ASU/ACS/99

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

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AnalysisLoads considered

Gravity Hydrostatic Pressure Temperature Seismic

Joint element incorporatedCollision between elements modeledAnalysis accounts for additional flexibility

provided by non-linear joints

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Alkali Silica Reaction

Reaction of Aggregates with CementCauses Extensive Cracking and

Fragmentation

Extensive testing is performed

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Results of testing analysis

Interior concrete was still strongNo further deterioration due to ASR expectedTotal replacement is not requiredEpoxy coated Post-tensioning

Best remedy for seismic safety Least expensive Used 62 cables, 22 wire 15.24mm diameter

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Modeling of the joint elements

Three Dimensional ElementAccount for the following effects

Friction Loss of contact between different pours Impact between disjointed elements Loss of joint material

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Joint types

Shear component of the joint force force is in the joint plane determined by frictional interaction

Normal componentNo inertial propertiesNonlinear (Piecewise linear) force-

displacement relationship

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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

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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

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

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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

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Expansion Mechanism

Breakdown of the silica structure by hydroxyl ions

Adsorption of alkali ions on new productThis alkali-silicate gel swells in presence of

water through the process of osmosis

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Case Histories

Buck Hydroelectric plant on New River (Virginia, US)

Arch dam in California crown deflection of 127 mm in 9 years

Railroad Canyon DamMorrow Point Dam, Colorado, USAStewart Mountain Dam, ArizonaParker Dam (Arizona)

expansion in excess of 0.1 percent

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Factors influencing the reaction

Alkali content of cement and other sourcesAmount, size and reactivity of alkali-

reactive material present in aggregateAvailability of moistureAmbient temperatureExpansive effects of MgO and CaO

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Measures for prevention

Low alkali content cement and mildly reactive aggregate

Sweetening of aggregate using limestoneControl of access of water to concreteReplacing part of cement by pozzolanic

admixturesMgO content should not exceed 6 percent

(ASTM C 150-83)

ASU/ACS/99

International Congress

Creating With Concrete

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:

University of DundeeDundee, Scotland, UK

ASU/ACS/99

REHABILITATION AND RETROFITTING OF AN ARCH DAM

By

Dr. Avinash C. Singhal

Arizona State University

Tempe, Arizona, USA

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Overview

IntroductionAlkali-Silica reaction and its effectsSeismic StudyCase 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 recognizedLocal seismicity was unknown

ASU/ACS/99

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

Stewart Mountain Dam