normalmap rendering issue(?)

I’m quite new to UE4 materials. I watched some startercontents and video tutorals about materials and thought I create my own material.
Actually I created a photo of asphalt to create a normalmap later on.

I made a seamless texture and created normalmap from it in GIMP 2.8
I imported the normal (in 2k resolution).

Without making any precaution I hooked up the normalmap to a completely blank material and added a dark gray color.
The result was satisfying but the bumps were too big since I created the picture from a close distance.
Therefore I added textcoord and tiled the normalmap 5x5 in each direction.

The result is now a little awkward. In a close distance it resembles asphalt which is nice and expected. If I watch the texturized object from a distance it looks like an object from the era of DOOM.
The asphalt is still gray but there are few completely black blocks appearing on the object which is the result of the normal map.

How should I handle such situation?