Saturday 29 March 2014

Surface Tension

I am by no means a materials expert, but with surfacing materials I know what I like as both an engineer and a user. so this week, some of my thoughts.

About ten years ago I had some development control work dumped on me after a colleague left. One scheme included a short walking and cycling link. It was a typical tarmac path with a white line along the middle. Half for pedestrians, half for bikes. Not great, but not bad. The thing that struck me was that the surface had been machine laid.

The contractor's next job was to coat the cycle side in green surface dressing and I suggested not as the machine-laid surface was far superior that the hand applied surface dressing could ever be in terms of ride quality. Sadly, I didn't get my way as the drawings were all agreed, I was a bit green myself on the development control side of things and the developer wasn't interested, so we got stuck with it. I have since learnt how to play the developers at their own game (when necessary) and the vision of a machine-laid cycling surface stuck with me as the best approach even though this was several years before I got serious about day to day cycling.

Blue resin anti-skid on CS2. It is not pretty and it is not that even.
Last week I posted my thoughts about the upcoming London - Surrey 100 and the need to get some miles in. So, last Saturday I hit the road. Part of my run took me through Stratford and lo and behold, the new protected start to CS2 was closed with cyclists being pushed into the live carriageway. As I went past, I noticed that the blue surfacing had been redone, but it looked like anti-skid rather than the smooth blue "gunk" that was there before.

I need to explain a bit more. Much of the London Cycle Superhighway network is coloured a rather hideous shade of blue which is either the corporate colour of the original sponsor, Barclays, or the Mayor copying what he saw at some junctions in Copenhagen. Whatever was put down originally on the carriageways appears to have failed in places and I am still not 100% sure on all of the materials used.

Blue paint over an already good surface on the A13. Look on the
other side of the road and you can see the original green of the route.
Truly an example of blue paint sold as a scheme.
TfL did test various surfaces in advance as shown in a Freedom of Information request. I suspect much of the surfacing was a kind of paint. Certainly, the section of CS3 by the A13 is a good example of what I mean - you can see that the stones of the tarmac surfacing are simply covered in a thin layer of paint because you can still make them out - probably epoxy modified acrylic.

There are also patches of blue thermoplastic here and there (hot, poured paint which is used for normal road markings) which pick out places which people on bikes may be found overtaking buses. We also have the on-carriageway "stripes" along the carriageway edge or in the cycle lanes (where present) which is a very smooth surface indeed. From my research, I think it is some kind of polymer concrete slurry which gets applied to the surface with a squeegee type arrangement.

The blue antiskid - see how rough it is in the low sun. Great for
stopping vehicles at 30mph in the rain, but not really an appropriate
treatment for a protected cycle track.
Anyway, much of this surface is smooth (apart from the odd pothole), but the anti-skid is nowhere near as good. Anti-skid normally comes in two flavours. Either a thermoplastic which is hot poured or a cold-applied resin which is squeegeed around and then coloured chippings applied. It is well worth reading up on this on the excellent Highways Maintenance website. Antiskid is usually used in areas of high vehicle braking such as on the approach to zebra crossings and is basically expensive stone and other materials which give high levels of grip. 

You have seen it everywhere, green or red antiskid
on a cycle lane. Beyond, you can make out the
ridges of grey thermoplastic antiskid approaching
the crossing.
On cycling schemes antiskid is often used to paint in areas of conflict. London had tended to use green stripes and other parts of the country red stripes in locations such as where cycle lanes pass junctions. the problem is that antiskid has a thickness and can make riding over it uncomfortable. It is also bugger all use at night unless the area is brightly lit with white light.

The thermoplastic option is often quite uneven and even worse to cycle on. Resin can be better, although the loose stones can be an issue in its very early life. The other thing is that if traffic is using it, then eventually it wears out and potholes appear with the coloured versions having a particularly poor design life (perhaps 5 - 7 years). Everyone has there own views about unprotected cycle lanes, but purely in surfacing terms, I am not a fan - I would rather ride on a machine laid surface.

A road planing machine. Image by David Wright.
And that is the point. When we surface a carriageway, we tend to do it with a machine-laid asphalt material. The old surface is planed off with a road planing machine and the new surface laid with a paving machine. Some of this kit is extremely sophisticated using GPS and computer controlled sensors to lay very smooth and flat surfaces. Some even continuously check the temperature of the material being laid for quality control.

Obviously, there is an awful lot more to it, but I wanted to give an idea of what goes on. Of course, even resurfacing works need some proper thought and a new scheme has to be designed to get the levels to work properly anyway. So, rather than all of the blue which went in to the Stratford section of CS2 and all the blue going in again, what would I have done?

Nice narrow pond you have there, Boris.
Well, assuming the levels were right (and some are not as can be seen in on CS2), I would have gone for a machine laid asphalt surface. There are two types of materials I would have looked like and they are known as AC10 and 55/10.

AC10 is an "asphalt concrete" with a maximum stone size of 10mm. It comprises of crushed rock (or recycled aggregates) of 10mm and smaller bound together with bitumen (the "binder"). It is suitable for relatively low stress areas (estate roads and of course cycle tracks). 

We often see AC6 (6mm stone and smaller) laid on footways and cycle tracks which is fine, it is just AC10 gives a little more grip. Then there is 55/10 HRA or Hot Rolled Asphalt with 55% of its makeup comprising of 10mm stone. HRA differs from AC in that it has an awful lot more finer materials within its matrix. Again, it is all bound together with bitumen. You will recognise "normal" HRA on the streets because stones get applied after the material is laid (but hot) and they are clear to see - they are for skid resistance. 55/10 HRA does not need anything applied for grip in the same way because of the high single sized stone content.

A cycle track machine laid in 55/10 HRA.
This is on a site on my patch and is not going
to be coated in rough green antiskid. Pretty
much surfacing perfection for me, although I
would have preferred a higher kerb separation
from the footway and perhaps with the
footway in a contrasting paving.
AC10 is cheaper than 55/10 HRA and I would always go for the latter given the choice because it is absolutely bullet-proof if properly laid and laying is the key. If you really need colour, both can be found in a variety of shades, although AC10 will have more choice.

Unfortunately, most of the cycle tracks I have ever ridden on have been laid by hand and I have been involved in the construction of some. This is essentially the road workers dumping a pile of the surfacing material, spreading it around with rakes and then compacting it with a roller. While some crews can do some fantastic hand-laying, they can never do as well a machine and on a bike, one can feel every bump and ridge - look at a cycle track surface with low sun and the shadows are a give away.

The reason we end up with hand-laid AC6 is generally because engineers and the contractors working for developers and local authorities don't often know better. 

To machine lay, we obviously need the paving machines and even small ones can be quite heavy and the lorries delivering the materials are also heavy. In the UK, we traditionally tip the asphalt from the lorry directly into the paver which is fine for roads designed to take heavy loads, but no good for more lightly constructed cycle tracks and footways. It costs money to make the cycle tracks more heavily constructed and that is often where we are let down - lack of proper budget.

It won't be a surprise that other countries have worked out that mini-pavers are the key. Some are mini-versions of the road going kit and have to have the material placed with a grab or digger (to keep delivery lorries off the cycle track). Some can be fed from the road with the laying part of the machine sticking out the side such as on this US video:

UK contractors are perfectly capable of doing this, it just needs to be insisted on by those running the schemes and it means that we should be writing this into our contracts, drawings and specifications. I had a conversation with a contractor a while back about laying cycle tracks. He was happy to use a mini-paver, but he didn't have a grab lorry he could use to lift in the asphalt (it would need to be dedicated to asphalt really). Also, as much of the asphalt is delivered by suppliers, they won't wait on site to be unloaded by a digger - emptying into standard road paver doesn't take long.

So, back to CS2 - although the blue is all about way-finding, I cannot see anyone getting lost on the protected sections, so why not use a machine laid black surface instead of expensive anti-skid. That is how they do it in Copenhagen. If we need a colour, then use red like the Dutch as UK suppliers often have it available.

Paving flags in the City of London. Fine for cycling over a short
distance cycle track link between two roads. The cobbles in the
foreground form a little ramp and are nice and flat, so a visual
reminder that we are moving to an area a little different (ie. there are
pedestrians here!)
Of course, we have other materials such as concrete and block paving. Both are fine for cycling on if they are laid properly, although concrete has joints which can be felt. Natural stone slabs and the like are fine if properly laid and are machine sawn - I cannot stand cobbles or anything rough because of the ride quality and actually some types of stone and/or their finish is slippery in the wet.

No, for me, simple machine-laid AC10 or 55/10 HRA is my idea of cycling utopia (well from a surfacing point of view anyway!)

Update 30/3/14
Oh and just to show that I am not just a one track HRA dinosaur (Andy R!), I will leave you with a street paved with multi-coloured granite blocks - Lower Marsh near Waterloo Station!


  1. Haha...a local authority engineer expressing a preference for HRA. It only surprises me one of you fellas could actually name another type of surfacing. ;-)

    BTW, anti-skid material for cycle lanes? This really does nothing to contradict my thoughts that TfL have money to burn.

    Andy R

    1. Heh - I have added some pretty granite just for you!

      It is entirely possible to "do cycling" with materials which compliment the urban realm, but obviously it costs. When you take CS2X through Stratford, there are quite a few design issues which others have blogged about, but the surfacing is a real let down.

      I have been involved with schemes using hand-laid AC6 which gives a better ride quality and the bright blue will simply not last - they should have planed the lot out, sorted out the drainage and then surfaced in AC10 or 55/10 HRA.

  2. You'd also think that by now we'd have settled on a single colour for cycle tracks nationally. Letting each Local Authority choose their own has not given those areas their own character, it's just led to a mess of different colours on the road (and some colours, like green, basically become invisible in wet weather).

    Andy R.

    1. Here West Sussex County Council can't even decide on their own colours. On National Cycle Network 2 between Worthing and Shoreham, half the route has:

      green = cycle lane
      red = shared use
      black = pedestrians

      and the other half has:

      green = shared use (in some places)
      black = cycle lane, or pedestrians, or shared use

      This obviously causes much confusion, especially since CYCLISTS DISMOUNT is advisory so most people riding bicycles ignore it.

      The other problem we have is on the green bit at the Worthing end. The contractors laid a layer of rough green "paint" on top of the lovely new smooth tarmac surface. The green "paint" was laid using a rectangular box thing in thousands of individual overlapping stripes, and since it has a thickness this results in regular annoying bumps where each stripe overlaps the next, every couple of feet. Comparing the smooth pedestrian side with the green painted cycle lane shows that the smooth side is significantly easier to cycle on, I generally can change up one gear on the smoother side.

      The paint is also starting to come off after a few years, with only cycle traffic on it. At least when the paint has all gone it'll be decently smooth for cycling!

    2. And that is the point. The colour doesn't mean anything and it is no use at night. Unless a certain colour is regulated for cycle track use, why bother.

      The green paint you saw was hot thermoplastic applied with a "shoe" and even with a skilled roadworker, that is the finish one gets. No good and as you state, it comes off eventually!

  3. I live in a Dutch town and I spoke with some planners a few months ago at a public feedback night. I asked them about red surfacing, and they told me they had some issues with using it on main carriageways because it was slightly more slippery and wore a bit faster. Apparently the concerns weren't enough to stop them using it for cycle lanes, cycle paths and cycle streets, but enough to abandon the policy of paving city-centre through streets in red tarmac for signaling reasons. I'm not aware of the technical details, but it does seem mixing in the red paint is slightly detrimental to the road quality. Just FYI.

  4. In the Netherlands it's not paint. The crushed rocks themselves are red.

  5. Yes, you can used red rock and red binder - I assume the Dutch problem may well be with the hardness of the rock rather than the coloured binder?

    In the UK, we have a very famous red road, but not very nice for cycling on other than about 7am on the weekend!

  6. Hi! I thought I would respond to your caption below the picture of the cycle track in 55/10 HRA, which perhaps speaks to how cycle tracks should look in general. Where possible, cycle tracks should be level with the footway, and in the same colouring. Level is important, because it creates fewer impediments to pedestrians crossing the cycle path and makes for a more inviting environment generally (this was brought home to me by the experience of clambering over high kerbs demarcating side-street cycle paths in Beijing). I think this post makes the point quite well:

    1. I guess it depends on the situation and so things need to be looked at on their merits. A kerb can be useful sometimes and a hindrance at other times.