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CON 124 - Session 2 - Proportioning Methods

  1. 1. CON 124 Basic Concrete Mix Design Proportioning Session 2 Concrete Durability
  2. 2. Causes of Concrete Deteriorations     Freezing and Thawing Corrosion Mechanism Recommendations
  3. 3. Concrete Durability   Definition - its ability to resist weathering action, chemical attack, w/cm ratio considerations, mass concrete, abrasion, or any other process of deterioration. Session 1 will discuss Freezing-thawing distress in detail
  4. 4. Concrete Durability Video
  5. 5. Freezing Distress caused by frost damage – Hydraulic Pressure photos courtesy of M. Thomas
  6. 6. Thawing Distress caused by Scaling Distress – Hydraulic & Osmotic Pressures photos courtesy of M. Thomas
  7. 7. Frost Damage by to cyclic F-T damage photos courtesy of M. Thomas
  8. 8. Frost Damage by to cyclic F-T damage photos courtesy of M. Thomas
  9. 9. Recommended Air Content Effective For Durability:  When volume of air in mortar fraction of concrete is about 9 ± 1% (or about 18% by paste volume).  Required air increases as coarse aggregate size is reduced (due to greater paste volume)  Required air increases as exposure conditions become more severe.
  10. 10. Weathering region according to climate conditions in ASTM C33
  11. 11. Recommended Average Air Content Percentage for Level of Exposure Nominal maximum sized of aggregates Exposure 3/8 in. (9.5 mm) 1/2 in. 3/4 in. (12.5 mm) (19 mm) 1 in. (25 mm) 1 – 1/2 in. 2 in. (37.5 mm) (50 mm) 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
  12. 12. Control of Air Content Materials • Cement content; Fineness, Alkali • Aggregates; Fineness, Shape • Admixtures • Pigments
  13. 13. Control of Air Content Mixture Parameters, Etc. • w/cm • Slump; <3-in. (75mm) >6-in. (150mm) • Temperature • Altitude • Mixing Speed, Time, Capacity
  14. 14. Placement Issues which Affect Control of Air Content Placement • • • • Transportation Placement; Pumping Consolidation Finishing
  15. 15. 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.
  16. 16. Air Entrainment Video
  17. 17. Acts at Air-Water Interface
  18. 18. Air-Entraining Admixture Function
  19. 19. Microscopic Air Bubbles
  20. 20. Mechanism of Protection by Air Voids Air-entrained 32oF 23oF As the temperature of saturated concrete(.91.7%) in service is lowered, the water held in the capillary pores of harden concrete as paste freezes. Saturation > 91.7%
  21. 21. Mechanism of Protection by Air Voids Air-entrained 32oF 23oF As microscopic air bubbles are filled with freezing of water causes an increase of volume by 9%,thus the excess water in the cavity is expelled by hydraulic pressure. Saturation > 91.7%
  22. 22. Mechanism of Protection by Air Voids Air-entrained 32oF 23oF 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. Saturation > 91.7%
  23. 23. Mechanism of Protection by Air Voids Air-entrained 32oF 23oF Each cycle of freezing and thawing causes a migration of water to locations where it can freeze. Subsequent freezing repeats the development of pressures and consequences of re-freezing, further expansion takes which has an cumulative effect. Saturation > 91.7%
  24. 24. Air Void Spacing & Volume • Spacing factor ( L ): the maximum distance of any point in the cement paste from the periphery of an air void. L L • Specific surface: the surface area of a quantity of air voids that have a total volume of 1 in.3 (25 mm3).
  25. 25. Characteristics of an Adequate Air Void System (ASTM C 457)  Test method ASTM C457 recommendations for an adequate air-void system.  Spacing factor < 0.008 in. (0.203 mm)  Specific surface > 600 in.2/in.3 (24 mm2/mm3)  Voids per linear inch: 1.5 - 2 times the percentage of air
  26. 26. Please return to Blackboard and watch the following videos:   Video 1: Durability Video 2: Air Entrainment
  27. 27. Questions? Email cemtek@netzero.net

Editor's Notes

  • Recommend Average Air Content for Levels of Exposure per ACI design methods
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