Thixotropy of SCC and Its Effects on Formwork Pressure

Eric P. Koehler, Ph.D. Vice President

Verifi

eric.koehler@verifitechnologies.com

ACI Spring 2013

Convention

Minneapolis,

Minnesota

April 14,

2013

SCC would be impossible without thixotropy.

Due to thixotropy, concrete exhibits higher viscosity

when at rest than when flowing.

2

Flows Under Its

Own Mass

When SCC is flowing, a

low viscosity means

minimal resistance to flow.

Formwork

Pressure

When SCC is at rest, a

high viscosity reduces

formwork pressure…

Segregation

Resistance

…and prevents particles

from settling.

+ +

Outline

3

01 What is Thixotropy?

02 Measurement of Thixotropy

03 Role of Thixotropy in Formwork Pressure

04 Field Data

05 Conclusions

What is Thixotropy?

4

Thixotropy is the reversible, isothermal, time-dependent

decrease in viscosity when a fluid is subjected to increased

shear stress or shear rate.

Reversible

Viscosity will increase to

its original value when

the shear stress or shear

rate is decreased to its

original value.

Isothermal

Independent from

changes in viscosity due

to changes in

temperature.

Time-Dependent

Change in viscosity

occurs over a period of

time, rather than

instantaneously.

What is Thixotropy?

5

Shear Rate,

m = plastic viscosity

1

Bingham Model t = t0 + m

.

.

Sh

ear

Str

ess,

t

t0 = dynamic

yield stress

What is Thixotropy?

6

Shear Rate,

1

Bingham Model t = t0 + m

.

.

happ = apparent viscosity

Sh

ear

Str

ess,

t

t0 = dynamic

yield stress

apparent viscosity is used in the definition of thixotropy

What is Thixotropy?

7

Shear Rate,

m = plastic viscosity

1

.

Sh

ear

Str

ess,

t

t0 = dynamic

yield stress

t0(S) = static

yield stress

Up Curve

from rest

Down Curve

after breakdown of

thixotropic structure

What is Thixotropy?

8

Shear Rate,

m = plastic viscosity

1

.

Sh

ear

Str

ess,

t

t0 = dynamic

yield stress

t0(S) = static

yield stress

Up Curve

from rest

Thixotropy

Down Curve

after breakdown of

thixotropic structure

Static Yield Stress Minimum shear stress to

initiate flow from rest

Dynamic Yield Stress Minimum shear stress to

maintain flow

Plastic Viscosity Change in shear stress per change in

shear rate, above dynamic yield stress

9

What is Thixotropy?

A

B C

D

Thixotropy Should Not Be Confused With…

10

Hydration

An increase in viscosity

due to hydration. Not

reversible.

Segregation

An change in viscosity

due to a denser or less

dense concentration of

aggregates.

Shear Thinning

A decrease in viscosity

with increased shear

rate. Not time-

dependent.

Shear Rate

Sh

ea

r S

tre

ss

Why is Concrete Thixotropic?

11

At Rest

Flowing

Cement particles flocculate

to form a three-dimensional,

networked structure

van der Waals attraction

Brownian motion

An equilibrium is achieved

between:

Shear breaking apart flocs

Flocculation between

contacting cement particles

Viscosity

Viscosity

Outline

12

01 What is Thixotropy?

02 Measurement of Thixotropy

03 Role of Thixotropy in Formwork Pressure

04 Field Data

05 Conclusions

Measurement of Thixotropy

Hysteresis Loop Test

Constant Shear Rate Test

Stress Growth Test

13

These tests

make use of

a rotational

rheometer

for concrete

In a rheometer…

1) Maintain concrete initially at rest.

2) Increase shear rate from zero to

maximum.

3) Hold shear rate at maximum to

fully break down thixotropic

structure.

4) Decrease shear rate from

maximum to zero.

5) Calculate area between up and

down curves.

Hysteresis Loop Test

14

Shear Rate, .

Sh

ear

Str

ess,

t

Up Curve

from rest

Down Curve

after breakdown of

thixotropic structure

Thixotropy

Area = Thixotropy

Constant Shear Rate Test

15

Time

Sh

ear

Str

ess,

t

In a rheometer…

1) Maintain concrete initially at rest.

2) Apply a constant shear rate.

3) Measure the change in shear

stress over time.

4) Compare the difference in shear

stress (or apparent viscosity)

initially and at equilibrium.

tini = initial shear stress

teq = equilibrium shear stress

Difference in shear stress = Thixotropy

Stress Growth Test

16

Time

Static Yield Stress

maximum stress from rest

In a rheometer…

1) Maintain concrete initially at rest.

2) Apply shear at a low, constant

rate.

3) Measure increase in torque.

4) Record maximum torque.

5) Convert torque to shear stress

to calculate static yield stress.

zone

sheared

Difference between static and dynamic yield stress = Thixotropy

Sh

ear

Str

ess,

t

Measuring Thixotropy

17

CAUTION Results are highly dependent on the test protocol.

Consistent sample preparation

Consistent rest period before test

Consistent shearing regime

Outline

18

01 What is Thixotropy?

02 Measurement of Thixotropy

03 Role of Thixotropy in Formwork Pressure

04 Field Data

05 Conclusions

19

Role of Thixotropy in Formwork Pressure

Formwork pressure is related to concrete rheology

Pressure increases with slump (or slump flow)

Concrete is at rest in forms; therefore, static yield stress is relevant

Static yield stress is affected by dynamic yield stress and thixotropy

SCC must be designed to flow under its own mass and exert low formwork pressure

Low dynamic yield stress (self flow)

Fast increase in static yield stress due to thixotropy (reduced formwork pressure)

20

Thixotropy in a Concrete Delivery and Placement

Change in yield stress from mixing through delivery and placement

Dynamic Yield Stress Full Breakdown of

Thixotropic Structure

Static Yield Stress of Non-Agitated SCC

No Breakdown of Thixotropic Structure

Time from Mixing

Yie

ld S

tress

Concrete is in formwork; at

rest structure rebuilds as

static yield stress increases

Concrete is discharged into

forms; resulting shearing causes

breakdown of at-rest structure

Concrete is partially agitated

during transit, preventing full

build-up of at rest structure

Static Yield Stress

of SCC During

Placement

Effects of Rheology on SCC Formwork Pressure

21

CAUTION Thixotropy is needed, but should not be too high.

x Cold joints

x Poor pumpability

x Restarting placement after rest (for example, bucket

placement)

Outline

22

01 What is Thixotropy?

02 Measurement of Thixotropy

03 Role of Thixotropy in Formwork Pressure

04 Field Data

05 Conclusions

23

Effects of Rheology on SCC Formwork Pressure

Reference: Koehler, E.P., Keller, L., and Gardner, N.J. (2007). “Field Measurements of

SCC Rheology and Formwork Pressure” Proceedings of SCC 2007, Ghent, Belgium

0

100

200

300

400

500

600

0 20 40 60 80 100 120

Time from Placement, Minutes

Dy

nam

ic Y

ield

Str

ess

(Pa)

Mix 1 (Base)

Mix 2 (IncreasedCA)

Mix 3 (Lower w/cm,Different Admix)

-0.1

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 20 40 60 80 100 120

Time from Placement, Minutes

Th

ixo

tro

pic

Bre

akd

ow

n A

rea

(Nm

/s)

Mix 1 (Base)

Mix 2 (IncreasedCA)

Mix 3 (Lower w/cm,Different Admix)

Peterborough Trial 2 - July 12, 2006

Concrete temperature 20C

-10

-5

0

5

10

15

20

25

30

35

40

11.0 11.5 12.0 12.5 13.0

Time (Hour + Decimal)

Lat

eral

Pre

ssu

re (

kP

a)

Cell 13 (Hyd.Pres. 36.1 kPa)

Cell 14 (Hyd.Pres. 63.5 kPa)

Cell 15 (Hyd.Pres. 91.1 kPa)

Cell 16 (Hyd.Pres. 98.7 kPa)

Peterborough Trial 3 - Sept 20, 2006,

Concrete temperature 21C

-20

0

20

40

60

80

100

10.0 10.5 11.0 11.5 12.0 12.5 13.0

Time (Hour + Decimal)

Lat

eral

Pre

ssu

re (

kP

a)

Cell 13 (Hyd.Pres. 36.1 kPa)

Cell 14 (Hyd.Pres. 63.5 kPa)

Cell 15 (Hyd.Pres. 91.1 kPa)

Cell 16 (Hyd.Pres. 98.7 kPa)

Mix 1 and 2: Fast increase in yield stress and thixotropy – low

formwork pressure

Mix 3: Slow increase in yield stress and thixotropy – high

formwork pressure

Results confirm that thixotropy

reduces formwork pressure.

Outline

24

01 What is Thixotropy?

02 Measurement of Thixotropy

03 Role of Thixotropy in Formwork Pressure

04 Field Data

05 Conclusions

Conclusions

SCC is not possible without thixotropy.

Thixotropy is the reversible, isothermal, time-dependent

decrease in viscosity when a fluid is subjected to increased

shear stress or shear rate.

SCC should have:

Low dynamic yield stress (self-flow)

Fast increase in static yield stress due to thixotropy (reduced

formwork pressure)

25

26

Thank You.