What sits under a concrete slab matters more than the concrete on top of it. A slab is only as steady as the ground beneath it, so a good pour starts with firm subgrade, a compacted aggregate base, and a plan for where the water goes. Minnesota Valley Concrete builds every flatwork job from the ground up, and the surface is the last thing you see.
Why the ground decides how a slab ages
Concrete is strong when you press on it and weak when it is asked to bend. A slab that sits on even, firm ground is pressed evenly and stays in one piece. A slab that sits on a soft spot is asked to bridge it, and concrete bridges poorly, so it cracks. Most of the cracking and sinking homeowners notice years later was decided before the truck arrived.
That is why two driveways poured from the same mix can look nothing alike a few winters on. The mix was not the difference. The ground was.
The layers under a slab, from the bottom up
A slab that lasts is a stack of layers, and each one has a job. Here is the stack we build on a typical driveway, patio or walk in the southwest Twin Cities metro.
Subgrade: the native ground
This is the soil the whole job rests on. Before anything is placed, the crew removes topsoil, roots and anything organic, because those rot and settle. Any soft or wet spot found at this stage gets dug out and corrected now. It is far cheaper to fix on the day than after the concrete has set.
Compacted aggregate base
On top of the subgrade goes a layer of crushed aggregate, compacted so it will not settle. It gives the slab a uniform bed, so every part of the slab is supported the same way, and it drains freely so water does not sit under the concrete. In a climate where the ground freezes, free drainage under the slab is the thing that most protects it.
The forms and the pitch
The forms set the width, the thickness and the slope. A flat slab holds water, and standing water finds every crack. We set the pitch so water leaves the slab and does not run toward the house.
The concrete and its joints
Concrete cracks as it cures and as it moves with the seasons. Control joints are cuts that tell it where to crack, so the cracks run in straight lines we planned instead of wandering across the surface.
What happens when the layers are skipped
A slab poured straight onto unprepared soil can look perfect on the day. The trouble shows up later, and it shows up in a few familiar ways:
- A dropped corner or a sunken panel, where soft ground settled under part of the slab.
- Cracks that run the full width, where the slab flexed over ground that was not even.
- A slab that heaves in winter, where water collected under it and froze.
- Water pooling on the surface, where the pitch was never set properly.
None of these is a surface problem, and none of them is fixed by sealing or patching the top. They are ground problems, which is why a quote that talks only about the surface tells you very little.
Reading an existing slab for clues about its base
If you already have a slab that is giving you trouble, it tells you a lot about what is under it. Walk it slowly and notice where the damage is, because the location points to the cause. A single low corner suggests a local soft spot. Cracks that repeat down the whole length suggest the support changed along the entire run. Water that lingers after rain suggests the pitch was never right. Take photos from a few angles before you call anyone, since a contractor can learn a great deal from them before the first visit.
Questions to ask any contractor about the base
You cannot see the base once the concrete is down, so you have to ask about it before. A contractor who builds it properly will be glad to explain it. Ask these before you sign anything:
- What is going under the slab, and how will it be compacted?
- What will you do if you find a soft spot when you dig?
- How is the slab being pitched, and where will the water go?
- Where will the control joints be cut?
- If an old slab is coming out, is it being removed completely?
Our guide to choosing a concrete contractor in the Twin Cities covers the rest of what to compare between bids.
The same rule applies to walls and foundations
Flatwork is not the only job that depends on what is underneath. A poured wall or foundation carries far more load than a driveway, and it is only as good as the footing and the ground below it. The principle is the same: prepare the ground first, and the concrete has a chance to do its job. Our poured walls and foundations page explains how we approach those jobs.
Plan the ground work into your pour season
Our pour season runs from March to mid October. The winter months are a good time to ask about the base, agree on the layout and get on the calendar for spring, so the ground work is settled before the crew arrives and not argued over on the day.
Frequently Asked Questions
What goes under a concrete driveway?
A concrete driveway sits on firm subgrade, with a compacted aggregate base on top of it. The base gives the slab even support and lets water drain away from underneath. The forms then set the width, thickness and pitch before any concrete is placed, so water leaves the slab.
Why does a concrete slab crack or sink?
Most cracking and sinking comes from the ground, not the concrete. Soft or uneven subgrade, a base that was never compacted, or water collecting under the slab lets part of it move. Concrete bends poorly, so it cracks where the support under it changes.
Can I pour new concrete over an old slab?
We do not recommend it. A pour over an old slab carries its cracks, movement and drainage problems into the new surface, and it hides the ground. Removing the old slab lets the crew correct the subgrade and build a compacted base before new concrete goes in.
What is a control joint in concrete?
A control joint is a cut made in the slab so that it cracks along a planned straight line. Concrete cracks as it cures and moves with the seasons, so the joints guide where that happens. Without them, the cracks run wherever the slab is weakest.
What should I ask a contractor about the base?
Ask what goes under the slab and how it is compacted, what happens if a soft spot turns up, how the slab will be pitched for drainage, and where the control joints go. A contractor who builds the base properly can explain each answer plainly.
Minnesota Valley Concrete pours driveways, patios, walks and walls across the southwest Twin Cities metro, and the ground work comes first on every one. You can see how we build a driveway on our concrete driveways page. If you want a straight answer about what your project needs under it, request a free estimate and we will look at your site.