Effect of High Early-Age Construction Loads on the Long Term Behavior of Slab Structures


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Title: Effect of High Early-Age Construction Loads on the Long Term Behavior of Slab Structures

Author(s): H.C. Fu and N. J. Gardner

Publication: Special Publication

Volume: 95


Appears on pages(s): 173-200

Keywords: age; concrete construction; concrete slabs; creep properties; deflection; loads (forces); multistory buildings; reinforced concrete; shoring.

Date: 10/1/1986

When the shore/reshore method of construction is used high early age short duration loads are imposed upon the supporting slabs. These loads can be calculated to be of comparable magnitude to the design service loads and are applied to concrete slabs which have not achieved their specified concrete strength. Due to the slab concrete being immature with a reduced modulus of elasticity, the immediate deflections due to the construction loads are relatively large. Creep effects are dependent upon the magnitude of the applied stress relative to the concrete developed strength. Hence creep deflections due to construction loads should be large. Deflection due to concrete shrinkage also must be considered. Five, nominally identical, one-way slabs were fabricated and subjected to different load histories to compare immediate and time dependent deformations. The load histories were modelled to represent different construction methods. All slabs were designed for a live load/dead load ratio of 0.5. Taking account of the actual moduli of elasticity, the immediate deflections were consistent. The time dependent deflections were significant and of the order of 5 to 7 times the immediate deflections. Shrinkage deflections were also greater than the immediate deflections. A method is given to determine the total long term deflection of one-way slabs in terms of the peak construction load relative to the slab strength.