It will continue to harden somewhat over time, but once it reaches the prescribed strength, it is judged strong enough for its intended purpose. We choose 28 days as a safe time period before allowing normal use. Is a minimum flexural strength of 4.5 MPA required for 7 or 28 days? No, 3.5 MPA is sufficient.
The required minimum flexure strength depends on the intended use of the concrete. For example, if the concrete is only being used to form steps and walkways on a building site, then a minimum flexure strength of 3.5 MPA (45 MPa) can be used for either 7 or 28 days. However, if the concrete is going into a place where heavy loads will be placed on it (such as a floor or bridge), then a minimum flexure strength of 4.5 MPA (60 MPa) should be used to ensure that it will not break under pressure.
Concrete's flexural strength increases as it continues to cure; thus, a lower minimum strength can be used after seven days if the concrete is still in its early stages of curing.
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.
Initial Power Because of early strength improvement, the strength of 53 grade does not increase much after 28 days. After 28 days, 33-grade and 43-grade cement continue to build strength. 33 and 43-grade cement will eventually achieve the same ultimate strength as 53-grade cement. Price Cement costs more for higher grades. Generally, low-cost cements are lower in quality. They may be less refined or have larger particles than high-quality cements. Low-cost cements tend to harden faster so they must be mixed with water before being placed in the mold.
Higher Final Strength The extra silicon added to 53-grade cement allows it to develop greater initial strength. This means that a sample of 53-grade cement placed in a test tube could support more weight than a sample of 33-grade cement. When exposed to air, both cements would harden but the silicon in the 53-grade cement would begin to dissolve, releasing more calcium silicate hydrate (C-S-H) gel into its interior. This extra cross-linking of molecules creates a stronger cement that can hold more weight before breaking.
The extra silicon in 53-grade cement also makes it more resistant to chemical attack. Chemicals such as sulfur dioxide gas used by some coal-fired plants to control acid rain would react with the calcium carbonate present in both types of cement to create calcium sulfate hemihydrates.
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