## 1 Introduction  
Corrosion induced by carbonation is the primary reason for the deterioration of concrete structures exposure to an atmospheric environment [1; 2; 3; 4; 5] and non-optimum environments [6; 7; 8]. The pH of Portland cement concrete is usually in the range of 12.5 to 13.8 [9; 10; 11; 12; 13; 14]. Under this high alkalinity environment, the rebar surface can form a passive film that prevents the rebar from corroding. The pH of concrete decreases due to the carbonation reaction; furthermore, this process can accelerate the destabilization of the passive film on the rebar [15]. Once pH decreases to around 9.0-9.5, the rebar will begin to corrode due to the passive film breakdown [16; 17; 18; 19; 20; 21]. Therefore, pH is a major concern for the corrosion initiation of rebars embedded in concrete. It is very important to measure the pH of concrete, and several methods have been developed for measuring pH values of concrete, such as the in-situ leaching method [22; 23], the ex-situ leaching method [24; 25], the halochromic sensor method [26; 27], and the expression method [28; 29]. There is general agreement that the expression method is the most reliable test procedure available for acquiring the pH of concrete. Much work has focused on the prediction model of the corrosion initiation period of rebars under atmospheric environments. A corrosion initiation period model has been proposed, in which a carbonation front penetrates the depth of concrete cover the corrosion of rebars initiated [30; 31]. The traditional carbonation-induced corrosion mechanism employs the carbonation depth to indirectly reflect the corrosion status of rebars. Carbonation depth is usually measured with a phenolphthalein indicator, which is a fast and convenient method [2]. However, the indicator only indicates a pH range of 8.0-9.8 [17]. Thus, it provides limited information regarding the actual carbonation status of concrete. Moreover, it cannot explain how the rebar becomes corroded when the carbonation front has not reached the surface of the rebar [30]. Therefore, it is urgent to study the relationship between the pore solution pH of concrete and the corrosion status of rebars embedded in concrete. To study the effect of pore solution pH on rebar corrosion initiated by carbonation, several methods are frequently employed, such as a simulated concrete pore solution, an accelerated carbonation test, and natural carbonation. A simulated concrete pore solution is frequently used to study the corrosion initiation of rebars for a short experimental period in spite of its limitations [32; 33; 34]. Carbonation tests show that the pH of uncarbonated concrete is in the range of 12.5 and 13.8 [13; 35; 36; 37]. However, the pore solution pH under accelerated carbonation is significantly lower than that at natural CO\({}_{2}\) concentrations, leading to underestimations of the service life of structures [37; 38]. There is a small amount of data that is available under accelerated exposure conditions in controlled environments [9; 39; 40; 41]. The pore solution pH of cover concrete is one of the most important parameters that can be used in the service life prediction of structures; however, only a few actual concrete pore solution pH tests have been conducted on reinforced concrete structures exposed to long-term natural environmental conditions. This paper presents an in-situ inspection and experimental investigation on the pore solution pH of cover concrete for corrosion initiation of rebars embedded in concrete under a long-term carbonation reaction. An old steel mill was inspected, and concrete cover samples were collected to test the pH of the concrete pore solution and phase compositions layer by layer. Pore solution was extracted from discrete depths of the in-situ concrete cover samples by cutting the specimens into slices. Concrete strength was determined with drilled cylinder specimens, which was drilled out from the same location of samples.