When is it safe for you to place a construction load on a concrete structure

When the concrete structure avails it maximum strength, it is safer to place construction load on it. Usually, the concrete gain it maximum strength after 28 days. Concrete when cast take initial settlement on 7 days. Curing is done continuously through the period so that there is no crack occur due to heat transfer. There are various method of concrete curing. On a horizontal surface/ roof/ floor, curing is done through ponding or spreading wet sheets. Testiment/ sample of the concrete are cured by immersion in water tank. After 28 days of curing, concrete gains it maximum strength and can be used for test or usage. Construction load limit is considered in designing/ proportioning the specimen. So the applied load is equal or less than the limit for which it is actually designed.

electric field is a function of 1/r^2

yes, the force per unit charge for a point charge is inverse square law.