CONCRETE SLAB CRACKS:Causes, Meaning and What To Do

Slab-on-grade foundations are one of the most common types of foundations used in residential construction. It is a single, continuous concrete slab poured directly on the ground, reinforced with steel and design to support the entire structure above it. They are popular because they are cost-effective, quick to build, energy efficient and well-suited for many climates.

However, while concrete is strong, durable and long-lasting, is not indestructible. Given that the slabs rets directly on soil, it is closely tied to what happens beneath it. Changes in moisture, soil movement and temperature will directly influence the way the slab behaves over time, and cracks might be the first thing you notice when this happens.

Now, cracks are not a sing that concrete has ‘failed’. As a matter of fact, cracking is part of the concrete’s natural life cycle. The questions you need to ask yourself are ‘What kind of crack is this? What might’ve caused it?’

Understanding the behavior of concrete will give you a clearer picture of things. With that in mind, let’s talk about pouring and curing.

When concrete is poured it is a mixture of cement, water and aggregates that hardens through a chemical process called hydration. During curing the concrete loses moisture and shrinks slightly while interna stress begins to form.

Now the shrink is completely normal. If the slab is restrained either by soil friction, reinforcement or nearby structures, the stress needs somewhere to go and more often than not, it release itself in the form of cracks. So cracking at this stage is expected.

Now let’s explore some of the common causes of concrete cracks:1. Shrinkage: Which we’ve already talked about as part of the curing process. As concrete cures and dries, it shrinks. If it can’t move freely, it cracks. The result of this is:

• Hairline cracks

• Spiderweb or map-like patterns

These usually appear within the first weeks of months, and are often cosmetic and will not affect structural performance.

2. Soil Movement: Concrete slabs rest directly on the soil beneath them. When the soil beneath a slab moves, the slab has no choice but to respond. In physics terms, it’s a bit like Newton’s third law: the ground applies force, and the structure reacts. The problem is that concrete doesn’t react evenly. It’s strong under compression, but weak in tension, so instead of bending, it relieves stress by cracking.

Remember that the soil can compress, shrink if it dries out or expand if moistened or even wash away due to poor drainage. And as a result we get:-Wider cracks

-Uneven or offset edges

-Cracks that change over time.

And….. this is where concrete overlaps with foundation movement.

3. Temperature Changes. Like most things on earth when subjected to heat, concrete expands and contracts when cold. Repeated temperature cycles create stress, specially in large slabs. 

• Straight cracks running across slabs

• Cracks near edges or corners

• Cracks that open and close seasonally.

This is exactly why control joints exist. To guide where cracks form, making them essential for durability. Which takes us to:

4. Improper Joint Placement. Or no joints at all, for that matter. Control joints are intentional weak spots designed to manage the inevitable cracking and  tell concrete where to crack, instead of letting them appear randomly. Best practices require them to be cut to a depth of 1/4 of the slab thickness and within 6 to 18 hours of pouring, while spaced between 4 to 12 apart. If the joints are

• Too far apart

• Too shallow

• Missing entirely

then the slab will decide for itself, resulting in random cracking patterns and long straight cracks across open areas.

5. Overloading or Point Loads. Another common cause of cracking is overloading the concrete beyond strength capacity. Heavy loads such as vehicles, large appliances or structural additions can overstress areas not designed to carry that weight and will result in:

• Cracks radiating from load points

• Corner cracking

• Settlement-related fractures.

Now, let’s take a look at the different types of concrete cracks:

Hairline Cracks: are very thin (less than 3mm wide) are typically nothing to worry about. They are usually shrinkage-related and common in new slabs. They are often referred to

as cosmetic in nature and can be sealed and monitored. However wider cracks may require professional inspection to determine cause and appropriate repairs.

Settlement Cracks: Are usually cased by the soil under the concrete settling or compressing over time. Usually they will be wider at the top and narrower at the bottom, kind of in a ‘V’ shape. While smaller settlement cracks pose no danger, larger ones may require structural repairs.

Drying Shrinkage Cracks: As discussed above. Happen naturally in irregular shapes or patterns. Usually superficial. Keeping concrete moist as it cures can minimize them.

Restraint Cracks: Again, as stated above, these happens when concrete is restrained from shrinking freely while it dries. These often appear in straight, regular lines.

Vertical Cracks: are often related to shrinkage or minor settlement. They are usually of less cancer if stable, so monitor them over time.

Diagonal Cracks: are often associated with uneven movement. They are common near corners or openings and may indicate differential movement

Horizontal Cracks: are less common in slabs, more serious in walls and often linked to lateral pressure, usually usually worth professional evaluation

Offset or Uneven Cracks: One side is higher than the other. It indicates movement beneath the slab and is of structural concern.

Freeze-Thaw Damage: this ones are quite pertinent in the DFW area or any other place with cold climates. A freeze-thaw cycle is an environmental process where temperatures fluctuate above and below freezing, causing water trapped in materials to freeze, expand by 9% and melt. This repeated expansion and contraction is what causes damage. In the case of concrete, the water in cracks and pores can create stress and enlarge cracks upon freezing. These cracks tend to be wider at the surface, getting narrower as they go in deeper. Waterproofing and keeping concrete seal can help prevent this damage.

Next time we will explore the different things you can do for both prevention of serious risks derived from cracks in concrete slabs as well as a guide of things you can handle yourself.

Remember that cracks are not automatically bad news, but In the meantime, if you are worried about cracks don’t hesitate to give us call. At iFix  we don’t just look at cracks; we look at what caused them. Soil conditions, moisture, drainage, load paths, and foundation behavior all matter. If you’re unsure about what you’re seeing, a professional assessment can bring clarity and peace of mind before small issues turn into larger ones.