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

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Presentation on theme: "Courtney Collins. Jason Ideker. Gayle Willis. Jessica Hurst Alkali-Silica Reaction: “The Cancer of Concrete”"— Presentation transcript:

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

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

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

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

5 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 CO 2 emissions (a greenhouse gas). Why is it important to study ASR?

6 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:

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

8 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 and non-reactive aggregate 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?

9 Creation of alkali-silica gel

10 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

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

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

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

14 1. Aggregate in solution, pre-ASR damage Creation of alkali-silica gel

15 2. Surface of aggregate is attacked by OH - H 2 0 + Si-O-SiSi-OH…OH-Si Creation of alkali-silica gel

16 3. Silanol groups (Si-OH) on surface are broken down by OH - into SiO - molecules Si-OH + OH - SiO - + H 2 0 Creation of alkali-silica gel

17 4. Released SiO - molecules attract alkali cations in pore solution, forming a gel around the aggregate Creation of alkali-silica gel

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

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

20 7. When cracks reach the surface of the structure, “map cracking” results Creation of alkali-silica gel

21 Images of ASR damage

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24 How to prevent ASR damage

25 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 Alkalis + Reactive Silica + Moisture ASR Gel

26 ANY QUESTIONS?


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