When should concrete samples be tested after their initial setting?

Concrete samples are typically tested for strength at 7 and 28 days after casting because these time frames align with the development of concrete's compressive strength during its curing process.

At 7 days, concrete has gained a significant amount of its strength, which can be as much as 70% of its design strength depending on the mix and curing conditions. Testing at this point provides useful information regarding the early strength gain and can help to assess whether the concrete is achieving the desired performance specifications.

The 28-day mark is traditionally recognized as a standard time for concrete strength testing because it's when concrete reaches a substantial portion of its potential compressive strength, often around 95%. By this point, the hydration process has significantly contributed to the overall strength, making it a critical indicator for quality control and structural integrity.

When testing concrete, it is essential to follow these standard time frames to ensure that the samples reflect the age-related strength properties reliably, which is fundamental for meeting design specifications and construction timelines.