The following are the differences between bitumens and tars.
The weathering properties of bitumens are superior to those of tars. Generally greater deterioration is produced in tar than in bitumen when exposed to equal weather conditions. Bitumens have a better durability and resistance to weathering than tars.
Tars are more susceptible to temperature changes than bitumens. Hot weather will soften a tar surface more than a surface made with bitumen of the same viscosity, and it will become more brittle at low temperatures than bitumen. In other words tar becomes brittle in cold weather and the surface treated with tar is apt to bleed in hot weather if a little extra quantity has been used. Tar is therefore considered unsuitable for locations with wide temperature changes. Gritting or surface dressing can be delayed a little where bitumen has been used but not with tar.
Surface dressings with bitumen are more prone to failure by water displacement than those made with tar. Tars generally adhere better than bitumens on wet aggregate.
Tars can be brought to a spraying condition at lower temperatures than those needed for bitumens; stones need not be heated to high temperatures.
Setting time for tars is more than that of bitumens and this property is useful in the production of pre-coated aggregate which can be transported to large distances or kept for sometime before spreading.
Tars harden much quicker than bitumens.
Tars have higher specific gravity than bitumens and lower viscosity and these properties give them greater penetrating power and which are more marked during summers. Higher viscosities can generally be used with tars than with bitumens.
Tars produce a less slippery surface than bitumens.
Bitumens have a tendency to stay at or just near the surface resulting in a rich and fat surface.
Roads built of bitumen need constant traffic to be maintained in good order; otherwise the surface will crack and reduce the life of the road.
Tars make harder surfaces (but such surfaces are brittle) than bitumens and should be preferred for roads in areas where bullock carts or other hard tyred traffic predominates. Bitumens make more elastic surfaces and are better suited for pneumatic traffic. Hardening of bitumens is very gradual.
Tar is more suitable for dense fine grained surfaces and bitumen for more open surfaces.
Volume of tar required is about 10 percent less than that of bitumen for the same type of road work.
Tar is cheaper than bitumen.
A primer is not generally needed with tar.
Road tars do not dissolve in a petroleum distillate such as petrol, kerosene, diesel oil. As such tar carpets have proved to be good material for parking sites as it remains unaffected by spillage of oil and petrol from automobiles.