ICAR Rheometer
Eric Koehler
W.R. Grace & Co.
eric.koehler@grace.com
Outline
 What is Rheology?
• Definition
• Measurement
 ICAR Rheometer
• Description
• Operation
 Applications
• Mixture proportioning
• SCC
• Production control
• Formwork pressure
• Segregation resistance
• Pumpability
2
Concrete Rheology
 Rheology is the scientific description of
flow.
 The rheology of concrete is measured
with a concrete rheometer, which
determines the resistance of concrete
to shear flow at various shear rates.
 Concrete rheology measurements are
typically expressed in terms of the
Bingham model, which is a function of:
• Plastic viscosity: the resistance to flow once
yield stress is exceeded (related to
stickiness)
 Concrete rheology provides many
insights into concrete workability.
• Slump and slump flow are a function of
concrete rheology.
 
Shear Stress,  (Pa)
• Yield stress: the minimum stress to initiate
or maintain flow (related to slump)
Results
Flow Curve
The Bingham Model
   0  
slope = plastic viscosity ()
intercept = yield stress (0)
   (1/s)
Shear Rate,
3
Workability and Rheology
 Workability: “The ease with which
[concrete] can be mixed, placed,
consolidated, and finished to a
homogenous condition.” (ACI
Definition)
ACI 238.1R-08 report describes 69
workability and rheology tests.
 Workability tests are typically
empirical
• Tests simulate placement condition and
measure value (such as distance or
time) that is specific to the test method
• Difficult to compare results from one test
to another
• Multiple tests needed to describe
different aspects of workability
 Rheology provides a fundamental
measurement
• Results from different rheometers have
been shown to be correlated
• Results can be used to describe multiple
aspects or workability
4
Concrete Flow Curves (Constitutive Models)
 Flow curves represent shear stress vs. shear rate
 Bingham model is applicable to majority of concrete
 Other models are available and can be useful for specific
applications (e.g. pumping)
 Very stiff concrete behaves more as a solid than a liquid. Such
mixtures are not described by these models.
   0  a b
   0  
b
  0  a0 
a b
  
b
  0  a0 
a b
5
Concrete Rheology: Non-Steady State
Concrete exhibits different rheology when at rest than when flowing.
6
Concrete Rheology: Non-Steady State
Flow Curve Test
Concrete exhibits different rheology
when at rest than when flowing.
Static Yield Stress
minimum shear stress to initiate flow from
rest
Shear Stress (Pa)
concrete sheared at various rates
area between up and down
curves due to thixotropy
slope = plastic viscosity
Dynamic Yield Stress
minimum shear stress to maintain flow after
breakdown of thixotropic structure
intercept =
dynamic
yield stress
Plastic Viscosity
Thixotropy
reversible, time-dependent reduction in
viscosity in material subject to shear
Stress Growth Test
concrete sheared at constant, low rate
Torque (Nm)
change in shear stress per change in shear
rate, above yield stress
Shear Rate (1/s)
maximum stress from rest
= static yield stress
Thixotropy is especially critical in highly flowable concretes.
Time (s)
7
Rheology Measurement: Typical Geometry
 Rheometers continuously shear concrete through rotational
movement.
 Rheometers must be uniquely designed for concrete (primarily
due to large aggregate size)
 Results can be expressed in relative units (torque vs. speed) or
absolute units (shear stress vs. shear rate)
Typical Rheometer Geometry Configurations
Coaxial Cylinders
Parallel Plate
Impeller
8
Concrete Rheometers
Tattersall Two-Point Rheometer
BTRHEOM Rheometer
IBB Rheometer
ICAR Rheometer
BML Viscometer
9
ICAR Rheometer
 Portable concrete rheometer
• Laboratory
• Jobsite
 Appropriate for moderately and
highly flowable concrete
• Measures slumps greater than 75
mm
• Especially well-suited for selfconsolidating concrete
 Flexible interface allows
measurement of Bingham
parameters, thixotropy, and
other protocols set by user
10
ICAR Rheometer: Operation
 Based on wide-gap, coaxial
cylinders design
Inner Cylinder
Apply Rotation,
Measure Torque
 Vane acts as inner cylinder
• Compact size
• Prevents slip
 Outer wall also has vertical
strips to prevent slip
 Vane is immersed in concrete
and rotated at different speeds
Outer
Cylinder
Fluid
Side View
Top View
 Computer software operates
test and computes results
 Single test complete in 60
seconds
 Vane can be replaced with any
other type of impeller
H: 5 in (125 mm)
D: 5 in (125 mm)
11
ICAR Rheometer: Portability
Rheometer Weight: 13 lb (6 kg)
[with accessories: 40 lb (18 kg)]
4.25”
(110 mm)
16” (400 mm)
Bucket size depends on aggregate size.
1” (25 mm) aggregate shown
12
Software Interface
All operations managed through single screen.
All data
automatically
written to text
and Excel file
Stress Growth
settings
start
real time
data
calculated
results
Flow Curve
settings
start
real time
data
calculated
results
13
Aggregate Size
 Vane is constant size for
all aggregate sizes
• Height: 5 in. (125 mm)
• Diameter: 5 in. (125 mm)
 Outer container is
selected based on
aggregate size
• Horizontal and vertical
gaps should be at least 4x
the maximum aggregate
size
• Larger container can be
always be used, but
smaller container should
never be used.
14
Stress Growth Test
 Stress growth test consists
of the following:
• Rotate vane at low, constant
speed
• Measure gradual increase in
torque
• Identify maximum torque and
convert to stress, which is
equal to static yield stress
• Note: reduction in torque after
peak value is associated with
further yielding of material and
is not typically analyzed further
 Material is previously at rest
for pre-determined period to
detect effect of thixotropy
 Vane speed is typically 0.01
to 0.05 rps
15
Flow Curve Test
 Flow curve test measures
shear stress at different
shear rates
 Raw torque vs. rotation
speed data are converted to
fundamental units of shear
stress and shear rate
 Can also be used to measure
thixotropy
Software Inputs
Test Units
16
Rheometer Test File
 All settings and results are
written automatically to a
summary text file.
 Raw data (instantaneous
torque and rotation speed) can
optionally be written to a file
for Excel
17
Thixotropy Testing: Flow Curve or Stress Growth
Flow Curve Test
 Flow Curve Test
concrete sheared at various rates
Shear Stress (Pa)
• Place concrete in container and allow to rest
for pre-determined time (to allow thixotropic
build-up)
• Run flow curve with speeds in ascending
order (low to high), exclude breakdown
period
• Immediately run second curve with speeds in
descending order (high to low), include
breakdown period at high speed to assure
intercept =
full breakdown of thixotropy
• Area between up and down curves is
indicative of thixotropy
dynamic
yield stress
• The difference between the static yield stress
and dynamic yield stress (flow flow curve) is
indicative of thixotropy
Torque (Nm)
• Run stress growth test, which measures the
static yield stress
slope = plastic viscosity
Shear Rate (1/s)
Stress Growth Test
concrete sheared at constant, low rate
 Stress Growth Test
• Place concrete in container and allow to rest
for pre-determined time (to allow thixotropic
build-up)
area between up and down
curves due to thixotropy
maximum stress from rest
= static yield stress
Time (s)
18
Applications: Mixture Proportioning
Both the mixture proportions and the admixture can adjusted to
tailor the rheology to the application.
• Precast vs. ready mix
• SCC vs. conventional concrete
• Formwork pressure
• Pumpability
• Segregation resistance
• Mixing
• “Stickiness” and “Cohesion”
• Form surface finish
• Finishability
19
Applications: Mixture Proportioning
Effects of Materials and Mixture Proportions on Rheology
Plastic Viscosity (Pa.s)
Aggregate max. size (increase)
Aggregate grading (optimize)
Aggregate angularity
Silica Fume
HRWR
Aggregate shape (equidimensional)
Paste volume (increase)
Water/powder (increase)
AEA
Fly ash
Slag
Water
Silica fume (low %)
Silica fume (high %)
Yield Stress (Pa)
VMA
HRWR
AEA
Yield
Stress
Plastic
Viscosity


























Reference: Koehler, E.P., Fowler, D.W. (2007). “ICAR Mixture Proportioning
Procedure for SCC” International Center for Aggregates Research, Austin, TX.
20
 SCC is designed to flow under its own
mass, resist segregation, and meet
other requirements (e.g. mechanical
properties, durability, formwork
pressure, pump pressure)
 Compared to conventional concrete,
SCC exhibits:
• Significantly lower yield stress (near zero):
allows concrete to flow under its own mass
• Similar plastic viscosity: ensures
segregation resistance
 Plastic viscosity must not be too high
or too low
 
Shear Stress,  (Pa)
Applications: SCC Rheology
Conventional
Concrete

0
Similar plastic
viscosity
Near zero
yield stress
SCC

0
   (1/s)
Shear Rate,
• Too high: concrete is sticky and difficult to
pump and place
• Too low: concrete is susceptible to
segregation
 Thixotropy is more critical for SCC due
to low yield stress
Yield stress is the main difference between SCC and conventional concrete.
21
Applications: SCC Rheology
Empirical workability tests are a function of rheology.
Rheology provides greater insight into workability.
Slump flow vs. yield stress for single
mixture proportion, variable HRWR
T20 vs. plastic viscosity
10
2
R = 0.90
9
8
T20 (s)
7
6
5
4
3
2
1
0
0
30
60
90
120
Plastic Viscosity (Pa.s)
Reference: Koehler, E.P., Fowler, D.W. (2008). “Comparison of Workability Test
Methods for Self-Consolidating Concrete” Submitted to Journal of ASTM International.
22
Applications: SCC Rheology
3 Different HRWRs | Same Slump Flow | Same Mix Design | Different Rheology
w/c = 0.35
w/c = 0.35
250
PC 068
20
15
10
PC 068
PC 059
PC 915
Dynamic Yield Stress (Pa)
Slump Flow (inches)
25
5
PC 059
200
PC 915
150
100
0
50
0
0
30
60
90
120
0
30
60
90
120
Elapsed Time (Minutes)
Elapsed Time (Minutes)
w/c = 0.35
120
0.45
PC 068
PC 068
PC 059
100
PC 915
Thixotropy (Nm/s)
Plastic Viscosity (Pa.s)
Reference: Jeknavorian, A., Koehler, E.P., Geary, D., Malone, J. (2008).
“Concrete Rheology with High-Range Water-Reducers with Extended
Slump Flow Retention” Proceedings of SCC 2008, Chicago, Illinois.
30
80
60
40
0.40
PC 059
0.35
PC 915
0.30
0.25
0.20
0.15
0.10
20
0.05
0.00
0
0
30
60
90
Elapsed Time (Minutes)
120
0
30
60
90
Elapsed Time (Minutes)
120
23
Applications: Production Control
 The workability box is an effective
way to ensure production
consistency
Example
50
Low Flow
 Mixture proportions affect
rheology; therefore, controlling
rheology is an effective way to
control mixture proportions
 Workability boxes are mixturespecific
• SCC encompasses a wide range of
materials and rheology
• Rheology appropriate for one set of
materials may be inappropriate for
another set of materials
Plastic Viscosity (Pa.s)
Definition: Zone of rheology
associated with acceptable workability
(self-flow and segregation resistance)
45
Good
40
Segregation
Requires Vibration
35
30
Good
25
20
15
Segregation
10
5
0
0
50
100
150
Yield Stress (Pa)
• Larger workability box corresponds to
greater robustness
24
Applications: Formwork Pressure
 Formwork pressure is related to
concrete rheology
• Pressure is known to increase with slump
• SCC often exhibits high formwork
pressure due to its high fluidity
 Concrete is at rest in forms, therefore,
static yield stress is relevant
• Static yield stress is affected by dynamic
yield stress and thixotropy
• SCC is placed in lifts, which takes
advantage of 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
(reduced formwork pressure)
25
Mix 2 (Increased
CA)
Mix 3 (Lower w/cm,
Different Admix)
500
400
300
200
100
0
0
20
40
60
80
100
Time from Placement, Minutes
120
40
0.8
Mix 1 (Base)
0.7
Peterborough Trial 2 - July 12, 2006
Concrete temperature 20C
35
Mix 2 (Increased
CA)
Mix 3 (Lower w/cm,
Different Admix)
0.6
0.5
30
Lateral Pressure (kPa)
Mix 1 (Base)
0.4
0.3
0.2
0.1
25
20
15
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)
10
5
0
0
-0.1
0
20
40
60
80
100
11.0
-5
120
Results confirm that high static yield stress
reduces formwork pressure.
12.0
12.5
13.0
-10
100
Peterborough Trial 3 - Sept 20, 2006,
Concrete temperature 21C
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
11.5
Time (Hour + Decimal)
Time from Placement, Minutes
80
Lateral Pressure (kPa)
Dynamic Yield Stress (Pa)
600
Thixotropic Breakdown Area (Nm/s)
Applications: Formwork Pressure – Case Study
60
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)
40
20
0
10.0
10.5
11.0
11.5
12.0
12.5
13.0
Time (Hour + Decimal)
-20
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
26
Applications: Segregation Resistance
 SCC consists of aggregates suspended in a thixotropic, Bingham
paste
 Paste must exhibit proper rheology to suspend a particular set of
aggregates
• Static yield stress > minimum static yield stress: no segregation
• Static yield stress < minimum static yield stress: rate of descent of aggregate
depends on paste yield stress and viscosity
Gravitational Force
-Aggregate density
-Aggregate size
Equations relating descent of sphere to rheology
Reference
Beris, A. N., Tsamopoulos, J.A., Armstrong,
R.C., and Brown, R.A. (1985). “Creeping motion
of a sphere through a Bingham plastic”, Journal
of Fluid Mech., 158, 219-244.
Buoyancy + Resisting Force
-Paste rheology
-Paste density
-Aggregate morphology
-Neighboring aggregates (lattice
effect)
Jossic, L., and Magnin, A. (2001). “Drag and
Stability of Objects in a Yield Stress Fluid,”
AIChE Journal, 47(12). 2666-2672.
Saak, A.W., Jennings, H.M., and Shah, S.P.
(2001). “New Methodology for Designing SelfCompacting Concrete,” ACI Materials Journal,
98(6), 429-439.
Equation
 0  (0.09533) g  sphere   fluid R
 0  (0.124) g  sphere   fluid R
0 
4
g  sphere   fluid R
3
Reference: Koehler, E.P., and Fowler, D.W. (2008). “Static and Dynamic
Yield Stress Measurements of SCC” Proceedings of SCC 2008, Chicago, IL.
27
0.20
50
Column Seg<10%
Column Seg>10%
45
40
35
30
25
20
15
10
5
Thixotropyy, 0 min. (Nm/s)
Plastic Viscosity, 0 min. (Pa.s)
Applications: Segregation Resistance
Column Seg<10%
Column Seg>10%
0.15
0.10
0.05
0.00
-0.05
0
0
20
40
60
80
100
Dynamic Yield Stress, 0 min. (Pa)
0
20
40
60
80
100
Dynamic Yield Stress, 0 min. (Pa)
Segregation resistance increased with:
• Higher yield stress (static and dynamic yield stress assumed equal initially)
• Higher plastic viscosity
• Higher thixotropy
Reference: Koehler, E.P., and Fowler, D.W. (2008). “Static and Dynamic
Yield Stress Measurements of SCC” Proceedings of SCC 2008, Chicago, IL.
28
Applications: Pumpability
 Concrete moves through a
pump line as a “plug”
surrounded by a sheared
region at the walls.
• Higher viscosity increases
pumping pressure, reduces flow
rate
sheared
region
flow
plug flow
region
• Unstable mixes may cause
blocking
 Pumping concrete in high-rise
buildings presents unique
challenges
• High strength mixes often have
low w/cm, resulting in high
concrete viscosity
• Blockage can result in significant
jobsite delays
shear stress = yield stress
Buckingham-Reiner Equation
4

PR
4  0  1  0  
1       
Q
8L  3   w  3   w  


4
P  pressure
Q  flow rate
L  tube length
R  tube radius
 w  shear stress at wall
29
Applications: Pumpability – Case Study
 Duke Energy Building, Charlotte, NC
• 52 Story Office Tower (764 ft) with 9 story building
annex
• 8 Story Parking Structure 95 ft below street level
 Concrete Mixture Requirements
• Compressive Strength

5,000 psi to 18,000 psi (35 to 124 MPa)
• Modulus of Elasticity

4.6 to 8.0 x 106 psi (32 to 55 GPa)
• Workability

27 +/- 2 inch spread (690 +/- 50 mm)
 To meet compressive strength and elastic
modulus requirements, the high strength
concrete mixtures were proportioned with:
• Low w/c
• Silica fume
• High-modulus crushed coarse aggregate
 The resulting mixture exhibited:
• High viscosity
• High pump pressure
Reference: Koehler, E.P., and Brooks, W., Neuwald, A., and
Mogan, E.. (2009). “Applications of Rheology Measurements to
Enable and Ensure Concrete Performance” NRMCA Concrete
Technology Forum, Cincinnati, OH.
30
Applications: Pumpability – Case Study
Duke Energy Building, Charlotte, NC
31
Applications: Pumpability – Case Study
Duke Energy Building, Charlotte, NC
 VMA and/or other changes in
mixture proportions were shown to
increase pumpability by reducing
concrete viscosity.
5.0
4.5
4.0
Torque (Nm)
3.5
#1: baseline
#4: Increase paste vol
#4: +VMA
#5: Increase w/cm
#5: +VMA
#6: Change agg
#6: +VMA
 Role of VMA in reducing viscosity:
• VMA results in shear-thinning behavior
3.0

Increased viscosity (thickens) concrete at rest
and at low shear rates: beneficial for reduced
formwork pressure and increased segregation
resistance

Decreased viscosity (thins) at high shear rates:
beneficial for improved pumpability
2.5
2.0
1.5
• Reduced pump stroke time confirmed
in field mix with VMA
1.0
0.5
0.0
0.00
0.10
0.20
Rotation Speed (rps)
0.30
32
Conclusions
 Rheology is the scientific description of workability.
 The ICAR rheometer enables portable rheology measurements in
the lab and field.
• Measures concrete greater than 75 mm slump
• Measures yield stress, plastic viscosity, and thixotropy
 Rheology was shown to provide insights into the following
applications:
• Mixture proportioning
• SCC
• Production control
• Formwork pressure
• Segregation resistance
• Pumpability
33
Thank You.
Questions?
34