## 2 Experimental Section
### Test Method and Procedure
#### 2.2.2 Pore Solution pH of Concrete Cover  
The experimental program consisted of testing the pH of 108 concrete slices from 35 cover concrete samples. To test the pH value of the concrete cover layer by layer, the first step was to prepare the concrete pore solution. The expression method, which is the most widely recognized technique for acquiring pore solution, was adopted to extract the pore solution of the concrete cover [10; 17; 28].  
A concrete pore solution expression apparatus was designed and manufactured, as shown in Figure 3. The expression device consists of a hollow steel cylinder where concrete particles are contained. A piston, which is located on the top of the concrete particles inside the hollow cylinder, and a base plate has fluid drainage channels for gathering the extracted concrete pore solution. As the concrete particles are compressed, pore fluid is forced into a circular channel and then goes through fluid drainage channels drilled in the base plate.  
Figure 1: Schematic diagram of drilling samples of cover concrete.  
Figure 2: Cover depths measurement.  
The pH measurement of the concrete cover pore solutions included the following procedures:  
1. Samples of concrete cover cutting and drying. Thirty-five concrete cover samples were cut from the outer surface at intervals of 7 mm along the depth of carbonation with a precision saw using water cooling. Because the sizes of the drilled concrete cover samples were different, the number of slices for different concrete cover samples were either 3 or 4. Thus, 108 7-mm-thick slices were obtained from the concrete cover samples, e.g., 0-7, 9-16, 18-25, and 27-34 mm. After the concrete slices were cut, they were dried in an oven at 60 \({}^{\circ}\)C for 8 h, as shown in Figure 4a,b.
2. Samples pre-conditioning. The dried concrete cover slices were crushed into a powder with a maximum particle size of 5 mm, and the coarse aggregates were removed. The particles were then saturated with distilled water spraying, as shown in Figure 4c.
3. Concrete pore solutions extraction. Approximately 20-30 g of the saturated particles were placed into the cavity of the hollow cylinder of the concrete pore solution expression device. The extraction tests were conducted at ambient temperature using a servo-hydraulic testing system, as shown in Figure 4d. Pressure was applied to the piston using a maximum pressure of 610 MPa in pressing the concrete particles with a mean rate increase of 2 kN/s. After the maximum pressure is reached, it was held for 10 min until no more pore solutions could be obtained. The pore solution was collected in a centrifuge tube. A minimum volume of 2-3 mL of pore solutions added to carry out pH measurements.
4. pH measurements. The pH of the expressed pore solution was measured directly with a pH meter and a micro combination pH electrode, as shown in Figure 4e. This electrode was designed to measure in the pH = 0-14 range. Before performing the pH measurements, the pH electrode system was calibrated by two standard buffer solutions. The pH measurements were conducted at an ambient temperature within a range of 20-25 \({}^{\circ}\)C. The calibrated micro-combination pH electrode remained in the solution until the pH value was constant.  
Figure 4: _Cont_.  
Figure 3: Concrete pore solution expression apparatus: (**a**) schematic diagram; (**b**) extraction on a compression testing machine.