M25 should have a compressive strength of 15 to 17.5 MPa after 7 days. Concrete strength at 28 days should not be less than 100% of its typical compressive strength. At 28 days, M25 should have a compressive strength greater than or equivalent to 25 MPa. The required minimum strength increases as the cube moves from the center to the edge. At the center, a cube of M25 should have a minimum strength of 15 MPa; at the edges, it should have a strength of at least 20 MPa. These strengths can be achieved by using high-strength concrete. For example, for a cube of 30 cm on each side, a strength of at least 45 MPa is required at 28 days. The strength of concrete at various ages depends on many factors such as type of aggregate, size of aggregates, ratio of water to cement, rate of hydration of cement, temperature, air quality, and so on. Generally, the minimum requirement decreases as the age of the concrete increases. For example, the minimum requirement at 7 days is more than twice that at 28 days. Concrete that does not meet the minimum strength requirements may be able to support itself against wind pressures for some time but will eventually fail. It is important to note that the required minimum strength of M25 concrete varies depending on the placement method used.