Thermal Expansion Coefficient of Concrete

Reinforced concrete only works because concrete and steel expand at nearly the same rate: about 10 µm/(m·K) against steel's 11.7. If they differed by much, every temperature swing would debond the bars or crack the cover, and the material would have been abandoned in the nineteenth century. The near-match is a genuine coincidence of materials science, and it is worth appreciating.

The value depends mostly on aggregate, which is 70 % of the volume: siliceous gravel mixes run 11–13 µm/(m·K), limestone 6–9, and the difference is large enough that pavement design and bridge-deck crack prediction now call for a measured coefficient rather than a default. Moisture state moves it too. Separately, the reason concrete cracks is rarely thermal expansion alone but drying shrinkage — 400–800 µm/m over months, equivalent to a 40–80 °C temperature drop — which is what control joints, shrinkage reinforcement and curing regimes are fighting.