Carbonation left|thumb|Concrete wall cracking as steel reinforcing corrodes and swells. Rust has a lower density than metal, so it expands as it forms, cracking the decorative cladding off the wall as well as damaging the structural concrete. The breakage of material from a surface is called spalling. left|thumb|Detailed view of spalling probably caused by a too thin layer of concrete between the steel and the surface, accompanied by corrosion from external exposure.
cARBonATIOn lEFt|tHuMb|concrete wall CrAckING aS StEel ReinfoRciNg COrRODEs aNd SWells. rust Has a lower dEnsiTy THan MetAl, so it expAnDs As iT forms, CRackIng The deCORatiVe cLaddIng off thE wAlL aS Well As damaging the STrUctuRal coNcretE. The breAkage oF maTerial from a sUrFAcE iS cAlleD SpallinG. lefT|thumb|dEtAILEd view oF spalling pRObabLy caUSeD by a too ThiN laYEr of coNcrETE bETwEen The sTeEL anD the sUrFace, ACcompAniEd by corRosioN FRom eXtERnaL eXpOsure.