What's with the little road cuts?

December 8, 2005 9:38 AM

Roadconstructionfilter: Why do I see so many highway crews replacing only a 10' by 2' section?

It seems like they cut a section only wide enough to get a hammer drill and drill rebar holess into the unmolested part, then pour it back in with concrete. Then the make another cut 20' down the road!

Is this for research purposes - to see how various roadbed recipes hold up, or something else? Why not replace a whole section if there's a problem - I imagine the labor costs on a small cut are much more than the concrete to do, say, a 10'by 30' section?

posted by notsnot to travel & transportation (13 comments total)

It's one of several things:

If the roadway is damaged in just one place it probably indicates subbase damage. This can be repaired by sawcutting and excavating that portion, recompacting the subbase and replacing the base material, either concrete or crushed stone. If it's on a bridge with a concrete deck, they'll just chip out the damaged concrete, sandblast the rebar and replace the concrete patch.

or

It could be utility work and not road replacement. If this is the case, it's much cheaper to just replace the area where you have to open the roadway. Obviously it's nicer to repave the width of the road, but it may be scheduled for repaving on a future project or it's been determined that it doesn't require repaving.

10' x 2' is large enough for some utility work, or it could be a test pit to see if there are utilties present.

The labor costs per unit of paving are higher on the small patch, but the overall costs are lower. It makes for a nicer road if you replace a larger patch, but the small patch works just as well for cars. Not so much for bikes, skateboards, etc.
posted by electroboy at 10:05 AM on December 8, 2005

I'm sorry, I wasn't clear. The cuts are usually 10' *wide* (one whole lane) and only 2'down the length of the road. i've seen rural interstates with cuts on regular intervals, maybe 30-40', for a mile or more.
posted by notsnot at 10:19 AM on December 8, 2005

They might be installing induction loops under the road. They're used for detecting cars at traffic lights (you see them a lot in the UK). There's an article at HowStuffWorks.

If they have two of them 20' apart, I guess they might be for measuring the speed of traffic.
posted by ciaron at 10:31 AM on December 8, 2005

My guess is it's to allow for expansion and contraction during temperature changes. If you poured a one-mile length of highway as one chunk of concrete, it would break up in short order, because it would be constantly changing size, and the ends would be moved a long way.
posted by Kirth Gerson at 10:36 AM on December 8, 2005

I think Kirth Gerson is correct -- it's to allow road segments to shift without cracking in cold and icy weather. I've seldom seen this type of road repair in the south and never on asphalt highways, but it seems common on concrete highways in northern climes.
posted by naomi at 10:59 AM on December 8, 2005

Kirth Gerson and naomi have the right idea. Expansion joints in concrete highways are placed every 20 - 30 feet. They are a 1/2" wide cut, right through the concrete, and the cut is then filled with tar, which is flexible yet waterproof. In the summer, you can see the tar squeeze out from the slits and feel the regular bumps as you drive over them.

It frequently happens that these wear out from the incessant bumping, or they get frost damaged. In either case, they must be repaired.

More and more, I see the old concrete Interstates being covered over with asphalt, which requires less repair, is smoother, but only lasts about 10 years before it must be ground off and replaced. I called up a county engineer about this a while back, and was surprised to learn how much research is done on this subject and how cost effectiveness is the major concern.
posted by RMALCOLM at 11:10 AM on December 8, 2005

But why do they make a 10' x2' hole, and fill the whole damn thing with concrete? even with dowelling, if it's a solid block, poured flush with the old segments, there's still no room for expansion.
posted by notsnot at 11:17 AM on December 8, 2005

1/2" seems a little large for a control joint, and I've never heard of joints being put in after the concrete has been paved over. It makes sense for a concrete road, but not one that's been paved over with asphalt. I suppose if the original contractor hadn't put in the control joints, you might need to put some in after the fact. Here's some details on how control joints are installed.
posted by electroboy at 12:28 PM on December 8, 2005

Also, Fig 6.1 gives a good idea of how the joints are located in the different pavement types.
posted by electroboy at 12:33 PM on December 8, 2005

I've seen what notsnot's talking about. They're two-foot wide cuts, extending across an entire lane of an interstate, filled in with asphalt. I'm pretty sure the road itself was also asphalt, so I really don't think they had anything to do with expansion.

They're a little like flattened speed bumps, but uneven enough to still give you a bumpy ride at high speed. I assumed they had something to do with road construction, but I could never figure out exactly what.
posted by driveler at 12:46 PM on December 8, 2005

If these cuts run across the road from shoulder to shoulder, there is a good chance that they are made to replace a culvert, which drains water from one side of the road to the other. Usually these are marked with short posts and colored reflectors off the shoulder. Two feet seems a bit narrow for a culvert job, but that's my guess.
posted by Hobgoblin at 4:34 PM on December 8, 2005

In Pinellas and Hillsborough counties in Florida, along interstate 275, they are doing precisely this, though it's concrete, not asphalt.

Basically, they are cutting out the cracks and failures and just replacing them. Once they cut them out and refill them for a large enough stretch, they bring in this big diamond-tipped planer on wheels and grind the whole surface down smooth.

This is much quicker and cheaper and less intrusive to traffic than doing an entire lane or entire section of all of the lanes.
posted by tomierna at 8:53 PM on December 8, 2005

Not too many answers on the mark here. The repairs are made at the expansion joints of a concrete road. These are typically the first to have problems especially on older roads that didn't use current specs on reinforcement bars. These problems happen mostly in colder climates with freeze thaw cycles. Joints are also typically every 20' which would explain why you see the replacements done in those locations.

Properly placed rebars help to ensure that there is no vertical deflection of the pavement from vehicular loads which will cause early deterioration. Many times in the paving process rebars are not placed properly, especially in the old days. When a joint starts to fail, the concrete around it will break up or crack apart. It will cease to have a connection with the adjacent concrete panel and may settle. Once there is a lip between two adjacent panels of concrete, then the vehicle loads continuously hammer the pavement further weakening the concrete. It's like a jackhammer effect every time a car or truck drives over the joint. Eventually there will be total failure and potholes.

On the best concrete pavements, you will feel no bump when driving over joints. Many highway departments will even go so far as to grind the concrete pavement smooth after paving. It may cost more initially but the pavement will last for a very long time.
posted by JJ86 at 3:05 AM on December 9, 2005