FAILED THIN 6MM. BITUMINOUS WEARING COURSE ON A PARTICULAR SITE,
6 YEARS AFTER LAYING

Although with the second site that is discussed it may be fairer to say that it has just come to the end of the projected life for this type of material.
This page cannot be read in isolation, you must read the previous two connected pages !

This site, at the time of laying the surface course
This site, four years after laying

To recap, this picture shows the nature of the surface at the time of laying. It is a 6mm. medium textured surface course as specified in BS 4987, with rubber added as latex at the time of mixing to modify, enhance, the qualities of the bitumen.

	These photographs were taken in the Spring of 2005, this surfacing was laid in August 1999, so here it is not quite six years old. It can be seen that the failure of the entrance and exit points is extensive, with areas of total material failure in places. By total failure I mean all the applied surface course has been removed, and you can see where the first areas of failure have been patched. The increase in failure of this area of the site has been dramatic over the last twelve months, especially during the last winter, areas so large as to make patching pointless. Shortly after this picture was taken the junction received a forty millimetre planing and a new surface course of forty millimetres of 10mm. stone mastic asphalt was laid.
	It is interesting to note that there has been no failure of the first rip laid, seen on the left hand side of the picture. This begs the question is this because the material was of a better quality than the two rips laid later in the day, or is it  because it does nor experience any turning traffic. It is probably a combination of both factors. It is known that  one load of material was rejected later in the day as not having had the rubber latex added. Anybody who has worked with materials that contain rubberised bitumen will know that it is quite easy to tell the difference between rubberised and none rubberised bituminous mixtures.  Rubberised bituminous mixtures that have been correctly mixed, stored and transported are very "sticky", ask any laying gang member who has laid this material. It is when material contains some but not the specified amount of latex that on site judgment can be difficult.
	When an area begins to fail, as shown in the picture to the left, in isolation before the rest of the mat, with everything else being "equal", it is reasonable to assume that this material is in some way of a lower quality than the rest of the material. This in turn suggests variability at the time of mixing, i.e. poor distribution of the binder modifier (latex). I would point out that this material provided four years of reasonable life on a difficult site, some would regard this as acceptable. However a similar site, having received exactly the same surfacing one week earlier, just two hundred metres along the same road and at a more highly stressed junction is only now, Spring 2007, showing signs of material failure. With some local patching this junction should be sufficiently sound to last another year before resurfacing is required.
This means this particular thin (20mm.) overlay surfacing has currently achieved eight years life, a life expectancy which I believe is reasonable for this material at this site, and at the thickness it was laid.  I hope this simple case study is sufficient to make engineers, technicians  and "management" realise that the real world of highways maintenance is a lot more "interesting" than most of them are aware. Organisations and authorities need engineers with similar experience in "materials observation" if appropriate decisions are to be made in the choice of materials for future highways maintenance.  This means observing the performance of known materials, i.e. the composition/specification, have seen them laid, and have sampled and tested them to know that the material complied with the design/specification. Without this thorough engineering approach engineers are not going to know what is really happening. To my knowledge engineers and technicians are not being, trained, employed and supported in this role, and I personally cannot see how the maintenance of the highway network of this country can be professionally undertaken without such engineers and technicians. It is my opinion that the role of the Materials Engineer and his Materials Laboratory need to be supported and reintroduced in many areas if the term " intelligent client" is to have any meaning at all. It is also my opinion that large organisations should not make very important, and expensive decisions, without such support, and be seen to demonstrate how this support is instrumental in their decision making.

For more information on British Standard bituminous mixtures that can be laid thinly, press ----------------> HERE

For more information on proprietary "Thin Surfacings", press ----------------------------------------------------------------> HERE

For access to a table comparing and contrasting many bituminous surfacing materials, press --------------> HERE 

[ Top of Page ]