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A Concrete Arch Dam in Arizona (USA)

A Concrete Arch Dam in Arizona (USA). Stewart Mountain Dam Deterioration of Dam Analysis: Unsafe under Earthquake load Measures: Complete replacement Epoxy coated Post-tensioning. Analysis. Loads considered Gravity Hydrostatic Pressure Temperature Seismic Joint element incorporated

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A Concrete Arch Dam in Arizona (USA)

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  1. A Concrete Arch Dam in Arizona (USA) • Stewart Mountain Dam • Deterioration of Dam • Analysis: Unsafe under Earthquake load • Measures: • Complete replacement • Epoxy coated Post-tensioning ASU/ACS/99

  2. Analysis • Loads considered • Gravity • Hydrostatic Pressure • Temperature • Seismic • Joint element incorporated • Collision between elements modeled • Analysis accounts for additional flexibility provided by non-linear joints ASU/ACS/99

  3. Alkali Silica Reaction • Reaction of Aggregates with Cement • Causes Extensive Cracking and Fragmentation • Extensive testing is performed ASU/ACS/99

  4. Resultsof testing analysis • Interior concrete was still strong • No further deterioration due to ASR expected • Total replacement is not required • Epoxy coated Post-tensioning • Best remedy for seismic safety • Least expensive • Used 62 cables, 22 wire 15.24mm diameter ASU/ACS/99

  5. Modeling of the joint elements • Three Dimensional Element • Account for the following effects • Friction • Loss of contact between different pours • Impact between disjointed elements • Loss of joint material ASU/ACS/99

  6. Joint types • Shear component of the joint force • force is in the joint plane • determined by frictional interaction • Normal component • No inertial properties • Nonlinear (Piecewise linear) force-displacement relationship ASU/ACS/99

  7. Alkali Silica Reaction • Reaction of alkali ions present in Portland Cement and siliceous material in aggregates in the presence on hydroxyl ions • Leads to expansion, cracking, loss of strength, durability and elasticity • Cause of distress for structures exposed to humid environment ASU/ACS/99

  8. Chemistry of Alkali Silica Reaction • Cement production involves raw materials that contain alkalis in the range of 0.2 to 1.5 percent of Na2O • This generates a pore fluid with high pH (12.5 to 13.5) • Strong alkalinity causes the acidic siliceous material to react ASU/ACS/99

  9. ASTM specification • ASTM C150 designates cements with more than 0.6 percent of Na2O as high-alkali cements • Even with low alkali content, but sufficient amount of cement, alkali-silica reactions can occur • Investigations show that if total alkali content is less than 3 kg/m3, alkali-silica reactions will not occur ASU/ACS/99

  10. Contribution of Calcium Hydroxide • Ca(OH)2 is present in sizable proportions in cement • Even if alkali content is small, there is a chance of alkali-silica reaction due to • alkaline admixtures • aggregates that are contaminated • penetration of seawater • deicing solutions ASU/ACS/99

  11. Expansion Mechanism • Breakdown of the silica structure by hydroxyl ions • Adsorption of alkali ions on new product • This alkali-silicate gel swells in presence of water through the process of osmosis ASU/ACS/99

  12. Case Histories • Buck Hydroelectric plant on New River (Virginia, US) • Arch dam in California • crown deflection of 127 mm in 9 years • Railroad Canyon Dam • Morrow Point Dam, Colorado, USA • Stewart Mountain Dam, Arizona • Parker Dam (Arizona) • expansion in excess of 0.1 percent ASU/ACS/99

  13. Factors influencing the reaction • Alkali content of cement and other sources • Amount, size and reactivity of alkali-reactive material present in aggregate • Availability of moisture • Ambient temperature • Expansive effects of MgO and CaO ASU/ACS/99

  14. Measures for prevention • Low alkali content cement and mildly reactive aggregate • Sweetening of aggregate using limestone • Control of access of water to concrete • Replacing part of cement by pozzolanic admixtures • MgO content should not exceed 6 percent (ASTM C 150-83) ASU/ACS/99

  15. Concrete Durability and Repair Technology Repair Materials and Methods Thursday 9 September, 1999; 14:00-17:30 G.G.T. Masterton and M. Walker Conference 5: Theme 4: Date: Chair: International Congress Creating With Concrete University of Dundee Dundee, Scotland, UK ASU/ACS/99

  16. REHABILITATION AND RETROFITTING OF AN ARCH DAM By Dr. Avinash C. Singhal Arizona State University Tempe, Arizona, USA ASU/ACS/99

  17. Overview • Introduction • Alkali-Silica reaction and its effects • Seismic Study • Case Study: Stewart Mountain Dam • Problems encountered • Remedial measures • Analysis • Post-tensioning of dam structure ASU/ACS/99

  18. Dam deterioration • Bond within dam structure was not intact • Caused due to formation of laitance • Cleaning of horizontal construction surfaces was not recognized • 13 out of 16 joints unbonded (core-drilling) • Alkali-silica reaction was not recognized • Local seismicity was unknown ASU/ACS/99

  19. Stewart Mountain Dam • Located fifty miles east of Phoenix, Arizona on the Salt River • Double curvature arch dam • 64.6 m high • 2.44 m thick across the crest • 10.36 m thick across the base • 177.7 m long along the crest ASU/ACS/99

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