How is the compressive strength of concrete typically determined?

The compressive strength of concrete is primarily determined by testing concrete cubes or cylinders according to ASTM standards. This method provides a precise and standardized approach, allowing for consistent and reliable results that can be compared across different tests and projects.

In the testing process, concrete samples are prepared and cured under controlled conditions. Once they have reached the appropriate age, typically 28 days, these specimens are subjected to a compressive load until failure. The maximum load the specimen can withstand is measured, and this value is then used to calculate the compressive strength, typically expressed in psi or MPa. This systematic testing procedure ensures that the results reflect the material's true performance under load, minimizing variability and enhancing the reliability of the data.

Other methods, such as estimating strength based on the water-cement ratio or visual inspections, do not provide the same level of accuracy and should not be relied upon for definitive strength assessment. Observing surface cracks or applying manual pressure are also insufficient for measuring compressive strength, as these methods lack the rigor and precision required to obtain valid strength data.