Asphalt Permeability In-service: How Can It Be Evaluated Before and After Moisture Damage?

The first bituminous road surface was purportedly placed in Paris, France, more than 150 years ago. The primary purpose of a surfacing layer was to protect the underlying pavement from moisture ingress, confirming the innate cognisance of waterproofing and low permeability to enhance pavement performance. Over the decades various methods have been implemented to evaluate asphalt permeability, including volumetric properties e.g. voids in mix, field testing e.g. using a falling head permeameter and laboratory tests on specimens or cores e.g. air or water permeability (constant or falling head). The evaluation methods each have elements of empiricism e.g. overlooking lateral versus vertical permeability or underestimating hydrostatic pressures in-service. This paper describes the development of a laboratory permeameter that evaluates vertical permeability at high pressures i.e. representative of traffic induced hydraulic pressures. The apparatus, termed “High Pressure Permeameter” HPP, is able to sustain pressures above 200 kPa. The research is based on specimens retrieved from surfacing seals and asphalt. Cores were acquired from different sources all over South Africa, including different designs and climates. Testing includes laboratory Marvil (low pressure) permeability and HPP tests at 100, 150 and 200 kPa, correlated with volumetric mix properties and CT scans. In addition, moisture damage using the Moisture Inducing Simulating Test (MIST), induces representative hydrostatic pressures over sustained periods. Findings reveal inconsistent correlation between Marvil and HPP asphalt permeability values at 100 and 150 kPa testing pressure; however, at 200 kPa higher variability is apparent. The range of mixes that have been evaluated allows for preliminary guideline limits for HPP permeability method to be developed, which can pre-empt identification of mixes with unacceptably high permeability.