Thixotropy of SCC and Its Effects on Formwork Pressure
Eric P. Koehler, Ph.D. Vice President
Verifi
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.