Post on 12-Feb-2017
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
Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues3
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
Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues4
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.
Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues5
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.
Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues6
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.
Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues7
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
Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues8
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
Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues9
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
Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues10
Concrete RheometersTattersall Two-Point Rheometer IBB Rheometer ICAR Rheometer
BML ViscometerBTRHEOM Rheometer
Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues11
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.
Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues12
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.
Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues13
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
Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues14
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.
Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues15
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.
Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues16
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 Issues17
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
Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues18
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
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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
Elapsed Time (Minutes)
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
Elapsed Time (Minutes)
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.
Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues20
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.
Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues21
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
Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues22
SCC Case Studies
Formwork pressure
Segregation resistance
Pumpability
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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 Issues24
SCC Case Study: 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.
Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues25
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 Issues26
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.
Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues27
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.
Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues28
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
Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues29
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 Issues30
SCC Case Study: Pumpability
Duke Energy Building, Charlotte, NC
Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues31
SCC Case Study: Pumpability
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
Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues32
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