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Synopsis The only property of the concrete normally known to the designer at the calculation stage of a design is its specified characteristic strength. This paper considers the consequence of this fact for the reliability of deflection calculations by investigating the uncertainties introduced by the essential assumption that there are unique relationships between cube strength and modulus of elasticity and between cube strength and tensile strength. It is shown that, for members having low percentages of reinforcement, the likely range of differences attributable to this assumption can be very large under service loads. Some philosophical questions concerning the relationship between ‘research’ and ‘design’ formulae are also raised.Synopsis The only property of the concrete normally known to the designer at the calculation stage of a design is its specified characteristic strength. This paper considers the consequence of this fact for the reliability of deflection calculations by investigating the uncertainties introduced by the essential assumption that there are unique relationships between cube strength and modulus of elasticity and between cube strength and tensile strength. It is shown that, for members having low percentages of reinforcement, the likely range of differences attributable to this assumption can be very large under service loads. Some philosophical questions concerning the relationship between ‘research’ and ‘design’ formulae are also raised.

Concrete structures: stresses and deformations