Effect and Optimization of Silica Fume in Pervious Concrete
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Transcript of Effect and Optimization of Silica Fume in Pervious Concrete
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Effect and Optimization of Silica
Fume in Pervious Concrete
John Kevern and Tony Kojundic
Pervious in Paradise 2014
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Motivation• Anecdotal evidence
suggests that silica is
beneficial for pervious
concrete workability and
durability
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Project TasksPhase 1) Dosage optimization based on rheology
Phase 2) Effect on properties and durability using
independent void samples
Phase 3) Effect on properties and durability using fixed
void samples
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Phase 1 - Rheology• The properties of the film
of paste surrounding the
pervious concrete
aggregate highly
influences workability.
• The capillary paste
strongly influences
strength and draindown
• Both are described using
rheological tests and
models
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Phase 1 - Rheology• Rheology – Study of the flow of matter in
a liquid state
• Yield stress – Energy required to get a
fluid moving (controls the paste thickness
around the aggregate)
• Viscosity – Resistance of the fluid to
continued moving (controls the workability
of a pervious mixture)
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Phase 1 - Results
• Once completely mixed, paste properties held for 75 minutes
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Phase 1 - Results
• Yield stress increases linearly with dosage rate.
• Viscosity increases in a bi-linear fashion with the pivot at or around 5%.
• 5% was selected as the optimum dosage.
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Phase 2 – Fixed Compaction Energy,
Variable Voids
- Rounded river gravel aggregate to highlight
void changes
- 5% sand
- 564pcy cementitious
- 0.32 w/cm
– HRWR 5 oz/cwt
– Hyd. Stab 4 oz/cwt
– AEA 1 oz/cwt
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Effect of Dosage on Voids
• Additional workability was observed at 5%
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Effect of Dosage on Strength
• Higher tensile strength was observed at 5%
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Effect of Dosage on Freeze-Thaw Durability
• Increased durability was observed at 5%
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Phase 3 – Fixed Voids @ 20%• Aggregates
– Mixes with both limestone and river gravel
• Cementitious Materials
– 600 pcy
• w/cm
– 0.32
• Admixtures
– HRWR 5 oz/cwt
– Hyd. Stab 4 oz/cwt
– AEA 1 oz/cwt
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Phase 3 – Fixed Voids @ 20%• All fresh sample materials weighed prior
to placement so everything had
statistically the same voids
• Tested voids (ASTM C1754) for all mixes
ranged from 19.6% to 21.4%
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Effect of Dosage Rate
• Washed – 9% improvement at 5% SF
• Unwashed – 15% to 18% improvement at 3-5% SF
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Effect of Dosage Rate
• Washed – 11% improvement at 3% SF
• Unwashed – 17% to 22% improvement at 3-5% SF
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Abrasion (ASTM C944)
?
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Abrasion (C1747)
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Interfacial Transition Zone
Paste/agg ITZ failure Agg ITZ failure
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Freeze Thaw
SF does not improve
bad aggregate
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Summary of Findings
• Rheology testing indicates that 5% is the
optimum dosage
• At higher void contents (>25%) the effect
of silica fume not as pronounced as at
lower (20%) voids
• When void ratio was not fixed, 5%
showed improved workability, tensile
strength, and freeze-thaw durability
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Summary of Findings
• When void ratio was fixed at 20%, 3-5%
showed the greatest improvement in
compressive strength
• Greater increases in strength were
observed for the unwashed samples,
indicating that the silica fume is able to
improve the paste-to-aggregate bond
when conditions are not ideal
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Summary of Findings
• Surface abrasion improved for unwashed
limestone, washed limestone or gravel
• Raveling resistance improved for gravel,
no difference for limestone
• ITZ was improved for gravel
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Contact Information
John T. Kevern, PhD, PE, LEED AP
Associate Professor of Civil Engineering
370A Flarsheim Hall, 5110 Rockhill Rd.
University of Missouri – Kansas City
Kansas City, MO 64110
Phone: 816-235-5977
Fax: 816-235-1260
Email: [email protected]