| Summary: | In some sections of railroad, many prestressed concrete railway ties were found to
be cracked after being in service for only a few months, because of impact loading. The
dynamic properties of concrete ties were thus studied in this work. Two typical types of
impulse encountered in track, due to rail abnormalities and "wheel flats" of trains, were
successfully simulated by the use of a 578 kg impact machine and a 60 kg drop weight
impact machine, respectively. The previously developed "single-blow" impact technique
has been extended into a "multi-blow" impact technique, in order to better simulate the
repeated impact loading on the concrete tie in track.
It was found that the stiffness of the rubber support played an important role in the
dynamic response of the ties. Using the soft rubber support caused a low maximum load,
low loading rate, a higher fracture energy and a ductile flexural fracture mode. Using the
hard support caused a brittle flexure-shear or shear failure mode. Crack mode analysis
showed that the cause of the crack mode changing from flexural under quasi-static loading
to shear under impact loading is that the shear to moment ratio at the mid-span of the tie
changes under impact. A crack mode prediction method was proposed.
In the second series of tests, twelve types of modified ties were tested. The effects
of the concrete strength, steel fibre additions, changes in prestressing force, the presence
of stirrups were examined. The crack opening length and residual crack length were
detected by crack detection gauges. Steel fibres greatly improved tie behaviour, leading to
shorter and finer cracks in the concrete. Stirrups can, particularly when used in
conjunction with fibres, effectively retard the deterioration of the concrete tie. The ties
with a 40 MPa compressive strength and 30 mm fibres behaved very well. They were
markedly better than the ties which had the same fibre content but a 65 MPa concrete
compressive strength. The reason for this is that reducing the concrete compressive strength or prestressing level of the tie resulted in a reduction of the dynamic flexural
stiffness and hence the magnitude of the impact loads. It is believed that if these measures
were combined with the use of steel fibres in the concrete, a new type of concrete tie, with
improved ductility and high resistance to impact load could be developed.
Dynamic analysis of the ties showed that a different impulse duration or frequency
may lead to a very different concrete strain response. This may need to be considered in
the wheel truing program in service.
Thirteen types of commercial pads were tested and ranked. Soft pads may act as a
low-pass filter, leading to lower amplitudes of the concrete strain vibration. However, it
may structurally deteriorate more quickly, leading to an even worse influence on the ties
after a period of time in service.
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