Taking The Rough With The Smooth

The asphalt fairies have been out round my way and it gave me an opportunity to have a look at what they were up to a few nights ago.

The road in question is a 50mph trunk road which cuts through the community of my corner of London, and as such, it carries lots of motor traffic. It's built on good old London Clay which means it's susceptible to long term movement from getting saturated and drying out annually and for this road, a couple of sections had been on the move making the road surface a bit of a rollercoaster twisting lorries as they went over the defects.

The maintenance response was to relay the kerb lines to the correct levels and then resurface to suit and it is the resurfacing that caught my ear - yes I do mean "ear". Let's have a quick video and you'll see what I mean.

You might need to listen to this clip a couple of times to get your ear in, but the change in noise level between the new surface to the right and the old surface to the left is noticeable. Some of you might even be able to pick up the little thump as the vehicles go across the tie-in point between the two.

So what's happening here? For the answer to that, we need to take a closer look, courtesy of my creaking knees.

Above is a photograph of the old surface (taken where it has been ended in a side road - I wasn't going to bend down on a high speed road!) The black and white squares on the top of the reference card are 10mm so you can get a idea of the size of the pink and black stones that cover the surface which I'll come back to in a minute.

The surface itself is called "hot rolled asphalt" (HRA) and it is a mix of a bitumen binder, stone, sand and fillers (such as limestone dust). It is laid by machine and when properly compacted, forms a dense matrix which is very durable and waterproof. As a result, it has a long design life and in the scale of things is reasonable cost effective. Quite good for a trunk road carrying lots of lorries.

The problem with HRA however, is it isn't that great at attaining skidding resistance unless one chooses a variant which has a high proportion of high quality stone to leave a "rough" surface, but that puts the cost up. Instead, we add "pre-coated chippings" (PCC). PCCs are the pink and black stones you can see in the photograph above and come with a thin coating of bitumen to help them stick when they are laid. The pink stones are granite and the black are probably basalt - two very hard stones and as they are quite expensive, they are used sparingly on top rather than in the mix itself.

HRA surfaces are usually laid by machine with the PCCs added while the surface is still hot and prior to being compacted by roller (but not too much otherwise PCCs will be pushed right in). This leaves all the PCCs sticking up a little bit above the general surface leaving a rough texture. The PCCs sticking up creates a "positive macrotexture". The stones are also rough when you look at them at the microscopic level which gives us a "microtexture" which is also part of the grip story.

The action of tyres over an HRA surface is such that the stones grab and deform bits of the tyre in contact as they roll over the surface and the noise comes from the rubber "pinging" back as the stones lose grip as the tyre rotates. It's all happening very quickly, constantly and at a small scale, but all of the little pings add up to generate the noise we can hear in the video and of course, frictional forces are gradually wearing the tyres out. Asphalts generally absorb some noise from tyres, but the type explained above is fairly impervious and so much of the noise is reflected up.

PCC laying machines "chippers" were almost becoming museum pieces a few years back as there was a trend to use surfacing that didn't require them because they are often awkward to use because they are wider than the section of surface being laid, which usually led to road closures. Have a close look at this video to see the chips falling out of a chipper from a grooved roller fed by the hopper.

From a maintenance point of view, getting a chipper in for smallish scale work isn't economically viable and hand-laying PCCs isn't a great option for a high speed road from a quality perspective which is probably why for the bit of work I'm talking about, a different choice was made.

The new surface (above) is quite different to the HRA. In this case we are dealing with what is generally termed as "asphalt concrete" (AC) which is comprised of higher quality stone with fewer fillers and more stones of a similar size to give an "open" texture, but which still has bitumen to bind the mix. Whereas HRA provides grip with the PCCs sticking up, AC provides this by having lots of voids in the surface otherwise known as "negative texture" and of course the stones we see on the surface have a rough microtexture.

From a noise perspective, the voids in the AC are much better at absorbing the noise from tyre action than the reflective nature of HRA and so they are often favoured where there are residential areas as a result. The new surface here is probably a "thin surface course system" (TSCS) which is a more advanced type of AC where the bitumen binder has been modified using polymers to provide greater strength and durability, because basic AC is generally less durable than an HRA equivalent and which is an important consideration for areas with high traffic flow and lorry movements.

If we wanted to get even better at reducing noise, we could go for a porous asphalt which has more voids, but we need to take care that water entering the pores can drain away. This can be properly integrated with drainage design to provide a surface that throws up very little spray and even surfaces which will drain to engineered lower layers and sub-drainage systems.

So there you have it. A little bit of maintenance works can send us down the start of an asphalt rabbit hole which is a whole branch of engineering in its own right. It's a pity that the same amount of care wasn't applied to the uneven and cracked footway I was walking along.