It has been found that the durability of bridges is dependent on the effectiveness of the bridge deck waterproofing and the movement joints. Aggressive salt water must be prevented from entering the structural fabric to avoid unseen corrosion from damaging the concrete and steel elements. The waterproofing and expansion joints must be continuous to prevent water percolation through permeable road surfacing and resist the hydraulic water pressure developed by the traffic movement. There should be a positive link between the bridge deck waterproofing and the expansion joints with combined sub-surface drainage outlets to discharge water and prevent a build-up of water behind the joint. These drainage systems can take the form of slotted box sections or cast-in pipes which collect the infiltrated water in front of the joint.
Bridge expansion joints have to function as "riding plates" to carry the imposed traffic loads and also accommodate the thermal movement, shrinkage, pre-stress creep and rotation of the deck. These joints can be simple flexibilised asphalts or complex mechanical or elastomeric elements, according to the range of movements to be accommodated. The expansion joint should give good riding characteristics without generating excessive noise from traffic, especially in urban areas where adjacent residential property may need careful consideration. It must also be functional for the road-user whilst having good skid resistance and be suitable for the road curvature and alignment. If pedestrians, animals and cyclists use a bridge the expansion joint should be of a design which does not cause safety problems. Footpaths may need cover plates slightly recessed below the surface to provide safe access.
It is essential to use materials which are durable and offer a maintenance-free operation. Any elements subject to wear must be replaceable using simple techniques since traffic management schemes and lane closures are costly and need special authorisation as well as causing public irritation. Therefore, it may be expedient to replace bridge expansion joints prematurely while other maintence work, such as re-surfacing, is carried out so that future road closures are minimised.
The bridge designer must clearly set out the desired operating standards and define the total movements related to the imposed loadings, temperature range, deck shortening and rotation. In this way the manufacturer or supplier can provide the correct technical solution which can be incorporated into the working drawings. It is essential to embrace the particular design features of various bridge expansion joints so that box-outs and plinths can be formed without resorting to changes in reinforcement, etc, at a later state. It is vital that the bridge expansion joint is formed continuously from parapet to parapet taking into account footpaths, kerbs, central reserves and skew angles. The aggregated longitudinal movement and skew movement should be used to select the correct size and performance of the joint. The fixing and bonding of different types of joints should not cause or propagate damage to the road surfacing or the supporting bridge structure.
Installation of the joints should be delayed for as long as possible to allow for shrinkage and creep of the deck and settlement of the supports. The gap widths should be formed to suit the bridge deck temperature in relation to the mean deck temperature. Further information on this relationship can be obtained from TRL Report SR479 "Bridge temperature for setting bearings and expansion joints" by Mary Emerson.
Where segmental types, elastomeric elements in metal runners or cantilever comb/tooth joints are used, a separate drainage membrane should be used to collect and discharge any infiltrated water into outlets to increase the security of the joints.
For further information refer to the Highways Agency Departmental Standard BD 33/94 and Advice Note BA 26/94.
Highways Agency publication SA/1 List of Approved/Registered Products includes bridge expansion joints. It is revised annually.