The Importance of Time in Understanding Concrete Behaviour

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Title: The Importance of Time in Understanding Concrete Behaviour

Author(s): S.J. Alexander

Publication: Special Publication

Volume: 246

Issue:

Appears on pages(s): 293-300

Keywords: contraction; cracking; creep; design; flow chart; restraint; shortening; shrinkage; tensile strength; tensile stress

Date: 9/1/2007

Abstract:
Concrete cracks when the tensile stress exceeds the tensile strength. Tensile stress arises from restrained contraction. The main sources of contraction are early age effects, temperature drop and drying shrinkage. Creep offers significant relief to early-age contractions, but only a little to shrinkage. Tensile strength reduces under sustained stress, so the duration of the contraction is important. A new graph is presented to show that although cracking can occur at any age, it is most likely either during the first 3-10 days or after some years. This is because the creep relief provides a margin for temperature variations and some shrinkage to occur without exceeding the critical tensile stress. However this margin diminishes with time. A new flow chart is then presented which shows what controls are necessary at each of these two key stages. This differentiates between avoiding cracking and controlling cracking, an important distinction that is often confused in the approach to design. The case study of a feature wall where this was not appreciated is described.