What is the recommended age for testing concrete compressive strength using cylinder tests?

The recommended age for testing concrete compressive strength using cylinder tests is 7 and 28 days because these timeframes are established as standard practice in concrete construction. The 7-day test provides an early indication of the concrete's strength development, which can be important for making decisions about forms, shoring, and other construction activities. The 28-day test is considered the standard benchmark for assessing the concrete's final compressive strength, as most concrete mixtures are designed to achieve 28-day strengths based on factors such as mix design, curing conditions, and environmental factors. These testing times help ensure that engineers and contractors can evaluate the performance of concrete accurately and are aligned with industry standards and specifications, such as those from the American Concrete Institute (ACI).