ICAR Rheometer

Eric KoehlerW.R. Grace & Co.eric.koehler@grace.com

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.

7

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.

8

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

10

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)

12

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

13

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

14

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.

15

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

16

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

18

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

19

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

21

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.

22

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

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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)

26

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.

29

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.

31

Applications: Pumpability – Case Study

Duke Energy Building, Charlotte, NC

32

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

33

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?