Photograph of a cracked concrete slab from frost damage How to Repair Cracks or Settlement Damage in Poured Concrete Slabs & Floors
     

  • CONCRETE SLAB CRACK REPAIR - home - CONTENTS: How to repair cracked concrete floors or slabs. Types of cracks that occur in concrete slabs, shrinkage cracks, settlement cracks, frost heaves; Causes and problems of floor slab cracking; Slab on grade construction or "patio home" construction cracks
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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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Repair Methods for Cracks in Concrete Slabs & Floors

Cracks in a concrete slab in an Alaska home over thawing permafrost (C) D FriedmanTypes 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.

Article Series Contents

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.

Photograph of a classic shrinkage crack in poured concrete.

  • Shrinkage cracks in a slab are unlikely to be of any structural concern but can be a source of water entry or radon entry in buildings and may form a tripping hazard. The photo at left shows a typical concrete slab shrinkage crack.

    Details are at Shrinkage Cracks in Slabs.

    Also see SHRINKAGE vs EXPANSION vs SETTLEMENT.

  • Settlement cracks in a slab indicate inadequate site preparation, such as failure to compact fill on which a slab was poured.

    See Settlement Cracks in Slabs for details.

  • Frost heaves or expansive soil damage can cause substantial damage to basement, crawl space, or garage floor slabs in some conditions.

    See Frost Heave/Expansive Soil Cracks in Slabs for details.

List of Slab Crack Repair Methods Articles

Crack at glass block in wall (C) D Friedman B ObrienPlease see the individual slab crack repair articles listed 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."

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.

Standards for Repair of Cracks in Concrete Floors

Concrete slab cracks heaves form trip hazards (C) Daniel FriedmanSettlement 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.

 

Continue reading at CONTROL JOINT CRACKS in CONCRETE

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).

More Reading

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