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Aggregates

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Title: Aggregates


1
Aggregates
2
Introduction
  • Aggregates typically occupy 70 to 80 percent of
    concrete by volume
  • Most often derived from natural rock (crushed
    stone or gravel) but can also be obtained from
    industrial byproducts (slag) or produced to
    create lightweight or heavyweight concrete
  • For most normal-weight concrete, the strength of
    the concrete is independent of aggregate type
  • High strength concrete being the exception

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Introduction
  • Aggregates should be hard and strong, free of
    undesirable impurities, and chemically stable
  • Soft, porous rocks can limit strength and wear
    resistance, may break down during mixing, and
    make produce fines
  • Aggregates should be free of silt, clay, dirt, or
    organic manner

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Aggregate Grading
  • Determines paste requirement
  • Desirable to reduce paste to a minimum
  • Meet workability, strength, and durability
    requirements
  • The amount of paste is related to the space
    between the aggregate particles
  • Uniform grading requires the most paste
  • Smaller aggregates require more paste
  • The densest packing will not create a workable
    mixture

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Moisture Content
  • Very important as it influences the w/c
  • The SSD state is most often used as a reference,
    although it is more difficult to obtain than the
    OD state
  • Absorption is very important
  • Most natural aggregates have an absorption
    capacity of 1 to 2 percent
  • Effective absorption versus surface moisture
  • Bulking of fine aggregate

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Aggregate Durability Problems
  • Freeze-thaw durability
  • Alkali-aggregate reactivity
  • Alkali-silica reactivity
  • Alkali-carbonate reactivity
  • Deleterious materials

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F-T Soundness
  • Sulfate soundness test (ASTM C 88)
  • Unconfined freeze-thaw test (CSA A23.2-24A)
  • Testing of aggregates in concrete (ASTM C 666)
  • Effective at preventing D-cracking, but does not
    model the real world very well

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Alkali-Silica Reactivity
  • First noted in the 1920s (we have looked at
    concrete from the 1890s)
  • Reaction between silica found in some aggregates
    and alkalis found in cement
  • The reaction produces a gel like substance called
    ASR gel, that imbibes water and swells
  • The swelling leads to expansion and cracking of
    the concrete
  • Commonly observed 5 to 15 years after
    construction, but may not be noted for 40 years

16
What is Required?
  • Reactive aggregate
  • Sufficient reactivity and quantity to cause
    damage
  • Sufficient alkalinity
  • High alkali cements, high alkali fly ash, and/or
    high cement contents
  • Water
  • Concrete needs to be near or at saturation

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Reactive Constituents
  • Opal, silica glass, chalcedony, cristobalite, and
    highly strained or microcrystalline quartz
  • Amorphous, poorly crystalline, or disrupted
    structure
  • Reactive minerals can be found in all types of
    rocks including carbonates
  • Most problems in Michigan associated with
    reactive chert in fine aggregate

22
How Do We Detect It?
  • Visual assessment
  • Not conclusive
  • Staining techniques
  • Uranyl acetate and sodium cobaltinitrite/rhodamine
    B for ASR
  • Not conclusive
  • Microscopic evaluation
  • Stereo
  • Petrographic
  • Scanning Electron

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Staining With Sodium Cobaltinitrite For ASR Gel
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Stereo Microscopy
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ASR affected aggregate before and after staining
With sodium cobaltinitrite and rhodamine B
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Petrographic Examination
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Thin Sections
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Thin Section
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The Environmental SEM
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Scanning Electron Microscope Tests
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ASR Mechanism
  • Formation of two-component gel
  • Non-swelling calcium-alkali-silicate-hydrate
    C-N(K)-S-H
  • Swelling alkali-silicate-hydrate N(K)-S-H
  • If both gels form, reaction is deleterious
  • The key factor appears to be the relative amounts
    of alkali and reactive silica

40
ASR Mechanism
  • In the presence of pore solution (H2O and Na,
    K, Ca2, OH-, and H3SiO4- ions), reactive silica
    undergoes depolymerization, dissolution, and
    swelling
  • Depends on alkalinity of solution, not on alkalis
    per se, although they control alkalinity
  • The greater the alkalinity, the more soluble the
    silica

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ASR Mechanism (Continued)
  • Alkali and calcium ions diffuse into swollen
    aggregates creating non-swelling C-N(K)-S-H,
    which is like C-S-H with some alkalis
  • The solubility of CH is inversely proportional to
    the alkali concentration
  • Pore solution diffuses through C-N(K)-S-H to the
    silica
  • Depending on concentration of alkalis and the
    rate of diffusion, results can be deleterious
  • If CaO concentration is 53 or more of anhydrous
    C-N(K)-S-H (weight basis), non-swelling gel will
    form
  • For high alkali concentration, solubility of CH
    is depressed, and swelling low to no calcium
    N(K)-S-H gel forms

56
ASR Mechanism (Continued)
  • N(K)-S-H is low viscosity, and would easily
    diffuse away if not for the C-N(K)-S-H
  • Together they form a viscous, composite gel with
    decreased porosity
  • N(K)-S-H attracts water due to osmosis,
    increasing volume and local tensile stresses,
    eventually leading to cracking

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Approach To Prevention
  • Selection of Constituent Materials
  • Aggregate
  • Cementitious materials
  • Admixture
  • Proportioning and Mix Design
  • Testing of Hardened Concrete

61
Aggregate Selection
  • Performance history
  • Petrographic evaluation (ASTM C 295)
  • Alkali-silica reactivity
  • Rapid mortar bar (ASTM C 1260)
  • Concrete prism (ASTM C 1293)
  • Reject or Mitigate If Aggregate Source Does Not
    Pass

62
Selection of Portland Cement
  • Modern cements are more finely ground, contain
    more C3S, and have higher alkali and sulfate
    content than those of the past
  • If ASR potential exists, limit total alkalinity
    of mixture
  • Low alkali cement (lt 0.6 percent Na2O eq.)
  • Total alkalis limited to 3 kg/m3
  • Cements blended with fly ash or ground granulated
    blast furnace slag can also be used

63
Supplementary Cementitious Material
  • Low calcium fly ash (commonly Class F) is in
    general more effective because the pozzolanic
    reaction converts CH to C-S-H, ties up alkalis,
    and make the paste less permeable
  • GGBFS also reduces permeability and CH
  • Silica fume not commonly used in pavements, but
    common in bridge decks, parking structures, etc.
  • High calcium fly ash (commonly Class C) less
    effective and may contribute significant alkalis
  • Use with caution if ASR is a problem

64
ASR Inhibiting Admixture
  • Lithium admixtures have been found to be very
    effective in mitigating ASR
  • Lithium nitrate is very effective and easy to
    handle
  • Problem is that they are relatively expensive and
    other mitigation methods may be more cost
    effective
  • Also they do not reduce permeability like
    supplementary cementitious materials

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Alkali-Carbonate Reactivity
  • More rare than ASR, involving dolomitic
    limestones containing unique characteristics
  • Very fine-grained dolomite
  • Considerable amounts of fine-grained calcite
  • Abundant interstitial clay
  • Dolomite and calcite crystals even dispersed in
    clay matrix
  • The rocks have a fine-grained texture and
    structure characterized by relatively larger,
    rhombic-shaped dolomite crystals CaMg(CO3)2 set
    in a finer-grained calcite matrix (Ca(CO3)),
    clay, and commonly silt-sized quartz
  • Not well understood

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ACR Reactions
  • Dedolimitization of dolomite in calcite and
    brucite
  • Alkali carbonate reacts with CH to regenerate
    alkali hydroxide

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ACR
  • Almost impossible to treat or mitigate
  • Very low alkali cement may help prevent it, but
    deicers will aggravate reaction
  • Avoid use of susceptible aggregates
  • Service records
  • Testing (ASTM C 586 and C 1105)
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