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Various defects in APJs have been identified。 Typical damages include rutting and shoving in summer, cracking through the joint material and debonding at the joint-pavement interface in winter。 Rapid changes in temperature may be a factor that results in joint cracks [32]。 The damage of expansion joint will result in leak prob- lem or reduced ride quality over the joint。 The performance survey done by Johnson indicated that the most frequently observed de- fects of APJs included cracking (11。9%), rutting (36。4%) and inter- face adhesive failure (27。1%) [12]。 The poor performance and relative short service life was believed to do with the constituent materials, especially the binder used。
In total of 64 APJs were inspected in New England state and found that the major joint distresses and percentages of occurrence
were debonding 60。9%, joint cracking 39。1%, ravelling 37。5% and rutting 31。3% [33]。 The useful life span was reported to be about 5 years。 Material distress and installation in unsuitable locations leaded to the premature failure。
Literature review indicates some problems with APJs as men- tioned above。 However, these problems could be minimized by using good quality material, testing the material before installa- tion, checking whether the joint displacement is within acceptable ranges or not, installing the joints in the locations with a low vol- ume of heavy trucks [4,31]。
3。 Materials and testing methods
3。1。 Binders
APJs in most cases consist of stone aggregates and a high content polymer mod- ified bituminous binder。 As reported in Hean’s study, the used modified bitumen contained 11% SBS polymer modifier by weight of the binder。 Furthermore, high binder content of about 25% by weight of the total mixture was used to prepare the joint filling mixtures [5,19]。 Compared to traditional hot mix asphalt, the APJ mixture is a bitumen-rich mixture and thus the mixture performance is strongly dependent on the binder properties。 Typical joint damages including rutting and cracking are related to the binder’s flow resistance at high temperatures and relax- ation and elongation at low temperatures。
ASTM D6297-01 Standard Specification for asphaltic plug joints for bridges pro- vides a detailed specification to the binder used for APJs, see Table。 1。 More informa- tion on binder requirements of various manufacturers who were approved to supply APJ materials for the New England states can be found in [33]。
In APJ Standard produced by UK bridge joint association, the used binders in- clude rubberized binder and polymer modified binder。 Performance characteriza- tion is carried out using standard penetration, softening point, flow resistance and extension test at low temperatures。 The requirements on the related binder properties are listed in Table 2。
In 2005, the Swiss Guidelines of the Swiss Federal Road Office (ASTRA) released a new guideline on APJs。 The guideline contains joint system and material require- ments including appropriate test procedures as well as instructions on quality man- agement and practical execution of the construction [26]。 Table 3 gives some important requirements on binder properties。
Compared with the ASTM D6297-01, the binder properties as required by Swiss guidelines are more relevant to the field performance of APJs, especially on the elongation at low temperature and aging resistance。 As indicated, some important performance based properties are considered including the elastic recovery, aging resistance, storage and high temperature stability。 The elastic recovery test is an effective measurement widely used to test polymer contribution to the binder per- formance。 A degree of elastic recovery is desirable to reduce the risk of permanent deformation。 Dynamic modulus measurements at a wide range of temperature and frequency are specified for rheological analysis。 Examination on polymer dispersion state and change of polymer content before and after aging is also included。 A 5- year monitoring project on 18 joints done by EMPA showed that the key require- ments for the durability of APJ systems include a strong bond between joint-pave- ment interface, strong adhesion between aggregate and binder, maximum extensibility at low temperature and flow resistance at high temperature [5,14]。