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Home /  Home Maintenance / What You Should Know about Cracks in Slab Foundation
last_updated_date Last updated: September 11, 2020

What You Should Know about Cracks in Slab Foundation

PUBLISHED: September 15, 2017 6 MINS READ

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. Read this post to know how to fix cracks in slab foundation

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. This article will give you answers on how to fix cracks in cement slab foundation.

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

Is a Crack in a Slab Foundation Bad? Can this be fixed?

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.

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Common Signs of Foundation Problems

  • Foundation cracks, wall/floor cracks and other forms of fractures
  • Foundation settling or sinking
  • Foundation upheaval
  • Doors that stick or don’t open and close properly
  • Gaps around window frames or exterior doors
  • Sagging or uneven floors
  • A damp crawl space in a pier and beam house
  • Counters and cabinets separating from the wall

Warning Signs of Foundation Problems

1. Exterior Cracks

There could be small cracks on the exterior walls or on the steps which are nothing that you should make you worry. While if you notice large and zig-zag pattern cracks then there is something definitely wrong with your foundation. Brick cracks or bricks that protrude should be checked from time to time.

2. Interior Sheetrock Cracks

You should inspect your home and check whether there are interior sheetrock cracks that are zig-zagging and end at the top of the wall. Ensure that you call a contractor if the wallpaper is pulling away from the wall and if there are cracks at the junction of the ceiling and the wall.

3. Doors Out of Square and Uneven Floors

There are only a few homes that are plumb after the settling but you should look out for doors out of square and cracks in the wall above it should be looked into. There are lots of owners that have spilt water on uneven floors only to see the water roll in one direction. But, you should only be worried if the gradient of the floor is more than one or two inches for every 15 feet.

4. Door Frame/Window Frame Separation from Brick

Another thing that you should look out for is whether the window or door frame pulls away from the brick wall as this is a sign of a trouble in the foundation.

5. Rotten Wood – Pier & Beams

A bad foundation could be seen in the rotten wood of piers and beams which is never a good sign. When the humidity of the basement or crawlspace is too high, a flood has taken place or beams are attacked by termites then it is never a good sign.

6. Bouncing floors – Rotten Wood

You should know that your foundation is in trouble if there are bouncing floors caused by rotten wood. In case the floors resemble trampolines then you should get them professionally checked.

7. Tile Cracks

Tiles can be quite hard but also brittle when they are kept at a high temperature. There are several things that can cause tiles to crack but too many cracks in the bathroom, kitchen or other places could mean that there is a crack in slab foundation problem.

8. Expansion Joint Separation

When there is expansion joints installed in structures to compensate for movement caused by shifting of the earth, humidity, temperature, events and wind. A failing foundation could cause expansion joint separation that causes joint fails. This causes widespread caracking and this could cause the foundation to fail.

9. Nails Pop Out of Drywall

Even though the sight of the nails popping out of drywall could be harmless and easily fixed. In case it is widespread then you should call a professional in.

10. Walls Pull Away From the House

Usually, a building contractor should be contacted immediately if you notice that the exterior wall is actually pulling away from the house. In case the wall is not fixed then the wall would collapse.

Is My House Sinking or Settling?

If the soil underneath the foundation starts to shift then there are parts of the foundation that could sink into the ground. This leads to the foundation tilting and no longer being flat against the ground. In time, cracks would appear around areas that have been affected by the shift in the soil. There would be a number of reasons why there are cracks in the foundation which includes seasonal climate changes. This makes it not safe to assume that your house is settling simply because there are cracks in the foundation.

Are Cracks in Slab Foundations Normal?

You can expect shrinkage and cracks in the slab foundation and these are very common. They usually do not compromise the structural integrity of the home. Due to humidity, things like wood floors, trim, wood framing can shrink and get acclimated to lower inside humidity. Similarly with temperature changes, expansion and contraction can happen daily and seasonally.

Homes are designed for nearly 1 inch of soil movement. But, in highly expansive soil areas, slab foundations can be designed for soil movement up to 4 inches.

If the foundation moves, the structure moves and this causes a degree of cracking, distortion and racking. Here, a foundation should be created to maintain structural integrity but cracks are normal.

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 in slab foundation 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 in slab foundation 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. This could mean a crack in slab foundation.

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 in slab foundation and 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.


How to Fix a Crack in 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 in Slab Foundation Right at Home

Repairing Minor Crack

Step 1

Clear the area of any loose chips. For a small crack in slab foundation, 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. This is the best way to fix cracks in cement slab foundation. 

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.

This should solve all issues pertaining to a small crack in slab foundation. Similarly, this is procedure required to rectify a hairline crack in concrete slab foundation. 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?

Slabjacking

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). Usually, residential foundation repairs take as long as 2 to 3 days while some jobs take significantly longer. The cost of structural failure and repairs is based on the type of foundation. This makes the cost range from $20,000 to $100,000 on the initial cost of lifting.

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. Homebuilders, 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.
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