Why did my concrete slab sink?

Why did the slab sink when the concrete itself is still good?

A slab sinks because the ground under it lost volume. Soil washes out from under a gravel base, loose fill keeps compressing under its own weight, or clay shrinks back in a dry stretch and leaves air where it used to touch the underside. The slab is then bridging a gap, and concrete is bad at bridging. It drops.

A settled panel is usually in one piece, surface intact, edges not crumbling, stepping cleanly off its neighbor at a joint cut there on purpose. A good slab lying on bad ground. That is a different problem from a slab coming apart, and the read by damage type is on the page for concrete repair and replacement in Henderson.

What made the void under the slab?

Three things make voids on Henderson lots. Moving water carries the fines out of a base a little at a time. Fill that was dumped in one go and never compacted keeps settling for years. And clay pulls back from the underside of a slab in a dry August. Which one you have is worth knowing before anybody quotes a fix.

Where to look, in the order they turn up:

• A downspout that lets out three feet from the slab edge puts the most concentrated water on the property right where the base is thinnest. The panel beside that outlet is usually the one that dropped.
• Sump discharge arrives in bursts, often to a line that stops a foot past the foundation. In a wet March that pump runs dozens of times a day into one patch of ground beside the walk.
• A service trench gives itself away by shape. Look for a sunk strip about the width of a trench, crossing the slab at an angle that has nothing to do with the joints.
• Porch and stoop fill turns up on the 1950s and 1960s houses, where the dirt from the footing excavation went back against the foundation and the stoop got poured on it. Still settling, and the stoop leans back toward the house.

Geography sorts the rest. On the bottomland along the Green and the Ohio, water and a high spring table do the work, and settlement is the common failure. Up on the ridge ground toward Zion and Hebbardsville the clay does it. The soil read is in why concrete cracks here.

How can you tell a sunk section from a heaved one while standing on it?

Walk the joint and notice which way it steps. Going away from the house, if your toe catches on the near edge of the next panel, that panel is high. If it drops away under your foot, it sank. Settlement shows at one or two joints on the low side of a slab. Heave shows up where the ground has something to push against, at the street and alongside trees.

The water stain is the next tell. A panel collecting runoff for a few seasons carries a fine gray silt edge where the puddle keeps drying, and it stays damp for days after a hard rain while the rest of the slab has gone pale. Slow to dry means a void under it holding water.

Then go stand in the garage doorway. Daylight under the door seal where it used to sit tight means the apron dropped away from the threshold. Out at the street it is usually the opposite, a lifted lip you catch a shovel blade on.

What does slab lifting actually put under the concrete?

Slab lifting is sold around here as concrete leveling or concrete lifting, and the two materials behave nothing alike. Mudjacking pumps a slurry of cement, sand and soil through holes an inch and a half to two inches across, and the pressure floats the slab back to grade. Polyurethane foam goes in through holes closer to five-eighths of an inch, expands in the void, and lifts as it grows.

What each one leaves behind:

• Slurry is heavy, on the order of a hundred pounds per cubic foot. Cheap material, and it fills the void solidly, but it adds weight to ground that already failed under less, and it wants a day or two before a car goes on it.
• Foam weighs a few pounds per cubic foot, cures in minutes, and carries a vehicle the same afternoon. Being closed-cell, standing water does not dissolve it the way it can erode a slurry.
• Neither one compacts anything. Both fill a void and float the concrete back up. You get the elevation back. You do not get a base back.
• Neither one is waterproof in the way that matters. Water still running under a slab goes around a foam block and cuts a new void beside it.

Which is why lifting answers one problem well. A sound slab sitting at the wrong height over a void that has finished growing.

When is lifting the wrong money, and who does that work?

Lifting is the wrong call when the slab will not come up as one piece, when the void is still being made, or when height was never the complaint. Pumping a broken slab gives you the same pieces at slightly different heights and a set of patched holes to look at. Pumping over live water buys a season or two.

The cases where it does not pay:

• Concrete already cracked through in two or three places. Each piece lifts on its own, and the joints between them end up worse than the settlement did.
• A slab too thin to take the pressure. A builder-grade pour under three inches can crack from the pump before it lifts.
• A base that is still washing, because the downspout, the sump line or the grade has not been touched.
• A walk riding a tree root. The root is still growing, and lifting concrete off a moving root is a yearly appointment.

Slab lifting is a specialty with its own pump rig and a crew that does nothing else. Removal and re-pour is flatwork. If your slab is a lifting candidate we will say so, and you will be hiring a lifting outfit yourself. Nobody here takes a cut of that call.

What does replacing the settled section by itself involve?

The panel gets saw-cut free along the existing joints, broken out and hauled off, the base underneath gets rebuilt and compacted in lifts, and a new panel goes back to the same joint lines. Concrete removal runs $2 - $4 per sq ft on its own, and taking out a whole two-car drive runs $1,200 - $2,800 when all of it is going.

The base rebuild is the reason to choose this over a lift, and the reason a section replaced over a corrected downspout tends to be the last time that panel gets touched.

The catch is color. A new panel next to a thirty year old slab reads lighter for years and never quite blends. Sealer evens the sheen, not the shade. If that bothers you more than the step did, replacing a longer run is the better conversation.

Settled slabs turn up in the same few places. A shed pad or detached garage floor that dropped at a corner is on slabs and pole barn floors, and a stoop that pulled away from the house is on sidewalks, steps and porches. The ranges are gathered on what concrete repair and removal cost.

Does the water have to be fixed either way?

Yes, and it is the part that gets skipped. Lifting and replacing both put something into a void. Neither one touches the thing that made the void. If roof water still lands at the slab edge, or the sump line still daylights beside the walk, the new void starts forming the week the crew drives away.

What actually changes the outcome:

• Run the downspout past the edge of the concrete, onto ground that falls away. Ten feet of extension costs less than any of the fixes above and returns more.
• Pipe the sump discharge somewhere. A line that stops at the foundation is a line aimed at the slab. Carry it to the ditch or the low corner of the lot.
• Look at the dirt beside the slab, not only the slab. The ground along a patio edge settles below the concrete over the years, so runoff finds the edge and goes under. Build it back to slab level, sloped away.
• Seal the joints and the edge where water has been getting in.

A lifted slab over an unfixed outlet sinks again, and the second drop comes faster, because the fill under it is looser than the original ground ever was. The easiest day to read your own lot is the one after a hard fall rain. Stand at the low corner, watch where the roof water goes, then look at which panel is dark.