CT4860 STRUCTURAL DESIGN OF PAVEMENTS

3.4.3 Load transfer in joints/cracks
In 3.4.2 equations and charts have been given for the calculation of flexural
tensile stresses and deflections due to traffic loadings in a single concrete
slab. However, in reality a concrete pavement consists of a number of
concrete slabs with joints (plain concrete pavements) or cracks (reinforced
concrete pavements) between them. The load transfer in these joints and
cracks is dependent on the joint or crack width (which depends on the slab
length), the amount of traffic and the type of joint or crack construction, which
means: aggregate interlock, reinforcement, ty bars and dowel bars.
First the last mentioned four influence factors will be discussed separately and
then the total load transfer in the joint or crack (i.e. the joint or crack
efficiency).
Aggregate interlock is the phenomenon that both rough sides of a joint or
crack stick into each other. The load transfer due to aggregate interlock is
dependent on:
a. the joint or crack width: the greater this width, the lower the load transfer
b. the effective thickness of the concrete layer (= total thickness minus the
depth of the saw cut for a contraction joint): the greater this thickness, the
higher the load transfer
c. the rate of support of the concrete layer by the substructure: the higher the
modulus of substructure reaction k, the smaller the deflections w of the
slab edges due to traffic loadings P (see equations 47 and 49), the lower
the shear forces at the joint or crack sides, the lower the polishing of the
concrete, and thus the higher the load transfer
d. the magnitude of the traffic loadings: the deflections of the slab edges are
proportional to the magnitude P of the traffic loadings (see equations 28
and 30), and therefore the smaller the traffic loadings, the higher the load
transfer (similar to c)
e. the aggregate shape in the concrete mixture: the more rough (the higher
the angle of internal friction) the aggregate, the higher the load transfer.
Both the amount and the diameter of ty bars in longitudinal joints of plain
concrete pavements are such small, that they will not have any significant
direct influence on the load transfer. However, indirectly they have a
considerable effect because due to the ty bars the joint width is very limited
and therefore the load transfer by means of aggregate interlock will be
maintained.
For the reinforcement in reinforced concrete pavements the same applies as
for ty bars.
The amount and diameter of dowel bars in contraction joints of plain concrete
pavements are such that they yield a considerable load transfer. Load transfer
by means of dowel bars mainly occurs by shear forces rather than by bending
forces. The stiffness Sd of one single dowel bar is defined as (18):
40