InstroTek
26
9. Determination of Sample Porosity/Permeability
using the CoreLok Machine ASTM D7063
Introduction:
In recent years, design and use of open graded mixes has gained popularity. It
is important to consider that the use of % Air Voids for design and quality control
of these mixes might be misleading for determination of pavement durability.
While % Air Voids is a viable design and quality control criteria for fine graded
mixes, a more adequate and meaningful method has to be used for open
graded mixtures designed for the highway system.
Air void content is of concern primarily for stability and durability of asphalt
mixtures. Air void determination during construction is used to protect against
excessive water permeability that can cause premature failure. For durability,
only the air voids that are accessible to water, the “% Porosity”, is of concern.
The concerning fact with the present method is that two samples with 7% air
voids can have completely different permeability characteristics depending on
the void structure within the sample. However, two samples with the same
porosity will have the same permeability. We believe that for open graded
mixtures, determination of % Porosity is a better design and pavement quality
indicator as compared to the currently determined % Air Voids measurement.
Percent Porosity is defined as the percentage of water permeable voids in the
compacted mixture.
This parameter can be calculated by using a bulk specific
gravity and an apparent maximum gravity of any compacted sample. This
measurement relies completely on the material composition, gradation and
structure of the compacted mixture under test. It does not require a previously
determined Gmm value, which in most cases is not representative of the
gradation of a randomly selected coarse graded compacted sample.
Porosity can be used as a direct indicator of mix durability and will have a strong
correlation to mixture permeability and segregation. This test is easy to perform
and can be completed in approximately 7 minutes using the CoreLok system. A
detailed procedure for conducting this test is attached.
Knowing the total porosity of compacted samples is helpful in determining the
performance of pavements with respect to permeability. The present tests for
determination of permeability are time consuming and the measurements are
based on number of assumption that cannot be defended, physically and
theoretically.
