Selection and Design of Concrete Mixes

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Selection and Design of Concrete Mixes

• Concrete Technology
• George R. White

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Objectives of properly mixed concrete

• Required Qualities of Hardened Concrete
• Workability of Fresh Concrete

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Assumptions

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

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Water to Cement Ratio

• Initial selection based on environmental exposure
  and type of structural member being cast

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Water to Cement Ratio

• Secondary selection based on required strength at
  specified break ( e.g. 3000 psi _at_ 28 days)
• Add 15 for revised compressive strength

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Water to Cement Ratio

• Chose the LOWER (more conservative) W/C Ratio
  that satisfies BOTH criteria

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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

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Air Content

• Assume maximum aggregate Size
• Determine whether concrete is air-entrained
• Find recommended air content

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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

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Given/ Determined

• 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

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Mix Design Table (1CY)

• Water 280
• Cement 560
• Fines 1020
• Coarse 1940

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Admixtures for Concrete
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