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CON 122 Session 4 - Water Reducing & Set Controlling Admixtures

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CON 122 Session 4 - Water Reducing & Set Controlling Admixtures

  1. 1. CON 122 Concrete Admixtures Session 4 Water Reducing Admixtures & Set Controlling Admixtures
  2. 2. Standard Specification for Chemical Admixtures for Concrete ASTM C494  Type A - Water Reducing  Type B - Retarding  Type C - Accelerating NOTE: Type B & C provide no water reduction  Type D - Water-Reducing & Retarding  Type E - Water-Reducing & Accelerating  Type F - High-Range Water Reducing (HRWR)  Type G - HRWR & Retarding
  3. 3. Standard Specification for Chemical Admixtures for Concrete ASTM C494 Type Description Type A Water Reducing Type B* Retarding Type C* Accelerating Type D Water-Reducing & Retarding Type E Water-Reducing & Accelerating Type F High-Range Water Reducing (HRWR) Type G HRWR & Retarding *Note: Types B & C provide no water reduction
  4. 4. Liquid Admixture Dispenser  Liquid admixture dispenser at a ready mix plant provides accurate volumetric measurement of admixtures.
  5. 5. Set-Control Admixtures  Modify the setting and strength development characteristics of concrete  Accelerators  Retarders
  6. 6. Effect of Cool Temperatures on Time of Set Temperature Approximate Time of Setting 100 F (38 C) 2 hours 90 F (32 C) 3 hours 80 F (27 C) 4 hours 70 F (21 C) 6 hours 60 F (16 C) 8 hours 50 F (10 C) 11 hours 40 F (4 C) 14 hours 20 F (-7 C) Set does not occur (concrete will freeze)
  7. 7. Accelerating Admixtures ASTM C 494 or AASHTO M 194, Type C Accelerate the rate of:  Hydration (setting)  Early-age strength gain Calcium chloride accelerators:  Increase drying shrinkage, potential reinforcement corrosion, potential scaling  Darken concrete
  8. 8. ACI 306R – Cold Weather Concreting  1.3.1 ... ―The degree of saturation of newly placed concrete will be reduced as the concrete hardens and water is combined in the hydration process.‖  ―Under such conditions, the time at which the degree of saturation becomes reduced below a level which would cause damage by freezing, corresponds roughly with the time at which the concrete attains a compressive strength of 500 psi (3.5 MPa).‖  ―At temperatures of 50 ºF (10 ºC), most well proportioned concrete will reach this strength during the second day.‖
  9. 9. Accelerating Admixtures ASTM C 494 Physical Requirements Compressive Strength; Minimum % of Control Type C Type E 1 day -- -- 3 days 125 125 7 days 100 110 28 days 100 110 90 days n/a n/a 6 months 90 100 1 year 90 100
  10. 10. Maximum Chloride-Ion Content Maximum water soluble Type of member chloride-ion (CI¯) in concrete Pre-stressed concrete 0.06 Reinforced concrete ex- posed to chloride in 0.15 service Reinforced concrete that will be dry or protected 1.00 from moisture in service Other reinforced concrete 0.30 construction
  11. 11. Four Classes of Accelerating Admixtures  Calcium Chloride  Accelerating admixtures containing calcium chloride  Non-chloride accelerating admixtures  Non-chloride accelerating admixtures for use in concrete placed in sub-freezing temperatures
  12. 12. Accelerating Admixtures  Non-chloride  Non-corrosive  Dosage rates of up to 60 fl oz/cwt (3.9 L/m3)
  13. 13. Hot Weather as Defined in ACI 305R  "Any combination of high ambient temperature, high concrete temperature, low relative humidity, wind velocity, and solar radiation that impair quality of concrete by accelerating the rates of moisture loss and cement hydration”
  14. 14. Potential Problems: Freshly Mixed Concrete in Hot Weather  Increased:  water demand  rate of slump loss  potential for jobsite re-tempering  potential plastic shrinkage cracking  potential for cold joints  Fast setting  Placing, consolidation & finishing problems  Control of air entrainment
  15. 15. Plastic Shrinkage Cracking Wind
  16. 16. Potential Problems for Hardened Concrete in Hot Weather  Increased  Drying shrinkage and thermal cracking  Permeability  Decreased  28-day and later age strength  Durability
  17. 17. Retarding Admixtures  An admixture that causes a decrease in the rate of hydration of the hydraulic cement, and lengthens the time of setting (ACI 116)  ASTM C 494/C 494M (AASHTO M 194) Classifications  Type B: Retarding  Type D: Water-Reducing & Retarding  Type G: Water-Reducing, High-Range & Retarding
  18. 18. Retarding Admixtures ASTM C 494 Physical Requirements Type B Type D Min. Water Reduction, % -- 5 Initial Time of Set*: At Least……….. 1:00 later 1:00 later Not More Than….. 3:30 later 3:30 later Final Time of Set*: At Least……….. -- -- Not More Than….. 3:30 later 3:30 later
  19. 19. Retarding Admixtures ASTM C 494 Physical Requirements Compressive Strength; Minimum % of Control Type B Type D 1 day -- -- 3 days 90 110 7 days 90 110 28 days 90 100 90 days n/a n/a 6 months 90 100 1 year 90 100
  20. 20. Retarding Admixtures ASTM C 494 Physical Requirements Flexural Strength; Minimum % of Control Type B Type D 1 day -- -- 3 days 90 100 7 days 90 100 28 days 90 100
  21. 21. Types of Retarding Admixtures  Conventional Retarders  Extended-Set Control (ESC) Admixtures  Extended-Set Control Admixtures are very potent retarders that facilitate long hauls, and effective management of returned concrete.
  22. 22. Effect of Retarders on Time of Set Penetration Resistance, MPa 40 35 30 25 20 No Admix, 32°C 15 No Admix, 23°C Retarder, 32°C 10 Retarder, 23°C 5 0 2 3 4 5 6 7 8 9 10 11 Time, Hours
  23. 23. Effect of Retarders on Strength 7000 Compressive Strength, psi 6000 5000 4000 3000 2000 Plain Retarder @ 3 fl oz/cwt 1000 Retarder @ 4 fl oz/cwt Retarder @ 5 fl oz/cwt 0 1 7 28 Time, Days
  24. 24. Benefits of Retarders  Minimize potential for cold joints  Provide sufficient time for finishing operations in hot weather  Permit full form deflection before initial set of concrete
  25. 25. Potential Issues with Retarders  Over-retardation  Rapid stiffening & slump loss with some cements Time of Set should be determined from trial mixtures using local materials
  26. 26. Retarding Admixtures …DOT Applications  Retard set time:  Hot Weather Concreting  Deck Pours  Long Hauls  Particularly, with Extended-Set Control Admixtures  Slump retention & control of temperature rise
  27. 27. Water Reducing Admixtures ASTM C 494 or AASHTO M 194  Type A ―  reduces water content at least 5%  tends to retard ― accelerator often added  Type D ―  reduces water content 5% min.  retards set  Type E ―  reduces water content 5% min.  accelerates set
  28. 28. What are Water-Reducing Admixtures? ―Admixtures that either increase slump of freshly-mixed mortar or concrete without increasing water content OR maintain slump with a reduced amount of water, the effect being due to factors other than air entrainment.‖ (ACI 116R) 28
  29. 29. In Short…  Water-reducing admixtures increase the slump and workability of concrete without an increase in water-cementitious materials ratio.
  30. 30. Benefits of Water Reduction  Improved Slump & Workability  Without reducing water content  Increased Strength  by reducing total mix Strength water content while maintaining slump & workability  Economy  by lowering cement & water contents
  31. 31. Example: Reference Concrete Cement = 500 lb/yd3 Water = 300 lb/yd3 Slump = 5 in. Water 300 lb = Cement = 500 lb 0.60 Strength = 3500 psi
  32. 32. Add Water Reducer & Increase Slump Cement = 500 lb/yd3 Water = 300 lb/yd3 Slump = 7 in. Water 300 lb = Cement = 500 lb 0.60 Strength = 3500+ psi
  33. 33. Reduce Water Content to Maintain Slump Adjust agg. to yield Cement = 500 lb/yd3 Water = 280 lb/yd3 Slump = 5 in. again Water 280 lb = Cement = 500 lb 0.56 Strength = 4000+ psi
  34. 34. Reduce Cement & Water Contents: COST Adjust agg. to yield Cement = 467 lb/yd3 Water = 280 lb/yd3 Slump = 5 in. again Water 280 lb = Cement = 467 lb 0.60 Strength = 3500+ psi
  35. 35. Water Reducers are Dispersants  Water Reducers (all types)  belong to a group of chemical compounds known as dispersants
  36. 36. Dispersants Dispersant (Dispersing Agent): Material added to a solid in liquid suspension to prevent the individually suspended particles from flocculating. Dispersant Shear Flocculated State Dispersed State 36
  37. 37. Water Reducer Chemistries (categorized by primary ingredients) 1. Lignosulfonic acids & their salts (tree & pulping) 2. Hydroxylated polymers/carbohydrates (corn syrup) 3. Hydroxylated carboxylic acids & their salts 4. Sulfonated melamine (MSFC) and naphthalene formaldehyde (NSFC) condensates 5. Polyether-polycarboxylates (engineered comb-shaped polymers)
  38. 38. How Water Reducers Work + + + Water + Cement + + + Charged Cement Particles Cling Together and Form Flocs that Trap Water
  39. 39. Floc Busting Action of Water Reducers Dispersant (water reducer) molecules imparts negative charge on surface of cement particles when absorbed, causing the particles to repel each other (ELECTROSTATIC REPULSION) Water Reducer Freed Water Cement Water reducers separates flocs into individual grains. Thus, trapped water is released and the grains slip by each other like ball bearings, improving the workability of the concrete.
  40. 40. Benefits of Dispersion  Better dispersion of cement grains results in increased workability.  Increased dispersion results in more efficient hydration which increases compressive strength.  Increased dispersion maximizes benefits of reduced water-cementitious materials ratio.
  41. 41. Benefits of Water Reducers  Fresh Concrete properties:  Lower water-cementitious materials ratio  Improved workability, flowability, pumpability, and placeability  Influence time of setting  Improved finishing
  42. 42. Benefits of Water Reducers  Hardened Concrete properties:  Increased strength  Improved durability
  43. 43. Water Reducer Types  Conventional  Mid-Range  High-Range (superplasticizers) Water-reducing admixtures are used in most concretes produced today !
  44. 44. Conventional Water-Reducing Admixtures  Effects of Increasing Dosage...  Diminishing returns.  Excessive retardation / bleeding.  Increased air content (lignins).  Slow strength development. 44
  45. 45. Water-Reducing Admixtures ASTM C 494 Physical Requirements Compressive Strength; Minimum % of Control Type A Type F 1 day -- 140 3 days 110 125 7 days 110 115 110 110 28 days (120) (120) 90 days (117) (117) 100 100 6 months (113) (113) 1 year 100 100
  46. 46. Mid-Range Water-Reducing Admixtures  First Introduced in 1986*  Dosages of 3-15 fl oz/cwt (175-975 mL/100 kg)  Most Widely Used Water-Reducer Type  Based on lignosulfonate salts, polycarboxylate ether, other with set-, strength-, & finishability- enhancing ingredients
  47. 47. Mid-Range Water-Reducing Admixtures  Provide mid-range water reduction (6- 12%)  Used in production of concrete with slump between 5 - 8 in. (125 - 200 mm).  Used in concretes with moderate w/cm.  Also used in combination with HRWR at low w/cm.
  48. 48. Benefits of Mid-Range Water Reducers  True Mid-Range Performance  Excellent Finishability Characteristics  Fly Ash, Slag Cement, Silica Fume Mixtures  Lean & Harsh Mixtures (Manufactured Sands)  Flatwork, Curb & Gutter, etc.
  49. 49. Water Reducer and Slump Loss  Slump loss at 23 C (73 F) in concretes containing conventional water reducers (ASTM C 494 and AASHTO M 194 Type D) compared with a control mix.
  50. 50. Retardation (Initial Set)  Retardation of set in cement-reduced mixtures relative to control mixture. Concretes L and H contain conventional water reducer, concretes N, M, B, and X contain high- range water reducer.
  51. 51. Retardation (Final Set)  Retardation of set in cement-reduced mixtures relative to control mixture. Concretes L and H contain conventional water reducer, concretes N, M, B, and X contain high- range water reducer.
  52. 52. High-Range Water-Reducing Admixtures ASTM C 494 or AASHTO M 194  Type F― Water Reducing  Type G ― Water Reducing and Retarding  Reduce H2O content 12% -30%  Reduced W/C produces conc. with:  Compressive Strength > 70 MPa  Increased early strength gain  Reduced Cl ion penetration
  53. 53. Why Use High-Range Water Reducers? Highly workable, non-segregating, very pumpable concrete
  54. 54. Benefits of High-Range Water Reducers  Significant water reduction  high-early & high ultimate strengths  low permeability & increased durability  more effective use of cementitious materials  High slump & workability  faster discharge, pumping & placement  less consolidation effort
  55. 55. Low water to cement ratio concrete with low chloride permeability—easily made with high-range water reducers—is ideal for bridge decks.
  56. 56. Potential Issues with High- Range Water Reducers  Segregation potential if dosage & total water content are out of balance  Impact on air-void system  Retardation potential @ high dosages  Erratic performance* at lower slumps (Production Nightmare !!!) * Typically with non-Polycarboxylate Ether chemistries
  57. 57. High-Range Water Reducers …Impact on Air-Void System & Freeze Thaw  Studies show that freeze-thaw durability is typically not affected even though spacing and size of air voids may be altered.
  58. 58. High-Range Water Reducer and Slump Loss  Slump loss at 23 C (73 F) in mixtures containing high-range water reducers (N, M, B, and X) compared with control mixture (C).
  59. 59. HRWR and Air Loss Final air Percent Rate of air Initial air content, air loss, Mixture content, % %* retained %/minute Control C 5.4 3.0 56 0.020 Water L 7.0 4.7 67 0.038 reducer H 6.2 4.6 74 0.040 High-range N 6.8 4.8 71 0.040 water M 6.4 3.8 59 0.065 reducer B 6.8 5.6 82 0.048 *at point where slump falls below 25 mm Loss of Air from Cement Reduced Concrete Mixtures Whiting and Dziedzic 1992.
  60. 60. Compressive Strength Development  Compressive strength development in cement-reduced concretes: control mixture (C) and concretes containing high-range water reducers (N, M, and X).
  61. 61. Plasticizers for Flowing Concrete  Also known as – SUPERPLASTICIZERS (ASTM C 1017)  Plasticizers for flowing concrete  Type 1: Plasticizing  Plasticizing and Retard  Essentially High-Range Water Reducer
  62. 62. Plasticizers for Flowing Concrete  Superplasticizers (ASTM C 1017)  Produce flowing concrete with high slump (≥ 190 mm [7.5 in.]  Reduce bleeding  Extended-slump-life plasticizer reduces slump loss.
  63. 63. Flowing Concrete Flowable concrete with a high slump (top) is easily placed (middle), even in areas of 63 heavy reinforcing steel congestion (bottom)
  64. 64. Flowing Concretes and Slump Loss  Slump loss at 32 C (90 F) in flowing concretes (TN, TM, TB, and TX) compared with control mixture (TC).
  65. 65. Retardation of Flowing Concrete (Initial Set)  Retardation of set in flowing concrete with plasticizers (N, M, B, and X) relative to control mixture.
  66. 66. Retardation of Flowing Concrete (Final Set)  Retardation of set in flowing concrete with plasticizers (N, M, B, and X) relative to control mixture.
  67. 67. Compressive Strength Development in Flowing Concrete  Compressive strength development in flowing concretes. C is the control mixture. Mixtures FN, FM, and FX contain plasticizers.
  68. 68. Bleeding of Flowing Concrete  Bleeding of flowing concretes with plasticizers (N, M, B, and X) compared to control (C).
  69. 69. Retarding Admixtures ASTM C 494 or AASHTO M 194, Type B  Delay setting or hardening rate for:  Hot-weather concreting  Difficult placements  Special finishing processes
  70. 70. Slump Loss at Various Temperatures  Slump loss at various temperatures for conventional concretes prepared with and without set- retarding admixture.
  71. 71. Retarding and Water-Reducing Admixtures Please return to Blackboard and watch the following videos:  Video 1: Retarding Admixtures  Video 2: Water-Reducing Admixtures

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