An argument for using alizarine yellow R and indigo carmine to determine in situ the degree of alkalinity in reinforced concrete

The commonest way of determining the passivation of steel in reinforced concrete is to spray a pH indicator onto the surface of a reinforced concrete specimen. To do so, the traditional indicator used has been phenolphthalein in ethanol, which shifts from colourless to purple in the pH range of 8.5-10.0. If the concrete encasing the rebar turns purple, it indicates it is passivated; that is, it has a film of iron oxide impermeable to oxygen that protects it from oxidation. However, the reduction in the pH of the concrete due to carbonation (between 11.4 and 8.5) indicates, despite the reddish-purple colour, the disappearance of the passive layer of the steel. Consequently, we propose a novel in situ technique for determining with a high degree of reliability, the passive state (pH over 11.7) and the active state (pH under 11.4) using two reactants: alizarine yellow R and indigo carmine.