REMOVING THE FORMS. The length of time that should be allowed to elapse before removing the forms depends upon the weather and the load to which the member will be subjected when the form is removed.
With reference to the effect of the weather it should not be for gotten that concrete sets comparatively slowly in cold weather. To an experienced person, scratching the concrete with a knife or striking it with a hammer gives a rough idea of the amount of set, and therefore of its strength; but the only sure way to determine when the forms may safely be taken down is to make test specimens— either cubes or beams—at the same time the concrete is placed in the structure, taking care to get identical mixtures and to store the test specimens under similar conditions to those obtaining for the structure, and then test the specimens from time to time to determine the growth in strength. Such tests would also show whether the
materials and the mixing were uniform.
As to the effect of the load upon the time to elapse before removing the forms, it may be said that the nearer the load to be immediately sustained approaches the load for which the member was designed, the longer the forms should remain in position. For example, roof forms should remain longer than floor forms, floor forms longer than col umn forms, column forms of an upper story longer than those of a lower story, and column forms longer than footing forms. In this connection see the last paragraph of § 497.
When the forms are removed, the work should be done gradually with close attention to the results, so that if there is any sign of weakness the supports may be replaced, or if imperfect workmanship is discovered it may be repaired.