According to my colleagues, who are licensed structural engineers, "...all concrete is cracked concrete." For the purpose of running structural calculations of mechanical concrete fastening systems. Manufacturers like Hilti publish load tables with data for cracked and uncracked concrete. The structural engineers go with the load ratings for cracked concrete for all applications. They say the uncracked concrete is just a hypothetical situation; it is impractical to engineer something to the minimum standard for uncracked concrete. This is because virtually all concrete will develop hairline cracks as it cures. It may take several weeks or months, but eventually the hairline cracks will appear.

A diagonal crack like this is definitely one from differential settlement. As Worth say, notify the contractor in writing (as a future record), and request he excavate the soil in the vicinity of the crack and determine: 1) the depth and extent of the crack; 2) the condition of the soil beneath the foundation in this area to see if it has been properly backfilled and compacted. Continue to monitor this crack, with tape and or permanent weatherproof marks to see if it is moving and/or increasing in width. Good luck with your project.

No,do not put water in the void. Simply use a soaker hose around 3 sides of porch slab. Any streches of bare ground adjoining foundation that doesn't have grass within 6 feet that;s watered should be watered with soaker hose placed 3 to 5 feet from foundation. Back to the void between porch and foundation. If water from roof floods the void you should fill it with urathane expansion joint compound. You want to prevent sudden wide fluxiations in soil moisture anywhere near foundation. If a home levels up during rainy season that indicates the need to do as you are in watering landscape during dry weather. Don't waste money on foundation repair for such home. If home moves but never returns to normal there's little choice but make foundation repair.

Not a pro. Worked with someone with a concrete floor in two homes. Her husband said they included the plastic (and insulation board) atop the packed layer of gravel used to fill in some of the depth between the foundation blocks before the concrete was poured beneath the second home. They had regretted not doing so in the first home's floor after a pro explained to them why the floor of that first home felt cold and damp and the windows had a serious condensation problem. The thick plastic and insulation board made a positive difference beneath their second home. Yes, definitely replace the plastic (same kind and thickness overlapping and you should probably tape it to the adjoining plastic to create a seal before pouring concrete flooring patch.