Alkali-Silica Reaction: “The Cancer of Concrete”

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 ASR Gel which expands Concrete cracking

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 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).

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 and non-reactive aggregate 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 silica = most chemically reactive Common reactive rocks: opal obsidian cristobalite tridymite chelcedony cherts cryptocrystalline volcanic rocks strained quartz

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. Aggregate in solution, pre-ASR damage

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 a gel around the aggregate

18. Creation of alkali-silica gel 5. Alkali-silica gel takes in water, expanding and exerting a force against surrounding concrete.

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 the structure, “map cracking” results

21. Images of ASR damage

22. Images of ASR damage

23. Images of ASR damage

24. How to prevent ASR damage

25. 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

26. ANY QUESTIONS?