3.3. Low anchorage capability
A screw spike is primarily used to hold down the baseplates
that attach rails to sleepers and prevent vertical and lateral movements
between them. A hardwood timber sleeper has a screwspike
resistance of 40 kN [28,42] whilst at least 60 kN is required
for the modern design of prestressed concrete sleepers supporting
heavier and faster trains [43]. The low anchorage capacity of holding
screw (Table 3) is another problem for Type-1 composite sleeper.
It has been reported that the modified compound of a natural
rubber composite sleeper (Type-1) showed a very stiff and inelastic
performance when holding the spike for a fastening system [21].
This is due to the nature of plastic materials that cannot grip the
screw firmly like concrete, especially under dynamic loading conditions.
When using composite sleepers, the loosening of a fastener
over time makes a track unstable due to stress relaxation [40]
which has been considered the most likely reason for the derailment
failures of track systems constructed with composite sleepers
[44,45]. However, the anchorage capacity has been improved
in Type-2 and Type-3 sleepers due to the usage of more quality
and high performing materials.