RAAC panels can suddenly collapse before any warning of corrosion-induced surface cracking

Abstract

The collapse of reinforced autoclaved aerated concrete (RAAC) panels has attracted considerable public and academic interest. As detailed experimental data are not yet available and replicating the natural corrosion process requires years or decades, computational modelling is essential to understand under which conditions corrosion remains concealed. The very high porosity of RAAC is widely suspected to be a major contributing factor. However, current corrosion-induced cracking models are known to struggle with capturing the role of concrete porosity. To remedy this critical deficiency, we propose to enrich corrosion-induced cracking modelling with the analytical solution of reactive transport equations governing the precipitation of rust and a porosity-dependent description of diffusivity. With this, the corrosion concealment in RAAC panels is studied computationally for the first time, revealing that RAAC panels can suddenly collapse before any warning of corrosion-induced surface cracking and allowing to map the conditions most likely to result in sudden collapse.

Figures (3)

• Figure 1: The impact of concrete porosity on critical corrosion penetration. (a) Critical corrosion penetration $t_{crit}$ for the first cracks on the concrete surface increases with concrete porosity $\phi$. This is because iron ions diffusivity increases with porosity and thus the normalised flux of iron ions into pore space $j_{n}$ decreases more slowly with corrosion penetration $t_{cor}$ (b), hindering the accumulation of a dense rust layer in the process. Pressure evolution comprises the stress-free period when accumulated rust fills the volume vacated by steel corrosion so that the thickness of the rust layer $t_{r}$ is smaller than the available thickness of the corroded layer $t_{cor}$. In the subsequent period, when $t_{r}/t_{cor} > 1$, corrosion-induced pressure on concrete increases causing cracking in the vicinity of steel rebar until the dominant crack rapidly propagates to the concrete surface. In (a) and (b), concrete cover $c = 30$ mm and rebar diameter $d = 10$ mm were considered.
• Figure 2: The critical corrosion penetration for RAAC panels. (a) The critical corrosion penetration $t_{crit}$ increases exponentially with concrete porosity. A larger $c/d$ ratio (concrete cover to steel rebar diameter) and the thickness of a concrete cover also increases $t_{crit}$ (b). Excessively large $t_{crit}$ 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 ^2 Otieno2012aOtieno2016aAndrade2023Walsh2016andrade2023role (b). We can see that $t_{crit}$ in aerated concrete can easily exceed 100 microns at which rebar-concrete bond in standard concrete was found to deteriorate to the extent that the ultimate limit state may be affected Andrade1993. Although the exact threshold value of $t_{crit}$ 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.
• Figure 3: Graphical illustration of the reinforced concrete specimen with the geometry of a thick-walled concrete cylinder solution domain and polar coordinates.