Every slab in Green Bay has a few lines in it. Some showed up the first summer and never changed. Some open a little more every spring. One might have a lip you catch with the snowblower.
This is the "why" article. To identify a crack by its look, use our field guide to concrete crack types. To fix one, see how to fix concrete cracks. Here we go one layer down, to the physics, because once you know what's pulling on your slab, the cracks start explaining themselves.
Great at Pushing, Bad at Pulling
Concrete is rated by compressive strength: how hard you can squeeze it before it crushes. Industry guidance calls for 4,000 psi on outdoor slabs that stay wet, freeze, and get deicers. Each square inch could carry about 4,000 pounds, roughly the weight of a midsize car.
Now bend it. Concrete's strength in bending runs only about 10 to 15 percent of its compressive strength, so that 4,000 psi slab gives out somewhere around 400 to 600 psi. Pulled straight apart, it fails sooner still.
That gap is the whole story. Driveways and sidewalks almost never fail by being crushed. They fail when something stretches or bends them. Nearly every crack in a slab is a tension crack.
Movement Plus Restraint: The Recipe Behind Most Cracks
A slab that could slide around freely would rarely crack from shrinking or temperature. It would just get a little smaller in January and a little bigger in July. But nothing lets a slab move freely:
- The ground grips the bottom. Friction with the base resists every bit of shrinking and growing.
- Things pin the edges. The foundation wall, the garage floor, the stoop, a light pole, a drain grate.
- Shapes trap it. An L-shaped patio can't shrink in two directions at once without pulling hard at the inside corner. That notch is where the crack starts, which is why joint guidance says to avoid L-shaped panels.
When concrete wants to move and something holds it, tension builds until the slab cracks. Joints don't stop that. They decide in advance where it happens. What to ask your installer for is in preventing concrete cracks.
The Forces, One at a Time
Drying shrinkage: the crack built into the recipe
Concrete is mixed with more water than the cement needs to harden, so it can be poured and finished. As that extra water leaves over the following weeks and months, the slab shrinks.
Industry shrinkage specs commonly target around 0.04 to 0.05 percent in a lab test after 28 days of drying. That sounds like nothing until you run it out at 0.05 percent:
| Length of concrete | Shrinkage at 0.05% |
|---|---|
| One 10-ft panel | about 1/16" |
| A 25-ft run | about 5/32" |
| A 50-ft driveway with no joints | about 5/16" |
Concrete can't stretch anywhere near that far without breaking, so with the ground holding the bottom, a long, unjointed slab pulls itself apart. Control joints, cut every 8 to 12 feet in a typical 4" slab, give that shrinkage a planned place to go. Water added at the truck makes it worse: more water to dry out means more shrinkage.
What it leaves: a thin, flush crack down the bottom of a joint (normal), or across a panel when joints were too far apart, too shallow, or cut too late. A cousin forms in the first hours, when a hot, windy day dries the surface before the concrete sets. More in new concrete cracking.
Temperature: your driveway breathes all year
Concrete grows when it warms and shrinks when it cools, by an amount that depends mostly on the stone in the mix. Federal highway research puts it at roughly 4 to 7 millionths of an inch, per inch of slab, per degree Fahrenheit. Call it 5.5 for a middle-of-the-road mix.
Based on 1991–2020 climate normals at the Green Bay airport, the coldest night of a typical winter bottoms out around −16°F, and the hottest afternoon of a typical summer tops out around 93°F. That's a swing of about 109 degrees, before counting the extra heat a slab soaks up in full sun.
Worked example: a 50-foot driveway
- Length: 50 ft = 600 inches
- Yearly temperature swing: about 109°F
- Movement: 600 × 0.0000055 × 109 = about 0.36 inch, a hair under 3/8"
- Across the full range of mixes: about 1/4" to just under 1/2"
A slab's own temperature doesn't swing quite as hard as the air, so treat that as a ballpark. Cut into 10-foot panels, each joint opens and closes about 1/16" a year from temperature alone. Three things follow:
- 1Joints and cracks have to be able to move. That's why rigid patch in a joint pops out and flexible sealant lasts (see best concrete crack caulk).
- 2Debris in a joint is a July problem. Stones packed into an open joint in winter keep it from closing when the slab warms, and the slab chips its own edges pushing against them.
- 3Slabs push on whatever they touch, like the garage floor or the stoop, every summer.
Lost support: when the slab becomes a bridge
A slab is built to rest fully on the ground. When soil under part of it compacts or washes out, that section turns into a small bridge, and bridges bend. Where the tension lands depends on where the hollow is:
- Hollow under the middle: the slab sags into it, the bottom stretches, and a crack starts underneath and works its way up.
- Hollow under an edge or corner: the unsupported part hangs out like a diving board, the top stretches, and you get a crack across the surface, often a corner breaking off.
Around here, hollows come from loose foundation backfill, downspouts and sump lines dumping in one spot, and snowmelt soaking clay-rich soil. A crack with one side dropped is settlement, and it won't stop on its own. See voids under concrete and why concrete settles.
Heavy loads: more than the slab was built for
Most residential driveways are about 4 inches thick, which is enough for passenger vehicles in the 3,000 to 6,000 pound range. Trucks and RVs call for 5 or 6 inches. Going from 4 to 5 inches adds roughly half again to the load a slab can carry, because resistance to bending climbs with the square of thickness (5 × 5 is 25; 4 × 4 is 16).
Where the wheel lands matters too. Mid-panel, concrete on every side shares the load. At an edge or corner, the same wheel stresses the slab far more.
Then there's timing. The base is weakest during spring thaw, when the ground thaws from the top down and traps meltwater above the still-frozen layer. That's why WisDOT posts seasonal weight limits on many roads, on average from about early March to early May. Your driveway has the same problem without the sign. A loaded shingle delivery rolling over a slab corner in April is a classic way to snap one off.
Water that freezes: damage in two places
Water expands about 9 percent when it freezes.
- In the concrete. Water in the pores freezes and pushes outward. Air-entrained concrete, with roughly 5 to 7 percent tiny air bubbles, gives that ice room and is standard for outdoor work here. Water in an existing crack acts like a wedge, prying it wider each cycle. Green Bay averages 68 days a year when the high stays at or below freezing, plus all the spring and fall days that freeze at night and thaw by noon. More in freeze-thaw concrete damage.
- Under the concrete. Wet, fine-grained soil freezes and grows ice layers that lift the slab. Clays and silts, which describes a lot of Brown County, are among the most frost-susceptible soils. A slab lifted unevenly bends, and bending is tension. See frost heave vs. settling.
Rust: the crack that starts inside
Plenty of residential slabs have wire mesh or rebar in them. Steel buried in concrete is normally protected, but chloride from road salt and deicers soaking in over the years can break that protection down. Rust takes up about three times the space of the steel it replaces, according to federal research, and that growth splits the concrete from inside.
What it leaves: a straight crack right over where the steel sits, often with a rust-brown stain, and eventually a chunk popping loose over the bar. Industry guidance calls for at least 2 inches of concrete over the steel to keep salt and water away from it. See concrete spalling causes.
The Cheat Sheet: Force, Timing, and the Crack It Leaves
| Force | When it acts | The crack it usually leaves | Read more |
|---|---|---|---|
| Plastic shrinkage | First hours, on hot, dry, windy pour days | Short, shallow, roughly parallel surface cracks | New concrete cracking |
| Drying shrinkage | First weeks through the first year or so | Thin, flush crack in a joint, or across an oversized panel | New concrete cracking |
| Temperature | Daily and seasonally, for the life of the slab | Joints and cracks that open in winter and close in summer; chipped joint edges where debris blocks movement | Best concrete crack caulk |
| Restraint | Whenever the slab moves against something that won't | Cracks from inside corners, around posts and drains, near the foundation | Preventing concrete cracks |
| Lost support | Usually after wet springs; near the house, downspouts, old trenches | One side lower, rocking slab, broken corners | Voids under concrete |
| Heavy loads | The moment the truck rolls on; worst during spring thaw | Corner breaks, edge cracks, cracks along wheel paths | Driveway crack repair |
| Freezing water in the concrete | Late fall to spring, every freeze-thaw cycle | Crumbling crack edges, surface flaking, curved cracks along joints | Freeze-thaw concrete damage |
| Frost heave | Deep winter, partly reversing in spring | Hump or crack across a lifted panel; cracks that open and close with the seasons | Frost heave vs. settling |
| Rust | Years to decades, faster with salt | Straight cracks over steel, rust stains, popped chunks | Concrete spalling causes |
| Roots | Years, slowly getting worse | Hump near a tree, crack over the root | Tree roots lifting concrete |
One Driveway, Fifteen Years: How the Forces Stack Up
Real cracks rarely have one cause. Here's a made-up but very typical story for a 4" driveway in Howard or Suamico:
- Month 2. A hairline shows up at the bottom of each control joint: shrinkage, right where it belongs. But one panel by the garage was laid out 14 feet long, past the roughly 12-foot limit for a 4" slab, and it cracks across the middle too.
- Year 3. The downspout at the garage corner has dumped on loose backfill since day one. The panel beside it settles half an inch and cracks.
- Year 5, April. A loaded delivery truck backs on while the base is soft from thaw. The street-side corner snaps off.
- Years 6 to 15. Nobody seals the joints or cracks. Every winter, water gets in, freezes, and chips the edges, and salty slush off the car speeds it up.
By year 15, that "cracked driveway" has four forces written on it, each needing a different response: seal the shrinkage cracks, lift and then seal the settled panel, patch the broken corner, and seal the chipped joints while their edges are still solid. That's why we ask what caused a crack before we talk about fixing it.
What You Can and Can't Control
You can't stop drying shrinkage, Wisconsin temperature swings, or concrete's weakness in tension, which is why some cracking is normal on every slab. You can control water reaching the soil underneath, whether joints and cracks stay sealed, which heavy loads go where and when, and how much salt soaks in. Preventing concrete cracks covers each one.
Where we fit: we seal joints and cracks with flexible sealant, lift slabs that lost support with foam, and apply penetrating sealers. We don't replace concrete, patch crumbling edges, or fix foundations, and we'll tell you from the photos if that's what your crack points to.