At what duration is concrete considered to have reached its desired working strength?

Concrete typically gains strength over time through the hydration process of the cement particles. Although there is an initial set time when the concrete can be walked on or finished, the desired working strength is generally achieved at the 28-day mark. By this time, concrete has reached a significant percentage of its ultimate compressive strength, often around 70% to 90% of its intended design strength.

While there can be noticeable strength gain at 7 days, and to a lesser extent at 14 and 21 days, these durations are typically considered early points in the curing process and are not sufficient to indicate that the concrete has achieved its desired working strength. The 28-day standard has long been used in construction and engineering as a reliable period to assess concrete's performance under load, as it allows for nearly full strength development.

Because of this established practice, focusing on 28 days as the benchmark provides consistency in design and construction protocols, ensuring that engineers and contractors can depend on concrete performing as expected in structural applications.