Concrete psi, which refers to its compressive strength in pounds per square inch, is a critical indicator of its ability to withstand loads. Measuring concrete psi involves a standardized laboratory test.
How Concrete PSI is Measured
According to standard practices, including the ASTM (American Society for Testing & Materials) standard C39, concrete psi is measured by assessing the concrete's compressive strength. This is done through a specific testing procedure that utilizes physical specimens and specialized equipment.
As the reference states, compressive strength is tested by breaking cylindrical concrete specimens in a special machine designed to measure this type of strength. The result is measured in pounds per square inch (psi).
The Standard Testing Process
The measurement of concrete psi typically follows these steps:
- Sample Collection: During concrete placement, samples are taken from the batch.
- Specimen Molding: These samples are used to cast cylindrical specimens of a standard size (commonly 6 inches in diameter and 12 inches high).
- Curing: The cylindrical specimens are cured under controlled temperature and moisture conditions for a specified period. Common test ages are 7 days and 28 days, as concrete strength continues to develop over time.
- Testing: At the designated test age, the cured cylindrical specimen is placed in a special machine called a compression testing machine.
- Applying Load: The machine applies a gradually increasing compressive load (pressure) to the top and bottom surfaces of the cylinder.
- Breaking the Specimen: The load is increased until the concrete cylinder fails or "breaks."
- Calculating PSI: The machine records the maximum load the cylinder withstood before breaking. This load (in pounds) is then divided by the cross-sectional area of the cylinder (in square inches) to calculate the compressive strength in psi.
Key Aspects of the Test
- Standardization: The entire process, from sampling and molding to curing and testing, is governed by standards like ASTM C39 to ensure consistency and comparability of results.
- Cylindrical Shape: Cylindrical specimens are used because they provide a relatively uniform distribution of stress under axial compression.
- Specialized Equipment: A calibrated compression testing machine is essential for accurately applying and measuring the breaking load.
This process provides a direct measurement of the concrete's ability to resist being crushed, which is its ultimate compressive strength, expressed in psi.
