SECTION XI. FORM REMOVAL
5-86. Careless workers can cancel out the value of good detailing and planning by indiscriminate use of the wrecking bar. A pinch bar or other metal tool should never be placed against exposed concrete to wedge forms loose. If it is necessary to wedge between the concrete and the forms, use only wooden wedges.
5-87. Wall forms should not be removed until the concrete has thoroughly hardened, but specified curing should begin as early as possible in warm weather. Ties may be removed as early as 24 hours after casting to loosen forms slightly and permit entry of curing water between form and concrete. Ornamental molds must be left in place until they can be removed without damage to the concrete surface. In cold weather, removal of form work should be deferred or form work should be replaced with insulation blankets to avoid thermal shock and consequent crazing of the concrete surface.
5-88. When stripping forms in the vicinity of a belt course, cornice, or other projecting ornament, begin stripping some distance away from the ornament and work toward it. If there is any tendency for the forms to bind around the ornament, the pressure of the forms against projecting corners will be relieved so that there will be less chance of spalling sharp edges. Forms recessed into the concrete require special care in stripping. Wedging should be done gradually and should be accompanied by light tapping on the piece to crack it loose from the concrete. Never remove an embedded form with a single jerk. Embedded wood forms are generally left in place as long as possible so they will shrink away from the concrete. The embedded items should be separate from or loosely attached to the main form so that they will remain in place when the main form is stripped.
STRIPPING TIME BASED ON CONCRETE STRENGTH
5-89. 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. Such approved load should not exceed the live load for which the member was designed unless provision has been made by the engineer architect to allow for temporary construction loads.
5-90. Forms for vertical members such as columns and piers may be removed before those for beams and slabs. The strength of the concrete necessary before form work is stripped and the time required to attain it vary widely with different job conditions, and the most reliable basis is furnished by test specimens cured under job conditions. In general, forms and supports for suspended structures can be removed safely when the ratio of cylinder test compressive strength to design strength is equal to or greater than the ratio of total dead load and construction loads to total design load with a minimum of 50 percent of design compressive strength being required.
5-91. For some applications, a definite strength must be obtained; for example, 2,500 psi or two-thirds of the design strength. However, even when concrete is strong enough to show no immediate distress or deflection under a load, it is possible to damage corners and edges during stripping and for excessive creep deflections to occur. If strength tests are to be the basis for the designers, instructions to the project officer on form removal, the type of test, the method of evaluating, and the minimum strength standards should be stated clearly in specification.
5-92. The number of test specimens, as well as who should take them and perform the tests, should also be specified. Ideally, test beams and cylinders should be job cured under conditions which are similar to those for the portions of the concrete structure which the test specimens represent. (The specimens must not be confused with those cured under laboratory conditions to evaluate 28-day strength of the concrete.) The curing record including the time, temperature, and method for both the concrete structure and the test specimens, as well as the weather record, will assist both the engineer and the project officer in determining when forms can be safely stripped. It should be kept in mind that specimens which are relatively small are more quickly affected by freezing or drying conditions than concrete in the structure.
5-93. 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, Table 5-8 shows the minimum time forms and supports should remain in place under ordinary conditions.
Table 5-8. Recommended form stripping time
FORM REMOVAL PROCEDURES
5-94. Forms are designed and constructed so that their removal does not harm the concrete. The form must be stripped carefully to avoid damaging the surface. Do not jerk the forms from the concrete after wedging at one end, or the edges will break. Withdraw all nails while stripping and immediately clean and oil all forms to be reused.
|David L. Heiserman, Editor||
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Revised: June 06, 2015