THIN ASPHALT MIXTURES A new top layer should ; be flat and rough drain fast and well cause the impact sound between tyres and road surface to stay within limits a good-looking surface especially in urban areas  

Conventional maintenance techniques, especially the surface treatment are hardly able to meet with all these wishes. In this cold technique the existing road surface is treated with a bitumous emulsion, after which chippings are spreaded liberally.

Disadvantages of this cold technique : ·        the treatment is only effective for a limited period of time. ·        spurting gravel may cause damage to vehicles and present nuisance and danger of skidding. ·        a fresh layer of gravel renders much impact noise. ·        the weather circumstances play a role in the execution, such as being too wet, too dry, too hot or too cold. During use damage can also easily be caused by torsional traffic, for example at crossroads and on slip roads.   So, surface treatment is somewhat falling out of favour, in urban areas. Alternative measures for surface improvement: hot thin top layers

DEFINITION Thin surface course materials are defined as being of indermediate thickness between veneer coats, which are very thin surfacings systems such as surface dressings and slurries, and materials laid as surface courses at the more traditional thickness of 40mm to 50mm, such as rolled asphalt and coated macadam. Thefore, the materials are those laid 15mm thick and upwards to a maximum of 35mm but, more generally 30mm.

Advantages: ·        Long life, a thin top layer will last 10 to 15 years (against 3 to 6 years for a surface or emulsion treatment). Naturally, the underlying asphalting structure should be of sufficient quality ·        Ability fill out, hot thin top layers can fill out any irregularities in the road surface to a reasonable extent ·        the limited preparation, a thin top layer requests only little preparation. It is not necessary to mill out completely ·        the noise reduction, because of a favourable texture thin top layers have good acoustic qualities. Whereas in a surface treatment the spreaded stones will be mainly pointing upwards, in a thin top layer the smooth sides will turn to the surface due to rolling. So less impact noise is generated. The fine grain size contributes to a decrease in noise level as well. ·        the right flatness and roughness ·        the nice appearance

Although the mixtures do not come cheaply, the final costs turn out better than expected. However, the composition of the mixture is a very precise matter; there are high demands for the quality of the ingredients, the processing requires great attention and as emphasized before: the underlying layer has to be sufficiently stable.

Categories of Thin Top Layers Thin SMA (Stone Mastic Asphalt) is a reduced thickness layer of interlocking crushed rock aggregate with a filler and binder. This has been developed from full thickness SMA used in Europe. VTSL (Very Thin Surface Layer) is a thin paver laid Polymer-Modified Asphalt Concrete bonded to the road with a thin tack coat THMAL (Ultra Thin Hot Mix Asphalt Layer) is a hot bituminous mixture spread with a paving machine directly on a sprayed bond coat.

Thin Stone Mastic Asphalt Stone mastic asphalt is a gap-graded mixture of high-quality coarse aggregate filled and cemented by a rich sand/filler mastic with penetration grade binder and stabilizing additivies, which may consist of fibres or polymer modifiers or combinations of both, to produce a comparatively thick binder film. The material is paver-laid onto a tack coat which may be polymer-modified, and can be laid as a thin surfacing course. A stabilising aggregate is necessary to prevent segregation. The high percentage of bitumen gives the asphalt mixture a long life. One very important quality of stone mastic asphalt is that the grain structure of the stone mixture is graded intermittently and that it can contain more bitumen than nonporous mixtures, which benefits the durability.

Very Thin Surface Layer The first objective is the regeneration of surface characteristics, and this technique is employed on all types of roadways, both for maintenance and new road construction or partial repair of slow lanes on motorways. Polymer modified asphalt (PMA) is the high performance grade of asphalt cement. It is manufactured by blending a SBS polymer with asphalt cement. This premium binder gives superior technological and mechanical characteristics which allow the manufacturing and application of road asphaltic mixes with higher performances such as : 1. Improved temperature susceptibility. 2. Resistance to permanent deformation. 3. Resistance to fatigue cracking. 4. Improved cohesion of the mix 5.  Improved durability of skid resistance 6.  Reducing the noise emission from tyre-road contact

Ultra Thin Surface Layer This type system essentially comprises a hot paver-laid of high quality stone of either 10 mm or 14 mm nominal size carrying a sand/filler mixture in penetration grade binder. This matrix is laid directly onto a hot-sprayed polymer-modified binder film and forms a closely packed mosaic of stone with a relatively high level of surface voids, securely held in place.The original variety is known in France as Euroduit, in the United Kingdom as Safepave and elsewhere as Novachip.

Novachip The method is used for re-surfacing in cases where damage has been caused by rutting, segregation or cracks. The depth of ruts in the road surface should not exceed 20 mm. The biggest irregularities must be milled before surfacing.  ·        The Novachip method results in good wear resistance, because the aggregate contains an exceptionally large proportion of coarse particles. ·        The surfacing is very homogeneous with good friction properties, reducing the risk of aquaplaning compared with conventional surfacing materials. ·        Novachip is a quick method and the noise level of the surface is lower than that of commonly used materials. 30,000 m2 can be layed in one working day.

The left side of pavement is novachip and the right side of pavement is the existing road surface.

This asphalt consists of chippings enveloped in a mastic rich in bitumen. The grading of this mixture is close to that of stone mastic asphalt; the percentages of filler and bitumen are both about 5% regarding volume, depending on the kind of stone applied. Because of this a relatively open structure forms at the surface.

TACK COATS Thin surface course materials depend critically on good bonding with the underlying support layers in order to achieve their full life potential. Therefore, a tack coat is always required, but the type and spread rates vary depending on the system in use. It should be noted that all French contractors use polymer-modified tack coat as standard and this is likely to be a sound investment for long-term durability.

MAINTENANCE Rutting and fall-off of skid resistance are unlikely to be major problems with these systems provided that they are competently formulated and that coarse aggregate with PSV(polished stone value) adequately matched to the traffic has been incorporated. However, should the texture deteriorate, these materials would lend themselves to re-texturing techniques such as high pressure water jetting because they are composed of skidding-resistant aggregate throughout their depth.  

CONCLUSION The surface structure of roads will have to be improved upon periodically in the future as well. In order to postpone major repairs, it will remain necessary in the road sector to increase the roughness, to improve safety and to preserve the asphalt construction. However, it will not be possible anymore to apply the surface treatment to a number of roads as a suitable maintenance measure, precisely because of the problems mentioned before. It is obvious that road maintenance authorities are in need of alternatives. That means that the substituting preserving maintenance should offer more that only the positive qualities of the conventional techniques: rapidly executionable, little traffic nuisance, relatively cheap and effective.