Asphalt sealers

[HomeBrew2]

Great info, as always, Toiyabe! Hopefully I'm not the only one that would like for you to elaborate a bit more on the free-draining material under the AC as of primary importance. Or put another way, if one does not have free draining material under the pavement, wouldn't getting water off of and away from the road as quickly and completely as possible be of primary concern? Thus making filling cracks and helping prevent future cracks pretty important, as well as proper grading away from the road. Obviously lack of grade and/or cross slopes would defeat much good done by fixing cracks cuz the water would just sit there and eventually move thru the pavement and undermine the base. Just wondering.
Cheers!

 

[_RaT_]

Interesting. Why do they constantly have crews both California and Nevada out sealing the many cracks that develop on all the many roads around the Tahoe area?

 

[Egon]

The cracks are sealed to prevent water from getting under the pavement and softening the subbase or freezing and making a bigger crack which can turn into a pothole.

Egon

 

[_RaT_]

Well thats what I figured but in reading Toiyabes comment, that would not appear to be what he explains.

 

[Toiyabe]

The main purpose of crack sealing/filling is to keep non-compressible material out. There are several different kinds of cracks that can develop in an asphalt pavement. The ones that respond best to crack filling are thermal cracks (generally transverse and regularly spaced). If debris gets into these cracks they can no longer freely shrink, you will get secondary cracking around them as the pavement heats and expands. Once you know what to look for, you'll see this secondary cracking quite often.

Filling cracks other than thermal cracks is a waste of money in my opinion, although it is done frequently. It can also be a saftey hazard if excessive sealant is used while trying to fill aligator cracking - decreases the friction.

Preventing moisture from infiltrating through the surface can't hurt. However, even with a perfectly sealed surface, you'll have a lot of water moving in underneath a pavement. It rises up through capillary action, and then is blocked from evaporating by the pavement itself. This is a problem that's often seen when a dirt road gets paved - you want a higher fines content on a dirt road to keep it knitted together, but when it's sealed off by a pavement the moisture those fines wick up can't escape and the pavement falls apart quickly.

A pavement needs a free-draining base layer (either daylighted to a ditch or with drain pipe) underneath it or it needs to be strong enough to rest on a saturated subgrade. Even here in Reno Nevada you can't count on the subgrade never being saturated.

 

[Toiyabe]

While I'm at it, I'll mention that asphalt can also "strip" in the presence of moisture. Basically the bond between asphalt and aggregate can weaken when it is wet for long periods of time. Different asphalt types and aggregate types are more suseptible to this.

Stripping is another arguement for sealing cracks - keeps the moisture from sitting in the crack itself.

There's several schools of thought on crack sealing. My general opinion is that only routing and filling of working cracks is effective, and then only if the number of cracks per mile is low. Otherwise an overlay is a better solution.

 

[Egon]

I have to dissagree. If there is capilliary action roads in below freezing climates would be very rough and nearly undrivable due to frost heaves.

A packed clay sub base should prevent capilliary action. The packed gravel layer above this should provide drainage to the ditches. The pavement on top keeps water out so the integrity of the subbase is maintained.

Filling cracks keeps water and other debri out. Chip coats and seal coats also help for this as well as allow more reflection from headlights makeing it easier to drive at night.

Egon /forums/images/graemlins/grin.gif

 

[Toiyabe]

</font><font color="blue" class="small">( If there is capilliary action roads in below freezing climates would be very rough and nearly undrivable due to frost heaves.
)</font>

The accepted theory for frost heaving relys heavily on capillary action. Basically hydrophillic soils like silts and clays develop a suction when the free water around them freezes. This suction draws water up from the water table, assited by capilary action. The heat of fusion from the water that freezes causes the freezing front to stall, resulting in a large buildup in ice at the front ("ice lenses"). It's an interesting thermodynamic problem.

Clays have poor permeability (but high water affinity), so it takes a while for the water to move up from the water table. Therefore it takes long periods of time with consistent, sub-freezing temperatures to get large heaves in clay. Silts have better permeability and can respond to weather conditions quicker, but they can't draw water up as far.

 

[Toiyabe]

</font><font color="blue" class="small">( A packed clay sub base should prevent capilliary action. )</font>

Capillary rise is a function of pore size. That means that the lower the permeability of a soil, the greater the capilary rise.

Clays can keep moisture from seeping down, but the better they are at that, the further they'll wick moisture up. Weird, eh?