The material used for road surfaces was traditionally Hot Rolled Asphalt, and it is a
very long-lived material, lasting 30 years or more. However its downside is road noise from traffic that gets worse as the surface wears. HRA is made up from bitumen, plus fine materials like sand and quarry fines, and has the key quality, (so necessary in our damp, wet, climate), of having a very low level of porosity, almost nil in good quality versions. However, if laid as made, it would be quite deadly to motor traffic because it gives no grip whatsoever. The "grip" is provided by rolling in, after the material has been laid but still hot, granite chippings coated with a thin layer of bitumen. These then bond into the asphalt layer using the heat retained in the asphalt after laying. So the laying of HRA is a skilled job, and has to be done very carefully to make sure the chippings bond into the HRA layer. The street outside my house is surfaced in HRA laid in 1988, and is still in excellent condition, although, as a cul-de-sac residential street, traffic levels are low. However, I could take you to see roads in and around Crewe with very heavy traffic laid in HRA in the 80s that are still in good condition, although most have had one surface dressing applied since, but that's all.
So what of SMA ?
Developed by the Germans, it is a matrix of stone chippings and bitumen with some fines and fibre filler added to prevent voids. So it is laid in a single pass, with no rolling-in of chippings, these being part of the material. German SMA has been very successful, and long lasting, in Germany, and is also far less noisy than HRA. Grip is provided by traffic wearing away the bitumen on the top to expose the chippings. This caused some to question its use due to the period required for traffic to wear away the top bitumen to expose the chippings that give the grip. This was the case in Ireland. But we all know the Irish weather is "It's raining", or "it's going to rain", so maybe this was a special case. Imposition of a speed limit for a period after laying it is one way of reducing the skid risk.
But what about British SMA ?
Well, in theory, it should be as good as the German stuff, right ? Well experience since its widespread introduction to the UK in the early 2000s is not one of success. It's introduction was forced by the UK government, after the Highways Agency ruled in 1998 that it was to be the only surfacing material on the Strategic Road Network for all renewals. All over my local county of Cheshire East, I could take you to sites relayed with SMA only a few years ago that are now breaking up. Good business for the contractors !
Another aspect of comparing HRA and SMA, is that with the heavy traffic of modern times, it is quicker to lay SMA than HRA, so road closures can be of shorter duration. This is because HRA needs two stages, (1) laying the HRA, and then (2) spreading and rolling in the chippings.
Anyway, have a read of this 2003 newletter by Mr C J Summers on Thin Layer Surfaces
https://web.archive.org/web/20080708213 ... AN2003.htm