why Hair lined Cracks r observed on concrete surfae

Visible hairline cracks on the surface of concrete, if there are a bunch of them, are usually caused from shrinking on the surface during curing. If the patio is relatively new, the cracks are usually caused by either overworking the surface (bringing to much cream to the surface) or the pad cured to quickly. Most pro's will cover a new concrete pad with plastic to retain moisture and prolong the curing process to avoid this kind of cracking. If you want to minimize the cracking, and it's been at least a month since the pad was poured, you can apply a concrete sealer.