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Flashcards

Hardened Concrete

Maddi Everton English 43 cards

Paste, Mortar and Concrete

Cards in this deck (43)

  1. 1Paste, Mortar and ConcretePaste, Mortar and Concrete
  2. 2Water to Cement Ratio (w/c)- very important for concrete
    - major factor for quality: strength and durability
    -water mass doesnt include that absorbed by agg
    - compressive strength increases with decreased w/c
    -inc permeability = higher w/c
  3. 3Particulate Composite- Concrete is particulate composite
    - single continuous phase or matrix containing a discontinuous particulate phase
    - math models to compute mech properties of particulate composites
    - E=elastic mod
    V=volume fraction of a phase
    P=particulate phase ex) Agg, filler etc
    M=Matrix phase ex) binder, cement paste etc
  4. 4Agg/Concrete/Paste stress, strain, elastic mod- E of agg depends on rock type (45-85Gpa)
    - E of paste is function of paste porosity(10 Gpa)
    - E of concrete is intermediate between paste and agg, more non linear (30-50Gpa)    Agg/Concrete/Paste stress, strain, elastic mod
  5. 5Different Agg types on E of concreteDifferent Agg types on E of concrete
  6. 6Elastic Modulus and \compressive strengthE=0.043w^1.5sqrt(f'c)
  7. 7EcEc
  8. 8Poissons ratio-gamma=0.15 to 0.2 for concrete
    -gamma = lateral strain/longitudinal strain
  9. 9Failure in Particulate Composites- failure influenced by nature of composite
    - in conc, a crack that develops and propogates through matrix will encounter a particle and be deflected, in process, crack loses energy and doesnt propogate anymore
    - several cracks are likely to form before failure
    - influences on failure: particulate content, elastic mod difference between particle and matrix, interfacial zone around particle, bond between particle and matrix, strength of both
  10. 10Failure in particulate composites continued- m = modular ratio = Eparticle/Ematrix
    - photo on left shows interfacial zone
    - photo on right shows stress concentration across interfacial zone
    -k(sigma) = factor by which applied stress is increased internally
    - ep and em (strains) are strain in particle and matrix
    - elastic modulus discrepancy between particles and matrix creates stress concentration at interface, magnitude of stress concentration dependent on "m"    Failure in particulate composites continued
  11. 11Concrete Voids-very complex due to the compex nature of concrete
    - concrete has voids that exist over wide range of sizes
    - each size and type of void can have different effects on concrete
  12. 12Entrapped Void- 1000 to 10000 micrometers
    - "rock pockets" "honeycomb", "voids of incomplete consolidation"
    - effects leakage, durability, strength
    - not tolerated in concrete structures
  13. 13Macrocrack-100 to 3000 micro meters
    - visible to naked eye
    - can have severe effect on concrete durability if numerous
    - strength loss due to cracks accounted for in structural design
  14. 14Entrained Air-70 to 400 micrometers
    - intended for freeze thaw
    - reduction in concrete strength but adjusted for in mix design by reducing w/cm
    - bubbles not interconnected - do not result in more permeable concrete
  15. 15Microcracks- <0.1 to 100 micrometers
    - naturally occur from restraint shrinkage forces around agg
    - inc with inc load
    - influence durability and strength
  16. 16Transition zone- 10 to 50 micrometers
    - weak porous zone around agg particles
    - large, oriented Ca(OH)2 crystals
    - reduced by SCM's
    - together with microcracks can form connected pathway for water and other chemicals to flow into concrete
    - influence durability and strength
  17. 17Capillary Pore- 0.01 to 1 micrometers
    - a residue of originally water filled spaces in fresh concrete
    - primary path by which water and other ions penetrate into concrete matrix, resulting in durability problems ex) rebar corrosion
    - significantly reduces strength
  18. 18Concrete Permeability- Concrete matrix is permeable because of capillary pores, transition zone, microcracks
    - absorption is a surface related measure of permeability
    - some agg is permeable but not most
    - many tests can measure permeability: true thru-put water permeability tests, absorption tests (not always best), gas permeability (drying may alter matrix), chloride diffusion tests, resistivity, strength
  19. 19Concrete Strength- brittle material, stronger in compression
    - strength of concrete is mostly function of strength of paste, nature of paste agg interface and nature of microcracks
    - aggs are generally dense and strong, lesser influence on strength
    - strength of paste is dominated by capillary porosity - governed by w/c and degree of hydration (measure of how much cement has reacted)
  20. 20Strength vs porosity and w/c- most materials have inverse relationship between strength and porosity (see equation in handwritten notes)
    - generally dont use this equation to predict concrete strength
    - w/c is best parameter to predict compressive strength    Strength vs porosity and w/c
  21. 21Concrete Strength Gain vs timeConcrete Strength Gain vs time
  22. 22Approximations of Concrete Strengths-compressive strength: 7 day - approx 75% of 28 day, 56 and 90 day - 10-15% >28dau
    -flexural strength: normal density: 8-12% of compressive strength
    - tensile strength: direect tensile - 8-12% of compressive, splitting tensile - 8-14% of compressive
  23. 23Compressive strength rangesCompressive strength ranges
  24. 24Compressive Strength of concrete- strength is function of: porosity which is also a function of: w/cm, degree of hydration (curing time, temp, moisture), entrained air content, degree of compation (entrapped air content)
    - also function of agg characteristics, cement charateristics, admixture chars, total cementing materials content, rate of loading and moisture content at time of testing
  25. 25Stress Strain Curve of concrete- normal aggs in normal strength concrete have very little influence on strength but greatly influence stress strain curve
    - stress concentrations: due to elastic modulus difference, they occur at agg/paste interface resulting in progressive internal cracking
    - stress vs strain curve is curvilinear    Stress Strain Curve of concrete
  26. 26Stress strain curve contd- microcracks start to occur at 25% of ultimate strength - results in non linear behavior
    - above 75% of ult strength cracks become unstable and static fatigue failure will occur
    - doesnt have any elastic strain due to microcracking
  27. 27influence of properties of paste and aggs on stress strain curve- extent of microcracking is a function of compatibility between agg and paste
    - similar in strength and E = more linear stress vs strain
    - ex) lightweight agg in normal strength matrix or normal agg in high strenght matrix
  28. 28influence of curing on strengthinfluence of curing on strength
  29. 29Maturity Concept- degrees C hours or degress C days
    - method that can be used to predict conc strength in field based on the time since it was poured and the environmental temp
    - based on idea that 2 concretes of same maturity should have same strength
    - assuming f'c=f (age, temp) only
    - then f'c = f (summation of time x temp)
    - datum temp = -10C
    - **** IMPORTANT!
    -limitations are that many other factors influence strength besides time and temp such as: mix variability, moist curing, heat generation and dissipation, early age curing temp
  30. 30Creep- time dependent strain under load
    - if concrete is simultaneously loaded and dried, creep deformation is greater than if the concrete dried before loading    Creep
  31. 31Creep continued- amount of creep that occurs when concrete is dried as well as loaded depends on relative humidity and time
    - higher relative humidity, lower creep
    - agg volume fraction influences because if paste is reduced there will be less creep
    - concrete loaded at a later age will creep less
  32. 32Crack types - chemical shrinkage- vol reduction due to chemical reaction of cement
    - volume of products is less than the vol of reactants (products = right side of chem eqn)
    - occurs in plastic and hardened states
  33. 33Autogenous Shrinkage (external chrm shrinkage)- results in external volume change
    - can result in cracking at early age
    - can result in cracking in particular for high cement content mixes - higher the w/c, lower the cracking
    - difference is that chem shrinkage occurs at initial set, where as autogenous can occur after final set too
  34. 34Plastic Shrinkage- evaporative loss when concrete is not yet hardened
    - water goes in tension creating tensile stress in concrete
    - occurs generally prior to or shortly after set
    - hot dry or windy conditions
    - when mixes with little bleed water
    - cracks may be random or oriented in specific pattern
    - cracks do not pass thru ag
    - v shaped, much wider at surface
    - plastic and autogenous shrinkage will act in combination to make cracking worse
  35. 35Plastic and Autogenous- can result in reduced durability of structure
    - aesthetic problem
    - mitigated cracking potential by: providing early moist curing, slowing rate of rxn, modifying mix proportions (less cement), do trial tests
  36. 36Drying Shrinkage- time dependent strain due to loss or gain of water
    - only partially reversible as concrete is rewetted
    - mostly complete by 6 months
    - patterns: tend to take shortest path (ex) between columns or coming off a corner), tend tobe more linear
  37. 37Structural Cracks (load0- at midspan
    - over support
    - not always through
    - also shear type
    - drying shrinkage can add to width of crack
  38. 38Thermal Cracks- heat generated during cement hydration, resulting in temp difference and then strain difference between interior and exterior of concrete
    - different thermal expansion between interior and exterior or between varying thickness of concrete
    - restraint of thermal expansion and contraction
  39. 39Crack basics - widthCrack basics - width
  40. 40Crack basics- movement- see photo
    - delta w = crack movement
    - static = no movement, dynamic = yes
    - movement due to: daily temp changes, yearly temp changes, moving loads, settlement    Crack basics- movement
  41. 41crack basics - depthcrack basics - depth
  42. 42Shrinkage and water content- most important parameter is amount of water in concrete
    - also paste content important
    - lowering w/c doesnt necessarily give lower water content
    - drying shrinkage largely increases with increasing water
  43. 43Influence of curing on shrinkage- water immersion may result in some swelling
    - moist curing will prolong onset of drying shrinkage
    - moist curing reduce total final amt of shrink
    - allow time for tensile strength to develop
    - net effect = less cracks

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Flashcards on Hardened Concrete, created by Maddi Everton on 12/12/2017.
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Maddi Everton
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