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Founded in 1904 and headquartered in Farmington Hills, Michigan, USA, the American Concrete Institute is a leading authority and resource worldwide for the development, dissemination, and adoption of its consensus-based standards, technical resources, educational programs, and proven expertise for individuals and organizations involved in concrete design, construction, and materials, who share a commitment to pursuing the best use of concrete.
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Home > Publications > International Concrete Abstracts Portal
The International Concrete Abstracts Portal is an ACI led collaboration with leading technical organizations from within the international concrete industry and offers the most comprehensive collection of published concrete abstracts.
Title: Managing Deflection, Shortening and Cracking Arising from Restrained Contraction
Author(s): S. J. Alexander
Publication: Special Publication
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
Abstract: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.
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