Download presentation
Presentation is loading. Please wait.
1
Eric Koehler W.R. Grace & Co. eric.koehler@grace.com
April 15, 2017 ICAR Rheometer Eric Koehler W.R. Grace & Co.
2
Outline What is Rheology? ICAR Rheometer Applications Definition
April 15, 2017 Outline What is Rheology? Definition Measurement ICAR Rheometer Description Operation Applications Mixture proportioning SCC Production control Formwork pressure Segregation resistance Pumpability
3
Concrete Rheology Shear Stress, (Pa) Shear Rate, (1/s)
April 15, 2017 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. Results Flow Curve The Bingham Model Shear Stress, (Pa) slope = plastic viscosity (m) intercept = yield stress (t0) Shear Rate, (1/s)
4
Workability and Rheology
April 15, 2017 Workability and Rheology ACI 238.1R-08 report describes 69 workability and rheology tests. 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
5
Concrete Flow Curves (Constitutive Models)
April 15, 2017 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.
6
Concrete Rheology: Non-Steady State
April 15, 2017 Concrete Rheology: Non-Steady State Concrete exhibits different rheology when at rest than when flowing.
7
Concrete Rheology: Non-Steady State
April 15, 2017 Concrete Rheology: Non-Steady State Flow Curve Test concrete sheared at various rates Concrete exhibits different rheology when at rest than when flowing. area between up and down curves due to thixotropy Shear Stress (Pa) Static Yield Stress minimum shear stress to initiate flow from rest Dynamic Yield Stress minimum shear stress to maintain flow after breakdown of thixotropic structure Plastic Viscosity change in shear stress per change in shear rate, above yield stress Thixotropy reversible, time-dependent reduction in viscosity in material subject to shear slope = plastic viscosity intercept = dynamic yield stress Shear Rate (1/s) Stress Growth Test concrete sheared at constant, low rate maximum stress from rest = static yield stress Torque (Nm) Thixotropy is especially critical in highly flowable concretes. Time (s)
8
Rheology Measurement: Typical Geometry
April 15, 2017 Rheology Measurement: Typical Geometry Rheometers continuously shear concrete through rotational movement. Rheometers must be uniquely designed for concrete (primarily due to large aggregate size) Results can be expressed in relative units (torque vs. speed) or absolute units (shear stress vs. shear rate) Typical Rheometer Geometry Configurations Coaxial Cylinders Parallel Plate Impeller
9
Tattersall Two-Point Rheometer
April 15, 2017 Concrete Rheometers Tattersall Two-Point Rheometer IBB Rheometer ICAR Rheometer BTRHEOM Rheometer BML Viscometer
10
ICAR Rheometer Portable concrete rheometer
April 15, 2017 ICAR Rheometer Portable concrete rheometer Laboratory Jobsite Appropriate for moderately and highly flowable concrete Measures slumps greater than 75 mm Especially well-suited for self-consolidating concrete Flexible interface allows measurement of Bingham parameters, thixotropy, and other protocols set by user
11
ICAR Rheometer: Operation
April 15, 2017 ICAR Rheometer: Operation Based on wide-gap, coaxial cylinders design Vane acts as inner cylinder Compact size Prevents slip Outer wall also has vertical strips to prevent slip Vane is immersed in concrete and rotated at different speeds Computer software operates test and computes results Single test complete in 60 seconds Vane can be replaced with any other type of impeller Apply Rotation, Measure Torque Inner Cylinder Outer Cylinder Fluid Side View Top View H: 5 in (125 mm) D: 5 in (125 mm)
12
ICAR Rheometer: Portability
April 15, 2017 ICAR Rheometer: Portability Rheometer Weight: 13 lb (6 kg) [with accessories: 40 lb (18 kg)] 4.25” (110 mm) 16” (400 mm) Bucket size depends on aggregate size. 1” (25 mm) aggregate shown
13
All data automatically written to text and Excel file
April 15, 2017 Software Interface All operations managed through single screen. All data automatically written to text and Excel file Stress Growth Flow Curve settings settings start start real time data real time data calculated results calculated results
14
Aggregate Size Vane is constant size for all aggregate sizes
April 15, 2017 Aggregate Size Vane is constant size for all aggregate sizes Height: 5 in. (125 mm) Diameter: 5 in. (125 mm) Outer container is selected based on aggregate size Horizontal and vertical gaps should be at least 4x the maximum aggregate size Larger container can be always be used, but smaller container should never be used.
15
Stress Growth Test Stress growth test consists of the following:
April 15, 2017 Stress Growth Test Stress growth test consists of the following: Rotate vane at low, constant speed Measure gradual increase in torque Identify maximum torque and convert to stress, which is equal to static yield stress Note: reduction in torque after peak value is associated with further yielding of material and is not typically analyzed further Material is previously at rest for pre-determined period to detect effect of thixotropy Vane speed is typically 0.01 to 0.05 rps
16
April 15, 2017 Flow Curve Test Flow curve test measures shear stress at different shear rates Raw torque vs. rotation speed data are converted to fundamental units of shear stress and shear rate Can also be used to measure thixotropy Software Inputs Test Units
17
April 15, 2017 Rheometer Test File All settings and results are written automatically to a summary text file. Raw data (instantaneous torque and rotation speed) can optionally be written to a file for Excel
18
Thixotropy Testing: Flow Curve or Stress Growth
April 15, 2017 Thixotropy Testing: Flow Curve or Stress Growth Flow Curve Test Flow Curve Test Place concrete in container and allow to rest for pre-determined time (to allow thixotropic build-up) Run flow curve with speeds in ascending order (low to high), exclude breakdown period Immediately run second curve with speeds in descending order (high to low), include breakdown period at high speed to assure full breakdown of thixotropy Area between up and down curves is indicative of thixotropy Stress Growth Test Run stress growth test, which measures the static yield stress The difference between the static yield stress and dynamic yield stress (flow flow curve) is indicative of thixotropy concrete sheared at various rates area between up and down curves due to thixotropy Shear Stress (Pa) slope = plastic viscosity intercept = dynamic yield stress Shear Rate (1/s) Stress Growth Test concrete sheared at constant, low rate maximum stress from rest = static yield stress Torque (Nm) Time (s)
19
Applications: Mixture Proportioning
April 15, 2017 Applications: Mixture Proportioning Both the mixture proportions and the admixture can adjusted to tailor the rheology to the application. Precast vs. ready mix SCC vs. conventional concrete Formwork pressure Pumpability Segregation resistance Mixing “Stickiness” and “Cohesion” Form surface finish Finishability
20
Applications: Mixture Proportioning
April 15, 2017 Applications: Mixture Proportioning Effects of Materials and Mixture Proportions on Rheology 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 Silica Fume HRWR Plastic Viscosity (Pa.s) AEA Water Yield Stress (Pa) Reference: Koehler, E.P., Fowler, D.W. (2007). “ICAR Mixture Proportioning Procedure for SCC” International Center for Aggregates Research, Austin, TX.
21
Applications: SCC Rheology
April 15, 2017 Applications: SCC Rheology Conventional Concrete 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 m t0 Similar plastic viscosity Shear Stress, (Pa) Near zero yield stress SCC m t0 Shear Rate, (1/s) Yield stress is the main difference between SCC and conventional concrete.
22
Applications: SCC Rheology
April 15, 2017 Applications: SCC Rheology 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 Reference: Koehler, E.P., Fowler, D.W. (2008). “Comparison of Workability Test Methods for Self-Consolidating Concrete” Submitted to Journal of ASTM International.
23
Applications: SCC Rheology
April 15, 2017 Applications: SCC Rheology 3 Different HRWRs | Same Slump Flow | Same Mix Design | Different Rheology 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.
24
Applications: Production Control
April 15, 2017 Applications: Production Control The workability box is an effective way to ensure production consistency Definition: Zone of rheology associated with acceptable workability (self-flow and segregation resistance) Mixture proportions affect rheology; therefore, controlling rheology is an effective way to control mixture proportions Workability boxes are mixture-specific SCC encompasses a wide range of materials and rheology Rheology appropriate for one set of materials may be inappropriate for another set of materials Larger workability box corresponds to greater robustness Example Requires Vibration Good Segregation
25
Applications: Formwork Pressure
April 15, 2017 Applications: Formwork Pressure Formwork pressure is related to concrete rheology Pressure is known to increase with slump SCC often exhibits high formwork pressure due to its high fluidity Concrete is at rest in forms, therefore, static yield stress is relevant Static yield stress is affected by dynamic yield stress and thixotropy SCC is placed in lifts, which takes advantage of thixotropy SCC must be designed to flow under its own mass and exert low formwork pressure Low dynamic yield stress (self flow) Fast increase in static yield stress (reduced formwork pressure)
26
Applications: Formwork Pressure – Case Study
April 15, 2017 Applications: Formwork Pressure – Case Study 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. 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
27
Applications: Segregation Resistance
April 15, 2017 Applications: Segregation Resistance SCC consists of aggregates suspended in a thixotropic, Bingham paste Paste must exhibit proper rheology to suspend a particular set of aggregates Static yield stress > minimum static yield stress: no segregation Static yield stress < minimum static yield stress: rate of descent of aggregate depends on paste yield stress and viscosity Gravitational Force -Aggregate density -Aggregate size Equations relating descent of sphere to rheology Reference Equation 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, Jossic, L., and Magnin, A. (2001). “Drag and Stability of Objects in a Yield Stress Fluid,” AIChE Journal, 47(12) Saak, A.W., Jennings, H.M., and Shah, S.P. (2001). “New Methodology for Designing Self-Compacting Concrete,” ACI Materials Journal, 98(6), Buoyancy + Resisting Force -Paste rheology -Paste density -Aggregate morphology -Neighboring aggregates (lattice effect) Reference: Koehler, E.P., and Fowler, D.W. (2008). “Static and Dynamic Yield Stress Measurements of SCC” Proceedings of SCC 2008, Chicago, IL.
28
Applications: Segregation Resistance
April 15, 2017 Applications: Segregation Resistance Segregation resistance increased with: Higher yield stress (static and dynamic yield stress assumed equal initially) Higher plastic viscosity Higher thixotropy Reference: Koehler, E.P., and Fowler, D.W. (2008). “Static and Dynamic Yield Stress Measurements of SCC” Proceedings of SCC 2008, Chicago, IL.
29
Applications: Pumpability
April 15, 2017 Applications: Pumpability Concrete moves through a pump line as a “plug” surrounded by a sheared region at the walls. Higher viscosity increases pumping pressure, reduces flow rate Unstable mixes may cause blocking Pumping concrete in high-rise buildings presents unique challenges High strength mixes often have low w/cm, resulting in high concrete viscosity Blockage can result in significant jobsite delays sheared region flow plug flow region shear stress = yield stress Buckingham-Reiner Equation
30
Applications: Pumpability – Case Study
April 15, 2017 Applications: Pumpability – Case Study Duke Energy Building, Charlotte, NC 52 Story Office Tower (764 ft) with 9 story building annex 8 Story Parking Structure 95 ft below street level Concrete Mixture Requirements Compressive Strength 5,000 psi to 18,000 psi (35 to 124 MPa) Modulus of Elasticity 4.6 to 8.0 x 106 psi (32 to 55 GPa) Workability 27 +/- 2 inch spread (690 +/- 50 mm) To meet compressive strength and elastic modulus requirements, the high strength concrete mixtures were proportioned with: Low w/c Silica fume High-modulus crushed coarse aggregate The resulting mixture exhibited: High viscosity High pump pressure Reference: Koehler, E.P., and Brooks, W., Neuwald, A., and Mogan, E.. (2009). “Applications of Rheology Measurements to Enable and Ensure Concrete Performance” NRMCA Concrete Technology Forum, Cincinnati, OH.
31
Applications: Pumpability – Case Study
April 15, 2017 Applications: Pumpability – Case Study Duke Energy Building, Charlotte, NC
32
Applications: Pumpability – Case Study
April 15, 2017 Applications: Pumpability – Case Study 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
33
Conclusions Rheology is the scientific description of workability.
April 15, 2017 Conclusions Rheology is the scientific description of workability. The ICAR rheometer enables portable rheology measurements in the lab and field. Measures concrete greater than 75 mm slump Measures yield stress, plastic viscosity, and thixotropy Rheology was shown to provide insights into the following applications: Mixture proportioning SCC Production control Formwork pressure Segregation resistance Pumpability
34
April 15, 2017 Thank You. Questions?
Similar presentations
