Bridges should last at least 100 years if they are well designed and maintained. The materials used to construct modern bridges far exceed the quality of materials from even 40 years ago. Many of the newer bridges have precast components which are created in a controlled environment and then shipped to the bridge site.
So why do bridges need to be repaired or rehabilitated? The single biggest reason why bridges deteriorate is due to joints on the bridge.
Why are bridge joints even necessary? Bridge joints are necessary to allow for thermal expansion and contraction of the bridge. Bridge joints also allow for beam rotation at the end of the beams.
How can we as bridge engineers eliminate bridge joints? There are several ways. The most common way to eliminate bridge joints is to eliminate the bridge joint over the interior bents by making the superstructure “continuous.” Eliminating the joint at the interior bents will create additional movement at the end bents. This can be dealt with by using joints that accommodate more movement.
Another way to eliminate joints is to build integral abutments. Integral abutments are an excellent example of a bridge design that keeps the surface water off of the beam ends and the bridge substructure. The integral abutment design relies on the movement of the end bents to facilitate the thermal expansion.
During the design process, we design the beam ends to be fully connected, or “integral” with the supporting bent cap. A single row of piles which support the end bent cap will move slightly as the bridge superstructure moves. This integral design eliminates the need for a bridge joint at the supporting end bents.
Getting back to bridge repair and bridge rehabilitation,the most common types of bridge rehabilitation include the following:
• Steel beam end repair
• Concrete repair to decks, piers, abutments, and concrete beams.
• Joint repair
Let’s talk about the two most common types of bridge repairs. Concrete and steel.
Concrete repair is typically accomplished by locating and chipping away the deteriorated concrete and replacing the old concrete with new concrete.
Over the past 30 years or so, a relatively new way of applying new concrete to existing structures has been used. This new method involves spraying the new concrete on the deteriorated areas of the bridge. This method is much more efficient than the traditional method of pouring concrete into forms.
Spraying the concrete also allows for the placement of concrete on the vertical and the bottom side of horizontal surfaces. This method of applying concrete is often referred to as shotcrete.
Steel Beam Repair
As bridge engineers, we employ two main methods for repairing steel beams. The first method is plating and the second method is replacement of the deteriorated areas.
Plating is a method of covering the deteriorated areas with a steel plate and welding the edges of the plate to the existing beam.
Replacement of deteriorated steel is a bit more involved because the bridge usually needs to be temporarily supported. We often develop jacking plans for this purpose. Once the bridge is temporarily shored, the deteriorated areas of the steel beams can be cut out and replaced.