## 1 Introduction  
Reinforced concrete (RC) bridge decks suffer from high deteriorations due to environmental attacks besides traffic loading, where corrosion, freeze and thawing, alkali silica reaction, and shrinkage and thermal cracking were reported to be significant on the reduction of life of RC decks [1; 2; 3; 4; 5; 6; 7; 8; 9]. On the other hand, in high seismic risk countries like Japan, thicknesses of RC decks were aimed to be thinner in order to reduce the inertia forces at earthquakes to satisfy earthquake resistant design requirements since bridge slabs are main source of seismic loads to bridges. These limited-thicknesses of decks (less than 200 mm) were constructed in 1960-70s, where enormous numbers of highway bridges were built. After around half a century, degradation of bridge decks has been observed from accumulated loads of daily traffics, where these deteriorations may reduce the safety of users. Previous research reported that RC slabs exposed to moving loads are extremely deteriorated compared to those exposed to fixed-point pulsating ones. The reversal cyclic-shear along crack planes induced by moving wheel type loading rapidly deteriorates the shear transfer of aggregates interlock along concrete cracks [10]. Finally, RC slabs speedily lose their stiffness until total failure. Thus, we have common issues in view of bridge deterioration where mechanical fatigue loads and environmental actions develop together with more or less interaction. It should be noted that the performance of RC decks can be upgraded by utilizing pre-stressing techniques [11] for newly constructed bridges. In fact, traffic-induced cracking is suppressed and water may not come inside crack gaps of concrete. However, we have to face serious problems such as that damaged RC decks of many bridges cannot be easily replaced since it will directly disturb the traffic flow leading to social problems. Therefore, old RC decks shall be retrofitted and/or limitedly replaced for extending the lifetime of RC decks on the basis of reliable maintenance plans. Moreover, stagnant water from rainfall may remain on RC bridge decks due to imperfect waterproofing works and/or insufficient maintenance. Stagnant water is well recognized to seriously influence fatigue life [12; 13; 14; 15; 16; 17], where performance of concrete is weakened and disintegration between aggregates and cement binder has been reported from experiments and site investigations, as shown in Figure 1. Accelerated wearing of the surface of cracks is demonstrated experimentally by cyclic shear tests in pure water, and the loss of the strength of concrete is qualitatively explained by changing the surface energy of Calcium-Silicate-Hydrate (CSH) binders. Under a high deformational rate, condensed water cannot easily disperse leading to a sharp rise and/or fall of the pore water pressure owing to its viscosity [18]. Fatigue loading experiments of water supply on RC decks demonstrate the negative impacts on their lives, and it was reported that the reduction in fatigue life is around 1/200 of the dry states [12; 13]. In recent research [17], predictive models were proposed for fatigue life of RC decks based on site inspected cracks under stagnant water, where artificial neural network model and mechanics-based correlation are introduced on the basis of an enormous number of investigated crack patterns. However, in the mentioned research, the RC decks are assumed to be in fully submerged states for safer and conservative assessment of RC decks at the current moment. It was found that cracks with the existence of stagnant water have more negative impacts on fatigue life of RC decks than the cracks in dry states, but it should be noted that the negative impacts of stagnant water on fatigue life are much higher than the one of the cracks as an overall. Most of these research usually has targeted RC decks in fully submerged conditions, but little attention has been directed to more realistic moisture states where the wet area is scattered in space.