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: kevernj@umkc.edu