Settlement Cracks in Slabs in Poured Concrete Slabs & Floors
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How to Identify and Evaluate Settlement Cracks in Slabs in Poured Concrete Slabs or Floors
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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.
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 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
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. 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.
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) 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!
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Additional technical contributors & reference sources for this article are listed below.
Use links just below or 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.
"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
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