Towards the development of a standard test protocol: Permeability of chip seals under pressure

Ockert Renaldo Grobbelaar, Ero Engineering

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In South Africa, the Marvil apparatus (SANS 3001-BT12:2012) is the primary method used to determine the permeability of a seal. The Marvil apparatus, however, does not consider the influence of the magnitude of pressure generated between the wet road and the wheels of the traffic, which led to this research study with three objectives. Firstly, design and build an apparatus able to test the permeability of seals under a pressure of up to 300 kPa. Secondly, introduce a preliminary test protocol, and thirdly conduct pressurised permeability testing on seals.

The objectives were achieved based on the successful testing of 26 cored seal specimens with newly designed High Pressure Permeability (HPP) test apparatus. The permeability was calculated by means of measuring the amount of water seeping through the 100mm seal core over a set time, under a constant pressure. Five different seal types were tested and permeability within the wheel tracks were compared to permeability between the wheel tracks and on the shoulder.
The data from the study suggests the following:

  • Permeability increases with the increase in applied pressure; In comparing traffic data to permeability, a trend is recognised which suggest permeability increases as traffic loading increase. There are however exceptions within the data, that does not follow the suggested trend and more testing is recommended;
  • Isolated defects, such as cracked stone or small holes, greatly influences the permeability which highlights the negative influence such as aggregate crushing or cracked stones found in seals;
  • Aged cores recovered are more permeable in the wheel tracks as opposed to between the wheel tracks, and on the shoulder. This suggests that ageing combined with fatigue damage caused by traffic increases the permeability of the seal;
  • Permeability was also compared to binder content, but there is no clear relationship between bitumen content and the permeability.
    An overview of permeability rates measured for each seal type is presented in the paper. It is however important to highlight that each seal functioned under different circumstances and that the number of cores tested per seal type varied. These permeability values should be viewed as preliminary test values and not exact seal type specific values, as more testing is recommended.

Cape Seals had the highest average permeability of 31.54 ml/min at 300 kPa. The other three seals were found to have an average permeability close to each other. The Single Seals was found least permeable with 2.50 ml/min at 300 kPa. The Double Seal had a permeability of 2.75 ml/min and the Multiple Seal a permeability of 2.81 ml/min at 300 kPa. For each seal type these values are the average values found within the wheel track only.

This study did not result in an ultimate measurement for permeability of seals under pressure. It did however produce a meaningful test apparatus and method which will serve as platform for further investigation into permeability of seals and asphalt under pressure.