Removal of forms, shores, and reshoring

In ACI318M-02, section 6.2 state that: 

6.2.1. Removal of forms 

Forms shall be removed in such a manner as not to impair safety and serviceability of the structure. Concrete exposed by form removal shall sufficient strength not to be damaged by removal operation. 

6.2.2. Removal of shores and reshoring 

The provisions of section 6.2.2.1 through 6.2.2.3 shall apply to slabs and beam except where cast on the ground. 

    6.2.2.1. Before starting construction, the contractor shall develop a procedure and schedule for removal of shores and installation of reshores and for calculating the loads transferred to the structure during the process. 

     (a) The structural analysis and concrete strength data used in planning and implementing form 

           removal and shoring shall be furnished by the contractor to the building official when so 

           requested; 

    (b) No construction loads shall be supported on, nor any shoring removed from, any part of the 

          structure under construction except when that portion of the structure in combination with 

          remaining forming and shoring system has sufficient strength to support safety its weight and  

          loads placed thereon; 

    (c)  Sufficient strength shall be demonstrated by structural analysis considering proposed loads, 

           strength of forming and shoring system, and concrete strength data. Concrete strength data  

           shall be based on tests of field-cured cylinders or, when approved by the building official, on 

           other procedure to evaluate concrete strength. 

   6.2.2.2. No construction loads exceeding the combination of superimposed dead load plus   

   specified live load shall be supported on any  unshored portion of the structure under construction,  

   unless analysis indicates adequate strength to support such additional loads. 

   6.2.2.3. Form supports for prestressed concrete member shall not be removed until sufficient pre-

   stressing has been applied to enable pre-stressed members to carry their dead load and anticipated  

   construction loads. 

(Commentary-R6.2. Removal of forms, shores, and reshoring: In determining the time for removal of forms, consideration should be given to the construction loads and to the possibilities of deflections. The construction loads are frequently at least as great as the specified live loads. At early ages, a structure may be adequate to support the applied loads but may deflect sufficiently to cause permanent damage. Evaluation of concrete strength during construction may be demonstrated by field-cured test cylinders or other procedures approved by the building official such as: 

    (a) Tests of cast-in-place cylinders in accordance with “Standard Test Method for Compressive  

          Strength of Concrete Cylinders Cast-in-Place in Cylindrical Molds”(ASTM C873). (This 

          method is limited to use in slabs where the depth of concrete is from 125 to 300 mm); 

    (b) Penetration resistance in accordance with “Standard Test Method for Penetration Resistance of  

          Hardened Concrete” (ASTM 803); 

    (c) Pullout strength in accordance with “Standard Test Method for Pullout Strength of Hardened 

         Concrete”(ASTM C900); 

    (d) Maturity factor measurements and correlation in accordance with ASTM C 1074. 

Procedures (b), (c), and (d) require sufficient data, using job materials, to demonstrate correlation of 

measurements on the structure with compressive strength of molded cylinders or drilled cores. Where the structure is adequately supported on shores, the side forms of beams, girders, columns, walls, and similar vertical forms may generally be removed after 12h of cumulative curing time, provided the side forms support no loads other than lateral pressure of the plastic concrete. Cumulative curing time represents the sum of time intervals, not necessarily consecutive, during which the temperature of the air surrounding the concrete is above 10 c0. The 12h cumulative curing time is based on regular cements and ordinary conditions; the use of special cements or unusual conditions may require adjustment of the given limits. For example, concrete made with type II or V (ASTM C150) or (ASTM C 595 ) cements, concrete containing retarding admixing (to lower the temperature of fresh concrete) may not have sufficient strength in 12h and should be investigated before removal of formwork. The removal of formwork for multistory construction should be a part of a planned procedure considering the temporary support of the whole structure as well as the of each individual member. Such a procedure should be worked out prior to construction and should be based on a structural analysis taking into account the following items, as a minimum: 

    (a) The structural system that exists at the various stages of construction and the construction     

          loads corresponding to those stages; 

    (b) The strength of the concrete at the various ages during construction; 

    (c)  The influence of deformations of the structure and shoring system on the distribution of dead 

           loads and construction load during the various stages of construction; 

    (d)  The strength and spacing of shores or shoring systems used, as well as the method of shoring, 

           bracing, shore removal, and reshoring including the minimum time intervals between the 

           various operations; 

    (e)  Any other loading or condition that affects the safety or serviceability of the structure during 

         construction. For multistory construction, the strength of the concrete during the various stages  

         of construction should be substantiated by field-cured test specimens or other approved  

          methods.

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