## 3 Structural Method of Reinforcement and Construction Stages
### Reinforcement Method  
Different engineering methods can be applied to the replacement of the pavement layer of the hollow slab girder bridge, and they have different effects. The existing methods for the direct replacement of the bridge pavement of ordinary hollow slab girder bridges mainly include the following two methods:  
1. The previous pavement layer is completely removed and replaced with UHPC and stone matrix asphalt (SMA).  
The removal of the pavement layer ensures that the structural weight is basically unchanged. Combined with the UHPC performance, this method is better to improve the structural performance. At the same time, the use of more UHPC leads to an increase in cost. Additionally, there is doubt as to whether the use of thick UHPC may produce large amounts of shrinkage in the upper part of the beam, which is equivalent to the positive moment effect, thus reducing the UHPC effect [24].  
In this method, it is necessary to consider the bonding and anchoring performance of the UHPC and the original structural plate and beam, and it is necessary to increase the rebar mesh and the anchoring rebar. For bridges with partial damage, it is also necessary to consider the damage of the hinge joint.
2. Partial removal of the previous pavement, which is followed by the addition of a thin layer of UHPC and SMA is added.  
This method is suitable for situations in which the damage to the hinge joint is not serious. Because it does not involve the replacement of the hinge joint, only the replacement of the upper surface is needed, the corresponding amount of work is small, and the construction is fast and inexpensive.  
Considering the leveling layer, the pavement removal depth and the bridge elevation, the leveling layer is combined with the reserved pavement to ensure that the replaced pavement has a consistent elevation.  
When the partial pavement is retained and the original rebar grid is exposed, it is relatively simple to remove and replace it. However, this type of project is relatively rare because most bridges that need the pavement layer replaced also have the problem of hinge joint damage. At present, there is no test or engineering verification that confirms that the partial replacement of pavement ensures that the overall performance of the structure and the stress on the hinge joint will not deteriorate.  
In the two construction methods mentioned above, the reliable bond between the UHPC and the existing beam concrete is the basis for the common loading bearing. Many researchers have conducted relevant studies and experiments on the bonding performance between UHPC and NC and proved that after surface chiseling and cleaning, good adhesion between UHPC and NC can be ensured, especially in the presence of rebar mesh.  
With reference to the conventional pavement structure, the pavement replacement in this project adopted the rebar mesh and hinge joint reinforcing methods at the same time. The specific design measures were as follows.  
On the basis of retaining the original hinged rebar, the rebar was implanted into the web of the hollow slab girder from the top according to \(\Phi\)12@200 and a rebar mesh was formed in the hinge area. In addition, a single layer of \(\Phi\)12@100 \(\times\) 100 rebar mesh was placed above the slab girder. The specific structure and layout are shown in Figure 5.  
The UHPC used was UC120 grade, and the mixing ratio was powder:water:admixture: steel fiber = 1000:90.5:6.05:104.8. The specific performance parameters are shown in Table 1.