ASPHALT ANSWERS

By: Prithvi (Ken) Kandhal, P.E

Most of us have heard the following before, three things are important for highway pavements: drainage, drainage and drainage. However, we do not generally consider the drainage aspect of highway pavements seriously when designing new pavements or rehabilitating existing ones. One type of hot mix asphalt (HMA), which is called ATPM (asphalt treated permeable material), remains under utilized although it provides positive rapid internal drainage as a means of extending pavement life.

According to a 1990 survey, only 21 of the 47 states responding reported construction of pavements with an ATPM layer while nine more indicated plans to do so.

The primary function of the ATPM is to remove infiltrated water from the highway pavement structural section quickly (typically 1/2­p;2 hours). This can be achieved because the lateral permeability of ATPM can be as high as 10,000 plus ft per day. When constructing new PCC or new HMA pavements, the ATPM is used as one of the layers in a two-layer subsurface drainage system. It is used between the sub-base and PCC pavement or between the sub-base and HMA base course. Since the vertical permeability of the sub-base is typically a fraction of a foot per day, rapid drainage is assured because the water, following the path of least resistance, flows into the ATPM which is connected to the pavement edge-drain system. The ATPM has also been used successfully between badly pumping PCC pavements and new HMA overlays to provide a positive drainage to the water coming from underneath and thus minimizing potential water damage (stripping) to the new HMA overlay.

Only a coarse aggregate, typically AASHTO No. 57 (1­p;1/2 in. maximum size) or AASHTO No. 67 (1 in. maximum size), is generally used in the ATPM.

The use of crushed aggregate is desirable. The asphalt content in the ATPM varies from 1.5 to 2.5% by weight of the mix. A stiffer (AC-20 plus) grade of asphalt cement than normally used, is desirable. The thickness of the ATPM layer is generally 4 in. Two or three roller passes are enough to compact the ATPM. The compaction temperature should be lowered if the ATPM exhibits initial instability problems at elevated temperatures. On a newly constructed ATPM layer, the water should disappear rapidly when poured from a bucket onto the ATPM surface. Although it is a crude test it works well.

The ATPM also contributes to the structural strength of the highway pavement. A structural coefficient corresponding to a stabilized base (AASHTO coefficient .20­p;.25) should be used for ATPM in the pavement thickness design evaluation.

About The Author: Kandhal is the Associate Director at the National Center for Asphalt Technology at Auburn University. You may write him in care of the editor.