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Engineer Concrete study materials
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Engineer Concrete: Concrete Materials and Mix Proportioning

Engineer Concrete · Work through your exam, one topic at a time.

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Concrete Materials and Mix Proportioning

This domain tests whether you can choose constituents and proportion a mixture that satisfies strength, durability and placeability at once, and whether you can defend the water-to-binder ratio, binder content and air content against ACI 211 proportioning, ACI 318 durability limits and the project specification.

Constituent selection

  • Cement type per ASTM C150: Type I general purpose, II moderate sulfate resistance and lower heat, III high early strength, V high sulfate resistance; ASTM C595 blended cements are now common.
  • Supplementary cementitious materials such as fly ash, slag and silica fume lower permeability and heat of hydration and raise long-term strength, but they increase early-age sensitivity and require longer moist curing.
  • Aggregates make up roughly 60 to 75 percent of the volume; grading and fineness modulus drive paste demand, and alkali-silica reactivity must be screened before use.
  • Admixtures cover air entrainment, water reduction, retardation, acceleration and corrosion inhibition.

Proportioning by absolute volume

Target strength is the larger of f'c + 1.34s and f'c + 2.33s minus 3.45 MPa. Water demand follows slump and nominal maximum aggregate size; binder follows the governing water-to-binder ratio, whichever is lower between strength and durability; air follows exposure; coarse aggregate volume comes from the ACI 211 table; fine aggregate fills the remaining absolute volume.

Worked example

A 30 MPa air-entrained mix with 25 mm aggregate, 75 to 100 mm slump and a maximum w/c of 0.45. Water is about 165 kg/m3, air 6 percent equals 60 L, so binder is 165/0.45 = 367 kg/m3. With 25 percent Class F fly ash: 275 kg cement plus 92 kg fly ash, w/cm still 0.45. Absolute volumes: water 165 L, binder 118 L, air 60 L, coarse aggregate 1050 kg at relative density 2.70 equals 389 L. Fine aggregate is 1000 minus 732 = 268 L, about 710 kg at 2.65 relative density.

Traps

Confusing w/c with w/cm once SCMs are added; ignoring aggregate moisture and batching by saturated-surface-dry weights; forgetting that each 1 percent of entrained air costs roughly 4 to 6 percent strength; specifying a minimum cement content but no maximum w/c.