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How to Distinguish Settlement Cracking from Frost Heave in Slabs InspectAPedia®  -

• How to distinguish frost heave cracking from settlement cracking in concrete slabs
• Photographs of types of poured concrete slab cracks
• Poured concrete crack diagnosis for concrete slabs and concrete floors

This article describes how to distinguish frost heave cracking from settlement cracking in concrete slabs - a division of our article on 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.

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Concrete Slab Settlement Cracks Compared with Frost Heaves in Poured Concrete Slabs

Where cracks appear in slabs in homes built in freezing climates, it can be difficult to distinguish between frost heaves and settlement since both types of movement can cause similar cracking in a slab or foundation.

Clues suggesting settlement cracks in a concrete slab

• Evidence of probable backfill or infill defects below a poured concrete slab: significant cracks in a garage floor slab that are not near nor tracking the footing across the garage entry, especially where the level of the garage floor is well above outside grade - fill was needed inside the garage foundation.
• Evidence of extensive poured concrete slab settlement such as noting that the level of the top of the slab is below a concrete trace line along the foundation walls where the top was located when the slab was originally poured is clear evidence of slab settlement. [We regret we have not photographed this condition though it is quite obvious - if any readers have photographed this condition please let us know.]

Clues suggesting frost heaved cracks in a concrete slab

Frost heave cracking is likely to be cyclical unless the building has gone through less than one cycle of seasons. Frost heave cracking can often be reasonably postulated as a cause or main factor in slab cracking if the inspector can identify site conditions that make it likely that there has been water around and under the foundation or slab, and that freezing conditions have occurred.

• Evidence of cyclical movement in poured concrete foundations and slabs such as crumbling broken edges of cracked sections may suggest recurrent movement.
• Evidence of a history of site or building water entry or poor site drainage may suggest (in areas of freezing climates) frost heaved damage to a poured concrete floor slab.
• Absence of evidence of backfill or site fill problems at a poured concrete structure: a garage slab poured at grade and cracked across its width near the entry is more likely due to frost heaves. But remember this basic principle of diagnostics: absence of evidence is not evidence of absence. It may be inconclusive.

Frost heave cracked basement slabs in older homes

A classic example of slab cracking due to frost heaving can be seen in some older homes (1885-1935) in which roof drainage was routed from gutters down into the home to a drain that ran under the basement floor slab and thence out to a storm drain in the street (urban houses).

In such homes you may find that the basement floor slab is seriously broken up and heaved, some sections pushed several inches into the air, along either side of a rather straight line which tracks exactly the route of one of these buried drains. [We regret we didn't take photos of this - if any readers have photographed this condition please let us know.]

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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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