Pavement Related Questions

[raydefan]

I have two quick pavement related questions.

1. When using a leveling course on an overlay project in order to increase the crown slope, can you place this leveling course directly on the existing surface (on top of tack coat) or should you coldmill an inch or so?

2. Do you still place a tack coat under full depth AC pavement?

Thanks,

Raydefan

 

[civilperson]

If the existing surface is in good shape then only cleaning and tack coat are required before overlay. Cold milling helps keep the elevation profile at design height or removes alligator cracking from sun exposure. If the subbase is aggregate or paving, then use tack coat for start of full depth pavement.

 

[Ron]

You should cold mill the existing surface. If you've ever seen the placement of tack coat, you will notice how inconsistent it can be. Further, the lateral stresses at the bottom of the leveling course/overlay are relatively high and will continually work on the tack coat interface.

Milling takes away bond problems between excessively oxidized asphalt and the tack coat, reduces the need to "clean" the existing surface (not easy to do properly), an mitigates the bond issue due to the high lateral stress at the interface.

After milling, apply tack coat and then install leveling course if you need to raise the crown.

 

[Drumchaser]

I would not plane (mill) unless the existing pavement is in distress, and certainly not just because someone does not take the time to apply a tack coat properly.

When planing: you either have to mill at a % slope or a certain depth, you cannot do both. Naturally this causes problems for some. Also there is the pavt. marking (daily) to deal with.

We place a tack coat (usually emulsion) under all Dense Graded ACP layers unless a relatively new under seal (chip seal) is present.

If you are placing ACP with-in air, surface, and mix temperature requirements, and the surface is power broomed, tack placed (and rolled in to distribute satisfactorily) you should be fine.

 

[Ron]

The poor application of tack coat is not the reason to mill....that's just a side benefit!

While we don't know any traffic information or category of the road, we have to assume that there is some reason to remediate the pavement other than changing the crown elevation.

We don't know the thickness of the existing pavement. Could the slope be obtained by milling, then overlaying with a structural course to the new slope, negating the need for a leveling course?

In high traffic areas with a reasonable percentage of trucks, non-milled overlays often fail early...just a fact. They don't usually fail in their entirety, they fail by delamination in spots, which increases maintenance, lowers ride quality, and creates complaints from the riding public. If the money's there, mill it and do it right.

 

[Drumchaser]

While it is all too common that tack coat is misapplied (diluting the already diluted emulsion, spray nozzles clogged, too low a rate) it is possible to apply this material correctly with little effort resulting in proper adhesion of the new ACP layer.

"non-milled overlays often fail early...just a fact"
This may be a routine occurrence in some areas; we apparently have not experienced this yet.

The cost of planing is very reasonable, and often provides (after fractionating) a reusable product that is recycled into the ACP for the same milled project and therefore reducing unit cost and being green simultaneously.

Milling is cheap and popular with local industry.

Milling "can" cause a multitude of problems, and we stay away from it until it is absolutely necessary. Our cores usually indicate whether milling is needed.


The level-up layer addition into the new pavement structure adds another opportunity for improved ride quality. A better IRI.

I have witnessed milled sections fall apart far more often than non-milled. As stated, this has a lot to do with the roadway and traffic mix/count.

Either way...milled section or level-up, a one coarse seal (chip seal) is always applied before the new ACP layers are added, in which case we do not use a tack coat over a fresh seal coat.

 

[WL123]

I don't know where your located, but here in the northeast almost every overlay job in the last two years that has not been milled is falling apart this winter. This is what I have noticed in my area. If it was up to me I think money is well spent milling before overlay, it just lasts longer because of the bond.

 

[volfanbob]

I would concur with with many others here in that it is almost impossible to equal the bond created by milling with any simple application (i.e. tack coat). The initial cost of milling can be quite costly, but in the overall life of the wearing surface, the cost should be much less than simply overlaying with a tack coat.

For full-depth pavement, a "Prime Coat" rather than a "Tack Coat" should suffice. Typically, prime coats are a 50/50 asphalt emulsion (50 percent water/50 asphalt) with an emulsifier added (often clay or detergent).