Enhancing the capacity of concrete bridges
The increasing demands placed on the UK bridge stock during the last decade, including the introduction of 40 tonne vehicles and a robust motorway widening programme, caused a major re-evaluation of bridge strengthening. As a result, the bridge industry came up with many innovative methods to enable appropriate strengthening processes to be carried out economically and to provide long-term integrity.
Enhancing the capacity of concrete bridges, published by the Concrete Bridge Development Group, covers reinforced and prestressed concrete highway and railway bridges and masonry arch bridges. The different types of strengthening available to industry are comprehensively covered. In many cases repair will be required before strengthening can be undertaken, but this report does not cover repair as such and reference must be made to other documents that discuss repairs in more detail. The main emphasis is on the contractor and site operations so that it can be used as a working document. The report covers buildability, contractual issues, traffic management and delay, safety, etc, which are often a major consideration in the choice of strengthening options. Design aspects are only covered briefly, but designers and owners will find a comprehensive list of strengthening methods that may provide more scope for solving their particular problems.
The total or partial replacement of a bridge is probably the ultimate solution, but it is also the most costly. The benefits and cost of various strengthening options are presented and contrasted with those of replacement. The strengthening of concrete structures was a relatively difficult operation until the developments of recent years, which have opened up a range of new possibilities. In any structure the quantity of prestressing steel and reinforcement is designed to meet the original loads to be applied, and until recently there were few options or strategies for changing that proportion. However, the introduction of advanced composite materials has opened up new strategies where the advantages of these materials can be exploited to the full.
The range of intervention using the strengthening options can extend from the minimal (do nothing) to complete replacement of the structure. Some of the early options contained in the report can be described more as gaining strength from the existing structure rather than as pure strengthening. At the other end of the scale methods are described which involve major reconstruction of parts of the bridge, where it is difficult to decide at what point strengthening stops and reconstruction begins. In many cases it is economic to save as much of the structure as possible, but more often the overriding factor is how the existing structure can be given additional strength to carry modern-day loads with minimal disruption to the traffic carried over or under the bridge.