Gramps416, A couple of times you mentioned the slab settling (I am wondering how much more can it possibly settle even with the extra weight if I mudjack). Can you explain what you mean by that?
The settling around a building is an ongoing process that can take many years to achieve. Where the soils consist of fatty or expansive clays, that time frame can be 60-90 years to settle at 23% of volume. Sandy soils settle much faster at a lower ratio of volume.
Water accelerates the settling. You see the greatest movement at the downspouts. This settling is happening all around the foundation... under the driveway, the front steps, the patio, sidewalks next to the building and especially the trench for the sewer and water lines serving the house. Normally, for the flower garden, we just throw in a little extra dirt to replace what disappears. However, nobody replaces the soil under concrete because of "out of sight out of mind".
This happens because of the way we build. The stages of construction mean different crews come and go and the continuity of craftsmanship gets broken. For instance, the concrete crew shows up after the soil is replaced around the foundation and they have no idea where holes where dug and settling will happen. The true fact of the matter is there are more holes under concrete than anyone knows about.
Your slab has settled, but doesn't mean it's touching the ground in all places. It's an almost certainty there is an underground reservoir next to the building and wherever someone dug a trench. When it rains, because these reservoirs are lower than everything in the vicinity, they fill with water. Over time the saturated soils will slough off (erode) to cavities way deep down and the process continues unabated.
All of this is magnified by where you live. If you live at elevation or above the thirtieth parallel, you can add in the worry of frost. Water freezes and expands and we bury our pipes deeper the further north you go. I don't know where you live, but the extra cracking at your sidewalk corner looks like frost cracking. The water under concrete contributes to it's demise.
With that said, I'll get to my point. Your choices are simple... I think. If the surface and overall condition of your slab is good, don't replace it , repair it. How it's repaired makes a big difference in how much damage is done to the slab and how long it lasts. The best, least intrusive, and longest lasting repair is sandjacking (costs vary widely from area to area, but usually under $3/sq ft). If not available, you should mudjack, which should be cheaper. Look for a mudjacker who drills small holes (1.5-1.75"). Use an electric hammer drill (less destructive) instead of air rock drill to drill holes in a grid no more than three feet apart. Stagger the grid. Forget the cementitous aspect, it's only a placebo to make you feel better and costs more. If they have crushed Limestone or lime sludge from water treatment plants, that will suffice. Keep the moisture level of the material at 40-50%. Too stiff doesn't fill, too wet will shrink from water loss. At 50% moisture you will lose 25% volume to water loss.
The biggest problem with mudjacking is most contractors buy into the industry boast of "when the void is full, the concrete lifts". Not true. The void is never full and most contractors come to know that if you pump in a stiff mix it lifts right away and the job goes quicker. Not all contractors are alike. Some can resist the urge to cheapen the work done to make more profit and some can't. Plus, nobody knows how common or how big the voids are because the voids aren't filled reliably.
As for longevity, a proper repair should last you 5-10 yrs or more and it won't drop 4" because if you fill it, that product will not disappear.
