# Nonlinear Stiffness of Semi-Fixed Dowel Joints in Semi-Integral Bridges
## 3 Experimental Program  
### Specimens  
A specimen consisting of the dowel joint components and the upper and bottom concrete blocks was fabricated to simulate the superstructure and the substructure of the bridge, as shown in Figure 3. The steel dowel in the dowel joint had the same diameter of 32 mm as in the real structure, and its 380 mm length was buried into the bottom block and 480 mm was extended into the upper block. The steel dowel was wrapped with a  
Figure 1: Semi-fixed dowel joints in the Jingpu Bridge.  
Figure 2: Elevation of the Jingpu Bridge before and after retrofitting.  
rubber sleeve, and a rubber pad was arranged between the upper and bottom block of the specimen.  
The wall thickness of the rubber sleeve, \(t\), and the thickness of the rubber pad, \(h\), were taken as the main parameters.  
The rubber sleeve had an inner diameter the same as the diameter of the steel dowel, that is, 32 mm. With 40 mm as the basic wall thickness as in the real structure, 20 mm thicker and 20 mm thinner of the wall was taken for the wall thickness variation, which results in three specimens with different wall thicknesses of rubber sleeves, i.e., 20 mm, 40 mm and 60 mm. The rubber pad had a plan dimension of 800 mm \(\times\) 600 mm, with its basic thickness of 40 mm, The thickness of another contrast specimen is 20 mm. Therefore, a total of 4 specimens were designed and fabricated, as listed in Table 1.  
The dimension of the upper concrete block is 800 mm \(\times\) 600 mm \(\times\) 800 mm, and the bottom block is 1300 mm \(\times\) 800 mm \(\times\) 710 mm for the specimen with a rubber pad thickness of 40 mm; it was 1300 mm \(\times\) 800 mm \(\times\) 730 mm when the rubber pad thickness was 20 mm. The specimen dimension is shown in Figure 3.  
Considering the concrete blocks of the specimens would work as rigid bodies in the test and only very small stresses would be produced, they were only reinforced with longitudinal steel bars with a diameter of 16 mm and stirrups, less than the actual reinforcement in the bridge. The reinforcement is not drawn in Figure 3.  
\begin{table}
\begin{tabular}{c c c} \hline
**Specimens** & **Rubber Sleeve** & **Size of Rubber Pad** \\
**Number** & **(Length \(\times\) Wall Thickness)** & **(Length \(\times\) Width \(\times\) Height)** \\ \hline JD1 & 520 \(\times\) 20 & 800 \(\times\) 600 \(\times\) 40 \\ \hline JD2 & 520 \(\times\) 40 & 800 \(\times\) 600 \(\times\) 40 \\ \hline JD3 & 520 \(\times\) 60 & 800 \(\times\) 600 \(\times\) 40 \\ \hline JD4 & 520 \(\times\) 40 & 800 \(\times\) 600 \(\times\) 20 \\ \hline \end{tabular}
\end{table}
Table 1: Specim parameters (unit: mm).  
Figure 3: _Cont._  
The concrete strength grade used for the specimens was C35 concrete, which means that its compression strength of cubic specimen with 150 mm side length is 35 MPa. The concrete sample prepared and cured under the same conditions as used for the corresponding specimens were tested, giving an average compressive strength of 36.7 MPa from three samples.  
Three samples of steel bars were taken for material property test. The measured yielding strength and tensile strength were 402.7 MPa and 510.8 MPa, respectively. According to JGJ D62-2004 [18], the elastic modulus is set to 2.0 \(\times\) 10\({}^{5}\) MPa. The rubber properties are provided by the manufacturer, as shown in Table 2.