Merged with duplicate record 10026.1/855 on 06.20.2017 by CS (TIS)A large proportion of the U.K. highway network constructed in the
1960's and 1970's contains lean concrete roadbase with bituminous
surfacing. Pavements containing relatively high strength lean concrete
have rarely required structural maintenance (thick overlay or reconstruction)
but have required maintenance because of reflection cracking
where the surfacing cracks above cracks in the lean concrete. The
time of appearance of this cracking is very variable (2-20 years).
Field observations indicate that roadbase transverse crack spacings
are often greater than 5m. Reflection cracking at these long spacings
can be caused by thermal stresses. This project identifies
conditions under which thermal reflection cracking will occur and
develops a predictive model that allows estimation of the combined
effect of thermal and traffic stresses. Finite element analyses
indicate that initial crack development is likely to be caused by
thermal stresses and final cracking will be assisted by traffic
stresses.
A temperature model has been developed to determine roadbase daily
temperature range and surfacing temperature on a mean monthly basis.
Thermal reflection cracking is considered to result from daily cycle
fatigue rather than an extreme low temperature mechanism. A test
rig has been developed to apply cyclic crack opening movements and
simulative tests have been accelerated to 0.1Hz by using a "bitumen
stiffness", fatigue criterion.
Finite element results, displacements recorded during tests and tensile
creep tests to determine mix stiffness, enable dc/dN and K1 values
and material constants (A, n) to be determined. This fracture mechanics
interpretation of test results serves as the basis of the predictive
model for thermal reflection cracking that is consistent with observations
from an untrafficked road.
The combined estimate of thermal and traffic stresses cannot however
explain reflection cracking at <5m spacings. This cracking apparently
initiates at the surface and is probably influenced by other mechanisms.University
of Birmingham, the Transport and Road Research
Laboratory (TRRL), U.K. and Devon and Cornwall
County Council Highways Department
