Why aren't roads "dual compound?"

OK this is kinda rhetorical as I know its likely to be cost but it occurred to me this morning when I noticed again that the outside of corners (especially where trucks swerve one way then the next) takes much more abuse and wears faster and is always patched up.
I bet they could use a harder compound in that area and it would save a good deal of the callouts and upkeep later on.

Any thoughts beyond this?

(I'm in an odd mood at the moment, lots of strange thoughts going on so have to vent them somewhere)

[MisterTommyH]

Surfacing is laid in bands as wide as possible (practically limited by the width of the plant and geometry of the road) in order to minimise the joints. This is why you often see the 'overlap' at the joint in the centre of a single carriageway road.

Joints are a weakness in construction and let the weather in (water gets in, freezes expands, pops the surface course and sub-base causing it to break up). By providing different surfaces you would be introducing more joints and these would be far more likely to break up.

Joints used to be sealed with overbanding which helped with the problem of lots of joints, but from personal experiance I know that it's not ideal to have a shiny raised band (with very little grip) running down the length of the road. Current thinking is that over-banding has potential to cause more harm that it does good.

Finally, with single compound the highways designer (whilst still considering the drainage and layout) doesn't necessarily have to analyse the numerous vehicle paths through the corner. If you did put a 'special' compound on the outside what would happen if a lorry took a different path, swerved on the joint, and opened it up. Straight away the road is buggered - and lorry drivers aren't known for their predictable driving patterns.

I'm not mentioning cost.

dIFFERENT COMPOUNDS? We have a road that looks like a patchwork quilt---we have more patches than original road.

8:53 thread started.
9:15 thread ended with one post.

Short and sweet.

pavement design is based on several factors, the classification of the road, the likely traffic using the road, the gradient and the predict X million axles over its design life.

Roads have 3-4 construction layers

1 sub base - compacted stone often between 150-600mm deep depending on the ground conditions

2 road base - this is the part that generally takes the load, generally between 200-400 28-40mm aggregate, a very high strength macadam, but would be totally useless to drive on, no skid resistance and very noisy, this is the strongest layer of the road and transfer the load into the sub base

3 Binder coures - now were getting into the sections of the road that are sacrificial in that they are intended to be replaced over time, for binder course youd hop to not have to replace it for maybe 15yr +
this layer will be around 60mm of a 20mm aggregate macadam.

4 surface course, between 25-50mm dependant on type this is a sacrificial layer, its purpose is to offer good ride quality and skid resistance, ie low noise, good drainage accross it, smooth surface, youd expect to replace a surface course after maybe 10-15 yr, hopefully if its specd properly it wont rut or pick out, but it will lose skid resitance over time.

Surface courses kind of already are dual compound, approaches to bends, roundabouts , junctions ped xings etc have an increased psv (skid resistance) so say a motorway might have a psv of 55-60, a downhill approach to a ped xing will likely be 68, teh prob with higher psv is that the aggregates are expensive and once laid they wear down, thats why a lot of the time you see secondary applied treatments like red or buff high friction laid.

it would be a bad thing to introduce longitudinal joints to change base course or binder course compunds accross the mat, that would just build in a weakness, when you see roads pushed or damaged at the edges if the roads been designed correctly 9probably hasnt as it wouldnt have been designed for the traffic it now has) the binder will be the toughest they could get, if its pushing it means that it just cant cope, its not like you could put something stronger in .

when you see cracking maybe 500mm in from the edge, thats a haunch failure, it means the chanel of the road isnt restrained and under load its bending down and propagating cracks from the top downwards, a way to stop that is to put a restrained edge in, ie a kerb, but thats not usually practical, if its a rural road its likely that it will be free draining, ie the water on the carriageway runs onto the verge and soaks in, sometimes into a french drain, but usually just free, if you put a kerb in you need gullies, pipework, outfalls etc, just gets too expensive.

one of the major problems is that roads were not designed to take the traffic loads theyre managing, they werent designed for 40t lorries, 1 lorry screwing on a bend will do more damage than 1000 cars, many none highways agency roads will never see a full reconstruction, its just too expensive. for example full reconstruction on a motorway will be well over £1million per kilometre, put that into context that a highways authorities entire highways budget for a year may be around 6-8 million.

Authorities have difficult choices to make regarding cost effective treatment to keep the network running and which routes to prioritise, theres no a highway authority in the land that couldnt spend 60m a year if they could get hold of it, then maybe after 10yrs of this the roads would be in a sustainable state

its lorries that wreck the roads, this is why i think that foreign companies filling with diesel in europe then running loads in the uk should have to make a significant financial contribution to highways funding

yeah , this IS my life, im hoping it will get better |-(

Sometimes the org is better than wiki itself. The first post pointed out why it wasn't a good idea (I wasn't thinking I was a genius BTW, just wondered why) and the second really detailed one explained why the phenomena that I noticed, happens.

The org never fails to amaze me at the cross section of people we have on here

Awesome answers, thank you guys for your time and effort, I've learned far more than I was expecting to from that!

[punyXpress]

But . .
With a slight redesign of the surface course laying tackle, you would not need additional longitudinal joints:
Have TWO hoppers for the material, nearside with harder compound.
Even better would be a third in the middle of each lane with super-grippy - just for bikes ( and the front wheel of Reliants.

you dont need two hoppers, we lay the entire width in the harder compound, but dont mix skid resitance up with the strength of the road, the skid resistance it totally controlled by the surface course which isnt really a structural part of the pavement. the strength of the road is in the sub layers and these are designed on the MSA (million standard axles) that that road is predicted to take.

it just wouldnt be practicle to lay different compounds accross a single lane width, part of the design standards are that sections of different skid resistances have to be minimum lengths and consistant, you cant start swapping them round all over as they have different properties at temperature or in the wet, it would give an inconsistant surface, add to that that people park at the side of the road, or over take vehicles , if you imagine when you ovetake a truck and you gun past it and you drift onto central hatching or rubbish between the lanes and you lose a bit of traction as youve gone from a 65psv road surface to a 55psv road marking and your ar5e tightens, that would happen every day.

and you cant simply make the entire width grippy as its too expensive and it wears too quickly, high friction probably needs replacin every 5-10 years, and its around tripple the cost of the standard stuff. The aggregates are getting harder to come by as well , i think that theres only one quarry left in the uk that can provide the highest PSV aggregates for use in surfacing,

its an interesting thought mind and in a way motorways (used to be) are constructed that way , the HS and L3 were of a lot lighter construction because L3 didnt take heavys and the hard shoulder wasnt a running lane, even now L1 and L2 can have a different surface course/binder construction to take the lorries and resist rutting, but thats on a motorway or Dual where running lanes are very defined, i dont think you could do it on LA roads. its a while since ive done a motorway reconstruction but i think now what with the managed motorways systems coming out, HS and L1-L3 are similar construction (higher spec) to give more flexibility.

[MisterTommyH]

Originally Posted by Brettus

The org never fails to amaze me at the cross section of people we have on here

And obviously from the time it took to bottom out the thread on a tuesday morning we're both working very hard......

ive got server probs. cant get onto our networks properly (cough)