Durability of a recycled aggregate concrete base course material under coupled environmental exposure and cyclic

Long term durability is a major concern for wide-scale use of recycled
aggregate materials in civil engineering construction. The purpose of this study is
to provide an insight into the damaging effects of combined wet-dry cycles and
repeated mechanical loading in a recycled aggregate concrete (RAC) base
course material made from recycled crushed concrete aggregate and cement. A
coordinated experimental program followed by a mechanistic pavement modeling
and life cycle analysis was conducted as part of this research study. This
laboratory investigation was divided into three phases each consisting of both
wet-dry exposed specimens (WD), and control or non wet-dry exposed
specimens (NWD). Phase I experiments involved monotonic loading tests under
compression and flexure to evaluate the strength properties. Phase II involved
testing a total of 108 cylindrical specimens in cyclic compressive loading at three different stress ratios. After each regime of cyclic loading, residual compressive
strengths were determined. In addition, the load-deformation hysteresis loops
and the accumulated plastic deformation were continuously monitored through all
loading cycles. Phase III included a flexural fatigue test program on 39 beam
specimens, and fracture testing program on 6 notched beam specimens, each
one having 19-mm initial notch. Traditional SR-N curves, relating the Stress Ratio
(SR) with the number of cycles to failure (N or Nf), were developed. Fatigue crack
growth rate and changes in Stress Intensity Factors were obtained to determine
Paris Law constants and fracture toughness. A mechanistic analysis of a typical
highway pavement incorporating RAC base was performed with KENPAVE
program, followed by a Life Cycle Analysis (LCA) using the GaBi software. It was
found that the specimens subjected to wet-dry cycles suffered significantly higher
damage expressed in terms of accumulated plastic deformation, and loss of
residual compressive strength, modulus, fatigue endurance limit, and design life,
compared to specimens not exposed to wet-dry cycles. Although such
degradation in material properties are important considerations in pavement
design, a concurrent Life Cycle Analysis demonstrated that recycled aggregate
concrete base course still holds promise as an alternative construction material
from environmental stand point.