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Effect and Optimization of Silica
Fume in Pervious Concrete
John Kevern and Tony Kojundic
Pervious in Paradise 2014
Motivation• Anecdotal evidence
suggests that silica is
beneficial for pervious
concrete workability and
durability
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
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
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)
Phase 1 - Results
• Once completely mixed, paste properties held for 75 minutes
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.
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
Effect of Dosage on Voids
• Additional workability was observed at 5%
Effect of Dosage on Strength
• Higher tensile strength was observed at 5%
Effect of Dosage on Freeze-Thaw Durability
• Increased durability was observed at 5%
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
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%
Effect of Dosage Rate
• Washed – 9% improvement at 5% SF
• Unwashed – 15% to 18% improvement at 3-5% SF
Effect of Dosage Rate
• Washed – 11% improvement at 3% SF
• Unwashed – 17% to 22% improvement at 3-5% SF
Abrasion (ASTM C944)
?
Abrasion (C1747)
Interfacial Transition Zone
Paste/agg ITZ failure Agg ITZ failure
Freeze Thaw
SF does not improve
bad aggregate
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
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
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
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]