Replacing pavement but not the sub-base

[wanderer2575]

A particular highway project raises a general question.

MDOT (Michigan) is replacing pavement on six miles of I-275 in Livonia.  The project website (revive275.com) is more or less upfront about why the pavement from the 1999 reconstruction started failing almost immediately.  Anyway, with limited available funds and the desire to get the project done in six months, all the concrete is being pulled up and replaced but the existing sub-base will remain.  MDOT stresses this is not a total reconstruction but essentially a large concrete patch, and they expect a service life of 15 to 20 years from this "rehabilitation."

What I don't understand, and this is my question:  What's the big deal about not digging out and replacing the sub-base?  What's there is *only* 16 years old and I would think nicely compacted.  Why is keeping that existing sub-base a detriment (I assume that's the reason the new pavement is expected to have a shortened lifespan)?  I know nothing about engineering or construction, so would really appreciate a primer.

[vdeane]

Perhaps the service life is simply the time until when it would next need work, not necessarily a full depth reconstruction?

[jjakucyk]

It's hard to say without knowing the exact details, because there's so many different road construction methods.  What I typically see a bit farther south here in Ohio when talking about (portland cement) concrete roads and highways is a sub-base of compacted gravel over the cleaned ground (sub-grade) of clay or rock that's been cleared of topsoil or other organic material.  The concrete road surface is poured over the sub-base in a single lift and given its finished texture on the surface.  In that case there's not really a base course compared to an all-asphalt cross-section where they'd do a rather thick base course (6" or so) of rough asphalt over the sub-base, and then the finer finished wear surface of 2" or so on top. 

Anyway, in theory if the sub-base is properly constructed it can last indefinitely.  Only the wearing surface of the road or possibly the full depth of the pavement itself (depending on circumstances) may need replacement.  It sounds like in the I-275 situation that the problem with the pavement is some unexpected chemical reactions in the concrete itself and a bad mix that has caused deterioration.  I didn't see an explanation of exactly what kind of deterioration that is, but I suspect it's along the lines of cracking, pitting, potholes, and the like.  If the sub-base had failed, then it would be shifting and actually moving around, compounding all those other issues. 

16 years isn't a very long service life for concrete, but they may be referring just to the wearing surface.  Being in a much colder and snowier climate there's going to be more freeze/thaw and salt damage than farther south.  I'd still expect about 30 years service life for a concrete wearing surface before it needs diamond grinding or an asphalt overlay to smooth it out.  US-52 near Winston-Salem has sported its original mid-1960s concrete until just a couple years ago, getting an asphalt overlay.  Much of I-71 between Cincinnati and Columbus is early 1960s concrete, most of which got an asphalt overlay in the late 1990s and is due for asphalt resurfacing.  As far as I know, little if any work has had to be done to that concrete, which is now technically the base course while the asphalt is the wearing surface, since it's on a good sub-base and the concrete isn't moving too much. 

There's also all-asphalt techniques where instead of a 6" lift of rough asphalt as the base course, they use something like 12"-18" of it right on the sub-grade.  It's basically a better sub-base, and only the wearing surface needs replacement from time to time.  That came out of the discovery that highways which got asphalt overlays (as opposed to having the wearing surface milled away) performed better than they had originally, since the thicker cross-section distributed vehicle loads better and reduced failures at the bottom of the pavement, which is where the worst deterioration happens. 

[vdeane]

I know in NY, it doesn't take long for things to get rough and for spalling/faulting to appear.

[RobbieL2415]

When I-84 through Hartford, CT was re-paved a couple years ago, they just re-surfaced above the original concrete.  It was cool, the original lane markings were still visible.

[codyg1985]

Anyway, in theory if the sub-base is properly constructed it can last indefinitely.  Only the wearing surface of the road or possibly the full depth of the pavement itself (depending on circumstances) may need replacement.  It sounds like in the I-275 situation that the problem with the pavement is some unexpected chemical reactions in the concrete itself and a bad mix that has caused deterioration.  I didn't see an explanation of exactly what kind of deterioration that is, but I suspect it's along the lines of cracking, pitting, potholes, and the like.  If the sub-base had failed, then it would be shifting and actually moving around, compounding all those other issues.

Based on what the website, that is exactly the reason. It isn't due to the deterioration of the base, but deterioration of the concrete due to harmful chemicals being used in the concrete mix and a lower air content in the concrete. Air inside the concrete gives water a place to go so that the concrete isn't as susceptible to freeze/thaw cycles.