Fig. 2: The critical corrosion penetration for RAAC panels.
From: RAAC panels can suddenly collapse before any warning of corrosion-induced surface cracking
a The critical corrosion penetration tcrit increases exponentially with concrete porosity. A larger c/d ratio (concrete cover to steel rebar diameter) and the thickness of a concrete cover also increase tcrit (b). Excessively large tcrit of aerated concrete means that steel corrosion in RAAC panels can be concealed well over 8 years, considering a typical value of corrosion current density about 1 μA/cm2 49,50,51,52,53 (b). We can see that tcrit in aerated concrete can easily exceed 100 μm at which rebar-concrete bond in standard concrete was found to deteriorate to the extent that the ultimate limit state may be affected48. Although the exact threshold value of tcrit for RAAC is not known yet, this indicates that RAAC panels can potentially collapse before any visually detectable warning of corrosion-induced surface cracking. The model allows to assess whether the panel with a given concrete density (which is directly linked to its porosity) and the thickness of a concrete cover is likely to be at the greatest risk of collapse before surface cracking (c) and thus narrow down and prioritise inspections or repairs. The safety map in (c) was calculated assuming steel rebar diameter d = 10 mm.
