Asphalt Base Course - Purpose?

Question

I know that common practice is to place a base course material (rough aggregate) down before placing bituminous asphalt. What would be the outcome (or potential problem) with re-paving over a trench after placing a sand-cement slurry and no base course?

I wanted to give a bit more information.

The location is a parking lot, and traffic is limited to about 5 mph. The location is in Southern California, so there's no freeze-thaw cycle. The ashpalt course we're matching is 4-inches thick. The old base course, visible in the sidewall, appears to be about 3 inches thick.

Answer 1

The asphalt would break up and, eventually, the entire trench would cave in. Base course gives the asphalt something to cling to and pack down on, which the sand slurry would not provide. Spend the money and do the job right the first time; you'd be spending LOTS more to redo it all later.......

Answer 2

Typically, the material will not last for more than 1 season.

The base material will eventually be pushed out enough and there will be enough movement to create cracks in the asphalt patch.

Depending on the thickness of the levelling and top course that you are matching, the method of patching in the levelling and top courses, also the ratio of cement slurry to sand will affect the life of the patch.

In a few projects I worked on, I seen full blow outs of the patches within 6-8 months due to water eventually getting into the base aggregate material, some freeze and thaw action adjacent to the patch but within the existing pavement, where seperation between the levelling and top course caused cracking and chunks of asphalt to pop out.

Also there will be reflective cracking prior to a blow out of the patch caused by the cement slurry. As the asphalt moves around and depending how hard the slurry has stiffened up, the asphalt would also be worked and torn from underneath.


Depending also if you are working on a state highway orcounty road.
Typical practice is 2-3" of base asphalt, 1.5-2" levelling course and 1-1.5" top course on state highways, excluding interstates.

County roads may just have 2 layers of bituminous asphalt.

There are a lot of variable factors that will affect what would happen in this type of situation you are describing. Unless I was out there to see what the conditions were (goes through wetlands, desert, etc...), traffic patterns and loads (state trunk line, industrial traffic, county road, etc), seasonal effects (alaska or michigan verses florida or arizona), just to name a few.

Answer 3

will there be traffic? how heavy and how many loading cycles. asphalt is known as flexible pavement and it does in fact flex. the idea of the base is to spread the load over a wider area of sub surface soils and lessen the stresses at any localized position. overstess equals cracking> cracking equals water infiltration, water infiltration equals soft sub grade and more cracking.

If there is no traffic, it will work fine. if you need something to hold up to traffic, then you need to design the sub grade and the base course. stabilized concrete slurry should work fine if everything else is right.

your state DOT should have standard specifications on what they expect for long life asphalt.