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Chapter 3. Obtaining Silica-Fume Concrete

Chapter 3. Obtaining Silica-Fume Concrete. Specifying Silica Fume and SFC Proportioning SFC Producing SFC. Specifying Silica Fume and Silica-Fume Concrete. Chapter Outline. Specifying Silica Fume. ASTM C 1240 AASHTO M 307 CSA A 23.5. Key Provisions: ASTM C 1240.

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Chapter 3. Obtaining Silica-Fume Concrete

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  1. Chapter 3. Obtaining Silica-Fume Concrete • Specifying Silica Fume and SFC • Proportioning SFC • Producing SFC

  2. Specifying Silica Fumeand Silica-Fume Concrete Chapter Outline

  3. Specifying Silica Fume • ASTM C 1240 • AASHTO M 307 • CSA A 23.5

  4. Key Provisions: ASTM C 1240 • SiO2 content, 85% min • Loss on ignition, 6% max • Retained, 45-µm sieve, 10% max • Specific Surface, BET, 15 m2/g min

  5. Specifying Silica-Fume ConcreteFollow the practices of your organization

  6. Minimum Specification Elements • Required concrete performance • Test placement requirements • Type of finish desired • Protection requirements • Curing requirements

  7. Proportioning Silica-Fume Concrete Chapter Outline

  8. General Rules for Proportioning Silica-Fume Concrete • There is no empirical method • Local materials, local proportions • Follow the step-by-step procedure • Test, test, test

  9. Proportioning Step 1: Determine project requirements Compressive strength Chloride exposure Freezing and thawing exposure Chemical exposure Abrasion resistance Other requirements

  10. Proportioning Step 2: Coordinate with contractor Special constructability requirements? Maximum size aggregate Placement method Maximum slump -- Concrete will be cohesive and won’t segregate -- increase slump by about 2 in. (50 mm)

  11. Proportioning Step 3: Select starting mixture Use SFA table to find a mixture with approximately your project requirements

  12. Proportioning Step 4: Determine volume of entrained air Air is absolutely required if silica-fume concrete will be exposed to freezing and thawing while saturated Use an industry standard document (ACI 211.1, ACI 318, or ASTM C 94) for recommended air content Air may be reduced 1% if compressive strength is over 5,000 psi (35 MPa)

  13. Proportioning Step 5: Add your local aggregates Calculate total aggregate volume to achieve desired yield Use ratio of fine to coarse aggregate that works works your materials

  14. Proportioning Step 6: Prepare laboratory trial mixtures, 1 of 2 Ensure that densified silica fume is adequately dispersed Don’t worry about water slump -- there may not be any! Batch at maximum allowed water content -- use WRA and HRWRA to develop workability

  15. Proportioning Step 6: Prepare laboratory trial mixtures, 2 of 2 • Does concrete meet fresh and hardened requirements? • Adjust mixture proportions as necessary

  16. Proportioning Step 7: Conduct full-scale testing Use same plant and trucks, if possible Batch enough concrete, at least 2 yd3 (1.5 m3) -- bad time to save $$$ Does concrete meet all requirements? Adjust mixture as necessary Work with the contractor

  17. Proportioning Example 1 Parking Structure

  18. Low chloride permeability Compressive strength of 5,000 psi (35 MPa) at 28 days Reduced heat and shrinkage Reduced rate of strength gain Protection against freezing and thawing Proportioning Example 1, Step 1: Project Requirements

  19. Proportioning Example 1, Step 2: Contractor Requirements • Use 1 in. (25 mm) maximum aggregate • 4 to 6 in. (100 to 150 mm) slump • Pump placement

  20. Proportioning Example 1, Step 3: Starting Mixture from SFA Table • Cement: 500 lb yd3 (300 kg/m3) • Fly ash: 135 lb/yd3 (80 kg/m3) • Silica fume: 40 lb/yd3 (25 kg/m3) • Maximum w/cm: 0.40 • Entrained air: required

  21. Proportioning Example 1, Step 4: Entrained Air Requirements • ACI 211.1 recommends 6.0% total air for severe exposure • Compressive strength > 5,000 psi (35 MPa) allows for reduction to 5.0%

  22. Proportioning Example 1, Step 5: Add Local Aggregates, (1 of 2) Total Paste Volume: 9.38 ft3

  23. Proportioning Example 1, Step 5: Add Local Aggregates, (2 of 2) • Coarse aggregate density: 2.68 • Fine aggregate density: 2.64 • Fine aggregate: 40% of total aggregate volume • Aggregate vol = 27.00 - 9.38 = 17.62 ft3 • Fine aggregate: 7.05 ft3, 1,160 lb/yd3 • Coarse aggregate: 10.57 ft3, 1,770 lb/yd3

  24. SI Proportioning Example 1, Step 5: Add Local Aggregates, (1 of 2) Total Paste Volume: 0.350 m3

  25. SI Proportioning Example 1, Step 5: Add Local Aggregates, (2 of 2) • Coarse aggregate density: 2.68 • Fine aggregate density: 2.64 • Fine aggregate: 40% of total aggregate volume • Aggregate vol = 1.000 - 0.350 = 0.650 m3 • Fine aggregate: 0.260 m3, 686 kg/m3 • Coarse aggregate: 0.390 m3, 1045 kg/m3

  26. Proportioning Example 1, Step 6: Prepare Lab Mixtures • You are now ready to go into a laboratory and begin making trial mixtures. • Control silica fume dispersion • Mix thoroughly • Conduct necessary testing on fresh and hardened concrete

  27. Proportioning Example 1, Step 7: Prepare Lab Mixtures • Adjust from lab mixes as necessary • Use large enough batches • Work with contractor to conduct placing and finishing trials as required

  28. Proportioning Example 2 High-Strength Columns

  29. Design compressive strength of 14,000 psi (98 MPa) at 28 days Proportion for 15,400 psi (108 MPa) in laboratory mixes No exposure to freezing and thawing Proportioning Example 2, Step 1: Project Requirements

  30. Proportioning Example 2, Step 2: Contractor Requirements • Use 1/2 in. (13 mm) maximum aggregate • 8 to 10 in. (200 to 250 mm) slump • Pump placement

  31. Proportioning Example 2, Step 3: Starting Mixture from SFA Table • Cement: 800 lb yd3 (475 kg/m3) • Fly ash: 175 lb/yd3 (104 kg/m3) • Silica fume: 125 lb/yd3 (74 kg/m3) • Maximum w/cm: 0.231 • Entrained air: none

  32. Proportioning Example 2, Step 4: Entrained Air Requirements • Entrained air not required

  33. Proportioning Example 2, Step 5: Add Local Aggregates, (1 of 2) Total Paste Volume: 10.58 ft3

  34. Proportioning Example 2, Step 5: Add Local Aggregates, (2 of 2) • Coarse aggregate density: 2.68 • Fine aggregate density: 2.60 • Fine aggregate: 38% of total aggregate volume • Aggregate vol = 27.00 - 10.58 = 16.42 ft3 • Fine aggregate: 6.24 ft3, 1,000 lb/yd3 • Coarse aggregate: 10.18 ft3, 1,700 lb/yd3

  35. SI Proportioning Example 2, Step 5: Add Local Aggregates, (1 of 2) Total Paste Volume: 0.393 m3

  36. SI Proportioning Example 2, Step 5: Add Local Aggregates, (2 of 2) • Coarse aggregate density: 2.68 • Fine aggregate density: 2.64 • Fine aggregate: 38% of total aggregate volume • Aggregate vol = 1.000 - 0.393 = 0.607 m3 • Fine aggregate: 0.231 m3, 600 kg/m3 • Coarse aggregate: 0.376 m3, 1010 kg/m3

  37. Proportioning Example 2, Step 6: Prepare Lab Mixtures • You are now ready to go into a laboratory and begin making trial mixtures. • Control silica fume dispersion • Mix thoroughly • Conduct necessary testing on fresh and hardened concrete

  38. Proportioning Example 2, Step 7: Prepare Lab Mixtures • Adjust from lab mixes as necessary • Use large enough batches • Work with contractor to conduct placing trials as required

  39. Statistical Approach to Determining Proportions If you have a very complex project with a number of requirements, simply making trial batches may not be the most efficient approach. Help is available to optimize concrete performance and cost.

  40. Producing Silica-Fume Concrete Chapter Outline

  41. Producing Silica-Fume Concrete • Measuring and batching

  42. Tank for storage of slurried silica fume

  43. Emptying bagged silica fume into truck mixer

  44. Adding repulpable bags directly to truck mixer

  45. Producing Silica-Fume Concrete • Measuring and batching • Adding HRWRA

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