Koehler_Rheology_v1.ppt (2.9 MB)

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 KoehlerW.R. Grace & Co.

Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues2

Outline

Rheology• Definition

• Measurement

SCC Rheology• Specification

• Design

• Management

Case Studies• Formwork pressure

• Segregation resistance

• Pumpability


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)

Shea

r St

ress

, (

Pa)

Results

The Bingham Model 0

slope = plastic viscosity ()

intercept = yield stress (0)

Flow Curve


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.


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.


Concrete Rheology: Non-Steady State

Static Yield Stressminimum shear stress to initiate flow from rest

Dynamic Yield Stressminimum shear stress to maintain flow after breakdown of thixotropic structure

Plastic Viscositychange in shear stress per change in shear rate, above yield stress

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

Shear Rate (1/s)

Shea

r Str

ess

(Pa)

Time (s)

Torq

ue (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.


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


Thixotropy Manifestation in Concrete Delivery

Change in yield stress from mixing through delivery and placement

Dynamic Yield Stress Full Breakdown, No Thixotropy

Static Yield Stress of Non-Agitated SCC No Breakdown, Full

Thixotropy

Static Yield Stress of SCC During

Placement

Time from Mixing

Yiel

d St

ress

Concrete is partially agitated during transit, preventing full build-up of at-rest structure

Concrete is discharged into forms resulting shearing causes fullbreakdown of at-rest structuretu

Concrete is in formwork; at-rest structure rebuilds and static yield stress increases


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


Concrete RheometersTattersall Two-Point Rheometer IBB Rheometer ICAR Rheometer

BML ViscometerBTRHEOM Rheometer


ICAR Rheometer

Example Test ProtocolsStress Growth TestProtocol: rotate vane at 0.05 rps, concrete maintained at rest before testResults: static yield stress (peak stress)Flow Curve 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 rpsResults: 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.


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)

Shea

r St

ress

, (

Pa)

0

0

Similar plastic viscosity

Near zero yield stress

Conventional Concrete

SCC

Yield stress is the main difference between SCC and conventional concrete.


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


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.


SCC: Specification

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)

T 20 (

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.


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 ½”)

• 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)


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

• Segregation resistance

• Mixing

• “Stickiness” and “Cohesion”

• Form surface finish

• Finishability


SCC: Design

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)

Plas

tic V

isco

sity

(Pa.

s)

AEA

Silica FumeHRWR

Water

Effects of Materials and Mixture Proportions on Rheology


SCC: Design

0

5

10

15

20

25

30

0 30 60 90 120

Elapsed Time (Minutes)

Slum

p Fl

ow (i

nche

s)

PC 068PC 059PC 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 068PC 059PC 915

w/c = 0.35

0

20

40

60

80

100

120

0 30 60 90 120


Plas

tic V

isco

sity

(Pa.

s)

PC 068PC 059PC 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


Thix

otro

py (N

m/s

)

PC 068PC 059PC 915

w/c = 0.35

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

Ref

eren

ce: J

ekna

voria

n, A

., K

oehl

er, E

.P.,

Gea

ry, D

., M

alon

e, J

. (20

08).

“Con

cret

e R

heol

ogy

with

Hig

h-R

ange

Wat

er-R

educ

ers

with

Ext

ende

d S

lum

p Fl

ow R

eten

tion”

Pro

ceed

ings

of S

CC

200

8, C

hica

go, I

llino

is.


SCC: DesignConcrete 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

AggregatesMaximum Size

GradingShape

Paste VolumeFilling Ability

Passing AbilityRobustness

Paste CompositionWater

PowderAir

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 calculating required paste volume.


SCC: Management 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)

Plas

tic V

isco

sity

(Pa.

s)

Low Flow

Good

Segregation

Example

Requires Vibration

Segregation

Good


SCC Case Studies

Formwork pressure

Segregation resistance

Pumpability


SCC Case Study: 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)



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 120Time from Placement, Minutes

Dyn

amic

Yie

ld S

tress

(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 120Time from Placement, Minutes

Thix

otro

pic

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)

Late

ral P

ress

ure

(kPa

)

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)

Late

ral P

ress

ure

(kPa

)

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.



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


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 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 34

0

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.


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)

Plas

tic V

isco

sity

, 0 m

in. (

Pa.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)

Thix

otro

pyy,

0 m

in. (

Nm

/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.


SCC Case Study: 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

31

341

8 wwLPRQ

Buckingham-Reiner Equation

sheared region

plug flow region

flow

shear stress = yield stress

wallat stress shearradius tuberateflow

w

RQ

length tube

pressure

LP



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



Duke Energy Building, Charlotte, NC



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 (N

m)

#1: baseline#4: Increase paste vol#4: +VMA#5: Increase w/cm#5: +VMA#6: Change agg#6: +VMA

Duke Energy Building, Charlotte, NC


Conclusions

Concrete rheology is a useful tool for specifying, designing, and managing SCC.

• Static yield stress – important for at-rest conditions

• Dynamic yield stress – important for flowing conditions

• Plastic viscosity – important for stickiness and cohesion

• Thixotropy – important 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 to thixotropy) and viscosity

• Increased pumpability: reduced plastic viscosity, stable mixture

Koehler_Rheology_v1.ppt (2.9 MB)

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