Comparison of alkali-silica chemical reaction of reactive ...
Alkali-Silica Reaction: “The Cancer of Concrete”
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Transcript of Alkali-Silica Reaction: “The Cancer of Concrete”
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Courtney Collins . Jason Ideker . Gayle Willis . Jessica Hurst
Alkali-Silica Reaction:“The Cancer of Concrete”
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• What is ASR and why is it important?
• How does ASR work?
• How can ASR damage be prevented?
Outline
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Alkali Silica Reaction (ASR)
Alkalis+
Reactive Silica
+Moisture
ASR Gel which
expands
Concrete expansion
andcracking
What is ASR?
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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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• 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 world’s CO2 emissions (a greenhouse gas)
Why is it important to study ASR?
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• Hydroelectric dam built in 1938• 180 mm of arch deflection due to alkali silica gel
expansion• Cracking and gel flow in concrete
Case Study: Parker Dam, California
http://www.acres.com/aar/Alkali-Aggregate Reactions in Hydroelectric Plants and Dams:
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• Possible ASR damage on concrete retaining wall - picture taken 1/2002
Case Study: I-85 - Atlanta, Georgia
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What we know:
What we don’t 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
• Mechanism of gel expansion• Lithium: it’s mechanism of inhibition, which
compounds work best, how much of each compound is needed to prevent expansion
How does ASR work?
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Creation of alkali-silica gel
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Reactants: alkalis, reactive silica, and water
Alkalis
Main cations: • Sodium (Na+) • Potassium (K+)
Common sources: • Portland cement• Deicing agents• Seawater
Creation of alkali-silica gel
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Reactive Silica
Silica tetrahedron:
Amorphous Silica Crystalline Silica
Creation of alkali-silica gel
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Reactive Silica
Creation of alkali-silica gel
Amorphous or disordered silica = most chemically reactive
Common reactive minerals: strained quartzopalobsidiancristobalitetridymitechelcedonychertscryptocrystalline volcanic rocks
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Water
Found in pore spaces in concrete
Sources: • Addition of water to concrete mixture• Moist environment/permeable concrete
Creation of alkali-silica gel
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1. Siliceous aggregate in solution
Creation of alkali-silica gel
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2. Surface of aggregate is attacked by OH-
H20 + Si-O-Si Si-OH…OH-Si
Creation of alkali-silica gel
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3. Silanol groups (Si-OH) on surface are broken down by OH- into SiO- molecules
Si-OH + OH- SiO- + H20
Creation of alkali-silica gel
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4. Released SiO- molecules attract alkali cations in pore solution, forming an alkali-silica gel around the aggregate.
Creation of alkali-silica gel
Si-OH + Na+ + OH- Si-O-Na + H20
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5. Alkali-silica gel takes in water, expanding and exerting an osmotic pressure against the surrounding paste or aggregate.
Creation of alkali-silica gel
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6. When the expansionary pressure exceeds the tensile strength of the concrete, the concrete cracks.
Creation of alkali-silica gel
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7. When cracks reach the surface of a structure, “map cracking” results. Other symptoms of ASR damage includes the presence of gel and staining.
Creation of alkali-silica gel
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8. Once ASR damage has begun:
Creation of alkali-silica gel
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
• Avoid high alkali content:– use low alkali portland cement: Na20eq < 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
Alkalis + Reactive Silica + Moisture ASR Gel
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ANY QUESTIONS?