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Use of Rheology to Design, Specify, and Manage Self-Consolidating Concrete

Use of Rheology to Design, Specify, and Manage Self-Consolidating Concrete

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Use of Rheology to Design, Specify, and Manage Self-Consolidating Concrete

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  1. 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

  2. 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

  3. 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 slope = plastic viscosity (m) Shear Stress, (Pa) intercept = yield stress (t0) Shear Rate, (1/s) Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues

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

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

  6. Concrete Rheology: Non-Steady State Flow Curve Test concrete sheared at various rates 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 Concrete exhibits different rheology when at rest than when flowing. 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) Thixotropy is especially critical in highly flowable concretes. Time (s) Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues

  7. 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 Issues

  8. Thixotropy Manifestation in Concrete Delivery Change in yield stress from mixing through delivery and placement Static Yield Stress of Non - Agitated SCC No Breakdown, Full Static Yield Stress Thixotropy of SCC During Placement Concrete is in formwork; a t - rest structure rebuilds Yield Stress and static yield stress increases Dynamic Yield Stress Full Breakdown, No Thixotropy Time from Mixing Concrete is partially agitated during transit , Concrete is discharged into forms preventing full build - up resulting shearing causes full of at - rest structure breakdown of at-rest structure t u Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues

  9. 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

  10. Concrete Rheometers Tattersall Two-Point Rheometer IBB Rheometer ICAR Rheometer BTRHEOM Rheometer BML Viscometer Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues

  11. ICAR Rheometer ICAR rheometer was used for the case studies described in this presentation. 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 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) Vane Geometry H: 5 in (125 mm) D: 5 in (125 mm) Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues

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

  13. 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 Issues

  14. SCC: Specification ASTM tests are available to measure the three SCC properties independently. Filling Ability Passing 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. 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

  15. 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 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

  16. 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

  17. 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 Issues

  18. SCC: Design Effects of Materials and Mixture Proportions on Rheology 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. Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues

  19. SCC: Design 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. Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues

  20. SCC: Design Concrete can be modeled as a concentration suspension. These model can be used to design mixture proportions. =viscosity of suspension =solid volume concentration =Huggins coefficient =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. 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

  21. 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 Example Requires Vibration Good Segregation Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues

  22. SCC Case Studies • Formwork pressure • Segregation resistance • Pumpability Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues

  23. 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

  24. SCC Case Study: Formwork Pressure 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 Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues

  25. 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

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

  27. SCC Case Study: 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. Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues

  28. 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 sheared region flow plug flow region shear stress = yield stress Buckingham-Reiner Equation Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues

  29. 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

  30. SCC Case Study: Pumpability Duke Energy Building, Charlotte, NC Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues

  31. 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 Duke Energy Building, Charlotte, NC Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues

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