Concrete (5) Durability of concrete

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Marciaho
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269832
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Concrete (5) Durability of concrete
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2014-04-08 19:28:11
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Concrete Durability concrete
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Concrete (5) Durability of concrete
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Concrete (5) Durability of concrete
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  1. Causes of Deterioration 
    Chemical
    Physical
    • Chemical :corrosion of steel
    • Alkali aggregate reaction
    • Sulfate attack
    • Acid Attack

    • Physical
    • Freeze thaw (scaling)
  2. Coefficient of permeability
    • K = (Q/A) . ( I/h) 
    • h = hydrostatic pressure
    • Q= outflow
    • A = cross section
  3. Effect of porsosity
    Higher
    Lower
    Higher= lower resistance to flow

    Lower = Greater resistance to flow
  4. Effect of connectivity
    Connected pores = resistance to flow

    Discrete pores = greater resistance to flow
  5. Effect Constricivity 
    One large pore
    Many small pores
    • One large pore = lower resistance to flow
    • Many small pores = greater resistance to flow
  6. Effect of Toruosity
    Straight pore
    Crooked pore
    Straight = Lower resistance to flow

    Crooked = Higher resistance to flow
  7. To decrease permeability: (pores)
    • Decrease total porosity
    • Decrease connectivity
    • Decrease pore size
    • Increase totuosity
  8. Most fluid flows through _____ not _____
    Paste not aggregate
  9. To lower permeability (paste)
    quantity and quality
    • Reduce quantity in paste 
    • decrease water and cement content
    • Improve aggregate grading

    • Improve quality of paste
    • Reduce water /cement ratio
    • Use SCMS
  10. Decreasing ______, improves _______ of paste, reducing total ________, decreasing _________ and increasing ________ and _______
    Decreasing water/cement ration improves quality of paste, reducing total porosity, decreasing connectivity, and increasing connectivity and tortuosity
  11. Concrete Microstructure
    ITZ has high proportion of
    Calcium hydroxide and ettringite and high porosity compared to bulk paste
  12. Factors affecting permeability of cncrete
    Principal and secondary
    • Principal : 
    • w/c ratio
    • water content
    • use of SCM 
    • curing

    • Secondary:
    • cement content
    • Chemical admixtures
    • Aggregate type
    • Air content
  13. Protection from freeze thaw through the use of:
    • air voids
    • air entrained concrete
  14. The rate of chemical reaction is dependent on:
    • Temperature
    • Accesibility
    • Other chemical and physical processes
    • Synergies between different mechanisms
  15. Sulphate attack reacts with ____ to form _____ which absorbs water and distrupts microstructure

    Dependent on:
    Sulphate attack reacts with  allumicate to form ettringite which absorbs water and distrupts microstructure

    Dependent on concrete permeability, concrete type, amount of water available
  16. Resistance of sulphate attack can be improved by:
    • Use of PC portland cement with low C3A content
    • Use low w/c
    • Use SCM
  17. Alkali aggregate reaction
    Aggregate reacts with ____ to form _____ 
    Effects of this
    Aggregate reacts with alkalis (sodium and potassium) to form silica gel

    Silica gel in wet concrete will absorb water causing swelling and ultimately leads to internal disruption of the concrete matrix
  18. Three necessities for ASR
    • Reactive silica
    • Sufficient Moisture
    • Sufficient Alkali
  19. How to prevent ASR
    • Use non reactive aggregate
    • Limit the alkali content of concrete
    • Use SCM
  20. ASR symptoms
    • Random crack orientation. 
    • Map cracking, pattern cracking
  21. Need to consider when designing for durability
    • Structural interactions (service stresses and serviceability) 
    • Environmental conditions 
    • Quality of construction
    • Level of durability required
  22. Most common durability design errors
    • Under estimating the environmental exposure
    • Inadequate cover depth
    • Reluctance to use SCMS
    • Now allowance for cracking
    • Lack of enforcing curing
    • Complete lack of supervision
  23. Important Design Factors
    Design process should consider the following (7)
    • 1. Service requirements
    • 2. Environmental exposure conditions
    • 3. Availability of concrete materials
    • 4. Proposed method of construction
    • 5. Economic constraints
    • 6. Consequences of failure
    • 7. Feasibility of repairs

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