The document summarizes research on the effect and optimization of silica fume in pervious concrete. Phase 1 found that 5% silica fume optimized workability by increasing yield stress and viscosity in a linear fashion. Phase 2 showed that 5% silica fume improved workability, tensile strength, and freeze-thaw durability when voids were allowed to vary. Phase 3 found that when voids were fixed at 20%, 3-5% silica fume provided the greatest improvement in compressive strength and bond between paste and aggregate.

1. Effect and Optimization of Silica
Fume in Pervious Concrete
John Kevern and Tony Kojundic
Pervious in Paradise 2014

2. Motivation
• Anecdotal evidence
suggests that silica is
beneficial for pervious
concrete workability and
durability

3. Project Tasks
Phase 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

4. 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

5. 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)

6. Phase 1 - Results
• Once completely mixed, paste properties held for 75 minutes

7. 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.

8. 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

9. Effect of Dosage on Voids
• Additional workability was observed at 5%

10. Effect of Dosage on Strength
• Higher tensile strength was observed at 5%

11. Effect of Dosage on Freeze-Thaw Durability
• Increased durability was observed at 5%

12. 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

13. 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%

14. Effect of Dosage Rate
• Washed – 9% improvement at 5% SF
• Unwashed – 15% to 18% improvement at 3-5% SF

15. Effect of Dosage Rate
• Washed – 11% improvement at 3% SF
• Unwashed – 17% to 22% improvement at 3-5% SF

16. Abrasion (ASTM C944)
?

17. Abrasion (C1747)

18. Interfacial Transition Zone
Paste/agg ITZ failure Agg ITZ failure

19. Freeze Thaw
SF does not improve
bad aggregate

20. 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

21. 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

22. 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

23. 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: kevernj@umkc.edu