The thesis investigates the mechanisms and restraints which influence
transverse crack propagation through the bituminous surfacings of
semi-flexible pavements. These pavements incorporate continuously
laid cement bound roadbases which, during curing, crack into slabs of
varying length, ranging from 4-25m.
Reciprocal crack growth can occur in the surfacing, known as
'reflection cracks', located through stresses concentrated at the
discontinuities within the roadbase .
Three mechanisms have been identified and are described as
contributing to reflection crack propagation. They have been
analysed independently although the majority of conclusions drawn are
applicable to their combined action. Their relative importance will
vary with respect to pavement geometry, material properties,
environmental conditions and traffic intensity.
The first mechanism, 'tensile fatigue', induces crack propagation
vertically upward through the surfacing. Tensile strains are
developed during daily and ru1nual fluctuations of temperature, which
cause expansion and contraction of the cement bound roadbase. This
mechanism is most prominent on pavements with thin surfacings and
long slab lengths. The rate of crack growth is dependent on the
range of temperature within the roadbase , slab length, thermal
characteristics of the roadbase material and resistance of the
surfacing to this form of fatigue .
A model has been developed based on a combination of results from an
extensive testing programme, the use of fracture mechanics theory and
computer simulation of the condition. The results quantify the
resistance shown by conventional bituminous mixes to reflection
cracking in terms of their mix parameters. Also considered are the
use of stress relieving membranes, reinforcement material and
modified binders to inhibit crack growth.
The second mechanism, 'tensile yield' is also thermally induced but
associated with cold weather conditions. Temperature gradients
through the pavement structure induce warping and contraction within
the uppermost layers. Tensile strains developed at the surface can,
under U.K. winter temperatures, exceed the ultimate yield strain of
the wearing course material.
Preliminary. investigations of four pavements constructed in the early
1970's to motorway specifications indicate that reflection cracking
will initiate at the surface if the yield strain, as defined through
tensile creep tests, is reduced through binder oxidization to a value
of 0.5%. This mechanism will operate on pavements with greater
structural layer thicknesses and is only partially dependent on slab
length.
The influence of a further mechanism, 'shear fatigue' induced through
trafficking of the pavement, has been shown to be confined to the
acceleration of crack growth in the final stages of propagation
unless a breakdown of interlock occurs between adjoining roadbase slabs .The Transport
and Road Research Laboratory (TRRL) U.
