Koehler Rheology v1

8/13/2019 Koehler Rheology v1

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Tenth CANMET/ACI International Conference on Recent Advances in

Concrete Technology and Sustainability Issues

Use of Rheology to Design, Specify, and

Manage Self-Consolidating Concrete

Eric Koehler

W.R. Grace & Co.


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Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues 2

Outline

Rheology

Definition Measurement

SCC Rheology

Specification

Design

Management

Case Studies

Formwork pressure

Segregation resistance

Pumpability


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Concrete Rheology

Rheology is the scientific description offlow.

The rheology of concrete is measuredwith a concrete rheometer, whichdetermines the resistance of concreteto shear flow at various shear rates.

Concrete rheology measurements are

typically expressed in terms of theBingham model, which is a function of:

Yield stress: the minimum stress to initiateor maintain flow (related to slump)

Plastic viscosity: the resistance to flow onceyield stress is exceeded (related to

stickiness) Concrete rheology provides many

insights into concrete workability.

Slump and slump flow are a function ofconcrete rheology.

Shear Rate, (1/s)

ShearStress,

(Pa)

Re

sults

The Bingham Model

0

slope = plastic viscosity ()

intercept = yield stress (0)

Flow Curve


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Workability and Rheology

Workability: The ease with which[concrete] can be mixed, placed,

consolidated, and finished to ahomogenous condition. (ACIDefinition)

Workability tests are typicallyempirical

Tests simulate placement condition andmeasure value (such as distance or

time) that is specific to the test method

Difficult to compare results from one testto another

Multiple tests needed to describedifferent aspects of workability

Rheology provides a fundamental

measurement Results from different rheometers have

been shown to be correlated

Results can be used to describe multipleaspects or workability

ACI 238.1R-08 report describes 69

workability and rheology tests.


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Concrete Flow Curves (Constitutive Models)

0

ba 0

ba

ba

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 specificapplications (e.g. pumping)

Very stiff concrete behaves more as a solid than a liquid. Suchmixtures are not described by these models.


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Concrete Rheology: Non-Steady State

Static Yield Stress

minimum shear stress to initiate flow fromrest

Dynamic Yield Stress

minimum shear stress to maintain flow afterbreakdown of thixotropic structure

Plastic Viscosity

change in shear stress per change in shearrate, above yield stress

Thixotropyreversible, time-dependent reduction inviscosity in material subject to shear

Shear Rate (1/s)

ShearStress(P

a)

Time (s)

Torque(

Nm)

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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Thixotropy Manifestation in Rheology Measurements

Increase in shear rate causes

gradual breakdown of

thixotropic structure

Decrease in shear rate allows

re-building of thixotropic

structure

Change in shear stress due to

change in thixotropic structure

must be taken into account

when:

Measuring rheology

Flow curve area

Stress growth

Proportioning concrete for

applications


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Thixotropy Manifestation in Concrete Delivery

Change in yield stress from mixing through delivery and placement

Dynamic Yield StressFull Breakdown,No Thixotropy

Static Yield Stress of

Non-Agitated SCCNo Breakdown, Full

ThixotropyStatic Yield Stress

of SCC During

Placement

Time from Mixing

YieldStress

Concrete is partiallyagitated during transit,

preventing full build-upof at-rest structure

Concrete is discharged into formsresulting shearing causes full

breakdown of at-rest structuretu

Concrete is in formwork;at-rest structure rebuildsand static yield stressincreases


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Rheology Measurement: Typical Geometry

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


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Concrete Rheometers

Tattersall Two-Point Rheometer IBB Rheometer ICAR Rheometer

BML ViscometerBTRHEOM Rheometer


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ICAR Rheometer

Example Test ProtocolsStress Growt h Test

Protocol: rotate vane at 0.05 rps, concrete maintained at rest

before test

Results: static yield stress (peak stress)

Flow Cu rve TestProtocol: Immediately after stress growth test, increase vane

speed in 8 increments from 0.05 to 0.50 rps, maintain 0.50 rps

for 20 s, reduce speed in 8 increments from 0.50 to 0.05 rps

Results: thixotropy (area between up and down curves), dynamic

yield stress (intercept of down curve), plastic viscosity (slope of

down curve)

Vane Geometry

H: 5 in (125 mm)

D: 5 in (125 mm)

ICAR rheometer was used for the case studies described in this presentation.


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SCC Rheology

SCC is designed to flow under its ownmass, resist segregation, and meetother requirements (e.g. mechanicalproperties, durability, formworkpressure, 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: ensuressegregation resistance

Plastic viscosity must not be too highor too low

Too high: concrete is sticky and difficult topump and place

Too low: concrete is susceptible tosegregation

Thixotropy is more critical for SCC dueto low yield stress

Shear Rate, (1/s)

ShearStress,

(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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SCC: Specification

SCC workability is described in terms of the following:

Filling ability

Passing ability

Segregation resistance (stability)

Static segregation resistance

Dynamic segregation resistance

Each property should be evaluated independently

Minimum requirements for each property vary by application


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SCC: Specification

Property Laboratory

(Pre-Qualification)

Field

(Quality Control)

Filling Ability

(Slump Flow)

Yes. Yes. Provides indirect measurement of yield

stress and plastic viscosity.

Passing Ability

(J-Ring)

Yes. No.Depends primarily on aggregates, paste

volume, slump flow.

Segregation Resistance

(Column Segregation)

Yes. Check robustness across typical changes

in materials (especially water)

No.Variations mainly depend on paste

rheology (water).

Slump FlowASTM C 1611

J-RingASTM C 1621

Column SegregationASTM C 1610

Filling Ability Passing Ability Segregation Resistance

Test requirements vary between lab and field.

ASTM tests are available to measure the three SCC properties independently.

By confirming robustness in lab and closely controlling materials, fewer tests may be needed in field.


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SCC: Specification

Slump flow vs. yield stress for single

mixture proportion, variable HRWR

R2= 0.90

0

1

2

3

4

5

6

78

9

10

0 30 60 90 120

Plastic Viscosit Pa.s

T20(s)

T20vs. 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.


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SCC: Design

Compared to conventional concrete, SCC proportions typically

exhibit:

Lower coarse aggregate content (S/A = 0.50 vs. 0.40)

Smaller maximum aggregate size (3/4 or less vs. up to 1 )

Higher paste volume (28-40% vs. 25-30%)

Higher powder content (cementitious and non-cementitious, >700 lb/yd3)

Low water/powder ratio (0.30-0.40)

Polycarboxylate-based HRWR (to achieve high slump flow)


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SCC: Design

Both the mixture proportions and the admixture can be tailored to

the application.

Precast vs. ready mix

SCC vs. conventional concrete

Formwork pressure

Pumpability


Mixing

Stickiness and Cohesion

Form surface finish

Finishability


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SCC: Design

Reference:Koehler, E.P., Fowler, D.W. (2007). ICAR Mixture Proportioning

Procedure for SCC International Center for Aggregates Research, Austin, TX.

YieldStress

PlasticViscosity

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)

PlasticViscos

ity(Pa.s

)

AEA

Silica FumeHRWR

Water

Effects of Materials and Mixture Proportions on Rheology


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SCC: Design

0

5

10

15

20

25

30

0 30 60 90 120

Elapsed Time (Minutes)

SlumpFlow

(inches)

PC 068

PC 059

PC 915

w/c = 0.35

0

50

100

150

200

250

0 30 60 90 120


D

ynamicYieldStress(Pa)

PC 068

PC 059

PC 915

w/c = 0.35

0

20

40

60

80

100

120

0 30 60 90 120


PlasticViscosity(P

a.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


Thixotropy(Nm/s)

PC 068

PC 059

PC 915

w/c = 0.35

3 Different HRWRs | Same Slump Flow | Same Mix Design | Different Rheology

Reference:Jeknavorian,

A.,Koehler,E.P.,

Geary,

D.,

Malone,

J.

(2008).

ConcreteRheologywithHigh-RangeWater-ReducerswithExtended

Slum

pFlow

RetentionProce

edingsofSCC

2008,

Chicago,

Illinois.


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SCC: Design

Concrete can be modeled as a concentration suspension. These model can

be used to design mixture proportions.

=Huggins coefficient

=solid volume concentration

=intrinsic viscosity

=viscosity of suspension

=viscosity of suspending medium

Factors Sub-Factors

Aggregates

Maximum Size

Grading

Shape

Paste Volume

Filling Ability

Passing Ability

Robustness

Paste Composition

Water

Powder

Air

Reference:Koehler, E.P., Fowler, D.W.

(2007). ICAR Mixture Proportioning

Procedure for SCC International Center for

Aggregates Research, Austin, TX.

ICAR Mixture Proportioning Procedure

Based on concrete as concentrated

suspension of aggregates in paste

Includes equation for calculatingrequired paste volume.


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SCC: Management

The workability box is an effectiveway to ensure production

consistencyDefinition:Zone of rheologyassociated with acceptable workability(self-flow and segregation resistance)

Mixture proportions affectrheology; therefore, controlling

rheology is an effective way tocontrol mixture proportions

Workability boxes are mixture-specific

SCC encompasses a wide range ofmaterials and rheology

Rheology appropriate for one set ofmaterials may be inappropriate foranother set of materials

Larger workability box corresponds togreater robustness

0

5

10

15

20

25

30

35

40

45

50

0 50 100 150

Yield Stress (Pa)

PlasticVisco

sity(Pa.s

)

Low Flow

Good

Segregation

Example

Requires Vibration

Segregation

Good


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SCC Case Studies

Formwork pressure


Pumpability


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SCC Case Study: Formwork Pressure

Formwork pressure is related toconcrete rheology

Pressure is known to increase with slump

SCC often exhibits high formworkpressure due to its high fluidity

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

Static yield stress is affected by dynamicyield stress and thixotropy

SCC is placed in lifts, which takesadvantage of thixotropy

SCC must be designed to flow underits own mass and exert low formworkpressure

Low dynamic yield stress (self flow)

Fast increase in static yield stress(reduced formwork pressure)


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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

DynamicYieldStress(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

ThixotropicBreakdownArea(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)

LateralPressure(kPa)

Cell 13 (Hyd.Pres. 36.1 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)

Lateral

Pressure(kPa)





Mix 1 and 2: Fast increase in yield stress and thixotropylow

formwork pressure

Mix 3:Slow increase in yield stress and thixotropyhigh formwork

pressure

Results confirm that high static yield stress

reduces formwork pressure.


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Options to Reduce SCC Formwork Pressure

Select concrete with fast build-up of static yield stress Attributable to thixotropy

Must achieve concurrent with low dynamic yield stress

Place concrete in lifts to allow build-up of thixotropic structure

Limit pour heights and rates based on concrete rheology

Do not vibrate concrete


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SCC Case Study: Segregation Resistance

SCC consists of aggregates suspended in a thixotropic, Bingham

paste

Paste must exhibit proper rheology to suspend a particular set ofaggregates

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 Equation

Beris, A. N., Tsamopoulos, J.A., Armstrong,

R.C., and Brown, R.A. (1985). Creeping motionof a sphere through a Bingham plastic, Journal

of Flu id Mech., 158, 219-244.

Jossic, L., and Magnin, A. (2001). Drag andStability 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.


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SCC Case Study: Segregation Resistance

0

5

10

15

20

25

30

35

40

45

50

0 20 40 60 80 100Dynamic Yield Stress, 0 min. (Pa)

Plastic

Viscosity,

0min.(

Pa.s

)Column Seg10%

-0.05

0.00

0.05

0.10

0.15

.

0 20 40 60 80 100Dynamic Yield Stress, 0 min. (Pa)

Thixo

tropyy,

0min.

(Nm

/s) Column Seg10%

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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SCC Case Study: Pumpability

Concrete moves through apump line as a plug

surrounded by a shearedregion at the walls.

Higher viscosity increasespumping pressure, reduces flowrate

Unstable mixes may cause

blocking Pumping concrete in high-rise

buildings presents uniquechallenges

High strength mixes often havelow w/cm, resulting in high

concrete viscosity

Blockage can result in significantjobsite delays

4

00

4

31

341

8 wwLPRQ

Buckingham-Reiner Equation

shearedregion

plug flow

regionflow

shear stress = yield stress

wallatstressshear

radiustube

rateflow

w

R

Q

lengthtube

pressure

L

P


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Duke Energy Building, Charlotte, NC

52 Story Office Tower (764 ft) with 9 story buildingannex

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 106psi (32 to 55 GPa)

Workability

27 +/- 2 inch spread (690 +/- 50 mm)

To meet compressive strength and elasticmodulus requirements, the high strengthconcrete mixtures were proportioned with:

Low w/c

Silica fume High-modulus crushed coarse aggregate

The resulting mixture exhibited:

High viscosity

High pump pressure


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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)

Torq

ue(Nm)

#1: baseline#4: Increase paste vol

#4: +VMA

#5: Increase w/cm

#5: +VMA

#6: Change agg

#6: +VMA



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Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues

Conclusions

Concrete rheology is a useful tool for specifying, designing, and

managing SCC.

Static yield stressimportant for at-rest conditions

Dynamic yield stressimportant for flowing conditions

Plastic viscosityimportant for stickiness and cohesion

Thixotropyimportant for at-rest conditions

Rheology can be optimized to ensure concrete performance.

Self-consolidating concrete: low dynamic yield stress, adequate plastic

viscosity and thixotropy

Reduced formwork pressure: increased static yield stress (due to

thixotropy)

Increased segregation resistance: increased static yield stress (due tothixotropy) and viscosity

Increased pumpability: reduced plastic viscosity, stable mixture

Koehler Rheology v1

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