If you’ve ever gone down into your basement or looked at your garage floor to notice some cracks spreading across the pristine concrete flooring, panic might be the natural reaction. Fortunately, many cracks, even cracks in a new concrete slab, are harmless. Even the ones that do require attention can usually be put right. Even armed with the knowledge that cracked cement isn’t always a disaster, you’ll probably find yourself wondering “why does concrete crack?”.
Understanding Concrete Cracks
Before we can set about answering the question “why does concrete crack?” we need a bit of grounding in the mechanics.
Concrete is an aggregate of sand, cement and water. It is, as you no doubt know, pretty durable, great at supporting heavy weights and has an almost unrivalled longevity. It’s easy to see why it’s so ubiquitous in basements, garages and other building tasks. Of course, while concrete certainly has a lot to recommend it, it’s not invulnerable.
Cracks in even new concrete slabs are not uncommon, but why does concrete crack? Well, the answer depends on what kind of cracks are forming. There are a few different descriptions which could apply here:
- Hairline
- Settlement
- Shrinkage
- Structural
- Heaving
- Hydrostatic Pressure
Each of these is caused by different conditions and not all of them are necessarily a cause for concern. The general rule is that if a crack is smaller than 1/8th of an inch wide, dry and doesn’t seem to be spreading, it can be safely ignored. If it’s bigger, wet or growing, you’ll need to investigate why.
Identifying Types Of Cracks In Concrete
Different types of cracks can normally be identified with a visual inspection. You’re looking for the following features when trying to work out which you’re facing:
- Hairline Cracks: Thin, spidery cracks across the surface of your concrete slab or walls. They’re particularly common in new build homes.
- Settlement Cracks: Wider than a hairline crack, but not by much, they generally run in roughly straight lines.
- Shrinkage :Wavy lines or even the appearance of crazing are most likely caused by shrinkage.
- Structural: If chunks appear to be missing or thick cracks are spreading through the concrete, this is a structural problem and needs addressing as a matter of priority.
- Heaving: If your cracks are raised, appearing to be pushed up from below, then they’re called ‘heaving’ cracks.
What Causes Cracks In Concrete
As we’ve alluded to above, there are different answers to “why does concrete crack” depending on what kind of cracked cement you’re facing. The most common causes are:
Shrinkage
Shrinkage cracks are caused by the natural process of concrete setting and drying. While they’re not always something to be worried about, they can suggest that there was a poor ratio of water to cement in the concrete mix. As the water evaporates, the volume of concrete reduces, so if the ratio is off, it can cause cracking in new concrete slabs.
Hydrostatic Pressure
Hydrostatic pressure occurs when water accumulates in the soil surrounding a concrete structure, such as a basement or foundation. This pressure builds up due to groundwater or poor drainage systems, creating a force that pushes against the concrete walls and floor. When the pressure becomes too great, it can cause the concrete to crack.
These cracks can start as hairline fractures but may expand over time, allowing more water to seep through and further increasing the pressure. Hydrostatic pressure is particularly concerning in areas with high water tables or during periods of heavy rainfall. Proper drainage systems, such as French drains or sump pumps, can help alleviate this pressure and prevent cracks caused by hydrostatic forces.
Thermal
Concrete is hardy stuff, able to withstand really pretty high temperatures (far higher than most homes will ever be subjected to). That being said, it is vulnerable to cracking when exposed to fluctuations in temperature. As it heats up it expands. As it cools, it shrinks. Over time, this movement may lead to cracked cement.
Structural
While the loadbearing qualities of properly mixed concrete are certainly impressive, they’re not limitless. Basement slab cracks can be caused by the sheer weight of your home pressing down causing stress on the concrete. Likewise, shifting weight caused by your house ‘settling’ is a common cause of cracks in new concrete slabs.
Alternatively, structural cracks might be caused by an external pressure being applied to your concrete. If the earth on which it rests is moved, this can often cause heaving cracks
Chemical Reactions
Concrete is so strong thanks to the chemical reactions which take place when its components are mixed together. Occasionally, contaminates or the wrong choice of admixtures can derail this process and you’re left facing an alkali-silica reaction (or ASR). This is where some contaminate in the aggregate reacts with the alkaline cement to produce an expansive gel after the concrete has set. As the gel expands, it pushes the concrete apart.
Environmental Factors
Sometimes, a cracked concrete slab isn’t really to do with the concrete at all. Environmental factors are a common cause of cracks. When exposed to repeated freeze/thaw cycles, moisture will eventually turn microscopic cracks into larger ones as water expands when frozen. As the cycle repeats, the cracks grow larger. This can start off with a very small amount of moisture and lead to some pretty serious issues down the line.
Impact of Concrete Cracks on Basements
Now we know why concrete cracks, what are the implications on our basements when it does happen? Well, they range from the minor annoyances of aesthetics to the serious issue of potential structural failure.
The biggest potential issues include:
Water Intrusion
Water intrusion is probably the scariest phrase in the homeowner’s lexicon. When water finds its way in you could easily find yourself facing problems ranging from mold and mildew all the way up to structural failure.
The soil around your home always contains a certain level of moisture, but it’s usually held at bay by your basement walls. When cracks appear in your basement slab, they provide an easy route through your walls and floors.
Foundation Health
While not always a cause for immediate concern, cracks in a basement slab can easily spiral out of control and threaten serious structural damage if not addressed in a timely manner. The foundations of your home bear the weight of your house and larger cracks can impair their ability to provide structural support.
If left unattended, this can lead to subsidence or even, in particularly extreme cases, collapse. It’s always recommended to have any larger cracks properly investigated by a professional as soon as they’re noticed.
Preventing Concrete Cracks
While there’s not much you as a homeowner can do to avoid cracked cement, there are steps that any builder should be taking to avoid them appearing in the first place:
- Proper mixing and curing: Ensuring that concrete is properly mixed, using the correct ratios is the most important step. It’s also worth considering whether curing techniques such as ponding, wet covering or curing with blankets fits the job.
- Suitable reinforcement: Particularly on larger slabs, the addition of fibers to the mix or steel reinforcement bars can boost the structural stability of a slab pour.
- Construction best practices: Ensuring the underlying foundations have been properly put in place and the locations of any joints have been carefully considered can reduce the risks of cracking.
- Environmental Controls: Using sealants and protective coatings can avoid contamination and regulating the temperature of the site may reduce the risk of thermal cracking.
Repairing Cracked Concrete
Now that we have a basic grasp on why concrete cracks, it’s time to discuss what can be done about it when it does happen.
Depending on the severity of the cracks, you may be able to ignore or remedy the problem yourself. Smaller cracks, under 1/8th of an inch wide that are dry can be filled, either with an epoxy injection or a thin layer of concrete. Larger, non-structural, ones will have to be drilled out but the process is otherwise the same. You can usually source the materials to do this from your local hardware store. If you want a really neat finish, it’s worth brining in the professionals, but it’s a task that most keen DIYers can successfully achieve.
In the case of more serious problems, it really is a matter for the pros. Depending on the underlying causes of your cracked slab, they may recommend a few different solutions. It’s important to tackle the cause as well as the symptom and this may involve installing braces or removing and replacing your cracked slab with an entirely new one.
Maintenance Tips
Of course, prevention is always better than a cure and a good maintenance routine is the best way of preventing cracks forming in the first place.
The first and most important step towards avoiding cracked cement is keeping an eye on your concrete slab. Regular visual inspections will warn you about problems which may be forming before they become serious.
Keeping a steady temperature in your basement and installing a dehumidifier or other waterproofing measures will also help. By avoiding moisture and swings in temperature, you’re giving yourself the best chance of avoiding problems down the line.
While it’s only natural to wonder why does concrete crack when faced with spreading lines across your basement floor, it’s not always a cause for concern. Keeping a close eye on it and knowing when to act can avert disaster.