Aggregates

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Aggregates
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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

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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

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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

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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

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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

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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

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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

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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

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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)