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Selection and Design of Concrete Mixes

Selection and Design of Concrete Mixes. Concrete Technology George R. White. Objectives of properly mixed concrete. Required Qualities of Hardened Concrete Workability of Fresh Concrete. Assumptions. Size of concrete member being cast and location and spacing of rebar within member

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Selection and Design of Concrete Mixes

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  1. Selection and Design of Concrete Mixes Concrete Technology George R. White

  2. Objectives of properly mixed concrete • Required Qualities of Hardened Concrete • Workability of Fresh Concrete

  3. Assumptions • Size of concrete member being cast and location and spacing of rebar within member • Environmental Exposure • Required strength of concrete

  4. Water to Cement Ratio • Initial selection based on environmental exposure and type of structural member being cast

  5. Water to Cement Ratio • Secondary selection based on required strength at specified break ( e.g. 3000 psi @ 28 days) • Add 15% for revised compressive strength

  6. Water to Cement Ratio • Chose the LOWER (more conservative) W/C Ratio that satisfies BOTH criteria

  7. Maximum Aggregate Size • No more than one-fifth minimum dimension of member • No more than three-fourths clear space between rebar or cover • For un-reinforced slabs, no more than one-third slab thickness • Choose maximum aggregate size that meets ALL above criteria

  8. Air Content • Assume maximum aggregate Size • Determine whether concrete is air-entrained • Find recommended air content

  9. Slump • Slump often specified • Slump an indicator of workability; with water-reducing admixtures, no longer a reliable indication of strength • See table for recommended slumps for various structural members

  10. Freeze-Thaw Minimum Concrete Member: 8” Cover: 1.5” Required 28 day strength: 3000 psi W/C Ratio 0.5 Maximum Aggregate Size: 1” Air Entrainment: yes Amount of Air Entrainment: 6% ( by volume) Expected Slump Given/ Determined

  11. Mix Design Table (1CY) • Water: 280# • Cement: 560# • Fines: 1020# • Coarse: 1940#

  12. Admixtures for Concrete

  13. END **

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