I have approximately 1200 sq. ft. of flooring that I am putting down 18" x 18" porcelain tile directly on concrete. I do not have the budget to lay a membrane throughout but there is a structural crack in the 15 year old slab. I was thinking of using something like nobleseal cis and just partially covering the floor where the crack is and surrounding it with a soft joint per nobleseal's installation instructions. I would like to know if anyone has any issues or advice on this type of installation. I also would like to know what type of modified thinset I should use that is most cost effective.