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 and cracking What is ASR?

Concrete failure due to ASR AASHTO Innovative Highway Technologies 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 CO 2 emissions (a greenhouse gas) Why is it important to study ASR?

Hydroelectric dam built in mm of arch deflection due to alkali silica gel expansion Cracking and gel flow in concrete Case Study: Parker Dam, California 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 quartz opal obsidian cristobalite tridymite chelcedony cherts cryptocrystalline 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 - H Si-O-SiSi-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 - + H 2 0 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 + H 2 0

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

How to prevent ASR damage

Avoid high alkali content: –use low alkali portland cement: Na 2 0eq < 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?