Transcript Research Tutorial - Georgia Institute of Technology

Alkali-Silica Reaction:
“The Cancer of Concrete”
Courtney Collins . Jason Ideker . Gayle Willis . Jessica Hurst
Outline
• What is ASR and why is it important?
• How does ASR work?
• How can ASR damage be prevented?
What is ASR?
Alkali Silica Reaction (ASR)
Alkalis
+
Reactive
Silica
+
Moisture
ASR Gel
which
expands
Concrete
cracking
Concrete failure due to ASR
AASHTO Innovative Highway Technologies
AASHTO Innovative Highway Technologies
AASHTO Innovative Highway Technologies
Georgia Tech School of CEE - Courtney Collins
Why is it important to study ASR?
• Concrete quality
• Loss of strength, stiffness, impermeability
• Premature failure of concrete structures
• Economic/Environmental impacts
• ASR lowers concrete lifetime
• Less reactive aggregates often expensive or
difficult to find
• Cement production creates 7% of the world’s
CO2 emissions (a greenhouse gas).
Case Study: Parker Dam, California
Alkali-Aggregate Reactions in Hydroelectric Plants and Dams: http://www.acres.com/aar/
• Hydroelectric dam built in 1938
• 180 mm of arch deflection due to alkali silica gel
expansion
• Cracking and gel flow in concrete
Case Study: I-85 - Atlanta, Georgia
• Possible ASR damage on concrete retaining wall picture taken 1/2002
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 and non-reactive aggregate
• Additives such as lithium compounds and
pozzolanic material help prevent ASR damage
What we don’t know:
• Mechanism of gel expansion
• Lithium: it’s mechanism of inhibition, which
compounds work best, how much of each
compound is needed to prevent expansion
Creation of alkali-silica gel
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
Amorphous silica = most chemically reactive
Common reactive rocks:
opal
obsidian
cristobalite
tridymite
chelcedony
cherts
cryptocrystalline volcanic rocks
strained quartz
Creation of alkali-silica gel
Water
Found in pore spaces in concrete
Sources:
• Addition of water to concrete mixture
• Moist environment/permeable concrete
Creation of alkali-silica gel
1. Aggregate in solution, pre-ASR damage
Creation of alkali-silica gel
2. Surface of aggregate is attacked by OHH20 + 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 a gel around the
aggregate
Creation of alkali-silica gel
5. Alkali-silica gel takes in water, expanding and
exerting a force against surrounding concrete.
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 the structure,
“map cracking” results
Images of ASR damage
Images of ASR damage
Images of ASR damage
How to prevent ASR damage
How to prevent ASR damage
Alkalis + Reactive Silica + Moisture
ASR Gel
• Avoid high alkali content:
– use low alkali portland cement
– replace cement with pozzolanic admixtures
• Avoid reactive aggregate (amorphous silica)
• Control access to water
• Use lithium additives prior to placement of concrete
or as a treatment in already existing concrete
ANY QUESTIONS?