Has anyone considered the importance of the Elastic Modulus? 1

[Contest]

In steel design the deflection of elements relies heavily on the E value or modulus of elasticity. In reinforced concrete elements this is a difficult value to determine as it is a composite material. The thickness of suspended slabs is very important when predicting deflection and is usually estimated by empirical methods. If the slab is made thinner than this estimate (and it usually is in the interests of economy) then the structure becomes increasingly sensitive to the modulus of elasticity.

Under these circumstances the modulus should be specified by the design engineer to obviate excessive deflection from occurring. Has anyone seen such a specification, what values were used and has the designer any right to anticipate a minimum value - even if he did not ask for it?

 

[dbuzz]

Since concrete is not an elastic material the values assigned to elastic constants, such as modulus of elasticity and Poisson’s ratio, must necessarily be somewhat arbitrary.

In my experience most design codes recognise this and provide some guidance for estimating the elastic constants for use in practice. The modulus of elasticity for concrete is often taken as the secant modulus, measured at a stress level equal to 45% of the ultimate strength, and Poisson’s ratio as 0.15. In Australian practice, modulus of elasticity (in MPa) is given by the equation E = [0.043 x (concrete density, in kg/m3)^1.5 x (mean concrete strength, in MPa)^0.5] ± 20% and Poisson’s ratio is given as 0.2.

As you can see, it would be unreasonable to provide a concreting Contractor with only the required 'modulus of elasticity'. We are the Engineers here. A Contractor can place reinforcement of the required size and spacing, order concrete at the required grade and pour elements to the required width and thickness. It is the Engineer's job to select the grade of concrete and design the size of the elements and the size and spacing of reinforcement, all the while remaining cognisant of serviceability issues such as deflection and vibration.