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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. 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 Koehler W.R. Grace & Co.

    2. 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

    3. 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.

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

    5. 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.

    6. Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues 6 Concrete Rheology: Non-Steady State 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

    7. 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

    8. Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues 8 Thixotropy Manifestation in Concrete Delivery

    9. 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)

    10. Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues 10 Concrete Rheometers

    11. Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues 11 ICAR Rheometer

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

    13. 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

    14. Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues 14 SCC: Specification

    15. Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues 15 SCC: Specification

    16. 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 ) 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)

    17. 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

    18. Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues 18 SCC: Design

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

    20. Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues 20 SCC: Design

    21. 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

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

    23. 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)

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

    25. 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

    26. Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues 26 SCC Case Study: Segregation Resistance

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

    28. 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

    29. 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

    30. Tenth CANMET/ACI International Conference on Recent Advances in Concrete Technology and Sustainability Issues 30 SCC Case Study: Pumpability

    31. 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

    32. 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

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