ACI Strength Testing cheat sheet

Cylinder molds: 4 in. (100 mm) and 6 in. (150 mm) diameter; length must be twice the diameter, so 4x8 in. or 6x12 in.

Cylinders are rodded in layers: 4x8 in. cylinders get 2 layers (25 blows each); 6x12 in. cylinders get 3 layers (25 blows per layer for 4 in. molds, 25 per layer for 6 in. molds per ACI/ASTM practice).

Tamping rod diameter: 5/8 in. (16 mm) for 6 in. cylinders; 3/8 in. (10 mm) for 4 in. cylinders.

Standard curing for strength specimens: 68 ± 2 °F (20 ± 2 °C) moist environment until testing; field-cured specimens follow job-site conditions.

Slump test measures consistency; the slump cone is 12 in. tall with 4 in. top and 8 in. base diameter; rod in 3 layers of 25 blows.

Air content by pressure meter or volumetric method; typical acceptance range for normal concrete is 4-8% depending on exposure.

Concrete temperature at placement: typically 50-90 °F (10-32 °C) per ACI 301; record at time of sampling.

Sampling: composite sample from mid-discharge of the mixer; test within 15 minutes of sampling for slump and air.

Cylinders are capped or ground flat before compression testing; cap thickness and planeness affect measured strength.

Compressive strength f'c is measured on standard 28-day moist-cured cylinders; design strength is typically specified at 28 days.

Compressive stress = load / area; 6 in. cylinder area = 28.27 in²; 4 in. cylinder area = 12.57 in².

Test loading rate for compression: 35 ± 7 psi/s (0.25 ± 0.05 MPa/s) for standard cylinders; maintain within tolerance.

Acceptance criteria (ACI 318): average of three consecutive tests ≥ f'c; no single test < f'c − 500 psi.

Splitting tensile strength fct = 2P/(π·L·d); typically 6-8% of compressive strength for normal-weight concrete.

Modulus of rupture (flexural): third-point loading; fr = PL/(bd²) for third-point; typically 7.5√f'c psi.

Elastic modulus Ec ≈ 57,000√f'c psi (normal weight); varies with aggregate and density.

Cylinder end condition: capped or ground; failure mode should be a proper cone or shear, not columnar or cap-related.

Field-cured cylinders assess structural adequacy; standard-cured cylinders assess mix design compliance.