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STRUCTURAL INSPECTIONS & DEFECTS
AGE of a BUILDING - how to determine
BRICK FOUNDATIONS & WALLS
BRICK STRUCTURAL WALL Loose Bulged
BRICK WALL THERMAL EXPANSION CRACKS
BRICK WALL DRAINAGE WEEP HOLES
BUCKLED FOUNDATIONS due to INSULATION?
CHIMNEY INSPECTION DIAGNOSIS REPAIR
COLD POUR JOINTS, CONCRETE
COLUMNS & POSTS, DEFECTS
CONCRETE FOUNDATIONS, PRE-CAST
DISASTER BUILDING INSPECTION & REPAIR
EARTHQUAKE DAMAGED FOUNDATIONS
FLOOD DAMAGE ASSESSMENT, SAFETY & CLEANUP
FLOOD DAMAGE TO FOUNDATIONS
FOOTING & FOUNDATION DRAINS
FOOTINGS EXPOSED, Repair Methods
FOUNDATION BULGE or LEAN MEASUREMENTS
FOUNDATION CONSTRUCTION TYPES
FOUNDATION CONTRACTORS, ENGINEERS
FOUNDATION CRACKS & DAMAGE GUIDE
FOUNDATION DEFECTS OF OMISSION - MISSING
FOUNDATION FAILURES by TYPE & MATERIAL
FOUNDATION FAILURES by MOVEMENT TYPE
FOUNDATION INSPECTION METHODS
FOUNDATION INSULATION OPTIONS
FOUNDATION MATERIALS, Age, Types
FOUNDATION REPAIR METHODS
FRAMING DAMAGE, INSPECTION, REPAIR
FROST HEAVES, FOUNDATION, SLAB
GRADING, DRAINAGE & SITE WORK
GUTTERS & DOWNSPOUTS
INSECT INFESTATION / DAMAGE
MOBILE HOMES, DOUBLEWIDES, TRAILERS
MODULAR HOME CONSTRUCTION
MOISTURE CONTROL in BUILDINGS
RETAINING WALL DESIGNS, TYPES, DAMAGE
RETAINING WALL GUARD RAILINGS
SINKHOLES, WARNING SIGNS
SLAB CRACK EVALUATION
SLAB CRACK REPAIR
STRAW BALE CONSTRUCTION
STRUCTURAL DAMAGE PROBING
STRUCTURAL WOOD ASSESSMENT
THERMAL EXPANSION of MATERIALS
TIMBER FRAMING, ROT
WATER BARRIERS, EXTERIOR BUILDING
WATER ENTRY in BUILDINGS
WINTERIZE A BUILDING
Concrete slab crack repair methods: this article describes methods for repair of cracks that occur in poured concrete slabs or floors and explains the need for accurate crack cause diagnosis and impact on structure before repairs are attempted.
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.
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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.
Our photo (left) shows cracks developing in a slab on grade in a homein Alaska. Built over thawing permafrost, the cause of these cracks needs to be understood before a proper repair can be designed.
Accurate Diagnosis of Slab Crack Cause, Impact on Structure, Repair Needs
Before attempting to choose a repair method for concrete floor slab cracks,
Diagnose the cause of the slab movement and cracking. Only by an accurate diagnosis of the cause of cracking or movement in a floor slab can we be assured that the crack repair will be durable and appropriate. In fact some types of floor cracks, such as hairline concrete shrinkage cracks may not need repair at all.
In contrast, a concrete slab may have settled over poorly-compacted or washed-out fill (such as in some garages) without cracking (tipping instead), but repair may still be necessary to correct slope, provide drainage, or to prevent further settlement or even collapse.
Types of Cracks in Slabs
Each type of basement slab, floor slab, or slab on grade crack is discussed and described in articles at this website. Understanding the differences among these concrete crack types is an important first step in diagnosing their cause and their significance to the structure.
List of Slab Crack Repair Methods Articles
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."
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.
Readers should also see FOUNDATION REPAIR METHODS and 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).
Standards for Repair of Cracks in Concrete Floors
While there are few construction acceptance standards for floor slab cracking, one, "Quality Standards for the Professional Remodeling Industry" NAHB, recommends that cracks in basement floors which exceed 3/16" in width or 1/8" in vertical displacement should be repaired.
The same standard provides that cracks in slab-on-grade floors shall not exceed 1/16" in width or in vertical displacement.
The same standard provides that cracks occurring in control joints in concrete slabs are normal and acceptable. [We suggest that this last criteria should apply to crack width but not to vertical displacement.
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).
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Technical Reviewers & References
Related Topics, found near the top of this page suggest articles closely related to this one.
Please 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 Entry
Happily 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
We 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.
We 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.
We 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!
Settlement cracks may form a tripping hazard even if they are not traced to a structural
concern. While there are few construction acceptance standards for floor slab cracking, one, "Quality Standards for the Professional
Remodeling Industry" NAHB, , recommends that cracks in basement floors which exceed 3/16" in width or 1/8" in vertical displacement should be repaired.
The same standard provides that cracks in slab-on-grade floors shall not exceed 1/16" in width or in vertical displacement. The same standard
provides that cracks occurring in control joints in concrete slabs are normal and acceptable. [We suggest that this last
criteria should apply to crack width but not to vertical displacement.]