ACI CQTM cheat sheet

Coefficient of variation: CV = (standard deviation / mean strength) × 100%; lower CV means tighter process control.

Standard deviation of compressive strength: s = sqrt( Σ(xi − x̄)² / (n − 1) ); use n − 1 for sample estimates.

ACI 318 acceptance: every strength test average of two 6×12 in. cylinders must equal or exceed f'c; no single cylinder below f'c − 500 psi.

ACI 318 strength criterion: averages of any three consecutive tests must equal or exceed f'c; below f'c by more than 500 psi triggers review.

Required average strength f'cr is set from standard deviation: f'cr = f'c + 1.34s (probability) and f'cr = f'c + 2.33s − 500 psi (absolute), whichever is greater.

When no prior data exist, default f'cr margins over f'c: 1000 psi for f'c < 3000 psi; 1200 psi for 3000–5000 psi; 1400 psi for 5000–7000 psi; 1600 psi above 7000 psi.

Slump test (ASTM C143) measures workability/consistency; inverted slump cone (ASTM C995) times the flow of freshly mixed concrete.

Air content by pressure meter (ASTM C231) or volumetric method (ASTM C173); entrained air improves freeze-thaw durability.

Curing standard: maintain moisture and temperature above 50 °F; ASTM C31 field cylinders are standard-cured in lime water or moist room before testing.

Slump limits for normal-weight concrete: 1–4 in. typical, up to 6 in. when vibration/consolidation is used, per ACI 301/318 guidance.

Minimum cover per ACI 318: 1.5 in. for cast-in-place beams/columns not exposed to weather; 2 in. for slabs/walls exposed to earth or weather.

Consolidation: internal vibrators should penetrate into the previous lift and be withdrawn slowly; over-vibration causes segregation.

Formwork removal: sides of beams/columns may be removed after concrete hardens sufficiently; soffits depend on strength gain and member span.

Cold weather: protect concrete from freezing until it reaches at least 500 psi; use heated enclosures and insulated forms.

Hot weather: cool aggregates/water, place at lower slump, minimize time between mixing and placement, and cure promptly.

Joints: construction joints are planned stopping points; contraction joints control cracking; isolation joints separate structures.

Finishing sequence: strike off, bull float, wait for bleed water to evaporate, then float and trowel; never finish with bleed water present.

Pumping: maintain continuous supply, prime the line, and avoid long vertical drops that cause segregation.