A%20Concrete%20Arch%20Dam%20in%20Arizona%20(USA)

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

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

3
Alkali Silica Reaction

• Reaction of Aggregates with Cement
• Causes Extensive Cracking and Fragmentation
• Extensive testing is performed

4
Results of 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

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

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

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

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

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

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

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

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

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

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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)

15

• International Congress
• Creating With Concrete

University of Dundee Dundee, Scotland, UK
16

• REHABILITATION AND RETROFITTING OF AN ARCH DAM
• By
• Dr. Avinash C. Singhal
• Arizona State University
• Tempe, Arizona, USA

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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

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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

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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