The strength of concrete depends upon the amount of cement content, quality and grading of aggregates, water of concrete ratio, compaction and curing Strength of concrete increase very fast in the initial stage.
In the 7 days, the strength gained is as much as 60 to 65present of 25 days strength it is customary to assume the 28 days strength as the full strength of concrete however, concrete gains strength beyond 28 days also.
The strength/grade of concrete is specified and measured in newtons/sq. mm., meganewtons/sq. meter or even megapascals, in fact the numerical figure will be the same in each case.
E.g. a strength of 20 newtons/sq.mm. is the same as 20 meganewtons/sq.metre
Compressive strength of concrete
The characterize strength of concrete is defined. As compressive strength of 150mm size cubes after 28 days.Concrete mixtures can be designed to provide a wide range of mechanical and durability properties to meet the design requirements of a structure. The compressive strength of concrete is the most common performance measure used by the engineer in designing building and other structures. The compressive strength is measured by cylindrical concrete specimens in a compression-testing machine.The compressive strength is calculated using the following formula.
The tensile strength of concrete is usually considered about one-tenth of the compressive strength; that is, concrete which has a compressive value of 2,000 pounds per square inch should have a tensile strength of about 200 pounds per square inch. Although there is no fixed relation between the two values, the general law of increase in strength due to increasing the percentage of cement and the density seems to hold in both cases.