Various joint materials and systems have been used to obtain the maximum benefit like watertightness,  maximum movements in both horizontal and vertical directions, ride quality and durabil- ity [1–4]。 APJs have gained more attentions because of their advan- tages of noise reduction and driving comfort [5,6]。 Other benefits of APJ application are their low cost, easy construction, maintenance and repair。 APJs are very common for small ranges of joint move- ment, typically smaller than 50 mm [7–10]。 They are also becom- ing popular for replacing or rehabilitating joints   [3,4]。

A typical APJ system consists of a backer rod in the gap, a steel plate on top of the gap, a block-out, and a special asphalt mixture that consists of polymer modified bitumen and aggregates [5,7,8,10]。 The structure of APJs seems simple, but their service behaviors are complex。 APJs may be damaged and failed due to dif- ferent mechanisms。 Typical types of damage include low tempera- ture and fatigue cracking, rutting and permanent deformation, and debonding of the joint-pavement interface [3,11,12]。 Because the joint filling material is a bitumen-rich mixture, the joint perfor- mance is strongly dependent on the type of asphalt binder。 The binder properties including flow/creep resistance at high tempera- tures, relaxation and tension elongation at low temperatures are directly related to the field performance of APJs。 Finite element analysis and laboratory validation tests confirmed that only a small length of APJ was effectively developing the strain field with bridge motion。 Strain/stress concentrations occurred at the edge of the gap plate and the joint-pavement interface [11,13,14]。 It indicated that the improved joint design can also lead to a better joint perfor- mance and longer service life [15–18]。 Research done by EMPA indicated that the introduction of movement aids allowed develop- ing APJs with larger joint movements of 70–100 mm [19–22]。 Lab- oratory tests and field trials confirmed that the embedment of the springs in joint materials enforced a more homogeneous longitudi- nal strain distribution within the material during joint movements。 The goal of this paper is to provide a review on the aspects asso- ciated with APJs including application and field performance, con- stituent materials and testing, installation and structure design。 The focus is to provide useful information to develop engineer- ing-based   design   guidelines   and   performance   based  material

specifications。

2。 Application and performance

APJ has become popular in many countries for accommodating bridge joint movements less than 50 mm because of its low cost,

ease of installation and repair, low instance of snowplow damage。 The benefits of noise reduction and driving comfort also become interesting especially in the Netherlands [6,23,24]。 However, the field performance of APJs worldwide varies significantly。 Fig。 1 gives an illustration of typical distress types of APJ reported by Part l [20]。 A survey of 250 joints on UK highway bridges done by Johnson indicated that half of bridge expansion joints were installed by APJs [1]。 50% of the APJs surveyed were leaking and tracking was com- mon, especially in heavily trafficked nearside lanes。 Cracking and debonding were found more on lightly trafficked road, particularly in winter。 An inspection survey of 150 expansion joints done by Lima indicated that the joints used more frequently included rein- forced elastomeric cushion joints (51%), elastomeric flexible strips (22%), and asphaltic plug joints (9%) in Brisa, Portugal。 APJs were very suitable for small ranges of movement, particularly replacing joints in old bridges。 For bridges older than 25 years, about 28% of the bridges have been installed by APJs [3,25]。 The average life of APJ was 7。7 years in Portugal。 In the Netherlands, the APJ systems only have an average service life of about 3 years。 In order to im- prove the joint performance and reduce the traffic noise caused by defected joints, the Dutch Ministry of Transport, Public Works and Water Management have started a large research project on