For hair line cracks, an epoxy like Pond Shield can be used to coat over the crack.  Coating over a crack will give you the most minimal resistance to the crack reappearing.  I suggest that you use a grinder with a cutting wheel to groove the crack and THEN fill it in with Pond Shield.

What you end up with at that repair is essentially a concrete stitch.  The epoxy holds the crack from both facing sides as well as across the top of the crack.  Pond Shield tensile bond strength (that exceeds the internal strength of concrete) combined with its elongation break strength of 9,500 psi will pull and hold the concrete as though it were once piece.