Fig。 2。  Improved geometrical design of APJ reported by    Park。

plate [11]。 Furthermore, thick joints are more susceptible to rutting in summer time。 Additional compaction on the filling mixture may improve the rutting resistance of thick joint   systems。

Laboratory tests and simulations confirm that the effective length of an APJ is less than the actual plug joint length。 Only a small length is effectively developing the strain field with bridge motion [11,13,14]。 This effective length is a function of the joint geometry, material stiffness and strength。 Because the material properties are strongly dependent on temperature,  this  makes the effective length a function of temperature as well。 Reducing temperature may result in shorter effective joint length and  lead to lower relaxation capability and elongation of the joint material。 All of these effects contribute to the joint cracking at low temper- atures。 The joint material close to the pavement does not   contrib-

ute to the effectiveness of the joint to share the joint movements。 This extra material not only adds to the expense of the joint but also increases the opportunities for serviceability failures, espe- cially rutting。 It indicates that the joint width also can be optimized if needed。

APJs for joint movement larger than 50 mm have been devel- oped [19,20]。The system consists of a row of springs which are embedded in the asphaltic joint filling material and anchored to two opposite L-shaped profiles。 Joint expansion and deformation simulation test at low temperatures indicated that the embedded springs enforced a homogeneous  longitudinal  strain distribution in the plug joint during joint movements。 L-shaped steel anchors prevented the tensile stress from transferring to the joint-pave- ment  interface。  Large  joint  horizontal  movements  may  result in

L。 Mo et al。 / Construction and Building Materials 45 (2013)  106–114 113

significant lateral contractions and lead to debonding at the cor- ners of joint-pavement interface。 Laboratory tests  showed  that the joint survived after  20  test  cycles  with  a  joint  opening  of 65 mm at —20 °C。 Site inspection also confirmed good performance of the developed APJs with large  movements。

6。 Summary and conclusions

Literature review shows that the service life of APJs is relatively short。 However, APJ is widely accepted because of easy installation, low cost of maintenance and repair, low traffic noise and high driv- ing comfort。 Various defects in APJs have been identified by field inspection。 Typical damages include rutting and shoving in sum- mer, cracking through the joint material and debonding at the joint-pavement interface in  winter。

Performance-based specifications on APJ binders can be devel- oped。 When Superpave binder performance grade is taken into ac- count, it is suggested that the high-temperature performance grade should be at less four levels higher compared to the bitumen used for local asphalt pavements while the low-temperature per- formance grade should be the same or one level lower。 Elastic recovery test can be used as an indicator of the polymer contribu- tion to binder performance。 DSR dynamic rheological testing can be an effective method for the purpose of ranking binder perfor- mance。 Changes of chemical composition due to effect of aging, polymer separation can be tested。 Creep and relaxation testing al- lows to get insight into the binder performance at high and low temperatures。 Extension test at low temperature is critical for bin- der selection to obtain strong adhesion bonding and maximum elongation。 An APJ binder with lower Tg is expected to have better performance at cold climates。