Assalamo Alaykom Dr. Mohamed,

Asphalt cracks happen due to different reasons depending on the cause of the crack, for example, Transverse Cracks that you can see across the road (transverse direction) and also the Block Cracks that you can see the road is divided into big blocks could be possibly according to Thermal Stresses. and this type of stress is produced in the asphalt pavement (expansion and contraction of the binding bituminous material) due to the big difference in temperature between night and day, and also the different seasonal temperature, you can see it clearly in Upper Egypt.

Another reason for cracks is Fatigue due to overloading causing a famous type of cracks called Alligator Cracks that looks like a crocodile back. Another type of cracks are the Longitudinal Cracks, These types of cracks predominately created between the existing pavement and new widening but may also be a result of expansive clay subgrade soils.

Regarding the Preventive actions, there are many actions depending on the density and severity of the crack, starting from Sealing and Filling of the crack with a a material that will fill the crack preventing the water (The most dangerous Asphalt pavement Enemy) from getting inside the pavement layers causing an accelerated deterioration of the asphalt pavement.

@Kamel Mahdy,this is a very comprehensive answer if the question raised, but the solutions that tackle the problem are a bit old have you find any new techniques such as doping with nano particles or finding a way to cool it down or absorb the thermal energy fallen on or generated within asphalt?

Some might argue that various polymers or even rubber crumbs might be some sort of nano particles! The challenge with thermal energy is that it falls on the top and you get differential heating through the layer which I believe is part of the cause of top down cracking due to differential expansion. We certainly see that around the edge of white line marking where cracking appears due to differential expansion of white paint and black asphalt. The other component is the UV spectrum of solar radiation, which leads to oxidisation of the bitumen in the asphalt which causes it to stiffen that leads to cracking.

Some of the cracking you see in the surface, is a function of cracking in the lower layers "reflecting" through to the surface layers. Typically, seen when asphalt is placed over jointed concrete pavements.

It usually cracks because of tension. The most important point is to control and repair it. I just tried to resolve it by carbon nanofiber and maybe you can analyze the mechanism theory behind it.