alkali-silica reaction: “the cancer of concrete”

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

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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?