A new apparatus for measuring the drainage properties of porous asphalt mixes: Fid-Bau Portal

A new apparatus for measuring the drainage properties of porous asphalt mixes

Tan, S.A. / Fwa, T.F. / Chuai, C.T.

Unlike temperate countries, Singapore and the countries in the tropics face frequent torrential downpours. Singapore's average annual rainfall is about 2500 mm, with intense rainfall during the monsoon months of October through January of the following year. To increase road safety in such inclement weather, the road authorities had embarked on a wide-scale application of porous asphalt drainage mixes for the new expressway surface wearing course. A new apparatus is designed to measure the permeability of porous asphalt road mixes under laboratory conditions. It consists of an inlet cylinder and outlet reservoir of the same diameter as the test specimen, and a highly precise high-speed submersible pressure transducer to measure the falling head with time at 0.01/s intervals. A special trap door (Singapore patent no. 9600363-7) enables the apparatus to start the falling head test at any desired initial hydraulic head in the inlet cylinder. Using the falling head data, the specific discharge through the specimen can be computed and the permeability of the material obtained from the velocity versus hydraulic gradient relationship. It is obvious that for the rapid flows in porous asphalt mixes, Darcy's law is no longer valid as Reynolds' number for the porous medium exceeds 10, and there is no simple linear relationship of falling speed v versus hydraulic gradient i as shown by the test results obtained. An empirical regression of v versus i on a log-log scale provides a means to obtain a pseudo permeability coefficient at i = 1 for comparative studies. The comparison of permeability k for cohesionless uniform glass beads showed that specimen thickness has little to no influence on the values of k. However, particle size and not porosity has the greatest influence on k, as the larger particles meant larger pore diameter, hence larger hydraulic conductivity. Tests with three types of common porous asphalt mixes were conducted and it is demonstrated that specification of void content alone is not adequate for describing the drainage property of a porous mix. The porosity alone is not a good indicator of hydraulic conductivity. Direct permeability measurement of porous asphalt mix specimens should be conducted.