesy of M. Thomas
  7. 7. Frost Damage Photos courtesy of M. Thomas
  8. 8. Frost Damage Photos courtesy of M. Thomas
  9. 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. 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. 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. 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. 13. Mechanism of Protection by Air Voids Air-entrained 32oF 23oF Saturation > 91.7%
  14. 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. 15. Mechanism of Protection by Air Voids Air-entrained 32oF 23oF Saturation > 91.7% 15
  16. 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. 17. Mechanism of Protection by Air Voids Air-entrained 32oF 23oF Saturation > 91.7% 17
  18. 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. 19. Mechanism of Protection by Air Voids Air-entrained 32oF 23oF Saturation > 91.7% 19
  20. 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. 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. 22. Effects of Weathering Effect of weathering on boxes and slabs on ground: Top are air-entrained
  23. 23. Effects of Weathering Effect of weathering on boxes and slabs on ground: Top are exhibiting severe crumbling and scaling
  24. 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. 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. 26. Acts at Air-Water Interface
  27. 27. Acts at Air-Water Interface
  28. 28. Mechanism of Protection by Air Voids Air-entrained 32oF 23oF courtesy of M. Thomas Saturation > 91.7% 29
  29. 29. Mechanism of Protection by Air Voids Air-entrained 32oF 23oF courtesy of M. Thomas Saturation > 91.7% 30
  30. 30. Mechanism of Protection by Air Voids Air-entrained 32oF 23oF courtesy of M. Thomas Saturation > 91.7% 31
  31. 31. Mechanism of Protection by Air Voids Air-entrained 32oF 23oF courtesy of M. Thomas Saturation > 91.7% 32
  32. 32. Spacing Factor and Air Content  Spacing factor as a function of total air content in concrete.
  33. 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. 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. 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. 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. 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. 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. 39. Water Reduction vs. Air Content  Reduction of water content obtained at various levels of air and cement contents.
  40. 40. Sand Reduction vs. Air Content  Reduction of sand content obtained at various levels of air and cement contents.
  41. 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. 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. 43. Measuring Air Content ASTM C 231 ASTM44C 173
  44. 44. Tests for Air Content Hardened Concrete (ASTM 457):  Linear-Traverse Method  Point-Count Method
  45. 45. Measuring Air in Hardened Concrete
  46. 46. Fresh Concrete Air Void Analyzer
  47. 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. 48. Air Entraining Please return to Blackboard and watch the following video:  Video 1: Air Entrainment

Hinweis der Redaktion

  • 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.
  • 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.

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