## 1 Introduction  
Reinforced cement-based structures are expected to remain intact for long periods of time with minimum service requirements. This demands the development and usage of long-lasting building materials. Lately, there is increasing attention towards incorporating nanoparticles in building materials for improvement of their mechanical and electrical performance as well as endowment of multi-functionality to the structure [1; 2; 3]. Relevant studies have explored the use, in cement-based materials, of such nano-reinforcements such as nano-CaCO\({}_{3}\)[4], nano-SiO\({}_{2}\)[5], as well as carbon-nanofibers (CNFs) [2], graphene nano-platelets [6; 7] and carbon-nanotubes (CNTs) [8; 9]. It has been reported that homogeneously-dispersed nano-particles can fill inherent voids in cement structures, hence lowering the porosity and increasing their strength and durability [10]. On the other hand, poor design and supervision, unsuitable construction, insufficient materials selection and harsh environments can severely downgrade cement structures [11; 12; 13]. One of the major durability issues that the construction industry is facing globally is the materials' structural degradation [14; 15; 16; 17]. This may lead to reductions in strength, reinforcement corrosion, as well as aesthetics issues, which would require early repair or replacement of the structure [18]. Corrosion, attacking mainly reinforcements via the pores in the material bulk, is primarily due to carbonation and chloride attack [12; 19; 20; 21; 22]. Dry environments favor corrosion because of carbonation and carbon oxide excess [23]. On the other hand, localized corrosion in different types of cement-basedstructures exposed to marine environments may lead to premature structure failure due to chloride ingress through the net of the material pores or via ion incorporation in the mixture [12; 24; 25]. In concrete, the corrosion process is completed in two stages: In the initial corrosion initiation stage, chlorides penetrate the concrete without causing damage to the material. In the second stage, termed corrosion propagation stage, corrosion elements accumulate until the material's ultimate failure [12]. For concrete structures in marine environments, the service life is the sum of the durations of the two stages, while durability and serviceability of the structures depend greatly on the duration of the initiation stage [12]. A variety of methods are currently available in the literature for prevention of deterioration due to chloride penetration, as are models for prediction of the time of corrosion initiation and total service life [19; 20; 21; 22; 23; 24; 26]. In order to better assess the phenomenon, knowledge of the different driving forces responsible for the penetration of chloride ions in concrete is indispensable. Possible such driving forces include absorption during wetting and drying cycles [27] and application of hydrostatic pressure, by means of an hydraulic head on the surface of the concrete [18]. However, the most common way in which chloride ions get in contact with concrete reinforcement is diffusion, _i.e._ the movement of chloride ions under a concentration gradient via a continuous liquid phase [28]. The penetration rate depends on the quality of the concrete, its chloride binding capacity, hydration degree, duration of exposure to chlorides, water to cement ratio, temperature and curing time, as well as the presence of supplementary cementing materials [12; 18; 24; 28; 29]. A sufficient understanding of these factors and their relationships could provide a good base for the limitation of degradation of cement-based structures. The degree of resistance of concrete to chloride ion penetration is characterized by the coefficient of diffusion in Fick's corresponding second law [30]. Its calculation requires exposure of the concrete sample to known concentrations of chloride solutions for specific durations followed by measurement of chlorides' concentration at successively larger depths.