Compression Test on Concrete Cylinder

The compressive strength test is a crucial procedure used to determine the ability of a concrete cylinder to withstand axial loads. This method follows the ASTM C39 standard, which specifies the requirements for testing concrete specimens to ensure consistency, reliability, and compliance with project specifications.

Equipment

The primary equipment required for this test is a compression test machine, capable of applying loads at a controlled rate.

Sample Specifications

The test specimens are concrete cylinders, which must adhere to the following dimensional and physical tolerances:

• Diameter Variation: The diameter should not deviate by more than 2%.
• End Perpendicularity: The ends of the specimen should be perpendicular to the axis within a tolerance of 0.5°.
• Planarity of Ends: The ends must be planed within 0.05 mm. If this criterion is not met, capping is necessary to ensure a flat and even surface.

Procedure

1. Preparation of Specimen: Remove the concrete cylinders from moist storage just before testing and ensure they remain in a moist condition throughout the test.
2. Cleaning: Wipe the bearing faces of the upper and lower bearing blocks, as well as the test specimen, to ensure no debris interferes with the test.
3. Positioning: Place the specimen on the lower bearing block of the compression test machine. Carefully align the axis of the specimen with the center of thrust of the spherically seated upper block.
4. Initial Loading: Lower the upper bearing block onto the specimen and adjust the load to ensure uniform seating.
5. Application of Load: Apply the load continuously at a controlled rate of 0.25±0.05 MPa/s, ensuring no shocks or interruptions during loading.
6. Recording Results: Monitor the load until the specimen fails. Record the maximum load (P) in Newtons and note the failure mode. Common failure modes include cone, cone and shear, cone and split, shear, and split failures.

Calculation of Compressive Strength

1. Formula: The compressive strength (f) is calculated as: f=P/A

Where P is the maximum load applied before failure (in Newtons) and A is the cross-sectional area of the specimen (in mm²).

2. Cross-Sectional Area: Calculate the cross-sectional area (A) of the cylinder using: A=πD²/4

where D is the diameter of the specimen.

3. Correction Factor: If the length-to-diameter ratio (L/D) of the specimen is less than 1.75, apply a correction factor based on the following table:
• L/D=1.75, Correction Factor = 0.98
• L/D=1.50, Correction Factor = 0.96
• L/D=1.25, Correction Factor = 0.93
• L/D=1.00, Correction Factor = 0.87

For intermediate values of L/D, interpolate the correction factor accordingly.

Key Considerations

• Ensure proper alignment of the specimen to avoid eccentric loading, which may produce unreliable results.
• Perform the test immediately after removing specimens from moist storage to maintain their conditioned state.
• Note the type of failure for quality control and diagnostics.