Specimen Preparation, Curing, and Compressive Strength Testing
Specimen preparation, curing, and compressive strength testing covers ASTM C31 field specimens, C192 laboratory specimens, C617 capping, C1231 neoprene pads, C511 curing, and C39 compression testing. The exam wants the details that make a cylinder valid.
Making cylinders
- 6 x 12 in cylinders: three layers, 25 rod blows per layer. 4 x 8 in: two layers, 25 blows per layer.
- Rod uniformly, tap the mold, strike off, and cover to prevent evaporation.
- Initial curing: 60–80°F, protected from moisture loss; standard moist curing at 73.5 ± 3.5°F.
Capping and testing
- C617 sulfur mortar caps: maximum thickness 1/8 in; ends must be flat and perpendicular.
- C1231 pads are an alternative when the specification permits and strength limits are met.
- C39: load at 35 ± 7 psi/s for 6 x 12 in cylinders; verify the testing machine within 12 months.
Worked example
A 4 x 8 in cylinder has an area of 12.57 in². It fails at 46,900 lb. Compressive strength = 46,900 ÷ 12.57 = 3,731 psi. Round to the nearest 10 psi: 3,730 psi. Do not apply a size correction factor; the result is reported as tested.
Traps
Caps that are too thick or not perpendicular; testing specimens that are dry or have been damaged; loading too fast; using neoprene pads outside their permitted strength range; and treating a single cylinder as the strength test when ACI 318 defines the test as an average of two 6 x 12 in or three 4 x 8 in cylinders.