Besides costing me $200 to replace my muffler after going over one too many snapped the old one, what are they doing to the roads when they create 1-6 foot trenches in the roads? They are a few inches deep and run the entire width of the roads.
This year is the first time I’ve ever seen them do this, and a huge number of roads now have them. I assume it has something to do with paving they intend to do, but they’ve left most of them for a couple months now, merely marked “Bump.” What purpose do they serve, beyond creating wear and tear on cars?
Here in New England newly paved roads are black, just as the term “blacktop” suggests. Less than a year later, though, they fade to grey. Do the roads in the deep south - places that get no snow - stay black longer? Or is the color related to the sun baking the road, rather than road salt?
I didn’t learn anything about those types of cuts in my limited civil engineering courses.
The roads in New Mexico turn grey pretty quickly. I think it has to do with environmental factors, including snow, sun, etc., and is not limited to any one in particular.
My buddy’s dad is a supervisor for NMDOT, and one of the things he mentions constantly is that the decision is often made to not use enough oil in the blacktop “chip seal”. This leads to cracking pretty quick, and most of our roads end up having all the cracks needing to be filled with tar (read: oil) within a year.
Roads in the South turn grey pretty quickly as well. I always assumed it was because the road isn’t truly dry when it is put down and it takes a while to cure.
Bituminous asphalt is never “wet” in the sense that concrete is. Even when concrete “drys”, it’s really getting hydrated. The water is used as part of an exothermic chemical reaction. Asphalt is more of an emulsion, and whereas concrete’s curing time is the chemical reaction, asphalt’s “curing time” is just the time it takes to cool enough to drive over. It’s already pretty close to its final state when laid down, and often times cars can drive over it within hours (or even tens of minutes) of it being laid down.
That being said, Shagnasty, I’m no expert in the field, but I think you may be on to something, although technically off track. I’m wondering how much of the greying can be attributed to environmental factors, and how much to the “aging” (as opposed to “curing”) of the asphalt.
The only thing I can picture is that they’re trenching across a road to lay either cable or pipe underneath the surface. They later backfill the trench with base course, and do either a patch or a full pave over top of the trench. Unless I’m not correctly picturing what you’re describing. . . Can you go a little further (i.e. how can a 1-6 foot trench be a few inches deep?)
Happens down here too (in middle Georgia). It’s just the bitumen wearing off (if at all) and exposing the aggregate. You wanna see a “quick greying” surface course? Look at a ‘chip seal’: they lay down a layer of adhesive tar, drop a lift of small gravel on top of it, and then maybe repeat. It’s just a wearing course, but it does look grey. . .
Tripler
I’m no expert, but I speak from a few years of experience.
It sounds like you are describing utility trenches. The trenches are used for utilities that run within or cross the road, including drainage pipes, water, sewer, gas, telephone, electric, etc.
A road reconstruction project is often performed in conjunction with a utility project, so you don’t have to later dig up a brand new road for utility work.
Those trenches are backfilled a few inches lower than the surrounding pavement so that they can install a few inches of “binder course” asphalt flush with the surrounding pavement. Then the whole road typically gets a curb-to-curb “top course” of asphalt.
Sorry to confuse you, but 1-6 feet is how wide they are, not how deep. Since they’re running all the way across the roads, imagine that they’re long, narrow rectangles (like a ruler, actually) carved out of the road to a depth of just a few inches.
I’m not convinced they have anything to do with utilities since they’re all over the place right now, and I can’t imagine a depth of less than a foot would be very useful for laying pipes of any sort.
Not sure it’s related, but just in case it’s a clue most of these roads were flooded multiple times over the past 2.5 years. The positions of these trenches are not close to the places bodies of water breeched their banks, though.
Argh. I don’t know the exact chemistry involved, but I’ll try to take a swing at this. As **Pygmy Rugger[\b] said, asphalt simply cools, it doesn’t ‘set’ the way portland cement does.
The main thing that happens, I think, is that sunlight acts a bleaching agent. You leave anything out in the sunlight long enough, and it’s going to turn pale.
For another thing, remember that asphalt concrete is a mixture of asphalt and crushed rock aggregate. When first layed, the aggregate is evenly coated with the very thick road oil asphalt. As the years wear along, more asphalt wears off the top of the surface aggregate; the ‘natural’ color of the aggregate comes through more, a lighter color than the black asphalt.
The effect of road salt, I can’t say. They use that in places where it gets cold enough to freeze, right? Er, I’ve never lived in such a place.
Without knowing slightly more, I don’t know what’s up with the trenches noted by **elfkin477[\b]. I gather that they’re not very deep, maybe an inch or so? Run pretty straight across the road? Wild speculation: an underlying portland cement concrete (PCC) road had been overlayed with asphalt concrete. Over the years, the joints in the PCC pavement have continued to move a little, and have risen through the AC to the surface, perhaps shoving the AC together to cause hillocks in places. The road department has run a milling machine across these joints to remove the faulted AC, itself otherwise in acceptable condition, in preparation to another round of AC overlay or patching.
What I can’t say is why they completed the milling, then left the job. If my speculation is correct to begin with.
Temperature is one of the key drivers in how fast it oxidizes. It usually oxidizes rapidly at first, then settles down and oxidizes more slowly. It does lead to surface failure.
I’m not sure what the trenches are (I can’t quite picture it from your description) but it does seem to be a bad idea if they are leaving them in such a state for months. If this was on concrete I would say that they were reconditioning some of the expansion joints, but I’m not sure what might be done with asphalt pavement.
As for turning grey, Civil Guy and Philster have identified some of what is going on. Another reason (of many) is the loss of the oil* from the surface of the pavement. This is lost slowly due to evaporation of the volatile components of the bituminous materials. This loss of the oil, which acts as the binding agent for the aggregate, results in the surface becoming more brittle as was indicated in an earlier response. So the combination of loss of some oil, oxidation of the oil and exposure of more of the aggregate portion of the pavement all work together to make the road surface turn a bit more grey.
(* The oil, or what is called “bituminous material for mixture” in our area, is what holds the aggregate rock in place in asphalt pavement. The oil mixture is typically 4-6% of the total weight of asphalt placed IIRC. )
Here we suffer from some very expansive clays which tend to heave the roadbed up in places. Many times the city/county/state will come in and mill the high spots down - more so when they are about to overlay. I don’t think NH suffers from this type subsoil, however. I am puzzled by the trenches described in the OP. Have you asked the City/County about it?
Regarding the color of asphalt - here in the Deep South, it all weathers to gray. Technical reasons for this are explained upthread.
I’ve seen the trenching that you mention on the Pennsylvania turnpike when they’re resurfacing. I can’t remember if they grade first or trench first, but there’s a process they use that involves digging the 6’ trenches ever 30’ or so and patching over them. The remaining surface is graded (but you can still see the trenched areas) and repaved. Once it’s repaved the trenches are gone. I’ve always assumed it has something to do with seams (and therefore problem prone spots) in the underlying material but that’s a WAG.
I wish I could take pictures, but there’s no place to safely park and do so, at least not near any of the five I encountered today.
As for leaving them… the city I work in put the first coat of new paving (I don’t know the techical terms, but it’s rough and not not evenly applied, often swerving away in odd places and where the double yellow lines are) in November on some of the same roads after the flooding before last, and have yet to finish paving them. I suppose we should be grateful the trenches haven’t been there as long :rolleyes:
While I hate to second guess the reasons anyone has for unusual behavior, it does seem to be overdue for the final lift of blacktop. In my area (Minnesota) it is very common to put the first lift (layer) of blacktop down one season and wait until after winter to put on the final lift. (The first blacktop layer is called the base course. The upper layer is the wear course. The names may change depending on locality, but these are the terms in my area.)
There are two reasons to wait, at least in the northern climates. One is to allow a winter freeze-thaw cycle to help settle the ground. Soils expand and contract with temperature variations, especially if there is moisture trapped. Waiting trough winter can allow the soils to dry out more and allow the soil to compact a bit more. This may result in cracks in the base course of the bituminous that can be corrected if you wait until later to put down the wear course. The other reason to wait is that in wintery places the bituminous plants shut down over winter months.
But, that all said, production plants open up again in April or May and the final lift is usually placed by now. A possible reason why it is not down yet is funding. If the repairs were due to an emergency, they may have run out of available funding and might need to wait until the next fiscal year to free up funds to complete the work. Of course we can’t rule out poor planning or run-of-the-mill incompetence.
Where in NH are you seeing these? If they are on the highways (The Everett turnpike has a few north of Exit 11), then they are likely marking the start/finish areas for sections of roadway that are to be “grooved.” The “grooving” (which I HATE) provides a rough surface for the new pavement to stick to. As I understand it, the “trenches” allow them to start the grooving machine at a fixed depth from the beginning of the cut.
As for the 1st layer last fall, with a layer to be placed later, this is pretty common up here. They lay down a cheap coat that sets the initial base, let that cure, then come by with the final coat. Of course, 20 minutes after they are done, a new house will be built, or utility work will commence, and a 3’ wide trench will be dug, and refilled and repaved poorly, leading to a dip in the road… usually on only one side. :smack:
I wouldn’t describe the bottom course of asphalt as a “cheap” coat. It generally costs about the same as the top course. The bottom course simply has larger aggregate mixed with the bituminous mixture. This make the bottom course (generally referred to here as the “binder course”) tougher and stronger than the top course, but also less smooth, with larger gaps in the surface. The top course has smaller aggregate, making it much less strong, but it’s much smoother and more aesthetically pleasing.
For a typical residential street, you might have 2 inches of binder course and 1-1/2 inches of top course.
For a state road, you might have 6 inches of binder course (laid down in two 3-inch lifts) followed by 2 inches of top course.
Most of NM has many days in the winter that cycle between well below and well above freezing. That plays hell with frost damage to the roads. Combine that with a low tax base per mile of road to be maintained, and you have all you need for NM roads to be in the shape they are in. Note that there are plenty of tar snakes even on roads that were paved with portland cement, which doesn’t get the chip seal that MacAdam does.