ian cooper
Hi, Unfortunately concrete will crack, there is not a lot that can be done to stop it. If they are only hairline cracks then it's more than likely down to shrinkage of the concrete. The more water that was added when it was poured the more it will shrink. Even steel reinforced concrete will crack. The key is to make it crack where you want it to crack in a straight line by putting in a crack inducement cut. Simply run a floor saw in a straight line about a fifth of the thickness of the slab then fill the joint with a flexible mastic. But this needs to be done as soon as the concrete is hard enough to walk on. Hairline cracks won't cause any structural issues with the concrete. Hope this helps.
