## 1 Introduction  
Reinforcement steel corrosion is widely acknowledged as the main degradation mechanism of reinforced concrete structures. This phenomenon occurs after the depassivation of the steel reinforcement, which, among others, can originate from chloride attack [1]. For a reliable and accurate prediction of the service life of concrete structures in marine exposure environments, the chloride-ion penetration needs to be perfectly understood and well simulated by representative expeditious methods for accessing the concrete resistance to chloride attack. However, the chloride-ion penetration in concrete is a complex phenomenon that involves a diverse set of transport mechanisms, namely diffusion, capillary absorption and permeation. Moreover, the chloride-ion penetration can also involve more than one of these mechanisms, besides physical and chemical interactions with the hydrated cement paste [2, 3]. In general, this is simplified by assuming that diffusion is the predominant mechanism, thus motivating the characterisation of this transport property by most existing tests. Current methods to access the concrete resistance to chloride attack may be classified as steady-state or non-steady-state, regarding the change in chloride-ion concentration in concrete, and as diffusion or migration tests, regarding the predominant driving transport mechanism [4]. Additionally, other tests based on indirect measurements of resistivity or conductivity can also be adopted [5, 6, 7]. In stationary tests, the diffusion coefficient is calculated by applying [PERSON]'s first law [8]. These tests do not consider the binding capacity of the cement paste and are affected by various factors, such as the specimen thickness, solution concentration, temperature and presence of other ions. Moreover, the time required to reach the steady-state can go from a few weeks to several months, depending on the depth of penetration and thickness of the specimens [2]. In non-stationary tests, the diffusion coefficient is calculated by considering [PERSON]'s second law [8]. In these tests, the chloride penetration can be evaluated by colorimetric methods, applying a solution of silver nitrate (AgNO\({}_{3}\)), or more rigorously, by extracting samples at different depths and building chloride profiles [2; 9]. In real exposure conditions, the penetration of chloride ions is affected by the chemical and physical interaction with the hydrated cement paste. Thus, the calculated diffusion coefficient is referred to as \"apparent\", given that it considers the total chloride concentration gradient as a driving force, even though the diffusion process only involves the flow of free chlorides in the pore solution. The major drawback of these tests is the fact that they require long exposure periods to obtain representative chloride profiles [3]. Among the existing non-stationary diffusion methods, the immersion test (IT) and the wetting and drying test (WDT) are those that better simulate the chloride ingress in submerged zone and tidal zone, respectively. In fact, the main advantage of these methods is their more accurate simulation of the chloride-ion penetration mechanism in natural environment, namely the diffusion mechanism in submerged conditions and the diffusion/absorption mechanisms in tidal regions (intermittent wetting and drying, leading to chloride penetration governed by absorption and diffusion mechanisms [10; 11]), respectively. Besides the long duration, the change of the pore structure throughout the test, as a consequence of the hydration process, affects the chloride diffusion coefficient calculation, conflicting with the assumptions of [PERSON]'s second law [12]. In addition, the complex procedure for the determination of the chloride-ion concentration also introduces further difficulty regarding this less expeditious method [1; 13; 14].