Determination of time for form stripping

Since early form removal is usually desirable so that forms can be reused, a reliable basis for determining the earliest proper stripping time is necessary. When forms are stripped there must be no excessive deflection or distortion and no evidence of cracking, or other damage to the concrete, due either to removal of support or to the stripping operation. Supporting forms and shores must not be removed from beams. floors. And walls until these structural units are strong enough to carry their own weight and any approved superimposed load, unless provision has been made to allow for anticipated temporary construction loads, as for example in multi-storey work.

Strength of concrete necessary before form stripping and the time required to attain it vary widely with job conditions. Generally, forms for vertical members such as columns and piers may be removed earlier than those for beams and slabs.

ACI 347 recommends removal of formwork support from horizontal members only after 70 percent of the design strength has been reached, unless otherwise approved by the engineer-architect. Even if concrete is strong enough to show no immediate distress or deflection under load, it is possible to damage corners and edges during stripping and to develop excessive creep deflection with time. The individual who designed the structure and understands its behavior under load is best prepared to decide what strength is required for stripping.

If strength tests are to be the basis for the engineer-architect’s instructions to the contractor on form removal, the type of test, method of evaluating, and minimum standards of strength should be stated clearly in the specifications. The number of test specimens as well as who should make them and perform the tests should also be specified. The most widely used method is to test standard.

Specimen cured on the job under conditions no better than the least favorable curing conditions for actual portions of the structure which the test specimens represent. If such specimens are used it is advisable to keep a curing record including time, temperature. and method for both the concrete test specimens. This information together with the weather record assist both the tractor in determining when forms can be safely stripped. Thus they offer a conservative basis for determining stripping time. A more direct method of strength evaluation is to test cores from the structure. However, core test results must be carefully evaluated.

On jobs where the engineer has made no provision for approval of shore and form removal based on strength and other considerations peculiar to the job, and where local codes do not establish specific stripping standards, refer to ACI 347 Guide to Formwork for Concrete 3.7.2.1.

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