Managing Deflection, Shortening and Cracking Arising from Restrained Contraction


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Title: Managing Deflection, Shortening and Cracking Arising from Restrained Contraction

Author(s): S. J. Alexander

Publication: Special Publication

Volume: 227


Appears on pages(s): 1-20

Keywords: composite beams; cracking; creep; deflection; design; early age contractions; overlays; reinforced concrete; restraint; serviceability; shortening; shrinkage; structural analysis

Date: 3/1/2005

Concrete shrinks. Steel doesn’t, and the resistance of reinforcement to shrinkage causes deflection of slabs and beams and shortening of columns and walls. A simple visualization is given, and used to derive formulae for analysis. Current methods of calculating shrinkage curvature and deflection in reinforced sections are examined and compared, concluding that the ACI method appears realistic while the UK and European methods significantly over-estimate the deflection. Restraint by differential contraction between an insitu concrete overlay and an older substrate produces tension in the overlay and curvature and deflection of the composite unit. A method for calculating this is given, and the resulting effects are found to be significant in certain circumstances. The method is extended to consider shrinkage in insitu slabs in steel-concrete composite construction. The deflection from shrinkage is found to be approaching span/750, and cracking in the slabs is predicted in some cases. External restraint to contraction induces tensile stresses, and a rational approach to providing sufficient reinforcement to control cracking in direct tension is given. It is particularly relevant to elements which need to be water-resisting, and a case study of a basement is presented. The reinforcement needed to control cracking reliably is found to exceed most current US, UK and European recommendations.