## 2 Field Test
### Cutting Test of Girders  
To maintain the desired engineering properties of the concrete bridges, the bridge durability, i.e., the ability to resist the abrasion, corrosion, and other deterioration actions, should be evaluated after certain years' service. Due to the hinge joint damage, the hollow slab girders may behave individually under the traffic load, leading to the difference of the durability of each girder. Unlike the conventional core sampling and testing of concrete, the studied bridges were demolished, and each girder was cut into six segments in the field to obtain an accurate assessment of the durability of aged bridges. The pavement was first demolished to reduce the bridge weight, and the hinge and bearing joints were cut off using the vibration-free concrete saw. Each girder was then lifted by the crane, and a diamond wire saw was used to cut the concrete girder into six segments. The whole cutting process is shown in Figure 4. Three spans, S1-S3, were included in the field test. In this way, a total of 75 concrete girder segments were obtained, as shown in Figure 5. The segments were numbered by the span, girder, and the cutting orders. Figure 3: Measurement based on the gauges and leveling instrument: (**a**) diagram; (**b**) installation of measuring rulers. After the cutting off of the concrete bridge, the service condition of the girder can be evaluated accurately. A total of six indicators were considered in the durability evaluation, i.e., surface cracks in longitudinal direction (\(u_{1}\)), surface cracks in transverse direction (\(u_{2}\)), compressive strength of the concrete (\(u_{3}\)), corrosion degree of the rebars (\(u_{4}\)), thickness of the reinforcement protective layer (\(u_{5}\)), and grout condition in the pre-stressed ducts (\(u_{6}\)). The surface cracks on the girders were first recorded and checked with the data in Section 2.2. The compressive strength of the concrete was obtained by the non-destructive rebound hammer test. The corrosion degree of the rebars was calculated by the measured and design value of the diameter. Grout defects in the ducts were also checked, and the thickness of the reinforcement protective layer was measured by a vernier caliper. Some of the indicators are shown in Figure 6.  
Figure 4: Process of cutting the concrete girders: (**a**) plan for cutting girders; (**b**) cutting test. According to the Chinese specification JTG H11-2015 [21], the condition of the whole bridge or bridge components can be classified into five grades, as shown in Table 2. For the specific parameters of the bridge components, the condition grade is quantified based on the evaluation criteria recommended by the specification. For the surface cracks, a complex calculation process was employed to obtain a comprehensive result. As shown in Table 3, the crack dimensions, crack location, and crack growth are included in the evaluation, and the grade of the surface crack is calculated by the values of different parameters. Figure 5: Girder segments. Figure 6: Diagram of indicators \(u_{2}\), \(u_{4}\), and \(u_{6}\). The compressive strength of the concrete in the field test is defined as \(S_{\rm c}\), and the design value is \(S_{\rm d}\). The concrete strength is then evaluated by the parameter \(D_{\rm b}=S_{\rm c}/S_{\rm d}\). The cross-sectional area of the rebars will decrease due to corrosion. The rebar diameter after service is measured, and the value is defined as \(d_{\rm c}\). The design value of the rebar diameter is \(d_{\rm d}\), and the corrosion degree \(D_{\rm c}\) can be calculated by Equation (1):  
\[D_{\rm c}=\frac{d_{\rm c}^{2}}{d_{\rm d}^{2}}, \tag{1}\]  
The reinforcement protective layer effectively prevents the rebars from the corrosive solution. If \(n_{t}\) points on the cross-sectional area of the concrete girder are selected in the measurement, the thickness of the layer for \(i\)th point is defined as \(t_{i}\).