Tech Talk : Asphalt – part 2

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We have seen how asphalts are made and laid on the ground, and also the different types of asphalts depending on the kind of stones (inert) and bitumen (binders) that are chosen. The different types of asphalts are selected according to the needs and particular uses that have to be made: asphalt for racing tracks, roads, highways, draining asphalts.

Characteristics of the asphalts

Draining asphalts are a great solution and invention for highways where high speeds really do not work well with water, especially when paddles are present. Aquaplaning can also become a problem, but also visibility is improved with draining asphalt.

The limit of such an asphalt is though that it has a lower compactness compared to traditional asphalts, this means that its resistance to wear is also reduced. In fact draining asphalts are called “open mixture” asphalts, because the empty spaces inside of them are equal to around 20% of the entire volume, whilst traditional “closed mixtures” have this percentage equal to less then 8%. This way these kind of asphalts have a better mechanical resistance. Draining asphalt was used once in the 80’s on the Formula 1 track in Belgium, Franco-Champs. The result was a real disaster, the track did not support the vertical and incredible lateral forces of the racing cars. The Grand Prix was cancelled that year.

Also, draining asphalts are permeable to water, but also greatly to air, which means that the positive effect with wet track conditions becomes a negative one considering the ground effect, so grip is greatly reduced for single-seaters.

A good compromise is to produce an asphalt with a rough surface that determines good grip, and a good drain of water, that will pass through the channels present between the peaks of the surface. Asphalts with completely smooth surfaces are no more made, but of course too rough surfaces generate high tyre wear and a weakness of the peaks on the plane of the asphalt. To avoid such limit binders of extremely high mechanical characteristics are used, such as bitumen with polymers.

To measure the roughness of the asphalt a particular procedure is used. A certain amount of sand is distributed on a circular area on the asphalt. The rougher the surface the more the sand will deposit inside the asphalt and the smaller the circular area will be. The area will have to generate in fact a smooth surface. Finally the radius of the area is measured. The greater the radius the smoother the asphalt is.

Racing circuits and roads

We might think that laying and compacting asphalt on a racing track is easier than on a road or highway since we have more time and no traffic passing on the track, except when having a race or testing. This is not actually true. Of course the perfection of a race track must be at maximum levels: no bumps at all! On the other hand though traffic and dirt on the asphalt help compacting. In fact when heavy cars, or even trucks, run along a road they press vertically on the asphalt. Also dirt avoids the new asphalt to stick to the tyres and be lifted and torn away. On the opposite side racing tracks are usually extremely clean, with no dust or dirt, and are also quickly cleaned by the new extremely soft rubbers of the tyres of racing cars. The forces on the asphalt are not so strong downwards along the vertical direction, since racing cars are not very heavy for obvious performance needs, so compacting of the asphalt does not really occur. Transversal-horizontal forces are on the other hand very strong since grip of the tyres is so high. This generates the tear of the asphalt sideways and an extremely strong wear of its surface.

Following this criteria karts are really the most destroying vehicles for what concerns tracks. The weight of a chassis plus engine plus driver is extremely reduced, whist the lateral grip is extremely high and the soft sticky rubber of the tyres pulls off bits and pieces of surface. Asphalt in curves is often, when new, torn away and worn quickly if the choice of the binder is not right and the compacting phase is not completed properly. Also motorbikes wear badly the asphalt surface since they are not heavy, but have a narrow area of contact between the tyres and the track surface. Once the deterioration of the asphalt starts the only solution is re-asphalting. Another partial solution often used in kart tracks is to put in the worn areas particular resins that harden quickly (within a night), but that are very expensive. You can recognize them because they are white coloured. They are also particularly smooth, but still have good grip characteristics.

Seasons and settling

One of the main problems in renewing a track surface is that work lasts quite long and the track has to close for such a period of time with great loss of money. The weather conditions such as rain, sun, temperature, humidity must be well considered to permit a good result of the asphalt laying and compacting phases. Also such conditions can increase or reduce the duration of work.

For what concerns the period of the year usually it is best to avoid the coldest seasons when the low temperatures make it difficult to work the asphalt with the right temperature (asphalt cools down quickly when transported or laid down). Also humidity acts negatively on the bitumen reducing the binding effect. In the rest of the year there are no real problems in renewing the asphalt. Times of rest after laying and compacting the asphalts are around 10 days. To increase the compacting effect what can be done is to spread all over the asphalt surface limestone sand to eliminate sticking characteristics and then to drive on the track with heavy vehicles.