Why is the strength of concrete specified as the standard strength of 28d

Concrete is to rely on cement cementation, gradually hardening, and improve the compressive strength. Because the knot of cement is not complete at a stroke, but with the increase of time and gradually perfect. Under normal curing conditions, the compressive strength increases rapidly in the first seven days, and slightly slowly between 7d and 14d, while after 28d, the strength increases more slowly. That is to say, the compressive strength after 28d is the standard strength, as the quality standard of design and construction inspection. Obviously, if the strength is less than 28d as the standard strength, the performance of concrete can not be fully developed. If the strength is greater than 28 days as the standard strength, the performance of concrete can be fully developed, but the construction schedule is affected because the time to reach the standard strength is too long.


Why does concrete have requirements for water quality

Industrial waste water containing fat, vegetable oil, sugar, acid and other sewage can not be used to mix concrete. Because these contain impurities of water will reduce the bonding force of cement, so that the strength of concrete declines, so can not use mineral water contains a lot of salts, so that cement can not resist water erosion. As for the chemical composition of mineral water, it can only be used if it meets the prescribed index or makes a comparative test with ordinary drinking fresh water to see that its strength does not decrease.

Tap water and potable water can be used to mix concrete. See gbJ63-89 for details.


Why are concrete test blocks in groups of three

Concrete test block is a standard to measure the strength of concrete components, that is, the strength of the test block compression damage, as a component can have the strength. Therefore, in addition to the test block should be made as components, the test block must also have a certain number. Because although we try to make the concrete of the test block is the same as the concrete of the component, but there are certain differences after all, if only rely on a test block to identify the strength of the component, it is difficult to be reliable. Therefore, in a group of three, the average strength of three test blocks is used as the strength of this group of test blocks (in special cases, part of the value needs to be removed), that is, it is set as the strength of the component.