Hardened Concrete Properties and Testing
This domain tests the relationship between specimen, test method and the number you are allowed to use in design: cylinder versus cube, standard versus field curing, and single results versus statistical acceptance.
Core relationships
- Compressive strength uses 150 by 300 mm cylinders (ASTM C39) moist cured at 23 plus or minus 2 degrees C and tested at 28 days. A 150 mm cube reads about 25 percent higher than a cylinder, so cylinder strength is roughly 0.8 times cube strength.
- Modulus of elasticity approximates 4700 times the square root of f'c in MPa.
- Modulus of rupture from third-point loading approximates 0.62 times the square root of f'c; splitting tensile approximates 0.56 times the square root of f'c.
- Nondestructive tests, rebound (ASTM C805) and ultrasonic pulse velocity (ASTM C597), correlate with strength only through a project-specific curve.
Acceptance (ACI 318)
Every average of three consecutive strength tests must equal or exceed f'c, and no single test may fall below f'c minus 3.5 MPa. Cores (ASTM C42) are acceptable if their average is at least 85 percent of f'c and no single core is below 75 percent.
Worked example
For f'c = 35 MPa, three consecutive results of 36.5, 34.0 and 31.0 MPa average 33.8 MPa, below 35 MPa, and the 31.0 MPa result is also below 31.5 MPa. The lot fails both criteria; investigate curing, batching and testing before accepting cores as a remedy.
Traps
Substituting cube strength into ACI equations; testing cylinders that dried in the field; averaging all results instead of consecutive sets; treating rebound numbers as direct strength.