Investigation of Water Uptake in Porous Asphalt Concrete Using Neutron Radiography

Porous asphalt (PA), a highly porous hydrophobic composite material, is subjected to water uptake and the process is documented with neutron radiography (NR). While the un-aged laboratory-prepared PA specimens do not show any water uptake, we observe uptake in aged PA even though the bitumen binder is a hydrophobic material. The moisture content distribution plots derived from the NR images clearly identify regions in the aged specimens where water uptake is active. Two-dimensional degree of saturation (DoS) distribution images, which are obtained by combining micro-computer tomography and NR images, identify those pores where saturated flow is certainly active. However, to clearly distinguish between saturated and unsaturated flows in the remaining wet pores, the DoS distribution images are read together with the three-dimensional PA microstructure obtained by micro-CT. It is observed that uptake begins mainly as unsaturated film/corner flow at large well-defined pores. As this uptaken water travels further into the material, the flow transforms into a combination of saturated flow and unsaturated film/corner flow. Saturated flow is seen to be mostly active in the small pores within the mastic. From the observed succession of unsaturated and saturated flows in an aged PA specimen, it can be concluded that years of environmental and mechanical loading has resulted in the stripping of binder from the aggregate surfaces and has consequently exposed patches of hydrophilic aggregate to water, which enables the capillary uptake of water. We also simulate an absolute permeability experiment and observe that relatively less tortuous and more connected paths play an important role in determining the preferential path of the uptaken water.ISSN:0169-3913ISSN:1573-163