The Idiots' Guide to Highways Maintenance
Copyright © 2000/08, C.J.Summers

55%/14mm. NEWLY LAID - HOT ROLLED ASPHALT WEARING COURSE,
( OFTEN REFERRED TO AS HIGH STONE CONTENT ASPHALT, HSCA 
Below is a newly laid 55%/14mm. High Stone Content Asphalt (HSCA), showing a fairly typical level of texture on initial laying, note there is not a high build of surface bitumen in this material, aggregate and sand surface texture is soon exposed by trafficking.

Below is close-up photograph of an established High Stone Content Asphalt (HSCA) surface course 

10mm. STONE MASTIC ASPHALT, (SMA)
Below is a 10mm. stone mastic asphalt surface course that is approximately 3 years old, located on a principal route carrying a a normal level of HGV's for a busy small town. 
The surface bitumen is now worn off and actual aggregate surface is exposed, texture depth is acceptable but will continue to improve with trafficking, and can now be seen to be of a "positive" rather than a "negative" nature.

14mm. STONE MASTIC ASPHALT
Below is a typical picture of a 14mm. stone mastic asphalt designed to have a low void content, i.e. 2% - 5%, this surface is five years old.
With this image a click on it will link to a larger file size picture to show more detail.
Note, this mixture is largely a single sized 14mm. aggregate held in a binder rich "mastic". As the significant initial "mastic" coating is worn of the surface aggregate by trafficking, a prominent macro texture is created. 
This transformation, over time, needs to be taken into account when regarding 14mm. SMA as a low noise generation surface.

10mm. SURFACE DRESSING
Below is a typical mosaic of a 10mm. single size surface dressing after several years of trafficking in a rural location.

10mm. CLOSE GRADED (DENSE) MACADAM AFTER APPROXIMATELY THREE MONTHS TRAFFICKING
Below is a picture of a 10mm. close grade (dense) bitumen macadam surface that is about three months old that still has some bitumen to be worn off by traffic. 

14mm. CLOSE GRADED MACADAM (CGM)
Below is a photograph of a typical 14mm. Close Graded Macadam after approximately two years trafficking.
With this image a click on it will link to a larger file size picture to show more detail.
This surface tends to have less depth of macro texture and increased micro texture, this is from the well graded nature of the mixture and the higher content of finer aggregate material, macro texture increase over the life of the material as the fines are removed by trafficking.
With a generic (BS 4987) CGM there is not a significant amount of bitumen build-up on the surface aggregate, and it is quickly remove by traffic.

10mm. NEWLY LAID OPEN GRADED MACADAM
The material below has about 20% voids and is porous and as such should only be laid on an impervious basecourse or bond-coat. The photograph above shows the material a few days after laying, you will note the well voided nature of the surface. Many of the proprietary Thin Surfacings will be similar in nature to this material.

10mm. OPEN GRADED MACADAM - AFTER TRAFFICKING
The same material as above after about 12 months trafficking, note how the voids have been filled with detritus. The flow of water through the material matrix will have slowed considerably and the initial low spray properties of this type of surface will have been considerably reduced.

30%/14mm. HOT ROLLED ASPHALT (HRA) WEARING COURSE AND 14mm. PRECOATED CHIPPINGS
14mm. precoated chippings is a permitted option, and is specified in BS 594. 
The 14mm. option seemed to lose out in favour of 20mm. chippings when greater texture depth was regarded as being the most important factor in a bituminous surfacing. 
This type of surfacing still gives good texture and is considerably quieter than HRA with a 20mm. chipping. 
This particular photograph was taken in 2001 in a busy urban street and I know the surface to be over 20 years old.  

35%/14mm. "DESIGN" HOT ROLLED ASPHALT (HRA) WEARING COURSE AND 20mm. HIGH POLISHED STONE VALUE (PSV) PRECOATED CHIPPINGS
The surface illustrated below was the bituminous surface that until recently was the material used on all motorways and trunk roads, it is a very durable material and has a high skid resistance when correct texture depths are achieved with the 20mm. precoated chippings obtained from a high PSV source.
Some lengths of this material have been prone to wheel-track rutting in hot summers, but if attention is paid to the nature of the fines used in the mixture, coupled with an appropriate binder modifier this problem can be overcome.
This material is no longer a permitted surfacing on motorways and trunk roads in England, although many hundreds of miles still remain to be replaced by proprietary Thin Surfacings. 
Many thousands of miles of this material are still to be found on the remaining (95%) of the road network, where it is still able to be used.
The main reason given for this material falling in to disfavour is that it is a surface on which high speed vehicle tyres may generate more noise, but the noise generated does need to be compared to that produced by other established surfaces before being ruled out.

IMAGES OF A TYPE OF PROPRIETARY "THIN SURFACING" ( 20mm. ASPHALTIC CONCRETE / PERVIOUS MACADAM)
The above title needs an great deal of qualification, because there are now many hundreds of proprietary thin surfacing bituminous mixtures offered for the highways construction and maintenance market, and very few of them will will have a "recipe" specification associated with them, available to the purchaser. 
Although a standard "recipe" is obviously employed in manufacture or you would have chaos in determining which "Thin Surfacing" mixture was which.
I do not know what nominal size designation this mixture has, but from comparison to my "scale" (one centimetre squares) it has the appearance of 20mm. nominal size, this meaning that the predominant size fraction is passing a 20mm. sieve retained on a 14mm. sieve.
It is obviously of a porous nature and appears to have relatively few "fines", so I would compare this mixture to that of a generic Porous Asphalt, which in turn was previously known as a Pervious Macadam. 
However this material will now have the broad generic description of Thin Aspaltic Concrete (TAC) because it will be laid less than 40mm. thick, and asphaltic concrete is now the term used in BS EN 13108 to describe coated macadams.
Laying a mixture of this nominal size only 40mm. thick will increase the porosity of the laid surface course.
I have included the following images on this page to increase the knowledge of those interested. 
The series goes from newly laid material to the nature of failure this type of surfacing tends to suffer from, i.e. the fretting out of the aggregate matrix due to oxidation of the bitumen and hydraulic scouring if the surface drainage is poor. 
These materials also tend to fail earlier than "conventional" materials if there is any underlying movement or weakness in the road pavement.
These pictures are all of one particular branded product from one particular supplier, but all major bituminous mixture suppliers will market a product that will be very, very similar in design ("recipe"), but differing in actual aggregate used, along with possible differences in bitumen supplier and bitumen modifier (polymer).
It is interesting to note that these differences could actually occur with a particular product from a particular supplier when purchasing from the same supplier, according to where in the UK the material is produced.
If the properties of the bitumen and modifier have been "damaged" during storage of the binder, during mixing, and/or the length of time in storage of the mixed material, the life of the laid mixture will be reduced.
This "damage" being temperature related. This deterioration in binder quality will of course be experienced by all bituminous mixtures during production, but porous materials depend on binder performance to retain road surface integrity to a greater degree than low void ("dense") bituminous mixtures.

The above image shows the nature of the failure that this type of "open" material experiences over time, i.e. the fretting out of the aggregate particles.