Poisson's Ratio of Concrete

Concrete in the elastic range spreads sideways less than steel does, and codes take 0.20 for uncracked normal-weight concrete, with measured values across mixes and aggregates running 0.15 to 0.25. It matters most in confined situations — thick slabs, shear walls, spirally reinforced columns, and any finite-element model where the transverse stiffness of a plate depends on 1/(1 − ν²).

The value is a fiction once microcracking begins. Above roughly 70–80 % of ultimate compressive stress, vertical cracks open parallel to the load and the apparent Poisson's ratio climbs steeply past 0.5 as the specimen dilates — this is not elastic behaviour at all but volumetric expansion from cracking, and it is exactly the mechanism that confinement reinforcement exists to restrain. That is why a spirally confined column is far more ductile than a tied one, and why quoting 0.20 anywhere near ultimate load is a mistake.