I have an elevated exposed aggregate concrete slab built on a steel structure. It is about 7 years old and measures 10'wide x 20'long. The contractor placed reinforcement wire in the slab when it was poured. Just noticed a hairline crack in the concrete about in the middle of the slab running lengthwise. The crack does not run the entire length but starts and stops along the same line. How can I repair the crack before it gets worse?