Title: 435.8R-85: Observed Deflections of Reinforced Concrete Slab Systems, and Causes of Large Deflections (Reapproved 1997)
Author(s): ACI Committee 435
Publication: Technical Documents
Appears on pages(s): 47
Keywords: concrete slabs;creep properties; deflection; flat concrete plates; form removal;
loads (forces); modulus of elasticity; reinforced concrete; shorinq; shrinkage; two-way slabs.
Note: 435.8R-85 is Out of Print. However, a PDF version is available.
This report is in two distinct parts . Part I is a summary of published studies on slab deflections (3 from Australia, 1 from Scotland, 1 from Sweden, 2 from U.S.). The summary focuses on construction practices and materials quality. Comparison of deflections calculated by various methods with actual long-term deflections is made in some cases. Part II summarizes several construction problems and material deficiencies which can contribute to large long-term deflections. Focusing on large construction loads, the authors showthat construction loads may be considerably higher than design loads and that high construction loads cause high initial deflections because concrete has a lower modulus of elasticity when loaded at an early age. Furthermore, concrete creeps more when it is loaded at an early age, thereby causing additional high long-term deflections, even when construction loads are sustained only for a few days. The authors then suggest a method of form removal and reshoring that has proved successful in the New York City area in preventing large slab deflections. Essentially, no more than an 8-foot slab span is left unsupported until a slab is mature.