When somebody calls us out to look at concrete that has broken up, they usually want to talk about the concrete. What mix was it, was it thick enough, did they use steel. Those are reasonable questions and once in a while one of them is the answer. But almost every failure we walk up on failed underneath, in the ground the slab was sitting on, and the concrete just reported it.
That is worth saying plainly. The base is the job. The concrete is the finish that goes on top of it.
Why the concrete gets blamed
Because the concrete is what you can see. The base is buried on day one and nobody ever looks at it again, so when the slab cracks and drops years later the visible thing takes the blame.
But a slab on grade is a plate laid on dirt. It does not span anything and it has nothing holding it up but what somebody put under it. Concrete is strong when it is evenly supported and it breaks when it is asked to bridge a gap, because it has almost no strength in bending. So if the ground under one part of it settles or washes out, that section is now spanning, and a slab that is spanning is a slab that is cracking. The mix has nothing to do with it.
Good base and average concrete outlasts great concrete on bad base every time. We have never seen it go the other way.
Cut to grade first
The first real step is excavating to the right depth, and doing it accurately, because everything after this depends on it.
Cut too shallow and there is no room for the base thickness the job needs, so somebody either shorts the base or shorts the concrete, and both of those come back. Cut unevenly and the slab ends up thick in some places and thin in others, and the thin places are where it will crack first regardless of what the plan said.
Cutting to grade also means getting rid of what should not be under a slab. Topsoil and organic material has to come out, because it decomposes and compresses and you cannot compact it to anything reliable. Old asphalt, chunks of a previous driveway, roots, buried debris from a remodel. If it is soft, organic, or unknown, it leaves. A lot of jobs are decided in that first hour, and it is the least visible work on the whole project.
Finding the soft spots and fixing them
Once the cut is open you find out what you are actually dealing with, and you almost always find something.
A utility trench that was backfilled and never properly compacted. A patch of ground that stayed wet. An area that was filled when the tract was graded decades ago and has been slowly compressing ever since. Around the San Jacinto Valley we also find clay that behaves differently across one lot, so the front of a driveway and the back of it are not the same problem.
You can feel it walking the subgrade and see it when a compactor goes over it, because ground that will hold still responds differently than ground that pumps under the plate. Soft spots get dug out and replaced with material that compacts, or stabilized, depending on how big and how deep.
The wrong move, and the common one, is to bridge over it. Put extra base on top of a soft pocket, get a surface that looks flat and reads fine, and pour. The pocket is still there. It compresses under load and the slab loses support right there, and the crack shows up in year three instead of year one, by which point nobody connects it to the day of the dig.
Place base in lifts, and compact every lift
This is the part that separates real base prep from something that looks identical.
Compaction equipment only reaches so deep. A plate compactor or a roller densifies the material near the surface effectively and its influence falls off below that. So if you dump base material to full depth and run a compactor over the top, you get a well compacted upper layer and loose material underneath it that never got touched. It looks and feels solid. It is a shell.
The way it is done is in lifts. Place a controlled depth, compact it, check it, then place the next lift and compact that, so every layer gets worked while it is thin enough for the equipment to reach the bottom of it. It takes longer and it is the difference between base and a pile of gravel that has been driven on.
Moisture matters too. Aggregate compacts best at a certain moisture content and summer material off a dry stockpile is usually well below it. Too dry and the particles never lock together no matter how many passes you make. Too wet and it pumps. That is why you see us watering base before we compact it, which people sometimes find strange to watch.
The material matters as well. A graded aggregate with a mix of particle sizes locks together and holds. Clean uniform rock does not compact into anything, it shifts. Native clay will compact and then swell when it gets wet, which puts you back where you started.
Dumped and raked flat is not base
Here is the reason this happens as often as it does. Once concrete is over it, you cannot tell the difference.
Properly compacted base and material that was dumped, raked flat and left look exactly the same from the surface. They feel the same underfoot the day before the pour, they take the same amount of concrete, and the slab comes out looking the same. The homeowner has no way to know, and skipping it is faster and cheaper, which is why it gets skipped.
Then years three through eight arrive and the two slabs behave completely differently. One stays flat. The other settles in the wheel path, cracks, lets water into the loose material, loses more support, and comes apart.
That is most of what we get called to look at. The concrete on those slabs is usually fine. We pull out chunks that are perfectly sound sitting on ground that was never prepared.
What you can do about it as a homeowner
You cannot inspect base after the fact, so the only real leverage is before the pour.
- Ask what depth they are cutting to and what base material is going in and how deep. A contractor who does it will answer immediately.
- Ask whether base goes in lifts and how many passes of compaction. Same test.
- Go look at the hole before the concrete is ordered. That is the one day the base is visible.
- Ask what happens if they find a soft spot, because the honest answer involves digging it out, not adding gravel over it.
Any of those questions being brushed off tells you what the plan is. The subgrade is the only part of the job you get to see, and it is the part that decides how long the slab lasts.
Why we spend the money down there
Base prep is unglamorous and nobody compliments you on it. It is dirt work, it takes a day or two that produces nothing the customer can photograph, and it costs real money in excavation, hauling, material and machine time before a yard of concrete gets ordered.
We do it anyway, because the alternative is pouring a nice looking slab onto ground we know will not hold it, and we would be the ones coming back. Fourth generation in this trade means we have seen what the shortcut looks like at year six. If you want somebody to walk the dirt with you before anything gets poured, call (951) 533-8821.