Alkali-Silica Reaction:

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Alkali-Silica ReactionThe Cancer of Concrete
Courtney Collins . Jason Ideker . Gayle
Willis . Jessica Hurst
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Outline

• What is ASR and why is it important?
• How does ASR work?
• How can ASR damage be prevented?

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What is ASR?
Alkali Silica Reaction (ASR)
Alkalis Reactive Silica Moisture
Concrete expansion and cracking
ASR Gel which expands
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Concrete failure due to ASR
AASHTO Innovative Highway Technologies
AASHTO Innovative Highway Technologies
AASHTO Innovative Highway Technologies
Georgia Tech School of CEE - Courtney Collins
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Why is it important to study ASR?

• Concrete quality
• Loss of strength, stiffness, impermeability
• Premature failure of concrete structures
• Economic/Environmental impacts
• ASR decreases concrete service life
• Reconstruction has both environmental and
  economic impacts. ex. cement production produces
  7 of the worlds CO2 emissions (a greenhouse
  gas)

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Case Study Parker Dam, California
http//www.acres.com/aar/
Alkali-Aggregate Reactions in Hydroelectric
Plants and Dams

• Hydroelectric dam built in 1938
• 180 mm of arch deflection due to alkali silica
  gel expansion
• Cracking and gel flow in concrete

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Case Study I-85 - Atlanta, Georgia

• Possible ASR damage on concrete retaining wall -
  picture taken 1/2002

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How does ASR work?
What we know

• Which reactants involved and their sources
• How alkali-silica gel is created
• ASR prevention can be achieved by using low
  alkali cement, non-reactive aggregate, and
  concrete with low permeability
• Additives such as lithium compounds and
  pozzolanic material help prevent ASR damage

What we dont know

• Mechanism of gel expansion
• Lithium its mechanism of inhibition, which
  compounds work best, how much of each compound is
  needed to prevent expansion

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Creation of alkali-silica gel
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Creation of alkali-silica gel
Reactants alkalis, reactive silica, and water
Alkalis

• Main cations
• Sodium (Na)
• Potassium (K)
• Common sources
• Portland cement
• Deicing agents
• Seawater

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Creation of alkali-silica gel
Reactive Silica
Silica tetrahedron
Amorphous Silica
Crystalline Silica
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Creation of alkali-silica gel
Reactive Silica
Amorphous or disordered silica most chemically
reactive Common reactive minerals strained
quartz opal obsidian cristobalite tridymite c
helcedony cherts cryptocrystalline volcanic
rocks
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Creation of alkali-silica gel
Water

• Found in pore spaces in concrete
• Sources
• Addition of water to concrete mixture
• Moist environment/permeable concrete

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Creation of alkali-silica gel
1. Siliceous aggregate in solution
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Creation of alkali-silica gel
2. Surface of aggregate is attacked by OH-
H20 Si-O-Si
Si-OHOH-Si
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Creation of alkali-silica gel
3. Silanol groups (Si-OH) on surface are broken
down by OH- into SiO- molecules
Si-OH OH-
SiO- H20
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Creation of alkali-silica gel
4. Released SiO- molecules attract alkali
cations in pore solution, forming an
alkali-silica gel around the aggregate.
Si-OH Na OH-
Si-O-Na H20
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Creation of alkali-silica gel
5. Alkali-silica gel takes in water, expanding
and exerting an osmotic pressure against the
surrounding paste or aggregate.
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Creation of alkali-silica gel
6. When the expansionary pressure exceeds
the tensile strength of the concrete, the
concrete cracks.
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Creation of alkali-silica gel
7. When cracks reach the surface of a structure,
map cracking results. Other symptoms of
ASR damage includes the presence of gel
and staining.
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Creation of alkali-silica gel
8. Once ASR damage has begun
Expansion and cracking of concrete
Increased permeability
More water and external alkalis penetrate concrete
Increased ASR damage
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Images of ASR damage
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Images of ASR damage
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Images of ASR damage
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How to prevent ASR damage
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How to prevent ASR damage
Alkalis Reactive Silica Moisture
ASR Gel

• Avoid high alkali content
• use low alkali portland cement Na20eq lt 0.69
• replace cement with low alkali mineral admixtures
• Avoid reactive aggregate (amorphous silica)
• Control access to water use low water to cement
  ratio, monitor curing conditions, use admixtures
  to minimize water contact.
• Use lithium additives prior to placement of
  concrete or as a treatment in already existing
  concrete

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ANY QUESTIONS?