Presentation on theme: "Courtney Collins. Jason Ideker. Gayle Willis. Jessica Hurst Alkali-Silica Reaction: “The Cancer of Concrete”"— Presentation transcript:
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 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 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?
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 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?
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 silica = most chemically reactive Common reactive rocks: opal obsidian cristobalite tridymite chelcedony cherts cryptocrystalline volcanic rocks strained quartz
Water Found in pore spaces in concrete Sources: Addition of water to concrete mixture Moist environment/permeable concrete Creation of alkali-silica gel
1. Aggregate in solution, pre-ASR damage Creation of alkali-silica gel
2. Surface of aggregate is attacked by OH - H 2 0 + 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 a gel around the aggregate Creation of alkali-silica gel
5. Alkali-silica gel takes in water, expanding and exerting a force against surrounding concrete. 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 the structure, “map cracking” results Creation of alkali-silica gel
Images of ASR damage
How to prevent ASR damage
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