Courtney Collins . Jason Ideker . Gayle Willis . Jessica Hurst
Alkali-Silica Reaction:“The Cancer of Concrete”
• What is ASR and why is it important?
• How does ASR work?
• How can ASR damage be prevented?
Outline
Alkali Silica Reaction (ASR)
Alkalis+
Reactive Silica
+Moisture
ASR Gel which
expands
Concrete expansion
andcracking
What is ASR?
Concrete failure due to ASR
AASHTO Innovative Highway Technologies
AASHTO Innovative Highway Technologies AASHTO Innovative Highway Technologies
Georgia Tech School of CEE - Courtney Collins
• 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?
• 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:
• Possible ASR damage on concrete retaining wall - picture taken 1/2002
Case Study: I-85 - Atlanta, Georgia
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?
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
Creation of alkali-silica gel
Reactive Silica
Silica tetrahedron:
Amorphous Silica Crystalline Silica
Creation of alkali-silica gel
Reactive Silica
Creation of alkali-silica gel
Amorphous or disordered silica = most chemically reactive
Common reactive minerals: strained quartzopalobsidiancristobalitetridymitechelcedonychertscryptocrystalline volcanic rocks
Water
Found in pore spaces in concrete
Sources: • Addition of water to concrete mixture• Moist environment/permeable concrete
Creation of alkali-silica gel
1. Siliceous aggregate in solution
Creation of alkali-silica gel
2. Surface of aggregate is attacked by OH-
H20 + Si-O-Si Si-OH…OH-Si
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
Creation of alkali-silica gel
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
5. Alkali-silica gel takes in water, expanding and exerting an osmotic pressure against the surrounding paste or aggregate.
Creation of alkali-silica gel
6. When the expansionary pressure exceeds the tensile strength of the concrete, the concrete cracks.
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.
Creation of alkali-silica gel
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
Images of ASR damage
Images of ASR damage
Images of ASR damage
How to prevent ASR damage
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
ANY QUESTIONS?