Strengthening of Bridges

Strengthening of Bridges

1 Methods of Strengthening of Sub Structures of Bridges 

The substructure of bridges can be strengthen by the following methods

1. Masonry structures: 

In the case of old masonry structure showing sign of disintegrations, all the loose material around the structure is removed and the extent of defect is ascertained. 

If large cavities is found in the structure they are filled with suitable grade of concrete and a wire net is stretched around the entire structure and fastened by driving nails, finally a sand and cement grout is forced  with the help of grouting gun

Strengthening piers of a bridge

 in order to strengthen an old or defective pier, a cofferdam is to be constructed around pier and water pumped out from the space between the cofferdam and pier. After dewatering, a thick casing of concrete is provided around the pier.

 In order to strengthen the weak foundation sometimes piles are driven in the space of pier and cofferdam and good cement concrete is filled around the old pier.

In case of unequal settlement of foundations under pinning of the base of the pier is resorted in deep and running water Under pinning at the base of the pier is done by sinking pneumatic caisson near the pier. 

The arch on the top of the caisson is extended upto the bed of the river and to prevent the flooding of the working spot, a cofferdam is constructed on the top of this arch. 

When the caisson reaches a hard strata below the foundation, the underpinning of the pier base is carried out.

Strengthening of Super Structure of The Bridges

 The method of strengthening of the super structure of the bridge depends upon the type of the bridge such as arch bridge or girder bridge etc. 

For  different types of bridges following methods may be adopted

1. RCC slab bridge: 

All existing RCC bridge can be strengthened by laying a layer of new concrete over the old concrete.

 Before laying the layer of new concrete, the old concrete is toughened either by sand Wasting or chiselling the old concrete After roughening the old concrete,it is cleaned by throwing water and bromming.

 After this a thin layer of neat cement is spread over the cleaned surface. 

After spreading the cement slurry, the new cement concrete is placed in the desired thickness compacted and finished in this way a good bond strength develops between the new and old concrete to act as a monolithic member

2. RCC beam and slab bridge: 

In such bridges, the beams of the bridge can be strengthened by providing steel i beams on each side of the existing beam.

 In case of larger spans, the load from the existing Beams are directly taken on cross steel girders supported on steel built-up girders.

In case a existing sound bridge is needed to be wider, by adding additional beams, it is preferable to do the enure widening on one side only for lateral stability considerations, provided road geometries allow it.

3. Masonry arch bridges: 

In such cases filling above the arch is removed and then the surface of extrados is roughened to provide bond between the new and old concrete.

 Now the roughened surface is washed with water and slurry of neat cement is spread over the surface. 

Over the prepared surface, the required thick layer of cement concrete is placed, compacted and finished. 

The new layer of concrete should be securely keyed into the abutments, as shown in ligure This methods adopted when the existing section is sound and can continue to contribute strength. The new RCC arch serves as a supplementary section only

In case the arch is too weak to bear any other  load than it's own weight in such cases, the new arch should be built several centimetres above the existing extrados with the help of removable shuttering

The existing arch serves the purpose supporting the form work during construction as shown in figure. 

 The shuttering is removed after casting and curing the new COach Continuous Bridges

These are also strengthened by methods similar to those adopted for single span bridges In continuous span bridges, care should be taken that when a span strengthened the adjacent span is not weaken

Steel Bridges

They are strengthened by providing extra steel plates anglesoangles concrete encasement etc.

Suspension Bridges They are usually strengthened by providing additional cables with fasteners.