When it says: "Characteristic cylinder compressive strength of concrete shall be minimum of 30 MPa and 25 MPa at 28 days," it is referring to the strength of the concrete after a specific period of curing, which is usually 28 days. Here's a breakdown of the statement:
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Characteristic Cylinder Compressive Strength:
This is the strength of concrete measured by a cylinder test, where a sample of concrete is molded into a cylindrical shape and then subjected to pressure until it fails. The result is the compressive strength, which tells us how much pressure the concrete can withstand before breaking. -
Minimum of 30 MPa:
This means that the concrete is required to have a compressive strength of at least 30 megapascals (MPa) when tested at 28 days. This is a standard measure of strength used for construction materials. 30 MPa is a relatively common strength requirement for concrete used in general construction. -
25 MPa at 28 days:
This part may seem like a repetition, but it's likely referring to the target strength or a slightly different specification depending on the context. It suggests that the concrete should have at least 25 MPa at 28 days, which may be the lower bound or a different value for specific circumstances. Often, specifications include a tolerance, and the strength could slightly vary but must meet the minimum requirement.
Summary:
This statement is saying that after 28 days of curing, the concrete must have a compressive strength of at least 30 MPa (characteristic strength) and no less than 25 MPa (minimum strength). The characteristic strength is generally based on a statistical 95% confidence level, meaning that 95% of the concrete samples tested should meet or exceed the 30 MPa strength.
In practice, the concrete mix is designed to achieve this strength by ensuring proper proportions of cement, water, aggregates, and curing conditions.