ASR

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ASR
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Subject Alkali-Aggregate Reactivity

• Certain constituents in aggregates can react
  harmfully with alkali hydroxides in concrete and
  cause significant expansion. There are two forms
  of this reaction
• Alkali silica reaction (ASR)
• Alkali-carbonate reaction (ACR)
• Alkali silica reaction (ASR)
• Develops by aggregates containing reactive silica
  minerals. This form is more serious and common
  than ACR.

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ASR

• ASR has been recognized as a potential source of
  distress in concrete since the late 1930s

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• Alkali carbonate reaction (ACR)
• The aggregates dolomitic (calcium-magnesium
  carbonate) have specific composition that is not
  very common.

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Alkali silica reaction (ASR)

• Mechanism
• The reaction can be visualized as a two-step
  process
• Alkali hydroxide reactive silica gel ?
  alkali-silica gel
• Alkali-silica gel moisture ? expansion

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Alkali silica reaction (ASR)

• The amount of gel formed in the concrete depends
  on
• Amount of and type of silica in aggregate.
• Alkali hydroxide concentration.
• Sufficient moisture.

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Alkali silica reaction (ASR)

• The ASR forms a gel that swells as it draws water
  from the surrounding cement paste (has great
  affinity to moisture). In absorbing water, these
  gels can induce pressure, expansion, and cracking
  of the aggregate and the surrounding paste.
• The alkali silica gels will fill the microcracked
  regions both within the aggregate and concrete.
  Continued availability of moisture to the
  concrete causes enlargement and extension of the
  microcracks which eventually reach the outer
  surface of the concrete. The crack pattern is
  irregular and referred to as map cracking (see
  Figure 5-20). Or fragments breaking out of the
  surface of the concrete (popouts) as in Figure
  5-21.

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Popouts
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Alkali silica reaction (ASR)

• List of most reactive substances
• Opal (SiO2 nH2O)
• Chalcedony (SiO2)
• Certain forms of quartz (SiO2)
• Cristobalite (SiO2)

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Alkali silica reaction (ASR)

• The most important harmful alkali reactive
  aggregates
• Opaline cherts
• Chalcedonic cherts
• Siliceous limestones
• Siliceous dolomite

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Alkali silica reaction (ASR)

• Identification of Potentially Reactive
  Aggregates
• Field performance history of structures in
  service for more than 15 years.
• Different tests can be conducted for initial
  screening and evaluating potential alkali-silica
  reactivity.

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Alkali silica reaction (ASR)

• Control of ASR
• Use of low-alkali Portland cement (less than 0.6
  equivalent Na2O) when alkali-silica reactive
  constituents are suspected to be present in the
  aggregate.
• If low-alkali cement is not available, the total
  alkali content can be reduced by replacing a part
  of high-alkali cement with supplementary
  cementitious materials such fly ash, ground blast
  furnace slag, and silica fume, or use blended
  cement.

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Alkali silica reaction (ASR)

• Control of ASR
• Wash beach sand and gravel with sweet water to
  insure that the total alkali content from the
  cement and aggregates in concrete does not exceed
  3 kg/m3.
• Control the access of water to concrete.
• Replacing 25 - 30 of the reactive sand gravel
  aggregate with crushed limestone (known as
  limestone sweetening).

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Alkali silica reaction (ASR)

• Utilization of silica fume, fly ash, and blast
  furnace slag as partial replacement of cement
  will reduce the expansion as shown in Figure
  5-23.

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