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Alkali-Silica Reaction: “The Cancer of Concrete”

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

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Alkali-Silica Reaction: “The Cancer of Concrete”

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  1. Alkali-Silica Reaction:“The Cancer of Concrete” Courtney Collins . Jason Ideker . Gayle Willis . Jessica Hurst

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

  3. What is ASR? Alkali Silica Reaction (ASR) Alkalis + Reactive Silica + Moisture Concrete expansion and cracking ASR Gel which expands

  4. Concrete failure due to ASR AASHTO Innovative Highway Technologies AASHTO Innovative Highway Technologies AASHTO Innovative Highway Technologies Georgia Tech School of CEE - Courtney Collins

  5. 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 world’s CO2 emissions (a greenhouse gas)

  6. 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

  7. Case Study: I-85 - Atlanta, Georgia • Possible ASR damage on concrete retaining wall - picture taken 1/2002

  8. 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 How does ASR work? What we know: 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

  9. Creation of alkali-silica gel

  10. 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

  11. Creation of alkali-silica gel Reactive Silica Silica tetrahedron: Amorphous Silica Crystalline Silica

  12. Creation of alkali-silica gel Reactive Silica Amorphous or disordered silica = most chemically reactive Common reactive minerals: strained quartz opal obsidian cristobalite tridymite chelcedony cherts cryptocrystalline volcanic rocks

  13. Creation of alkali-silica gel Water • Found in pore spaces in concrete • Sources: • Addition of water to concrete mixture • Moist environment/permeable concrete

  14. Creation of alkali-silica gel 1. Siliceous aggregate in solution

  15. Creation of alkali-silica gel 2. Surface of aggregate is attacked by OH- H20 + Si-O-Si Si-OH…OH-Si

  16. 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

  17. 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

  18. Creation of alkali-silica gel 5. Alkali-silica gel takes in water, expanding and exerting an osmotic pressure against the surrounding paste or aggregate.

  19. Creation of alkali-silica gel 6. When the expansionary pressure exceeds the tensile strength of the concrete, the concrete cracks.

  20. 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.

  21. 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

  22. Images of ASR damage

  23. Images of ASR damage

  24. Images of ASR damage

  25. How to prevent ASR damage

  26. How to prevent ASR damage Alkalis + Reactive Silica + Moisture ASR Gel • 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

  27. ANY QUESTIONS?

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