You fixed the sunken slab once and the dip is back

If your driveway or walk has sunk or tilted, the first decision is not which fix to buy. It is whether the water under the slab has been dealt with. A slab raised or replaced over the same wet, frost-prone ground will usually sink or heave again, and no surface treatment changes that.

Flaking, pitting and crumbling edges are surface problems. They come from freezing water, salt and the mix. A slab that has dropped, tilted or opened a gap at a joint is a ground problem, and the ground is where the cause lives.

You cannot reliably tell the two apart by eye, and neither can most owners. Look at the slab with a straight board or a level laid across a joint. If one side sits lower than the other, you have movement. Take a photo with a coin or tape beside the gap, and note the date.

That photo is the best thing you can hand a contractor. It shows what you saw before anyone started guessing. Send it with your request, and say whether the movement is new or has been creeping for years.

The popular explanation is that water in the ground freezes and expands under your slab. New York State's Department of Transportation points somewhere else: ice lenses, layers of ice that grow in the freezing zone as water is drawn in from below and from the sides.

The state says frost heave needs three things at once: freezing temperatures reaching the ground under the slab, a water supply into the freezing zone, and frost-susceptible soil. Remove any one and the heave stops. Ask your contractor which of the three they can change on your lot.

The cold cannot be changed. The soil is hard to change. The state names drainage, with ditches, culverts and underdrains, as the condition that can be reduced, so ask where the water around your slab goes.

The state says practically all surface soils in New York undergo some frost action. That does not tell you what is under your slab. It does tell you that frost-prone ground is the normal case here, not the exception.

That is also why two driveways on one street can behave differently. The soil and the drainage differ from lot to lot, so ask the contractor what the base under your slab is made of, and how they will check it.

The damage does not stop at the heave. On the thawing side, the state describes loss of support: meltwater cannot drain because the layers beneath are still frozen, the soil saturates, and it can no longer carry a load.

A slab that sat level in January can drop in March because the ground under it has gone soft. Heave that is uniform is not usually harmful, the state notes, and it becomes a problem when it is uneven. That is why one corner of a walk can sink next to a corner that did not.

The state also lists where uneven heave shows up. Over culverts, next to driveways that dam a roadside ditch because the driveway culvert is blocked or missing, and where the base changes abruptly. If your apron sits over a culvert, your next step is to look at that pipe and ask who keeps it clear.

Walk the property after a rain and watch where water goes. These are the common sources, and each one is something you can point to when you talk to a contractor.

• A downspout that empties onto or beside the slab.
• A grade that slopes toward the house or the garage instead of away.
• A blocked culvert or a full roadside ditch at the apron.
• A joint or crack that lets water run under the slab edge.
• A spot where the base was never compacted, so water collects.

Fix the ones within your reach first. If the ditch or culvert at the street is the problem, the road owner may be involved, so ask your town, village, city or county office whose it is before you pay anyone to dig.

There are three broad routes, and the right one depends on how far the slab has gone. The contractor does the work. Your job is to ask which route fits and why.

1. Lift the slab back to level. This suits a slab that is still sound but has dropped. It works only if the cause has been handled, or the slab drops again.
2. Patch the edges and joints. This suits small trip points where the main slab is level. It is a repair of the symptom.
3. Remove and replace. This suits a slab that is broken into pieces, deeply scaled, or sitting on a base that has failed.

A slab that has cracked into many pieces, or one with heavy and spreading surface damage, may be too far gone to lift. NRMCA says repairing a slab with excessive and progressing scaling may not be feasible at all. Ask the contractor to tell you plainly if your slab is past that point.

Ask what caused the movement, in the contractor's words. If the answer is "the ground," ask what about the ground and what will change so it does not repeat. A fix that does not name the cause is a gamble, and a good contractor will tell you so.

Ask where the water will go after the work. Ask whether the base is being rebuilt or only filled. Ask for the answer in the written scope, along with the mix and the pour month if concrete is going in.

New concrete placed to fix a settled slab needs the same first-winter care as any pour. NRMCA says to protect fresh concrete from freezing, and to use clean sand instead of de-icers in the first winter. Ask the contractor for the protection plan in writing, and remember that cold weather means 40F or below during the protection period, not on the day of the pour.

NRMCA lists the basics for a base: remove topsoil and soft spots, place compacted soil or well compacted granular fill, and slope the slab and the subgrade for drainage. In winter, snow and ice come off before the pour, and concrete must not go on frozen ground. NRMCA also says poor subgrade preparation is behind the majority of concrete cracks.

Joints help as well. NRMCA describes joints as planned cracks, because cracking cannot be prevented, only controlled. Isolation joints let a driveway or patio move separately from steps, walls and the garage slab. Ask where they will be, so movement shows up where it does no harm.

Take the straight-board photo, note where water goes after a rain, and write down when the movement began. Send us the slab and what it is doing. We help you work out whose job it is and what to ask for, then introduce you to an independent Syracuse-area contractor to quote it. The contractor does the work and names the price, and you decide.