Common Causes of Cracks in Fresh Concrete and When They Matter
Concrete might look solid and unshakable once it’s hardened, but in its early stages, it’s a lot more fragile than most people realize. Before it fully sets, fresh or “plastic” concrete is loaded with water and hasn’t yet built up its strength. That’s why you sometimes see cracks forming on brand-new surfaces, even before the concrete has a chance to settle into its long-term role. The tricky part is figuring out which cracks are harmless blemishes and which ones signal bigger problems down the road.
Here you’re going to read about the most common types of cracks in fresh concrete, why they happen, and when you should be concerned.
Plastic Shrinkage Cracks
One of the most common types of early cracks is called plastic shrinkage cracking. It sounds technical, but the idea is simple: concrete loses water as it sets, and if that water evaporates faster than it should, the surface pulls apart and small cracks appear. Picture a mud puddle drying out on a hot day; it starts to curl and split in different directions. Concrete works the same way.
These cracks usually look like fine, shallow lines across the surface, often in a random spiderweb pattern. While they might not go very deep, they can still leave the surface weaker than it should be. Over time, if those hairline openings allow water or chemicals to seep in, they can shorten the life of the concrete. The good news is, they can often be prevented with simple steps like proper curing, covering the surface, or slowing down evaporation on hot, windy days.
So, do they matter?
Not always.
If you’re only dealing with small, shallow cracks, it’s usually more of a cosmetic issue than a structural one. But if you notice them spreading or deepening, that’s when it’s worth keeping an eye out or calling an expert who knows concrete inside and out.
Plastic Settlement Cracks
Settlement cracks are another type you’ll often see in fresh concrete, but they happen for a slightly different reason. After pouring, the heavier materials like gravel or even the reinforcing steel inside can sink a little while the concrete is still soft. As everything shifts, the surface gets pulled apart, leaving long cracks that often show up directly above steel bars or along areas where the slab is thicker in some spots and thinner in others.
These cracks are more than just surface-level marks. If they run deep enough, they can expose the steel reinforcement to moisture, which is never a good thing. Once water finds its way to steel, rust and corrosion start creeping in, and that can lead to serious structural problems down the road.
What’s important here is paying attention to the location and direction of the cracks. A random hairline mark across the surface may be no big deal, but a straight crack lining up with the reinforcement pattern is worth a closer look. Sometimes sealing the surface quickly can protect against long-term damage, but in other cases, you may need a professional’s advice.
Thermal Cracking
Concrete goes through a natural chemical reaction as it hardens, and part of that process releases heat. In small slabs, you may never notice this, but in larger pours, like foundations, thick walls, or big floor slabs, that heat builds up. As the concrete eventually cools, it shrinks. If different parts of the slab cool at different rates, the stress can cause cracks to appear.
Think about a sidewalk on a freezing winter morning after a warm day. The sudden temperature shift makes it expand and contract, and the same thing is happening inside fresh concrete. The bigger the structure, the more uneven the temperature change can be, and the greater the chance of cracking.
Do these cracks always spell trouble?
Not necessarily.
Many are surface-level and can be controlled with good design, like adding joints in the right places to guide where the cracks will form. But if you’re looking at wide or deep thermal cracks, they can open the door to water damage, freeze-thaw cycles, and long-term weakening of the concrete. Catching them early makes all the difference.
Construction or Restraint Cracks
Sometimes cracks appear not because of the weather or evaporation, but because the concrete simply can’t move the way it needs to while it’s setting. This is what happens with restraint cracks. Picture concrete poured against an existing wall or tightly boxed in by a foundation form. As it tries to shrink during curing, those edges hold it in place. With nowhere else to go, the stress breaks out as cracks.
Restraint cracks usually show up at connections, where new concrete meets old, or where there are sudden changes in shape and thickness. While they might start small, these cracks can widen with time, especially if the structure keeps moving or settling. Unlike plastic shrinkage cracks, these aren’t just cosmetic or surface-level cracks. If they’re left alone, they can turn into pathways for water, salt, and other damaging elements to creep in.
That’s why restraint cracks deserve some attention. Good reinforcement, joint placement, and smart design can help prevent them, but once they’re there, it’s a good idea to seal them and keep monitoring for any signs of growth.
Drying and Early-Age Shrinkage Cracks
Even after the initial set, concrete is still losing moisture as it hardens. This longer-term process is called drying shrinkage, and it can create cracks if the concrete wasn’t cured properly in those early days. While plastic shrinkage cracks happen fast, drying shrinkage cracks take a little more time to show up.
You’ll often spot these as wider cracks that run across larger sections of a slab, sometimes straight and sometimes meandering. Left untreated, they can affect both the look and durability of the surface. And if they’re wide enough, they can cause trip hazards or allow water infiltration, both of which can shorten the life of the concrete.
Prevention is key here. Proper curing methods, like keeping the surface moist or using curing compounds help concrete hold on to its water long enough to gain full strength. If the curing process was rushed or skipped, though, these shrinkage cracks are a common outcome.
When Do Cracks in Fresh Concrete Really Matter?
Here’s the part that often confuses homeowners and even some contractors: not every crack in fresh concrete is a disaster. In fact, most new slabs will show some cracking at some point, no matter how carefully they’re poured. The question is whether those cracks are just surface blemishes or if they signal something deeper.
Hairline cracks from plastic shrinkage usually don’t matter much beyond appearance, especially if they’re shallow and not spreading. On the other hand, settlement cracks that trace along reinforcement, restraint cracks at joints, or thermal cracks that grow wider over time can compromise the strength and durability of the concrete. And that’s when it’s time to take action.
As a general rule, cracks that keep widening, allow water to seep through, or line up with reinforcement patterns deserve professional inspection. It’s always cheaper and easier to deal with them early than to wait until they’ve caused lasting damage.
Conclusion
Fresh concrete is a balancing act; it needs approximately 28 days, proper moisture, and the right conditions to reach its full potential. Cracks can and do happen, but understanding the different causes helps take the guesswork out of whether they’re harmless or a red flag. Some are just part of the natural curing process, while others point to deeper problems that could shorten the life of your project.
If you’re ever unsure, don’t ignore what you see. Keep an eye on how cracks develop, and when in doubt, talk to a professional. After all, concrete is supposed to be a foundation that lasts for decades. A little care and attention early on can make sure it does exactly that.
