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

Specifying Silica Fume and Silica-Fume Concrete Chapter Outline

 ASTM C 1240  AASHTO M 307  CSA A 23.5 Specifying Silica Fume

Key Provisions: ASTM C 1240  SiO 2 content, 85% min  Loss on ignition, 6% max  Retained, 45-µm sieve, 10% max  Specific Surface, BET, 15 m 2 /g min

Specifying Silica-Fume Concrete Follow the practices of your organization

Minimum Specification Elements  Required concrete performance  Test placement requirements  Type of finish desired  Protection requirements  Curing requirements

Proportioning Silica- Fume Concrete Chapter Outline

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

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

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)

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

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)

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

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

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

Proportioning Step 7: Conduct full-scale testing  Use same plant and trucks, if possible  Batch enough concrete, at least 2 yd 3 (1.5 m 3 ) -- bad time to save $$$  Does concrete meet all requirements?  Adjust mixture as necessary  Work with the contractor

Proportioning Example 1 Parking Structure

 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

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

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

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

Proportioning Example 1, Step 5: Add Local Aggregates, (1 of 2) Total Paste Volume: 9.38 ft 3

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 = = ft 3  Fine aggregate: 7.05 ft 3, 1,160 lb/yd 3  Coarse aggregate: ft 3, 1,770 lb/yd 3

Proportioning Example 1, Step 5: Add Local Aggregates, (1 of 2) Total Paste Volume: m 3 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 = = m 3  Fine aggregate: m 3, 686 kg/m 3  Coarse aggregate: m 3, 1045 kg/m 3 SI

 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 Proportioning Example 1, Step 6: Prepare Lab Mixtures

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

Proportioning Example 2 High-Strength Columns

 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

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

 Cement: 800 lb yd 3 (475 kg/m 3 )  Fly ash: 175 lb/yd 3 (104 kg/m 3 )  Silica fume: 125 lb/yd 3 (74 kg/m 3 )  Maximum w/cm:  Entrained air: none Proportioning Example 2, Step 3: Starting Mixture from SFA Table

 Entrained air not required Proportioning Example 2, Step 4: Entrained Air Requirements

Proportioning Example 2, Step 5: Add Local Aggregates, (1 of 2) Total Paste Volume: ft 3

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 = = ft 3  Fine aggregate: 6.24 ft 3, 1,000 lb/yd 3  Coarse aggregate: ft 3, 1,700 lb/yd 3

Proportioning Example 2, Step 5: Add Local Aggregates, (1 of 2) Total Paste Volume: m 3 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 = = m 3  Fine aggregate: m 3, 600 kg/m 3  Coarse aggregate: m 3, 1010 kg/m 3 SI

 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 Proportioning Example 2, Step 6: Prepare Lab Mixtures

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

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.

Producing Silica-Fume Concrete Chapter Outline

Producing Silica-Fume Concrete  Measuring and batching

Tank for storage of slurried silica fume

Emptying bagged silica fume into truck mixer

Adding repulpable bags directly to truck mixer

Producing Silica-Fume Concrete  Measuring and batching  Adding HRWRA

Producing Silica-Fume Concrete  Measuring and batching  Adding HRWRA  Mixing

Truck Mixer Rating Plate -- Don’t exceed rated mixing capacity when producing silica- fume concrete.

Producing Silica-Fume Concrete  Measuring and batching  Adding HRWRA  Mixing  Controlling temperature

Liquid nitrogen being used to cool silica-fume concrete

Producing Silica-Fume Concrete Follow established and documented procedures

End of Chapter 3 Main Outline