CON 124 - Session 7 - Concrete Durability

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CON 124 - Session 7 - Concrete Durability

  1. 1. CON 124 Basic Concrete Mix Design Proportioning Session 7 Concrete Durability
  2. 2. Session Topics        Dicers Mass Concrete Carbonation Abrasion Resistance Special Aggregate Reaction Mechanisms Curing Measures
  3. 3. De-Icers
  4. 4.   De-icing Salts for Snow & Ice Removal Mixture design
  5. 5.   De-icing Salts for Snow & Ice Removal Mixture design
  6. 6. De-Icing Chemicals Exposure Cementitious materials Fly ash (Class C up to 35%) and natural pozzolans Slag Silica fume Total of fly ash, slag, silica fume and natural pozzolans Maximum replacement , % 25 50 10 50 Cementitious Materials Requirements for Concrete Exposed to Deicing Chemicals (Ref ACI 318)
  7. 7. Fly Ash and Natural Pozzolans   Reduced Permeability & Diffusivity Resistance to ASR    Consumption of Ca(OH)2 Reduction in water mobility Resistance to Sulfate (Low CaO pozzolans)   Dilution of C3A Consumption of Ca(OH)2
  8. 8. Blast Furnace Slag    Reduced Permeability & Diffusivity Resistance to ASR (>35%) Resistance to Sulfate  Dilution of C3A
  9. 9. De-Icing Chemicals       Sodium Chloride Calcium Chloride Both in combination Magnesium Chloride Urea All of the above in solution or salts form
  10. 10. Disintegration Mechanism     Surface disintegration in the form of pitting or scaling High degree of saturation in the concrete is mainly responsible for their detrimental effect due to lower vapor pressure Development of disruptive osmotic and hydraulic pressures during freezing, principally in the paste Concentrations (3 to 4%) of deicing solutions most severe
  11. 11. Recommended Solutions    Benefit from entrained air in concrete exposure same as frost action Concrete surface should have received some drying, and minimum strength level specified and concrete cover Do not use in pre-stressed concrete or where steel reinforcement has been used due to corrosion effect
  12. 12. Mass Concrete Mass concrete requires minimizing heat generation in massive elements or structures such as very thick bridge supports, and dams.
  13. 13. Mass Concrete Hoover dam, shown here, used a Type IV cement to control temperature rise.
  14. 14. Mass Concrete Heat Generation    Control the generation of heat and resultant volume change within the mass will require consideration of temperature control measures Concrete temperature rise of 10 to 15 F per 100 lb of Portland cement/yd3 in 18 to 72 hours Temperature rise of the concrete mass creates thermal gradient causing Cracking
  15. 15. Thermal Cracking    May reduce the service life of a structure by promoting early deterioration or excessive maintenance Selection of proper mixture proportions is only one means of controlling temperature rise Additional aspects of the concrete work should be studied and incorporated into the design and construction requirements
  16. 16. Nominal Maximum Size Aggregate    Mass concrete is not necessarily larger aggregate concrete The minimum cross sectional dimensions of a solid concrete member approach or exceed 2 to 3 ft or when cement contents above 600 lb/yd3 are being used Larger aggregate provides less surface area to be coated by cement paste, a reduction in the quantity of cement and water can be realized for the same water-cement ratio
  17. 17. Nominal Maximum Size Aggregate ACI Recommendation
  18. 18. Cementitious Materials Impact    Reduction of heat of hydration, improved workability, improved strength and/or improved durability Fly ash, natural pozzolans meeting ASTM C 618 Slag cement meeting ASTM C 989
  19. 19. Portland Cement Impact    Fineness of cement is an important factor affecting rate of heat liberation, particularly at early ages ASTM C150 contains optional limits for the heat of hydration for Type IV and also includes Type II (MH) moderate heat cement that limits the C3S and C3A content Chemical optional requirements are less restrictive in ASTM C150, as compared to the optional Physical requirement when evoked; 60.cal/g is the maximum permitted for a Type V cement (7 days) or 70 cal/g (28 days)
  20. 20. Principle Phases of Portland Cement    Tricalcium aluminate (C3A) releases most of its heat in the first day or so Tricalcium silicate (C3S) in the first week Dicalcium silicate (C2S) and calcium aluminoferrite (C4AF) hydrate more slowly
  21. 21. Blended Cement Impact    Blended cements have lower heats of hydration than Portland cements Generally, most slag cements, fly ashes, and natural pozzolans will hydrate after 28-day thus lowering the maximum heat peak of a concrete mixture The volume occupy by supplementary materials like slag cement, fly ashes, and natural pozzolans is typically less than Portland cement.
  22. 22. Carbonation Affects    Carbon dioxide causes a reaction producing carbonates accompanied by shrinkage Carbonation during production can improve the strength, hardness, and dimensional stability of concrete products Carbonation can result in deterioration and a decrease in the pH of the cement paste leading to corrosion of reinforcement near the surface
  23. 23. Exposure to Carbon Dioxide (CO2)   During the hardening process can affect the finished surface of slabs, leaving a soft, dusting, less wear-resistant surface During the hardening process, the use of unvented heaters or exposure to exhaust fumes from equipment or other sources can produce a highly porous surface subject to further chemical attack
  24. 24. Dusting Surface Dusting of surface caused by the use of unvented heaters
  25. 25. Crazing Surface Crazing surface due to premature drying, shrinkage of concrete surface caused by exhaust fumes of heating equipment in an enclosed area
  26. 26. Reaction of Hydrated Portland Cement with CO2    Highly dependent on the relative humidity of the environment, temperature, permeability of the concrete, and concentration of CO2 Highest rates of carbonation occur when the relative humidity is maintained between 50% and 75% Below 25% relative humidity, the degree of carbonation that takes place is considered insignificant
  27. 27. Absorption of Ambient CO2    CO2 absorbed by rain enters the groundwater as carbonic acid CO2, together with humic, carbonic, acid, can be dissolved from decaying vegetation, resulting in high levels of free CO2 The rate of attack, similar to that by CO2 in the atmosphere, is dependent upon the properties of the concrete and concentration of the aggressive CO2
  28. 28. Abrasion Resistance     “Ability of a surface to resist being worn away by rubbing and friction” Abrasion resistance of concrete is a progressive phenomenon Closely related to compressive strength at the wearing surface degradation that is related to aggregate-topaste
  29. 29. Concrete Mixture Quality       Avoiding segregation; Eliminating bleeding; Properly timed finishing; Minimizing surface w/cm (forbidding any water addition to the surface to aid finishing); Hard toweling of the surface; and Proper curing procedures.
  30. 30. Abrasion Damage Wear of concrete surface, abrasion
  31. 31. Abrasion Damage Abrasion damage due to concrete baffle blocks and floor area of Dam sluiceway
  32. 32. Special Aggregates     Addition of high-quality quartz Traprock, or emery aggregates A blend of metallic aggregate Use of two-course floors using a high-strength topping is generally limited to floors where both abrasion and impact are destructive effects at the surface
  33. 33. Pavement Abrasion Resistance    Adequate texture and skid resistance for proper vehicular control Related to concrete’s compressive strength and to the type of aggregate in the concrete; harder aggregates resist wear better than softer aggregates Wear of pavement surfaces occurs due to the rubbing action from the wheels of vehicular traffic
  34. 34. Pavement Abrasion     Production Operations, or foot or vehicular traffic Wind or waterborne particles can also abrade concrete surfaces Abrasion is of little concern structurally, yet there may be a dusting problem that can be quite objectionable in some kinds of service Abrasion resistance of concrete is a progressive phenomenon
  35. 35. Wind Abated Surface Wind Abated Surface more potential for dusting
  36. 36. Testing for Abrasion Resistance     Los Angeles (LA) abrasion test (rattler method) performed in accordance with ASTM C 131 or ASTM C 535 / AASHTO T 96 ASTM C 418 subjects the concrete surface to airdriven silica sand, and the loss of volume of concrete is determined ASTM C 779, three procedures simulate different abrasion conditions ASTM C 944, a rotating cutter abrades the surface of the concrete under load
  37. 37. Alkali-carbonate rock reaction   Detrimental reactions are usually associated with argillaceous dolomitic limestones that have somewhat unusual textural characteristics Some carbonate rocks occurs in which the peripheral zones of the aggregate particles in contact with cement paste are modified and develop prominent rims within the particle
  38. 38. Alkali-Carbonate Reactivity    Brucite [Mg(OH)2], dedolomitization of Magnesium Feature is different from alkali-silica reactivity, in which the alkali is combined in the reaction product as the reaction proceeds Presence of clay minerals appears significant
  39. 39. Affected Concrete Characteristics   A network of pattern or map cracks Typically where the concrete has a constantly renewable supply of moisture    Waterline in piers Earth behind retaining walls wick action in posts or columns General absence of silica- gel exuding from cracks.
  40. 40. Map Cracking
  41. 41. Evaluation of Affected Concrete  Damage can be the result of:            Poor Design Faulty Workmanship Mechanical Abrasive Action Cavitation Or Erosion From Hydraulic Action Leaching Chemical Attack Chemical Reaction Inherent In The Concrete Mixture Exposure To Deicing Agents Corrosion Of Embedded Metal Or Another Lengthy Exposure To An Unfavorable Environment Guidance for examining and sampling hardened concrete in construction is found in ASTM C 823
  42. 42. D-Cracking Deterioration   D-cracking is damage that occurs in concrete due to expansive freezing of water in some aggregate particles The damage normally starts near joints to form a characteristic D-shaped crack
  43. 43. D-Cracking
  44. 44. D-Cracking Reduction    Selecting aggregates that are less susceptible to freeze-thaw deterioration Reducing the maximum aggregate size for marginal aggregates are used Providing drainage for carrying water away from the base may prevent saturation of the pavement
  45. 45. D- Cracking Aggregates Characteristics    Aggregate particles with coarse pore structure may be susceptible to freeze-thaw damage Particles become saturated and the water freezes, expanding water trapped in the pores cannot get out Aggregate particles cannot accom-modate the pressure from the expanding water; the particles crack and deteriorate
  46. 46. Identifying D-Cracking    Closely spaced cracks parallel to transverse and longitudinal joints Location where aggregate is most likely to become saturated Cracks multiply outward from the joints toward the center of the pavement slab
  47. 47. D-Cracking Corrective Measures    Designing a mixture it is critical to select aggregates that are not susceptible to freezethaw deterioration If marginal aggregates must be used, you may be able to reduce D-cracking susceptibility by reducing the maximum particle size Providing good drain-age for carrying water away from the pavement base
  48. 48. Please return to Blackboard and watch the following videos:   Video 1: Maximum Size Video 2: Minimum Size
  49. 49. Questions? Email cemtek@netzero.net

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