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Floor slab & tile crack diagnosis & repair: this article describes the types of cracks that occur in poured concrete slabs or floors and explains the risks associated with each, thus assisting in deciding what types of repair may be needed.> Cracks in concrete floors or slabs occur in poured concrete slabs may be found both in basement and in slab on grade or "patio home" construction and have a variety of causes and cures that we discuss here. 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. Also see FOUNDATION CONSTRUCTION TYPES. Readers should also see SLAB CRACK REPAIR and may also want to see SINKING BUILDINGS where we include case histories of both building settlement and slab cracking, heaving, settling: diagnosis and repair. Also see How to Diagnose & Evaluate Foundation Cracks since those pages also assist in distinguishing among types of cracking in concrete foundations (vertical supporting walls and footings). Green links show where you are. © Copyright 2013 InspectAPedia.com, All Rights Reserved. Author Daniel Friedman. Three Types of Cracks in Concrete Slabs & Floors
Each type of basement slab, floor slab, or slab on grade crack is discussed and described with photographs below. Cracks come to the job along with the concrete, riding in the same truck! At a Journal of Light Construction conference (Boston 1985) a lecturer informed us that "Every concrete truck that comes to your job to pour a slab has at least four cracks in it. It's up to you to either provide control joints, or not. If you leave out control joints the cracks will occur in a messier pattern at natural stress points in the slab." Three Types of Poured Concrete Slab Floors - Different Implications of Cracks & MovementBefore we describe crack, movement, and damage patterns and diagnosis in concrete slabs, we need to introduce three different types of poured concrete floor slabs. We use the three Carson Dunlop Sketches shown here to comment on the occurrence, causes, and significance of cracks and movement in poured concrete slab construction. We define types of foundations and their common vulnerabilities also at FOUNDATION CONSTRUCTION TYPES. The bad news about typical floating slab construction (where the soil is not compacted) is that anything that causes the soil to settle risks slab cracking and settlement. Flooding, leaks, or simply poor handling of roof and surface runoff can send water under a building where it causes loose soil to settle. The good news about cracks in floating slab construction is that the damage is to the floor, not to the structure that is supporting the building. Only if you see a floor slab crack that continues up in the foundation wall where the crack meets the wall would the structure be obviously involved. More good news: if there is significant soil settlement under a floating slab, the slab is likely to break and follow the settling soil downwards; a sudden precipitous collapse of a floating slab is less likely than the next case we describe. Supported poured concrete floor slab characteristics
Slab on Grade - Monolithic Concrete Slab Construction Characteristics
The articles listed below explain how we recognize and diagnose signs of cracking, damage, movement in these different concrete slab construction methods. Readers should also see SLAB CRACK REPAIR and may also want to see SINKING BUILDINGS where we include case histories of both building settlement and slab cracking, heaving, settling: diagnosis and repair. Also see How to Diagnose & Evaluate Foundation Cracks since those pages also assist in distinguishing among types of cracking in concrete foundations (vertical supporting walls and footings). Frequently Asked Questions (FAQs) about all types of cracks found in poured concrete floors or slabs: crack types, causes, evaluation, repairs.... Ask a Question or Search InspectApediaQuestions & answers or comments about cracked concrete slabs and floors. Question: Diagnose and repair cracks in ceramic tile covered slab in a nearly-new homeWe bought a house two years ago, after it sat empty as a model home for a few years, so now it’s about five years old. I believe it is a slab on grade foundation type--common in this area. We live in Austin, Texas and the drought has been really bad here, and recently we have discovered a few hairline cracks in a few different tiles. These tiles are on the first story and so are on top of the concrete. One crack is about 4 feet long and runs straight through about 4 different tiles, not following the grout lines. (Theses cracks are hairline, and are barely noticeable.) There is a lot of limestone and rock in the area, and so we never thought we’d ever have a problem here, unlike Houston, where foundation problems are everywhere. I kinda freaked out and started putting a level to everything in the house, and all the door frames are perfectly level and the countertops are very, very close to being perfectly level. There a few small hairline cracks throughout the house in the sheetrock, but these were there when we moved in and haven’t gotten worse, and seemed pretty normal to me. There is even a third story, which I know you’d see the most movement, (if there was sinking involved) but everything looks good up there(maybe a very slight slope to one side, like a ¼ inch over 10 feet) Well, mentioning the drought, a few neighbors have said they have a few cracks too, and they had a foundation company come out, and the company told them it was because of the drought, and told them to soak the foundation. I went out and bought soak hoses and are going to set that up, and I also bought a nice laser level and checked the slope of the foundation. The foundation slopes about 1 inch over about 20 feet towards the left side of the house, which happens to be the downhill side. But with almost everything else level in the house, I’m starting to think it was already like that, and I just never noticed it before. So this leads to my main question—how much slope in a foundation is generally acceptable to build on? Because I know brand new foundations won’t be perfectly level, but about how much can a new foundation be off-1 inch, 2 inches, 6 inches? Thanks, C.M. Reply: diagnose the crack type, impact on foundation, probable cause, and choose extent of repairsThanks for the interesting tile and slab crack question. A competent onsite inspection by an expert usually finds additional clues that help accurately diagnose a foundation, slab, or floor cracking problem because someone with experience might see clues that escape even a smart, careful, thorough homeowner performing a DIY investigation. That said, here are some things to consider: Decide if the floor and concrete cracks are structural or notI agree that the cracks you describe are more likely due to slab settlement or movement and that considering Austin has been in a drought, soil settlement (rather than rising due to suddenly newly-wet expansive clay soils) is probably at work. I also agree that other crack sources such as concrete slab shrinkage are probably not at fault. I think that slab shrinkage cracks appear early in building life, are not structural, and are less likely to suddenly telegraph through a ceramic tile floor. (To be accurate, severe shrinkage cracks in a slab could show up in a tiled floor if the floor were not properly installed to prevent those cracks from telegraphing through the tile, but that just doesn't sound like your case.) It's useful to distinguish between a crack that only affects the floor slab (usually not structural, the building is not threatened) and a crack that includes the foundations and footings (structural, the building might be threatened depending on extent of movement). Evaluate the extent of actual foundation movement vs. slab crackingWhile building codes expect footings to be poured level, I'm not surprised to read that a foundation and footing slope one inch over 20 feet in new construction. A key diagnostic step will be to convince yourself that this out of slope condition is as-built or that it is the result of settlement. Examine the sloping foundation walls wherever masonry foundation materials and surfaces are visible. A one-inch change in slope, if it happened after construction, would often be expected to produce vertical or stair-stepped cracks in the foundation wall. (While it's theoretically possible for an entire foundation or even building to settle or tip without cracking (I've found a few), you should not find upstairs floors, windows, doors, all dead level if the building had shifted one inch.) Also take a look at the location and pattern of floor cracks, If a crack line is more or less straight, and if it runs towards the foundation walls or more or less at right angle to the foundation wall, then if the foundation wall and footing had settled you'd expect to see wall cracks in the same area. Evaluate the extent of floor slab settlement cracksIf those clues of actual footing settlement and foundation movement are absent, you are more likely seeing settlement in the concrete slab itself. Often floor slabs are not poured on compacted fill (they should be). The result can be future slab settlement and cracking, exacerbated by changes in site conditions (more water, less water, freezing, drying) that may affect the soil below the slab. If the floor slab is poured with its perimeter sitting atop foundation footings (and presuming the footings are intact) the slab "hangs" on the footings, and perhaps also is "supported" by piers that may have been poured under the floor slab to support Lally columns that march down the center of a basement to support a main girder. In that construction settlement of the slab may produce cracks as the floor bends and dirt below it settles. Cracks tend to be away from and sometimes roughly parallel to the foundation walls, or to appear as islands around the Lally columns. If the floor slab was poured atop of dirt that covered the footings, or inside of the footings, the entire slab may settle or tip even at its perimeters. In any case, the combination of pouring a slab on soft fill and changes in soil moisture invite soil settlement and slab cracking. In fact my first job in construction was raking a huge dirt pile out to "level" inside of an already poured and built footing and masonry block foundation wall. Once the dirt was roughly "level" via my hand rake, the builder went ahead and poured his floor slab. "Compacted fill" was not in our vocabulary. See Settlement Cracks in Slabs for details about cracking floor slabs due to settlement. Repairing Slab Cracks and Cracked Floor TilesWe discuss several slab crack repair alternatives at SLAB CRACK REPAIR. Unfortunately there is no magic band-aid that will make the cracks disappear, especially where ceramic floor tiles are installed. If radon and water entry are not an issue in your area you might live with the cosmetic defect for a while. Repair of the cracked floor will require removal of the cracked ceramic floor tiles, including enough mastic and crud removal that you can bed replacement tiles smoothly in place. Before replacing the tiles that were removed you might want to also install mesh tape over the floor slab cracks to reduce the chances that those cracks telegraph again through the new tiles. Also see Guide to Installing Tile over Concrete Slab Floors
Ask a Question or Enter Search Terms in the InspectApedia search box just below. Technical Reviewers & ReferencesRelated Topics, found near the top of this page suggest articles closely related to this one.
How to Identify & Evaluate Shrinkage Cracks in Poured Concrete SlabsPlease see the main text version of this article at Shrinkage Cracks in Slabs We discuss the recognition and significance of concrete shrinkage cracks in detail at Shrinkage Cracks in Slabs. Just below you can read a summary of this topic.
Shrinkage Cracks in a Basement or other Slab May Be Severe and May Combine with Settlement
Depending on the mix and pour conditions as well as the site work preparation significant concrete shrinkage cracks can appear in a basement slab. On occasion we may also see vertical dislocation in a floor slab crack, that is, one side has settled or tipped away from the other side of the cracked concrete. Our photographs above show shrinkage cracking and ensuing minor settlement cracks occurring in the same poured concrete basement floor. We suspected that site drainage defects or possibly nearby site blasting for additional construction contributed to the 1/16" to 1/8" vertical dislocation across some of the cracks in this floor where the cracks were about 3/16" wide. Slab Cracks may Permit Basement Water Entry or Radon Gas EntryHappily the building owner at the site where we made the photos above did not report water entry through these openings, though it certainly might be expected as the home ages and its footing drains stop working, particularly if surface runoff and roof runoff are not kept away from the building. Photos of Concrete Floor Slab Shrinkage Points of Origin
How to Inspect & Evaluate Cracks at Control Joints in ConcreteWe discuss the recognition and significance of control joints in poured concrete slabs in detail at Cracks at Control Joints in Concrete. Just below you can read a summary of this topic.
The first photograph shows an expansion joint in a basement floor slab. Notice that we do not see other cracks in this slab. Shrinkage cracks that occur at control joints such as shown in the second, close up photo here, are occurring where they are supposed-to, although the width of this particular crack was surprisingly large. These cracks are not normally a defect in the slab but may be a source of water or radon entry into the building and may need to be sealed. Use a flexible sealant. Settlement Cracks in poured Concrete SlabsWe discuss the recognition and significance of settlement cracks in poured concrete slabs in detail at Settlement Cracks in Slabs. Just below you can read a summary of this topic.
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 above). 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. 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. Frost Heaves or Expansive Soil Cracks in Poured Concrete SlabsWe discuss the recognition and significance of frost heaving damage and cracks in poured concrete slabs in detail at Frost Heave/Expansive Soil Cracks in Slabs. Just below you can read a summary of this topic.
The combination of water under a garage floor (watch out for driveways and sites that slope towards the garage or home) and freezing can cause the portion of the slab which rests directly on the soil to move up and down during freeze/thaw cycles. Since a garage is often colder near the garage entry door than in other areas, there is extra risk of these cracks occurring there, but they can occur anywhere. When there is freezing and heaving of a slab, particularly one which omitted reinforcing steel, or did not extend the steel over the footing, these garage floor slab cracks may appear during freeze/thaw cycles. Similar floor slab damage might occur in areas of expansive clay soils if the proper moisture level is not maintained. Basement floor heave patterns - frozen floor drains: Basement floors can be frost heaved in other patterns in buildings which are unheated or which lose heat. We have found basement floor slabs broken and heaved above buried drain lines which ran below the basement floor of a home which remained unheated during freezing weather. A clogged drain sitting full of liquid combined with prolonged freezing weather was the culprit in most of these cases. The heaved concrete was raised following exactly the path of the frozen (and burst) buried, clogged drain line. This problem can be epidemic in older homes which were constructed using a downspout drain line extending below the basement slab.
Garage or basement floor sloped or semi-uniform settlement may also produce a tipped floor even if the concrete is not cracked, or the floor may settle uniformly. 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. My first construction job (for pay) 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! Readers should also see SLAB CRACK REPAIR and may also want to see SINKING BUILDINGS where we include case histories of both building settlement and slab cracking, heaving, settling: diagnosis and repair. Also see How to Diagnose & Evaluate Foundation Cracks since those pages also assist in distinguishing among types of cracking in concrete foundations (vertical supporting walls and footings). Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
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