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Settlement Cracks in Slabs in Poured Concrete Slabs & Floors StructAPedia © Enter your search terms Submit search form   Search This Website - InspectAPedia.com

• How to Identify and Evaluate Settlement Cracks in Slabs in Poured Concrete Slabs or Floors

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 website 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. © Copyright 2008 Daniel Friedman, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use links at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

Settlement Cracks in poured Concrete Slabs

Settlement 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]. 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

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 the sketch, (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

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

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. I've seen this line ranging from a fraction of an inch to six to eight inches above the current level of the slab!

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Technical Reviewers

Particular thanks are due to experts and also consumers who read these articles and suggest corrections, changes, and additions to the material. Content suggestions, technical corrections and content critique are invited for any of the content at our website.

• Daniel Friedman - principal author/editor of the InspectAPedia ^TM Website
• "Concrete Slab Finishes and the Use of the F-number System", Matthew Stuart, P.E., S.E., F.ASCE, online course at www.pdhonline.org/courses/s130/s130.htm
• "Best Practices for Concrete Sidewalk Construction," Balvant rajani, Canadian National Research Council
• "Design Considerations for Perlite Roof Slabs," a chapter in "Perlite Concrete Grade for Lightweight Concrete Construction", United Perlite Corporation
• "Quality Standards for the Professional Remodeling Industry", National Association of Home Builders Remodelers Council, NAHB Research Foundation, 1987. See our books at "Structure" at the InspectAPedia Bookstore
• Critique, contributions wanted: Contact Us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution.

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