Things You Should Know about Cracks in Slab Foundation

Concrete slabs are essential to the structural stability and design of modern buildings. A single, thick concrete slab is often used as the foundation for large buildings – called “Slab-on-Grade Foundation” – resting on compacted subsoil and ensuring the building stays level.

In terms of function, concrete slabs act as both the foundation for the ground floor, as well as the upper slabs that may act as ceilings. Concrete slabs are crucial and serve multiple purposes. Besides providing a flat surface, they also bear the load of the structure above them and act as sound insulators for heat and fire. In fact, the gaps between the slabs often provide for insulated and safe spaces for building facilities like electrical and plumbing systems.

Cracks in Slab Foundation

The slab is poured such that it is thicker at the edges, with reinforcing rods used to strengthen the extremities and retain its structural integrity. Depending on the weather and environment, there may also be a layer of crushed gravel underneath the slab to drain excess water, which if not properly attended to can cause cracking. Inserting a wire mesh in the slab at the time of pouring reduces the likelihood of cracking.

Types of cracks in slab foundation:

  • Hairline Cracks
  • Plastic Shrinkage
  • Pop-Outs
  • Crazing
  • Scaling
  • Spalling
  • D-cracking
  • Offset Cracking

Concrete is used in foundations because of its durability and strength, but it is not a fool-proof material. Factors ranging from the type of soil to the weather conditions can cause dangerous cracks to appear in foundational slabs.

Broadly speaking, cracks are classified as either active or dormant. Active cracks change over time, widening and moving in various directions, whereas dormant cracks stay the same. The danger in both cases is that cracks can channel moisture and cause damage, requiring increasing amounts of repair the longer they go unchecked. The danger posed by a crack depends on its direction, width, and depth. Further, the risk of cracking varies between cured, uncured and reinforced concrete. Below are some specific types of cracks that are found in slab foundations.

1) Hairline Cracks

These are extremely thin but possibly deep cracks. They are primarily caused by the concrete settling while it cures. If a hairline crack is deep, it can lead to wider, more severe cracking inside the slab over time.

2) Plastic Shrinkage

These cracks also occur when the concrete is cured incorrectly, i.e., if the surface dries much faster than the inner layers of the slab. The cracks typically reach halfway through the concrete, are quite short and seem to occur randomly across the surface.

3) Pop-Outs

These are depressions in the surface of the slab. They occur when the aggregate (the material originally mixed in with the cement, like sand) from a portion of the slab’s surface is absorbent enough to expand and ‘pop out’ of the surface of the concrete, leaving a gap behind.

4) Crazing

Another surface phenomenon caused by uneven drying during curing, but much shallower than other such cracks, so the damage is not very serious.

5) Scaling

Over time and with poor protection from water (delamination), the concrete absorbs the water and is forced to expand when temperatures drop below freezing. Alternatively, air pockets trapped near the surface can also cause this expansion. Pieces of the surface crack and are pushed out, causing small, shallow blisters that riddle the concrete surface.

6) Spalling

Yet another kind of surface depression, but larger and deeper than scaling. These can be linear when occurring along a rebar and are usually caused by poor joint construction or rusting rebars inside the concrete. Rust is expansive and can thus create pressure that causes damage to the slab. The presence of moisture exacerbates the corrosion, and this worsens if the spalling is severe enough to expose the metal.

7) D-cracking or Durability Cracking

These cracks take several years to form and occur well beneath the surface. The repeated freezing and thawing of moisture inside the foundation gradually wears on the aggregate and crumbles the concrete, making it quite vulnerable once the cracks are visible on the surface.

8) Offset Cracking

Offset cracks cause a difference of height in the concrete on either side of the crack. In most cases, this is due to an uneven seat for the slab itself, such as poorly compacted subsoil, invasive pressure from tree roots, previous concrete slabs that have not been removed, or repeated expansions and contractions in the rebar.

What Causes Cracks in a Concrete Slab Foundation?

While many causes for foundation cracking have been illustrated above, it is useful to know the common weather and environmental phenomena that result in such damage. This can help you head off cracks in the foundation by knowing what to expect in such circumstances.

1) Exposure to the Elements and Weathering

Most commonly, cracks in the foundation are out of your control and can be attributed almost wholly to sudden and extreme changes in weather.

A) Snow

Accumulated snow near the building can melt and create a sudden flood. If the foundation already contains cracks, even thin ones, the water will enter and widen them in time.

B) Drought

A reduction in the moisture of the subsoil can cause it to shrink and move further away from the foundation. This creates a gap between the soil and the foundation, leading to possibly dangerous slab movement if the foundation is not adequately supported by other means.

C) Expansive Soil

Some soils have a large capacity to absorb moisture and are heavily affected by it. High moisture could expand the soil below the foundation and heave the concrete slab. The damage can vary, and with evenly compacted soil, it could even be negligible.

D) Consolidating Soil

This phenomenon is the opposite of expansion in soil. While the result is similar to that of a drought – i.e., that the soil cleaves away from the foundation – the cause is a property of the soil itself, and can therefore occur even with normal weather patterns.

E) Storms and Heavy Rain

Flooding after by a storm can suddenly increase the moisture content of the soil and expand it, forcing the subsoil up against the foundation and causing it to become uneven or crack.

E) Tree roots

When tree roots extend under or around the foundation, they soak up moisture from the soil, causing it to shrink away from the slab or in some way alter the stability of the foundation.

2) Faults in Construction

Negligence on behalf of the constructors can also cause crack aand land your home in trouble.

A) Plumbing Leaks

Leaks in the house that make their way to the foundation tend to worsen any expansive soil problem, because this supplies more water to the soil, which in turn expands upwards.

B) Poor Construction

This category includes any use of substandard incompatible materials or a badly planned process of pouring out the slab foundation. Mixed brands or strengths of cement, incorrect cement-aggregate ratios, uneven curing or reinforcement of the concrete, all contribute to the appearance of cracks.

C) Poor Soil Preparation

The soil upon which the foundation slab is to be placed must be compacted as much as possible. In case the soil is itself unsuitable, crushed rock or gravel should be used to stabilize the foundation and ensure that moisture doesn’t cause any of the soil-related problems listed above. If this is not done, or if the soil is not properly compacted, the slab will likely become unstable over time.

D) Bad Drainage

Leaks or poorly planned drains can cause the slab and subsoil to come into contact with excess water. Regularly cleaning the gutters and ensuring that waste water is led far from the building are simple preventive measures to keep the slab and soil away from too much moisture.

Fixing a Cracked Slab Foundation

It is possible to repair a few small or shallow cracks in the foundation by yourself, but only after successfully identifying it. As discussed above, seemingly thin cracks can be indicative of a larger problem, and should then be left to professionals. Further, if the slab has a large number of small cracks or cracks wider than 1/4 inch, it would have to be inspected by a structural engineer. For extreme cases, it may be necessary to seek the advice of a geotechnical engineer, in order to tackle more fundamental problems.

It is important to remember that although concrete is primarily rigid, it does have some capacity to accommodate tension and flexibility, as it naturally expands and contracts according to the surrounding temperature. Hence, when repairing a crack, it’s best to use a similarly strong but flexible material.

Repairing a Minor Crack Right at Home

Repairing Minor Crack

Step 1

Clear the area of any loose chips. For small cracks, this can be done with a steel brush or any hard-edged tool. For wider cracks, you may have to use power tools or a large chisel to properly chip away loose material.

Step 2

Mixing the dry patch powder with latex instead of water gives it the necessary elasticity mentioned above. Such products tend to be fast-drying, so it’s best to mix small amounts at a time. If you don’t have material for a vinyl concrete patch, mix cement and sand in a 1:3 ratio, and add a concrete bonding agent until you have a smooth semi-solid mix. Add more bonding agent to small amounts of the mix as you apply it to the crack.

Step 3

Dampen the crack with water before applying the patch. This allows the patch to retain its moisture (and thus set properly), instead of having it soaked up by the surrounding concrete. If it loses its moisture too quickly, the patch may itself crack, being unable to bond properly with the concrete.

Make sure to clean your tools as soon as possible to avoid the patching compound drying on them.

Levelling and Major Repairs

1) How can an unbalanced foundation be leveled?


The main methods for fixing sunken foundations are slab-jacking and piering (also known as hydraulic jacking).

In slab-jacking, grout (a coarse mix of gravel, cement, and lime, for large scale use) is pumped under the slab through strategically made holes, to lift the foundation and restore it to its original or intended elevation. In piering, steel posts are inserted into the unstable soil for reinforcement, and hydraulic jacks are used to stabilize concrete slabs if the subsoil has caused movement.

Slab-jacking is better suited to leveling out smaller slabs of poured concrete because the placement of the holes is more likely to be accurate. Plus, the lime content in the grout will have a stabilizing effect on the subsoil around it. Piering or hydraulic jacking is a more expensive but sure-fire method of dealing with larger slabs, because the beams and footing are used independent of the soil.

2) What is the best repair method for a cracked concrete foundation?

A low-pressure injection using either epoxy or polyurethane resin is the ideal method. The important thing in both temporary and permanent solutions is to prevent more moisture getting into the crack. The injection procedure outlined above will fill the crack from end to end, thus completely sealing the crack. Going further, using polyurethane foam will help you fill any gaps beneath the surface.

Costs and Coverage

1) How much does it cost to level a concrete foundation?

There are several variables, so ultimately the best people to ask is usually the company you are planning to hire. The factors which affect the cost of repair are many and varied, but some things you will inevitably have to pay for are a structural engineer, a geotechnical (soil) engineer, and a building permit. Other costs depend on your location and details of the problem, such as seismic work (if your area is earthquake-prone), subterranean obstacles like tree roots or damaged footings. The number of holes you need drilled (for slab-jacking), or the number of piers you will need for the job (depending on the size of the building).

2) Does my Homeowner’s Insurance include foundation repair?

That largely depends on the cause of the problem. Foundation problems brought about by flooding due to household plumbing accidents usually fall under coverage. Read your policy or contact your provider to be certain.

Signs of Other Foundation Flaws

While there is always likely to be a harmless (negligible) amount of unevenness to the foundation slab, it is worth watching out for some tell-tale signs of instability. Inside the house, look for doors that used to close properly, but now get jammed, or windows that have a similar problem. Check if there is a gap between the door and frame at one end but not at the other. These could indicate an uneven foundation. If you have foundation walls or piers, check to see if they are perfectly straight and not bowed.

Home Warranties and Cracked Slabs

Usually, after cracks appear in your foundation, you begin to wonder if it is covered under home warranties. Home Warranties offer several services in their contracts. Home builders, however, provide the assurance for a year after purchase of a new home. Normally, it is builders’ warranties that cover foundation, bad wiring, plumbing and structural issues. But, in case of keeping your home appliances and systems in working condition, you must have a home warranty.
Do take a look at the reviews on home warranty companies to determine which home warranty fits the bill.

If you have any doubts, simply send us your questions in the comments section. We’re all ears to helping you figure out the best home warranty for you. Do take a look at those companies that have gone out of business to ensure that you sign up for the right one.

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