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Photograph of a cracked concrete slab, cracks around a Lally column Settlement Cracks in Slabs in Poured Concrete Slabs & Floors

  • SETTLEMENT CRACKS in SLABS - CONTENTS: How to Identify and Evaluate Settlement Cracks in Slabs in Poured Concrete Slabs or Floors. Concrete floor crack pattern analysis, evaluation, diagnosis, repair. Floor crack diagnosis
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Guide to concrete floor or foundation settlement cracking: this article describes How to Identify and Evaluate Settlement Cracks in Slabs in Poured Concrete Slabs or in concrete floors in basements, crawl spaces, or garages. This article series describes how to recognize and diagnose various types of foundation failure or damage, such as foundation cracks, masonry foundation crack patterns, and moving, leaning, bulging, or bowing building foundation walls.

Types of foundation cracks, crack patterns, differences in the meaning of cracks in different foundation materials, site conditions, building history, and other evidence of building movement and damage are described to assist in recognizing foundation defects and to help the inspector separate cosmetic or low-risk conditions from those likely to be important and potentially costly to repair.



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Guide to Diagnosing & Fixing Settlement Cracks in Poured Concrete Slabs

Sketch of a cracked concrete slab, cracks around a Lally columnSettlement cracks in a conventional concrete floor slab which has been poured inside a separate foundation wall (and often resting at its edges on the building's foundation wall footings) are usually not connected to the foundation wall and are not supporting any structure [except possibly Lally columns, discussed below].

[Click to enlarge any image]

So often cracks in a basement floor slab are not a threat to the structure.

Floor cracks can occasionally indicate a serious structural problem however, since there are exceptions to what we stated just above: significant settlement of a slab which is supporting an interior load-bearing partition or column could be a serous concern.

Settlement or movement of a slab on grade constructed building (the slab is forming a floor in the living space) may also be a concern.

Another serious concern suggested by a floor slab crack can be inferred if if the floor cracks track to corresponding cracks in the building foundation wall. If you follow a basement or slab floor crack across the surface to the foundation wall, and if you find a crack in the foundation wall which maps onto the wall from the end of the floor crack, there is risk of more serious foundation damage and further investigation by an expert is warranted.

Cracks in a floor slab around a Lally column may indicate settlement

Photograph of a cracked concrete slab, cracks around a Lally column

Settlement cracks in a concrete floor around a supporting Lally column might be indicative of a serious problem such as building settlement if the columns are settling.

Independent footings may have been provided supporting Lally columns in the building interior and those may be settling independently of the floor slab which may have been poured around and even over them (See photo and sketch above).

In our sketch above, (I) points to a roughly circular crack forming around the pier as the remaining slab settled away from the pier itself. [(E) is probably a shrinkage crack occurring at a natural stress point formed by the inside corner footprint of the foundation.]

But beware, where slab thickness and local building codes allow, supporting columns may bear directly on a poured floor slab without their own (deeper) pier or footing. In that case floor slab cracking and settling can cause column movement and may be a structural concern.

Also watch out for columns settling down through the slab - which may show up as sags in the floor above (supported by the column) where the slab itself may not show signs of movement.

Settlement cracks in a monolithic slab or floating slab floor may be more serious, depending on their extent since in this case the edges and other portions of the slab are, unlike the cases above) expected to support the upper portions of the building structure.

A monolithic concrete slab is one which includes the building footing as part of the slab, created in a single continuous pour of concrete.

A floating concrete slab is one which is poured at a (generally) uniform thickness on the ground without a separate footing. [Beware, in areas of wet soils, expansive clays, freezing climates, or unstable soils, floating slabs may be exposed to extra stresses and may tip or crack. Proper site work and drainage are important as is proper engineering design of such structures.

Random settlement or heave cracks in garage floors

Photograph of a cracked concrete slab from frost damage

Basement and garage floor random heave and crack patterns: Cracked and heaved concrete or settled concrete can occur in more random patterns in any concrete floor where there has been frost heaving, soil contraction/expansion, or simple soil settlement, as shown in this photograph.

Uniform or sloping settlement in garage floors or other floor slabs

Sketch of garage floor settlement cause (C) Carson Dunlop

Garage or basement floor s(C) Carson Dunlop Associates"m settlement may also produce a tipped floor even if the concrete is not cracked, or the floor may settle uniformly. Carson Dunlop's sketch shows how loose soil and gravel under a garage floor can combine with poor drainage to lead to serious slab settlement and a broken slab where no reinforcement was used in the floor slab.

If the floor slab was reinforced with steel the entire slab may pitch in the direction of settlement.

This condition occurs if the concrete was reinforced by steel or fiber cement, but was poured inside of a separate concrete or masonry block foundation. We see this condition more often in garages in which the slab was reinforced but poured on poorly-compacted soil. The problem may be worst if in addition to poor compaction, water runs under the slab, causing additional or more rapid soil settlement.

Slab bending on disturbed soil (C) Carson Dunlop Associates

Carson Dunlop's sketch shown here depicts slab bending, cracking and failure at opposing foundation walls due to construction on partially-disturbed soil independent of a drainage problem.

My first construction job (for pay - DF) was to rake level the backfill soil that the contractor had dumped inside of the newly-completed garage foundation in a series of homes.

No compaction of any kind was performed. When a lot of fill, several feet or more in depth, was required to bring the slab to the desired height, there was a good chance that the slab would settle or tip in the future.

Garage slabs which were poured inside of the foundation walls but which were pinned to the foundation sides (typically using re-bar set into holes punched into the masonry block foundation), the slab was resistant to settlement or movement even if there was modest soil settlement below.

In a garage where the slab has settled you can often spot the original level of the slab and thus can measure the amount of settlement. Look for a concrete line above the level of the top of the slab and found along the masonry block or poured concrete foundation wall. we have seen this line ranging from a fraction of an inch to six to eight inches above the current level of the slab!

Evaluating Garage Floor Slab Settlement

Question: how to separate slab settlement from foundation damage

(Aug 27, 2016) Jen said:

Thank you for this informative article! My Husband and I just bought a house in Grand Island, NY 4 weeks ago. It's a 3,000 Sq ft 2 story colonial, full basement. We noticed sloping settlement around the perimeter of our entire basement. 2-4 feet parallel to each wall, the cracks are about 1/4" wide. We recently had a radon test done, normal results and no water seepage. Our basement is very humid and moist so we just purchased a dehumidifier for it.

To make a long story short, I've been obsessively reading articles about basement slab cracks and have been measuring everything with a level. I'm terrorified that we purchased a lemon. Can you give me any advice? Our climate here ranges from 90° in the summer to -10° in the winter and we have tons of clay in our soil. Any feedback would be greatly appreciate!

Reply:

Jen

From just your e-text I can't guess at the cause nor the total effect of the cracks you describe. If the "cracks" are dead straight and parallel to building walls they might be deliberate control joints.

If the cracks do not extend up into foundation walls, that is if the house foundation is plumb, level, and not cracked, then the problems in the slab may still need to be addressed but we're missing evidence that the structure itself is being affected.

If the slab and house are brand new, moisture may still be coming off of concrete and other building products.

Question: how do I get an assessment of & recommendations about settlement in our Denver area garage floor slab?

2016/11/16 Kelly Dreilich said:

Our house underwent an inspection and the inspector noticed that the southwest corner of our garage slab has settled 3 in, Red line is apparent. The corner represents foundation to the outside wall to the left and the foundation to the entry to the house from the garage. There are not any cracks, pulling away, separation of any kind on the inside of the garage, outside on the brick wall, foundation etc. No cracks in dry wall both inside the garage and house. The house is a 13 year old custom home in colorado, just south of Denver.

Since that is the most inside part of the garage where there is never any water accumulation ever, and we never get any water in the garage at all. The buyers want someone to come out to inspect the slab and make recommendations. They are fearful that the foundation could be compromised in the the future.

We are the orginal owners and I have noticed that red line since we moved in and the house sat for one year before we purchased it and our inspector did not catch it. But I want to know who is the best and most qualified person to come and inspect this part of the garage, a structural/foundation company or a mudjacking company? I want to at least get two assesements to compare evals. Any advise or recomendations would be appreciated. Kelly my email is kdreilich62@comcast.net and cell phone is 3039139903

Reply: how to find the cause of slab settlement, look for expansive clay soil or unstable soil, assess impact, plan repair

Sorry, Kelly but I don't know what "red line is apparent" means. If you mean that a slab settled 3", leaving a visible mark along the garage wall, then I agree that is a significant settlement.

Without on-site expert work, we don't know if your slab settlement is due to construction errors (such as building a slab on poorly-compacted fill) or if unstable soil below the slab is a factor. In Colorado the Front Range area is at particular risk of expansive clay soil damage though expansive clay soil problems are found elsewhere in the state too.
See FOUNDATION FAILURES in CLAY SOIL for an explanation of foundation or slab damage due to expansive clay.

Readers living in other ares where there is construction on shale should also see FOUNDATION DAMAGE by MATERIAL or INCLUSIONS - heaving damage to foundation walls & cracking foundations or slabs due to Iron sulfide mineral (pyrrhotite) inclusions in concrete or due to building on Iron sulfide mineral (pyrrhotite) shale.

You're right to consider the effects of water, a factor that affects expansive clays as well as under-slab fill settlement in general. Though you don't see water inside the garage where the slab settled, there could still be an outside water cause or factor if surface or roof runoff are draining towards the building.

The question raised by your buyers is a reasonable one, though I doubt that anybody with any sense is going to give an absolute guarantee about the future conditions in your home. A reasonable approach is to follow something like these six steps to slab settlement evaluation and repair planning:

  1. Assess impact on structure: Assess the current extent of impact on structure. Is there movement of or damage to the foundation walls themselves? Often, infact usually a slab is structurally independent of the supporting foundation wall and footings.
  2. Assess Safety: Look for trip hazards in the garage: broken uneven slab with more than 1/8" difference across a crack. Other safety hazards can occur, such as damaged gas piping (fire and explosion risk) when a building suffers significant movement or dislocation.
  3. Find causes of settlement: Consider the most-likely causes of the movement as those will inform what repair is needed as well as the urgency of repair
  4. Consider history of settlement: Consider the probable history of the movement: is it original, occurring gradually over 13 years, or sudden and recent? The implications for diagnosis and repair would be different as would expectations of further movement. Not every settled garage slab is dangerous nor needs repair.
  5. Decide on justification for invasive inspections or tests: Decide whether or not the visual inspection, history, knowledge of local soil conditions and site factors does or does not justify the cost and trouble of further, more invasive investigation such asl borings outside or soil borings through the garage slab inside
  6. Make repair recommendations: Recommend watch and wait or repair, and if repair, recommend reasonable repair methods. Often slab jacking (pumping grout under a slab) and water runoff control are all that's needed but if your expert finds that there is a more serious site issue then other steps could be needed.

Before you do something expensive you're welcome to use the page top or bottom CONTACT link to send me your inspectors reports and photos for comment. I can't be as smart as an onsite expert but I might have some suggestions.

An experienced contractor, mason, foundation repair company may be quite sufficient for this investigation. Often those workers have seen hundreds or thousands of similar situations and have thus useful field experience.

Some home inspectors in your area may also have specific expertise with foundation damage and expansive clay soils. Carl Brahe is a Denver CO area home inspector who has written about expansive clays in Colorado. Tel: 303 816-5556. (We have no personal, business, nor any other economic relationship with Mr. Brahe nor with other consultants, products, or services discussed here at InspectApedia.com)

A civil or structural engineer would be useful if she or he has specific expertise and experience in residential construction and foundation diagnosis and repair work. If the problem found is traced to significant building structural damage or need for costly structural repair, I would certainly like the opinion of an independent design professional. Beware of conflicts of interest and loud arm-waving with no credible supporting observations and explanations of the situation.

If you or other readers face very costly repairs traced to unstable clay soils you might want to consider contacting one of the national experts such as Fredlund who has been writing about this issue for decades.

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Continue reading at SETTLEMENT vs. FROST HEAVE CRACKS or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see SETTLEMENT CRACK FAQs

Or see COLUMNS & POSTS, DEFECTS

Or see FOUNDATION CRACK DICTIONARY

Or see FOUNDATION FAILURES in CLAY SOIL about expansive clay and unstable clay soils

Or see FOUNDATION REPAIR METHODS - home

Or see SINKHOLES & BUILDING DAMAGE

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SETTLEMENT CRACKS in SLABS at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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