Koehler Rheology V1 6c412r
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Use of Rheology to Design, Specify, and Manage Self-Consolidating Concrete
Eric Koehler W.R. Grace & Co.
Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues
Outline Rheology • Definition • Measurement
SCC Rheology • Specification • Design • Management
Case Studies • Formwork pressure • Segregation resistance • Pumpability
Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues 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 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,
Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues 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
Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues 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 a0 a b
b 0 a0 a b
Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues 5
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) Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues 6
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 when: • Measuring rheology
Flow curve area
Stress growth
• Proportioning concrete for applications
Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues 7
Thixotropy Manifestation in Concrete Delivery
Yield Stress
Change in yield stress from mixing through delivery and placement
Static Yield Stress of Non-Agitated SCC No Breakdown, Full Thixotropy Concrete is in formwork; at-rest structure rebuilds and static yield stress increases
Concrete is partially agitated during transit, preventing full build-up of at-rest structure
Static Yield Stress of SCC During Placement
Dynamic Yield Stress Full Breakdown, No Thixotropy
Time from Mixing Concrete is discharged into forms resulting shearing causes full breakdown of at-rest structure tu
Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues 8
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)
Typical Rheometer Geometry Configurations Coaxial Cylinders
Parallel Plate
Impeller
Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues 9
Concrete Rheometers Tattersall Two-Point Rheometer
BTRHEOM Rheometer
IBB Rheometer
ICAR Rheometer
BML Viscometer
Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues 10
ICAR Rheometer ICAR rheometer was used for the case studies described in this presentation.
Vane Geometry
Example Test Protocols Stress Growth Test Protocol: rotate vane at 0.05 rps, concrete maintained at rest before test Results: static yield stress (peak stress)
Flow Curve Test
H: 5 in (125 mm) D: 5 in (125 mm)
Protocol: 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)
Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues 11
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)
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. Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues 12
SCC: Specification SCC workability is described in of the following: • Filling ability • ing ability • Segregation resistance (stability)
Static segregation resistance
Dynamic segregation resistance
Each property should be evaluated independently Minimum requirements for each property vary by application
Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues 13
SCC: Specification ASTM tests are available to measure the three SCC properties independently. Filling Ability
ing Ability
Segregation Resistance
Slump Flow ASTM C 1611
J-Ring ASTM C 1621
Column Segregation ASTM C 1610
Test requirements vary between lab and field. Property
Laboratory (Pre-Qualification)
Field (Quality Control)
Filling Ability (Slump Flow)
Yes.
Yes. Provides indirect measurement of yield stress and plastic viscosity.
ing 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).
By confirming robustness in lab and closely controlling materials, fewer tests may be needed in field. Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues 14
SCC: Specification 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. Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues 15
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)
Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues 16
SCC: Design Both the mixture proportions and the ixture 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
Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues 17
SCC: Design 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. Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues 18
SCC: Design 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
60
90
120 w/c = 0.35
120
0.45 PC 068
PC 068 PC 059 PC 915 80 60 40
Thixotropy (Nm/s)
100
30
Elapsed Time (Minutes)
Elapsed Time (Minutes)
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
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 90 Issues 120 0 30 60 90 Tenth CANMET/ACI International Conference on120 Recent Advances in Concrete Technology and60Sustainability
Elapsed Time (Minutes)
Elapsed Time (Minutes)
19
SCC: Design Concrete can be modeled as a concentration suspension. These model can be used to design mixture proportions.
=solid volume concentration =Huggins coefficient
=viscosity of suspension
=viscosity of suspending medium
=intrinsic viscosity
ICAR Mixture Proportioning Procedure • Based on concrete as concentrated suspension of aggregates in paste • Includes equation for calculating required paste volume.
Factors Aggregates
Paste Volume Reference: Koehler, E.P., Fowler, D.W. (2007). “ICAR Mixture Proportioning Procedure for SCC” International Center for Aggregates Research, Austin, TX.
Paste Composition
Sub-Factors Maximum Size Grading Shape Filling Ability ing Ability Robustness Water Powder Air
Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues 20
SCC: Management 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 encomes 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 Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues 21
SCC Case Studies Formwork pressure Segregation resistance
Pumpability
Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues 22
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) Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues 23
Mix 2 (Increased CA) Mix 3 (Lower w/cm, Different ix)
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 ix)
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)
SCC Case Study: Formwork Pressure
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 Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues 24
SCC Case Study: 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
Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues 25
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 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 Deg 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.
Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues 26
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)
SCC Case Study: 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. Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues 27
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
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 8L 3 w 3 w 4
P pressure Q flow rate L tube length R tube radius w shear stress at wall Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues 28
SCC Case Study: Pumpability 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 Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues 29
SCC Case Study: Pumpability Duke Energy Building, Charlotte, NC
Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues 30
SCC Case Study: Pumpability Duke Energy Building, Charlotte, NC
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
3.0
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
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 0.30 Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues
Rotation Speed (rps)
31
Conclusions Concrete rheology is a useful tool for specifying, deg, 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
Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues 32
Eric Koehler W.R. Grace & Co.
Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues
Outline Rheology • Definition • Measurement
SCC Rheology • Specification • Design • Management
Case Studies • Formwork pressure • Segregation resistance • Pumpability
Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues 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 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,
Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues 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
Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues 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 a0 a b
b 0 a0 a b
Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues 5
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) Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues 6
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 when: • Measuring rheology
Flow curve area
Stress growth
• Proportioning concrete for applications
Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues 7
Thixotropy Manifestation in Concrete Delivery
Yield Stress
Change in yield stress from mixing through delivery and placement
Static Yield Stress of Non-Agitated SCC No Breakdown, Full Thixotropy Concrete is in formwork; at-rest structure rebuilds and static yield stress increases
Concrete is partially agitated during transit, preventing full build-up of at-rest structure
Static Yield Stress of SCC During Placement
Dynamic Yield Stress Full Breakdown, No Thixotropy
Time from Mixing Concrete is discharged into forms resulting shearing causes full breakdown of at-rest structure tu
Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues 8
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)
Typical Rheometer Geometry Configurations Coaxial Cylinders
Parallel Plate
Impeller
Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues 9
Concrete Rheometers Tattersall Two-Point Rheometer
BTRHEOM Rheometer
IBB Rheometer
ICAR Rheometer
BML Viscometer
Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues 10
ICAR Rheometer ICAR rheometer was used for the case studies described in this presentation.
Vane Geometry
Example Test Protocols Stress Growth Test Protocol: rotate vane at 0.05 rps, concrete maintained at rest before test Results: static yield stress (peak stress)
Flow Curve Test
H: 5 in (125 mm) D: 5 in (125 mm)
Protocol: 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)
Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues 11
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)
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. Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues 12
SCC: Specification SCC workability is described in of the following: • Filling ability • ing ability • Segregation resistance (stability)
Static segregation resistance
Dynamic segregation resistance
Each property should be evaluated independently Minimum requirements for each property vary by application
Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues 13
SCC: Specification ASTM tests are available to measure the three SCC properties independently. Filling Ability
ing Ability
Segregation Resistance
Slump Flow ASTM C 1611
J-Ring ASTM C 1621
Column Segregation ASTM C 1610
Test requirements vary between lab and field. Property
Laboratory (Pre-Qualification)
Field (Quality Control)
Filling Ability (Slump Flow)
Yes.
Yes. Provides indirect measurement of yield stress and plastic viscosity.
ing 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).
By confirming robustness in lab and closely controlling materials, fewer tests may be needed in field. Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues 14
SCC: Specification 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. Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues 15
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)
Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues 16
SCC: Design Both the mixture proportions and the ixture 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
Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues 17
SCC: Design 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. Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues 18
SCC: Design 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
60
90
120 w/c = 0.35
120
0.45 PC 068
PC 068 PC 059 PC 915 80 60 40
Thixotropy (Nm/s)
100
30
Elapsed Time (Minutes)
Elapsed Time (Minutes)
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
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 90 Issues 120 0 30 60 90 Tenth CANMET/ACI International Conference on120 Recent Advances in Concrete Technology and60Sustainability
Elapsed Time (Minutes)
Elapsed Time (Minutes)
19
SCC: Design Concrete can be modeled as a concentration suspension. These model can be used to design mixture proportions.
=solid volume concentration =Huggins coefficient
=viscosity of suspension
=viscosity of suspending medium
=intrinsic viscosity
ICAR Mixture Proportioning Procedure • Based on concrete as concentrated suspension of aggregates in paste • Includes equation for calculating required paste volume.
Factors Aggregates
Paste Volume Reference: Koehler, E.P., Fowler, D.W. (2007). “ICAR Mixture Proportioning Procedure for SCC” International Center for Aggregates Research, Austin, TX.
Paste Composition
Sub-Factors Maximum Size Grading Shape Filling Ability ing Ability Robustness Water Powder Air
Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues 20
SCC: Management 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 encomes 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 Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues 21
SCC Case Studies Formwork pressure Segregation resistance
Pumpability
Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues 22
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) Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues 23
Mix 2 (Increased CA) Mix 3 (Lower w/cm, Different ix)
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 ix)
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)
SCC Case Study: Formwork Pressure
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 Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues 24
SCC Case Study: 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
Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues 25
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 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 Deg 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.
Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues 26
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)
SCC Case Study: 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. Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues 27
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
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 8L 3 w 3 w 4
P pressure Q flow rate L tube length R tube radius w shear stress at wall Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues 28
SCC Case Study: Pumpability 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 Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues 29
SCC Case Study: Pumpability Duke Energy Building, Charlotte, NC
Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues 30
SCC Case Study: Pumpability Duke Energy Building, Charlotte, NC
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
3.0
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
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 0.30 Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues
Rotation Speed (rps)
31
Conclusions Concrete rheology is a useful tool for specifying, deg, 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
Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues 32
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