icar rheometer eric koehler w.r. grace & co. [email protected]
TRANSCRIPT
ICAR Rheometer
Eric KoehlerW.R. Grace & [email protected]
2
Outline
What is Rheology?
• Definition
• Measurement
ICAR Rheometer
• Description
• Operation
Applications
• Mixture proportioning
• SCC
• Production control
• Formwork pressure
• Segregation resistance
• Pumpability
3
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:
• Yield stress: the minimum stress to initiate or maintain flow (related to slump)
• 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 Rate, (1/s)
Sh
ear
Str
ess,
(
Pa)
Results
The Bingham Model 0
slope = plastic viscosity ()
intercept = yield stress (0)
Flow Curve
4
Workability and Rheology
Workability: “The ease with which [concrete] can be mixed, placed, consolidated, and finished to a homogenous condition.” (ACI Definition)
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
ACI 238.1R-08 report describes 69 workability and rheology tests.
5
Concrete Flow Curves (Constitutive Models)
0
ba 0
ba 0ba 0
ba 0ba 0
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.
6
Concrete Rheology: Non-Steady State
Concrete exhibits different rheology when at rest than when flowing.
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Concrete Rheology: Non-Steady State
Static Yield Stress
minimum shear stress to initiate flow from rest
Dynamic Yield Stress
minimum shear stress to maintain flow after breakdown of thixotropic structure
Plastic Viscosity
change in shear stress per change in shear rate, above yield stress
Thixotropy
reversible, time-dependent reduction in viscosity in material subject to shear
Shear Rate (1/s)
Sh
ear
Str
ess
(Pa)
Time (s)
To
rqu
e (N
m)
concrete sheared at constant, low rate
Flow Curve Test
Stress Growth Test
concrete sheared at various rates
maximum stress from rest= static yield stress
area between up and down curves due to thixotropy
slope = plastic viscosity
intercept = dynamic
yield stress
Concrete exhibits different rheology when at rest than when flowing.
Thixotropy is especially critical in highly flowable concretes.
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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)
Coaxial Cylinders Parallel Plate Impeller
Typical Rheometer Geometry Configurations
9
Concrete Rheometers
Tattersall Two-Point Rheometer IBB Rheometer ICAR Rheometer
BML ViscometerBTRHEOM Rheometer
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ICAR Rheometer
Portable concrete rheometer
• Laboratory
• Jobsite
Appropriate for moderately and highly flowable concrete
• Measures slumps greater than 75 mm
• Especially well-suited for self-consolidating concrete
Flexible interface allows measurement of Bingham parameters, thixotropy, and other protocols set by user
11
ICAR Rheometer: Operation
Apply Rotation, Measure Torque
Fluid
Outer Cylinder
Inner Cylinder
Top ViewSide View
Based on wide-gap, coaxial cylinders design
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
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)
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ICAR Rheometer: Portability
Rheometer Weight: 13 lb (6 kg)[with accessories: 40 lb (18 kg)]
16” (400 mm)
4.25”(110 mm)
Bucket size depends on aggregate size.1” (25 mm) aggregate shown
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Software InterfaceAll operations managed through single screen.
Flow CurveStress Growth
settings
start
real time data
calculated results
settings
start
real time data
calculated results
All data automatically written to text and Excel file
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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.
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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
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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
17
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
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Thixotropy Testing: Flow Curve or Stress Growth
Flow Curve Test
• 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 full breakdown of thixotropy
• Area between up and down curves is indicative of thixotropy
Stress Growth Test
• Place concrete in container and allow to rest for pre-determined time (to allow thixotropic build-up)
• Run stress growth test, which measures the static yield stress
• The difference between the static yield stress and dynamic yield stress (flow flow curve) is indicative of thixotropy
Shear Rate (1/s)
Sh
ear
Str
ess
(Pa)
Time (s)
To
rqu
e (N
m)
concrete sheared at constant, low rate
Flow Curve Test
Stress Growth Test
concrete sheared at various rates
maximum stress from rest= static yield stress
area between up and down curves due to thixotropy
slope = plastic viscosity
intercept = dynamic
yield stress
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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
20
Applications: Mixture Proportioning
Reference: Koehler, E.P., Fowler, D.W. (2007). “ICAR Mixture Proportioning Procedure for SCC” International Center for Aggregates Research, Austin, TX.
Yield Stress
Plastic Viscosity
Aggregate max. size (increase) Aggregate grading (optimize) Aggregate angularity Aggregate shape (equidimensional)
Paste volume (increase) Water/powder (increase) Fly ash Slag Silica fume (low %) Silica fume (high %) VMA HRWR AEA
Yield Stress (Pa)
Pla
stic
Vis
cosi
ty (
Pa.
s)
AEA
Silica FumeHRWR
Water
Effects of Materials and Mixture Proportions on Rheology
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Applications: SCC Rheology
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
• 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
Shear Rate, (1/s)
Sh
ear
Str
ess,
(
Pa)
0
0
Similar plastic viscosity
Near zero yield stress
Conventional Concrete
SCC
Yield stress is the main difference between SCC and conventional concrete.
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Applications: SCC Rheology
Slump flow vs. yield stress for single mixture proportion, variable HRWR
R2 = 0.90
0
1
2
3
4
5
6
7
8
9
10
0 30 60 90 120
Plastic Viscosity (Pa.s)
T2
0 (
s)
T20 vs. plastic viscosity
Reference: Koehler, E.P., Fowler, D.W. (2008). “Comparison of Workability Test Methods for Self-Consolidating Concrete” Submitted to Journal of ASTM International.
Empirical workability tests are a function of rheology.Rheology provides greater insight into workability.
23
Applications: SCC Rheology
0
5
10
15
20
25
30
0 30 60 90 120
Elapsed Time (Minutes)
Slu
mp
Flo
w (
inc
he
s)
PC 068
PC 059
PC 915
w/c = 0.35
0
50
100
150
200
250
0 30 60 90 120Elapsed Time (Minutes)
Dyn
amic
Yie
ld S
tres
s (P
a)
PC 068
PC 059
PC 915
w/c = 0.35
0
20
40
60
80
100
120
0 30 60 90 120
Elapsed Time (Minutes)
Pla
stic
Vis
cosi
ty (
Pa.
s)
PC 068
PC 059
PC 915
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
0 30 60 90 120
Elapsed Time (Minutes)
Th
ixo
tro
py
(N
m/s
)
PC 068
PC 059
PC 915
w/c = 0.35
3 Different HRWRs | Same Slump Flow | Same Mix Design | Different Rheology
Re
fere
nc
e:
Jekn
avo
rian
, A
., K
oe
hle
r, E
.P.,
Ge
ary
, D
., M
alo
ne
, J.
(2
00
8).
“C
on
cre
te R
he
olo
gy
with
Hig
h-R
an
ge
Wa
ter-
Re
du
cers
with
Ext
en
de
d
Slu
mp
Flo
w R
ete
ntio
n”
Pro
cee
din
gs
of
SC
C 2
00
8,
Ch
ica
go
, Ill
ino
is.
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Applications: Production Control
The workability box is an effective way to ensure production consistency
Definition: Zone of rheology associated with acceptable workability (self-flow and segregation resistance)
Mixture proportions affect rheology; therefore, controlling rheology is an effective way to control mixture proportions
Workability boxes are mixture-specific
• SCC encompasses a wide range of materials and rheology
• Rheology appropriate for one set of materials may be inappropriate for another set of materials
• Larger workability box corresponds to greater robustness
0
5
10
15
20
25
30
35
40
45
50
0 50 100 150
Yield Stress (Pa)
Pla
stic
Vis
cosi
ty (
Pa.
s)
Low Flow
Good
Segregation
Example
Requires Vibration
Segregation
Good
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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)
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Applications: Formwork Pressure – Case Study
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
Dyn
amic
Yie
ld S
tres
s (P
a)
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
Thi
xotr
opic
Bre
akdo
wn
Are
a (N
m/s
)
Mix 1 (Base)
Mix 2 (IncreasedCA)Mix 3 (Lower w/cm,Different Admix)
Peterborough Trial 2 - July 12, 2006Concrete 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
ssur
e (k
Pa)
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 13.0Time (Hour + Decimal)
Lat
eral
Pre
ssur
e (k
Pa)
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 high static yield stress reduces formwork pressure.
27
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
Reference EquationBeris, 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.
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 Self-Compacting Concrete,” ACI Materials Journal, 98(6), 429-439.
Rg fluidsphere )09533.0(0
Rg fluidsphere )124.0(0
Rg fluidsphere 3
40
Buoyancy + Resisting Force-Paste rheology-Paste density-Aggregate morphology-Neighboring aggregates (lattice
effect)
Gravitational Force-Aggregate density-Aggregate size Equations relating descent of sphere to rheology
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: Segregation Resistance
0
5
10
15
20
25
30
35
40
45
50
0 20 40 60 80 100Dynamic Yield Stress, 0 min. (Pa)
Pla
stic
Vis
cosi
ty, 0
min
. (P
a.s) Column Seg<10%
Column Seg>10%
-0.05
0.00
0.05
0.10
0.15
0.20
0 20 40 60 80 100Dynamic Yield Stress, 0 min. (Pa)
Th
ixo
tro
pyy
, 0
min
. (N
m/s
) Column Seg<10%Column Seg>10%
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.
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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
• 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
4
004
3
1
3
41
8 wwL
PRQ
Buckingham-Reiner Equation
sheared region
plug flow region
flow
shear stress = yield stress
wallat stress shear
radius tube
rateflow
w
R
Q
length tube
pressure
L
P
30
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.
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Applications: Pumpability – Case Study
Duke Energy Building, Charlotte, NC
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Applications: Pumpability – Case Study
VMA and/or other changes in mixture proportions were shown to increase pumpability by reducing concrete viscosity.
Role of VMA in reducing viscosity:
• VMA results in shear-thinning behavior 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
• Reduced pump stroke time confirmed in field mix with VMA
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
0.00 0.10 0.20 0.30
Rotation Speed (rps)
To
rqu
e (
Nm
)
#1: baseline
#4: Increase paste vol
#4: +VMA
#5: Increase w/cm
#5: +VMA
#6: Change agg
#6: +VMA
Duke Energy Building, Charlotte, NC
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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
34
Thank You.
Questions?