So your driveway or path is uneven. Some sinking has been going on and your slab is looking worse for wear. If you have had concrete sink you may be thinking it needs to be replaced but there are more cost effective alternatives such as concrete leveling. Concrete levelling alters the foundation that the concrete sits upon helping to level out uneven surfaces. This is a far cheaper alternative to having concrete poured.
No sinking yet? If you are just worried about what may cause sinking but none has occurred yet you are in luck. Leveling measures can be done ahead of time to avoid sinking happening at all.
While extremely durable concrete will crack from time to time. Unfortunately water tends to flow through these cracks and sometimes quite forcefully. While it can and does occur, loss due the mechanical effect of running water is not the main problem, chemical effects are. Water dissolves ions and minerals it lacks such as calcium and leeches them into the water. This causes a loss of mass due to erosion.
Ground Movement Caused by Roots
Plant and tree roots push soil beneath the surface. This can cause multiple problems. Soil disruption can cause pockets of loose soil or even soilless pockets which are prone to cause leveling issues. Also the sheer strength of roots can lift a concrete upwards. While it is very rare roots will break through a slab driveway, walkways and smaller paved areas are known to have problems and are also more likely to buckle.
There is a moisture difference between the rainy and dry season. In winter the ground freezes in spring the ground contracts. This fluctuation results in soil contracting and expanding which as a negative effect on concrete solidity.
Damage caused by insects and animals
Animals such as rats, mice and voles create tunnels and burrows underground. These empty spaces weaken surface concrete. Once enough animal construction has occurred the voids cause the concrete to sink into the created holes.
Water needs to run downhill away from concrete surfaces not to pool. Clogged or poorly placed gutters, not enough downspouts or improper grading can all cause water build up below concrete. Saturated soil below concrete compromises a concretes base.
Poor excavation and backfilling practices
When land is excavated afterwards it needs to be back-filled. If there are issues with the back-filled land settling too much underneath concrete slabs sinking ensues. This occurs because the land around the excavation was very compact and backfill is not. Proper practices can compact the soil but when done wrong concrete will sink.