Rehabilitation of RC, Steel and Pre-stressed Bridges
Modern bridge infrastructure comprises of primarily concrete (reinforced or pre-stressed) and steel structures. Over the service life of a bridge, its constituent materials are continually subjected to fatigue and wear-tear due to dynamic vehicular loads. Overloading due to increase in wheel loads and regular exposure to aggressive external environment may aggravate the situation . Poor quality of construction and lack of regular maintenance could potentially lead to major retrofit in a bridge structure.
Components facilitating expansion/ contraction of bridge and load transfer to the sub-structure, such as expansion joints & bearings, may also require rehabilitation or replacement over time. Defects in the constituent materials may be manifested in the form of cracking – spalling of concrete, excessive deflection of structure, corrosion of steel components/ reinforcement etc. It is evident that rehabilitation of bridges involves addressing a myriad of problems and no single technique or retrofit method could offer a complete solution.
Reinforced / Pre-stressed Concrete Bridges
Most of the structural repair and retrofitting techniques used for ordinary reinforced concrete are applicable to reinforced concrete bridges as well. Structural repair techniques include crack injection with low viscosity epoxy and patch repair using an approved material such as polymer modified mortar or non-shrink grout. Strengthening techniques include
- concrete jacketing.
- steel plate or FRP (fibre reinforced polymer) bonding on the external surface of the member to be retrofitted.
For strengthening of pre-stressed concrete structures, external post-tensioning or FRP retrofit may be appropriate.
Involves increasing size of the existing reinforced concrete section by adding more reinforcement and concrete. It could be accomplished by either of the following methods:
- Conventional Concrete
- Sprayed Concrete (Shotcrete)
- Pre-Packed Aggregate Grouting
Regardless of the method deployed, jacketing results in increase in dimensions as well as dead weight of the retrofitted member.
Steel Plate Bonding
This technique involves enhancing strength (shear, flexure, compression) or improving stiffness of deficient reinforced concrete members by bonding steel plates of calculated thickness with adhesives and anchors to the existing sections. Steel plate not only acts as externally bonded reinforcement to the concrete section but it also improves the moment of inertia (stiffness) of the composite (concrete-steel) section. This technique is useful for the flexural and shear strengthening of bending elements such as beams and slabs and for compression capacity enhancement of columns. Resulting surface finish is unsightly and steel plate retrofit is prone to corrosion over time.
A Fiber Reinforced Polymer (FRP) typically consists of high tensile continuous fibers oriented in a desired direction in a specialty resin matrix. These continuous fibers are bonded to the external surface of the member to be strengthened in the direction of tensile force or as confining reinforcement normal to its axis. FRP can enhance shear, flexural, compression capacity and ductility of the deficient member. Aramid, carbon, and glass fibers are the most common types of fibers used in the majority of commercially available FRPs FRP strengthening is a quick, neat, effective, and aesthetically pleasing technique to rehabilitate reinforced and pre-stressed concrete structures.
Unlike steel plates, FRP systems possess high strength to self-weight ratio and do not corrode. It is imperative to be aware of the performance characteristics of various FRP systems under different circumstances to select a durable and suitable system for a particular application.
Steel/ Composite Bridges
Components of a steel bridge need to be continually protected against environmental corrosion. Proper care needs to be exercised to maintain critical elements such as piles, piers, decks, suspension cables etc. Piles and piers may be rehabilitated using bonded steel plates or FRP strengthening/ protection. Steel bridges are generally covered with a concrete decking, which gets worn off over time due to cyclic dynamic loads. It is a time consuming and cumbersome process to retrofit reinforced concrete decks with conventional methods. The following techniques are useful to rehabilitate a steel bridge deck:
- Bridge Deck Replacement
- Bridge Cable Protection
It is evident from the above discussion that bridge rehabilitation exercise involves input and assistance from a variety of specialist engineering fields. It is important to recognize that the solution to a particular problem lies in the selection of a suitable technique and a capable agency for the purpose. Proper diagnosis of the problem followed by an appropriate treatment would ensure a long-term durable retrofit of a bridge structure.
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