Why does concrete crack? The honest answer.

Three causes. Three different fixes.

When concrete cracks early — within the first year — it's almost always one of three things. Knowing which is which is the difference between a callback and a clean job.

1. Plastic shrinkage cracking (the preventable one)

This happens in the first couple of hours after placement, before the concrete has set. It's caused by surface moisture evaporating faster than bleed water can replace it. The surface tries to shrink, the slab below is still plastic, and you get short, random surface cracks — usually 1 to 3 inches long, parallel to each other, scattered across the slab.

You see this on hot days. You see it on windy days. You see it on dry days. Most of all, you see it on days that are all three. ACI 305R draws the line at 0.15 lb/ft²/hr evaporation rate — above that, plastic shrinkage cracking gets likely. With Type 1L blended cement, which has less bleed water than the old Type 1 we all grew up with, the threshold matters even more.

How to prevent it: Check evaporation rate before the pour. If it's too high, do one or more of: shift the pour to morning, set up windbreaks, fog the subgrade and forms, use an evaporation retarder during finishing, or push the pour to a different day. None of those work if you don't know the evaporation rate before you place the concrete.

2. Drying shrinkage cracking (the unavoidable one — but you control how much)

As concrete loses moisture over weeks and months, it shrinks. Roughly 1/16 of an inch per 10 feet for typical mixes. That shrinkage has to go somewhere. If you don't give it a planned place to crack — a control joint — it'll find its own place. That's the random crack running diagonally across your customer's patio.

How to control it: Cut control joints at 1/4 the slab depth, spaced no more than 24-30 times the slab thickness apart (so a 4-inch slab gets joints every 8-10 feet). Cut them within the first 6-12 hours — early enough that the slab hasn't already cracked on its own, late enough that the saw doesn't ravel the edge. Curing properly also reduces total shrinkage.

3. Restraint cracking (the design problem)

Concrete shrinks. If something stops it from shrinking — a column, a wall it's tied into, rebar that's too close to the surface, a footing that's holding the slab in place — the tension has to relieve somewhere, and concrete is weak in tension. So it cracks.

This is mostly a design and detailing problem. Isolation joints around columns, slip sheets where slabs meet walls, properly placed rebar with adequate cover. As a contractor in the field, you fix this by following the drawings carefully and flagging anything that looks like a restraint problem before you place. After the pour, you're mostly stuck with whatever cracks the design earned.

The crack that's easiest to prevent

Plastic shrinkage cracking is the only one of the three you can stop with a phone in your hand at 5am. Drying shrinkage requires good jointing — a finishing decision. Restraint cracking is mostly designed in or out. But plastic shrinkage cracking comes down to whether anyone checked the evaporation rate before the trucks rolled.

That's why we built PourDay. It's not the whole answer to cracking — but it's the answer to the one type of cracking you can actually decide on the morning of the pour.

What about "all concrete cracks"?

It's true and it's also a cop-out. Yes, every slab will eventually develop hairline cracks at control joints — that's by design. What customers complain about isn't that. They complain about random cracks crossing the slab in unplanned places, or surface crazing that looks like dried mud, or cracks that show up in the first month. Those almost always trace back to either bad jointing or bad pour conditions, both of which you control.