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Concrete shrinkage cracks:
How to identify, evaluate, & repair shrinkage cracks in concrete floors,slabs, or foundations. We address these questions: Are concrete floor cracks a structural problem? How to recognize and evaluate shrinkage cracks in poured concrete slabs or floors. Do shrinkage cracks in poured concrete walls or floor slabs always need repair?
This document explains how to recognize shrinkage, evaluate, and repair cracks in poured concrete walls or floors, and discusses a few (minor)
possible problems from shrinkage cracking such as water or radon leaks.
Guide to Shrinkage Cracks in Poured Concrete Slabs
This article series discusses concrete cracking in slabs, walls, floors, and foundations, and describes the types of cracks that occur in poured ("placed concrete" by some experts) walls, slabs or floors and explains the risks associated with each, thus
assisting in deciding what types of repair may be needed.
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.
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.
Shrinkage cracks such as shown in our photograph above are found in poured concrete, are easily recognizable, and can be distinguished from other types of cracks that occur
later in the life of a foundation wall or floor slab - as we explain here.
The photograph of cracks above were taken of shrinkage cracks in a concrete slab floor in a home built in 2006.
The cracks in this case ranged in width (measured across the crack) from "hairline" (less than 1/16") to about
3/32" in the basement floor slab of this particular home. They may appear larger.
What is unique about shrinkage cracks in concrete is that they usually appear to be discontinuous, as shown in
this photo. The crack will meander along in the concrete, taper to a stop, and then continue beginning in a parallel
line to the first crack, meandering again through the concrete.
This is characteristic of concrete (or mud) shrinking
while giving up its moisture.
You can see the shrinkage of even a perfect concrete floor slab with no visible cracks in the field of its surface if the floor was poured inside of an existing
foundation. Look for the gap between the edges of the slab and the foundation wall? Look also for the stains or concrete debris on the
wall at the slab level? These confirm that at the time the slab was poured it was touching the wall.
Causes of variation in width of shrinkage cracks in poured concrete
Why do concrete shrinkage cracks vary in width across a wall or floor? Probably because
concrete shrinkage cracks are meandering or "wandering" in their path, and are usually intermittent or
interrupted in their course, as you can see in the photograph above.
As the path of cracking caused by concrete
shrinkage wanders and stops and starts across an area of wall or floor, you will often see overlapping or
roughly parallel nearby cracks that represent the end of one crack line and the beginning of another.
The total width of the two close-by cracks is probably about the same as the total crack width
where a crack wanders in solitary along the concrete. The stresses producing the crack have simply shifted
slightly in the wall or floor as it cured.
Why cracks appear to originate in poured concrete walls at windows or at outside corners in concrete floor slabs
Why do concrete shrinkage cracks often begin near the corner of a window, or in a floor near a
corner in the foundation footprint?
If you think of the entire reinforced concrete wall or floor as a
rather uniform membrane, any discontinuity in the membrane, such as an opening for a window or the
placement of a corner projecting into the room in the case of a floor, creates a variation
in the distribution of (shrinkage during curing) forces in the wall or floor.
Shrinkage cracks may
have their origin at these points of discontinuity.
Watch out: other more varigated concrete crack patterns (shown below) may be ascribed to FOUNDATION DAMAGE by MATERIAL or INCLUSIONS producing crack patterns in concrete caused by inclusion of iron sulfide (pyrrhotite) particles in the concrete mix.
Above: a photograph of foundation cracks attributed to concrete that included iron sulfide (pyrrhotite), provided courtesy of CCACB - Connecticut Coalition Against Crumbling Basements.
This crack pattern is rather distinctive: somewhat random cracking pattern, with concrete cracks that are wider and more continuous than the usual concrete shrinkage cracks found at many jobs
Iron sulfide mineral (pyrrhotite) cracking may appear as variegated, "random pattern" cracking in concrete slabs, walls, and foundations whose concrete contains high levels of pyrrhotite. Moisture as well as oxygen react with this iron sulfide material, causing it to swell with tremendous force, causing varied-pattern cracking in the concrete.
Bryant in his thesis describes iron sulfide pyrrhotite cracking damage in several U.S. states including in Canada: Ottawa, in the U.K. in Derbyshire, and in the U.S. in Kentucky, Mississippi, Montana, Pennsylvania, Tennessee, Texas, Virginia, and more generally in the U.S. Great Plains area and in the Mississippian and Pennsylvanian shale beds. Bryant notes that cases of "... heave due to oxidation of pyritic shale have
been reported and continue to be reported worldwide. " (Bryant 2003).
What is the significance of differences in concrete height on opposite sides of a crack?
Why are some "shrinkage cracks" at different heights on either side of the crack?
The floor on one side of this 3/16" wide crack was about 3/32" higher than on the other. This may be due to settlement of the broken slab section on poorly-compacted fill in the building's basement.
Multiple forces and movements may be present as a poured concrete foundation cures, such as a combination of
shrinkage and settlement, or shrinkage and outside pressures on a wall from backfill.
It's best to let any
masonry wall cure before backfilling, though that's most critical with masonry block (CMU) walls where
early backfill before the first floor has been framed in place has been known to lead to a total collapse of the foundation.
In a poured concrete wall or floor if the surface of the concrete on opposite sides of a crack are also at different
elevations, that is if the concrete on one side of a crack is higher than the other, additional forces have been
at work and the crack is not a simple shrinkage crack.
Repair cracks that make trip hazards: If the height difference across a floor crack is 1/8" or more it forms a tripping hazard and it should be repaired.
Do we need to repair shrinkage cracks in foundation walls or slabs?
Small shrinkage cracks that are above ground level in a foundation wall, such as the cracks
in the inside corner of this poured concrete foundation are unlikely to be much of a problem except for
a few less common cases where water running down the wall is leaking in at the shrinkage crack.
[Do you think that the crack shown here is an initial footing or foundation settlement crack? Maybe so.]
Shrinkage cracks in concrete walls or floor slabs that are leaking into the building interior such
as the one shown in this photograph, should be sealed.
However a careful inspection in this particular
case revealed that the window flashing and exterior siding had been installed so as to direct wind-blown
rain into a concentrated runoff pattern that [unfortunately] passed over and then through this
crack in the foundation wall. Some flashing and siding adjustments outside cured this problem.
Concrete cracks often occur at natural stress points where the uniformity of a poured concrete wall or
floor has been interrupted by placement of a window (in walls of course) or inside corners (in floors). You may
often find shrinkage cracks that trace across the concrete to these stress points.
Shrinkage cracks in a concrete slab or floor might need to be repaired to avoid water leakage from below
or to stop radon gas entering the building. In the photograph shown here the floor was badly cracked from a combination
of concrete shrinkage, settlement, and frost heaves (the building had been left un-heated in a freezing climate).
can see that lots of water was leaking into the basement up through the floor.
In this case, while repairs to the basement
floor, or perhaps better, a new basement floor slab, were in order, the root cause of high water levels under the basement
floor needed to be addressed first.
How are concrete wall, foundation, or floor slab shrinkage cracks avoided or repaired
While shrinkage in poured concrete walls or floor slabs is a normal property of curing concrete, shrinkage cracks
can be controlled, or where they have occurred, in some cases repairs are needed. In addition to
reading about repairing concrete shrinkage cracks (if crack repair is needed at all) at
FOUNDATION REPAIR METHODS
see how we prevent shrinkage cracks
in poured concrete floors and walls by reading CONTROL JOINT CRACKS in CONCRETE
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Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
Bryant, Lee Davis, Matthew Mauldon, and James Kenneth Mitchell. Geotechnical problems with pyritic rock and soil. Virginia Polytechnic Institute and State University, Charles E. Via Department of Civil Engineering, 2003.
 "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
 A HREF="http://astore.amazon.com/inspectapedia-20?node=13&page=1">Quality Standards for the Professional Remodeling Industry, National Association of Home Builders Remodelers Council, NAHB Research Foundation, 1987.
Diagnosing & Repairing House Structure Problems, Edgar O. Seaquist, McGraw Hill, 1980 ISBN 0-07-056013-7 (obsolete, incomplete, missing most diagnosis steps, but very good reading; out of print but used copies are available at Amazon.com, and reprints are available from some inspection tool suppliers). Ed Seaquist was among the first speakers invited to a series of educational conferences organized by D Friedman for ASHI, the American Society of Home Inspectors, where the topic of inspecting the in-service condition of building structures was first addressed.
Defects and Deterioration in Buildings: A Practical Guide to the Science and Technology of Material Failure, Barry Richardson, Spon Press; 2d Ed (2001), ISBN-10: 041925210X, ISBN-13: 978-0419252108. Quoting: A professional reference designed to assist surveyors, engineers, architects and contractors in diagnosing existing problems and avoiding them in new buildings. Fully revised and updated, this edition, in new clearer format, covers developments in building defects, and problems such as sick building syndrome. Well liked for its mixture of theory and practice the new edition will complement Hinks and Cook's student textbook on defects at the practitioner level.
Sinkholes and Sudden Land Subsidence References, Products, Consultants
"A Hole in the Ground Erupts, to Estonia's Delight", New York Times, 9 December 2008 p. 10.
History of water usage in Estonia: (5.7 MB PDF) jaagupi.parnu.ee/freshwater/doc/the_history_of_water_usage_systems_in_estonia.pdf
"Quebec Family Dies as Home Vanishes Into Crater, in Reminder of Hidden Menace", Ian Austen, New York Times, 13 May 2010 p. A8. See http://www.nytimes.com/
"Quick Clay", Wikipedia search 5/13/2010 - http://en.wikipedia.org/wiki/Quick_clay
Florida DEP - Department of Environmental Protection, & Florida Geological survey (http://www.dep.state.fl.us/geology/default.htm) on Florida sinkholes: Effects of Sinkholes on Water Conditions Hernando County, Florida, Brett Buff, GIS in Water Resources, 2008, Dr. David R. Maidment, Photos - Tom Scott, Florida Geographic Survey - Web Search 06/09/2010 - http://www.dep.state.fl.us/geology/geologictopics/jacksonsink.htm
and - http://www.dep.state.fl.us/geology/geologictopics/sinkhole.htm also see Lane, Ed, 1986, Karst in Florida: Florida Geological Survey Special Publication 29, 100 p.
Foundation Engineering Problems and Hazards in Karst Terranes, James P. Reger, Maryland Geological Survey, web search 06/05/2010, original source: http://www.mgs.md.gov/esic/fs/fs11.html Maryland Geological Survey, 2300 St. Paul Street, Baltimore, MD 21218
"Frost Heaving Forces in Leda Clay", Penner, E., Division of Building Research, National Research Council of Canada, Canadian Geotechnical Journal, NRC Research Press, 1970-2, Vol 7, No 1, PP 8-16, National Research Council of Canada, Accession number 1970-023601, Quoting from original source
The frost heaving forces developed under a 1 ft. (30.5 cm) diameter steel plate were measured in the field throughout one winter. The steel plate was fixed at the ground surface with a rock-anchored reaction frame. heave gauges and thermocouples were installed at various depths to determine the position and temperature of the active heaving zone. The general trend was for the surface force to increase as the winter progressed. when the frost line approached the maximum depth the force was in excess of 30,000 lb (13,608 KG). Estimates of the heaving pressure at the frost line ranged from 7 to 12 psi (0.49 to 0.84 KG/cm) square during this period. The variation of surface heaving force was closely associated with weather conditions. Warming trends resulting in a temperature increase of the frozen layer caused the forces to decline.
Leda clay slopes in the Ottawa valley are vulnerable to catastrophic landslides. More than 250 landslides, historical and ancient, large and small, have been identified within 60 km of Ottawa. Some of these landslides caused deaths, injuries, and property damage, and their impact extended far beyond the site of the original failure. In spectacular flowslides, the sediment underlying large areas of flat land adjacent to unstable slopes liquefies. The debris may flow up to several kilometres, damming rivers and causing flooding, siltation, and water-quality problems or damaging infrastructure. Geologists and geotechnical engineers can identify potential landslide areas, and appropriate land-use zoning and protective engineering works can reduce the risk to property and people.
Deposits of Leda clay, a potentially unstable material, underlie extensive areas of the Ottawa-Gatineau region. Leda clay is composed of clay- and silt-sized particles of bedrock that were finely ground by glaciers and washed into the Champlain Sea. As the particles settled through the salty water, they were attracted to one another and formed loose clusters that fell to the seafloor. The resulting sediment had a loose but strong framework that was capable of retaining a large amount of water. Following the retreat of the sea, the salts that originally contributed to the bonding of the particles were slowly removed (leached) by fresh water filtering through the ground. If sufficiently disturbed, the leached Leda clay, a weak but water-rich sediment, may liquefy and become a 'quick clay'. Trigger disturbances include river erosion, increases in pore-water pressure (especially during periods of high rainfall or rapid snowmelt), earthquakes, and human activities such as excavation and construction.
After an initial failure removes the stiffer, weathered crust, the sensitive clay liquefies and collapses, flowing away from the scar. Failures continue in a domino-like fashion, rapidly eating back into the flat land lying behind the failed slope. The flowing mud may raft intact pieces of the stiffer surface material for great distances.
Kochanov, W. E., 1999, Sinkholes in Pennsylvania: Pennsylvania
Geological Survey, 4th ser., Educational Series 11, 33 p., 3rd printing April 2005, Pennsylvania Department of Conservation and Natural Resources / Bureau of Topographic and Geologic Survey, DCNR Educational Series 11, Pennsylvania Geological Survey, Fourth Series, Harrisburg, 1999 - web search 06/05/2010, original source: http://www.dcnr.state.pa.us/topogeo/hazards/es11.pdf - Quoting from the document introduction: The first 18 pages of this booklet contain an explanation of how sinkholes develop. In order to tell the sinkhole story, it is important to discuss a number of related geologic disciplines. The words used to describe sinkholes and these disciplines may be a bit unfamiliar. However, general explanations are given throughout the booklet to help clarify their meanings. Key words are printed in bold type for emphasis. The more important
ones are defined in a Glossary that begins on page 29. The remaining sections, starting with “Sinkholes in the Urban Environment” (page 18), deal with sinkholes and their impact on our environment.
This includes recognition of subsidence features and sinkhole repair.
 Sarah Cervone, [web page] data from the APIRS database, Graphics by Ann Murray, Sara Reinhart and Vic Ramey, Vic Ramey isthe editor. DEP review by Jeff Schardt and Judy Ludlow. The web page is acollaboration of the Center for Aquatic and Invasive Plants, University of Florida, and the Bureau of Invasive
Plant Management, Florida Department of Environmental Protection contact: email@example.com [A primary resource for this article
 Center for Cave and Karst Studies or the Kentucky Climate Center, both at Western Kentucky University
Vanity Fair - web search 06/04/2010 http://www.vanityfair.com/online/daily/2010/06/what-caused-the-guatemala-sinkhole-and-why-is-it-so-round.html
Sinkholes, Virginia Division of Mineral Resources,
Virginia Department of Mines, Minerals and Energy, www.dmme.virginia.gov Virginia Department of Mines, Minerals and Energy Division of Mineral Resources
900 Natural Resources Drive, Suite 500 Charlottesville, VA 22903 Sales Office: (434) 951-6341 FAX : (434) 951-6365 Geologic Information: (434) 951-6342
http://www.dmme.virginia.gov/ divisionmineralresources.shtml - Web search 06/09/2010
Newton, J. G., 1987, Development of sinkholes resulting from man's activities in the eastern United States: US Geological Survey Circular 968, 54 p.
Sinclair, W. C., 1982, Sinkhole development resulting from ground-water withdrawal in the Tampa Area, Florida: U.S. Geological Survey Water-Resources Investigations 81-50, 19 p.
White, W. B., 1988, Geomorphology and Hydrology of Karst Terrains: Oxford University Press, New York, 464 p.
Williams, J. H. and Vineyard, J. D., 1976, Geologic indicators of subsidence and collapse in karst terrain in Missouri: Presentation at the 55th Annual Meeting, Transportation Research Board, Washington, D.C.
Barry F. Beck, A. J. (1999). Hydrogeology and Engineering Geology of Sinkholes and Karst. Rotterdam, Netherlands: A. A. Balkema.
Beck, B. F. (2003). Sinkholes and the Engineering and Environmental Impacts of Karst. Huntsville, Alabama: The American Society of Civil Engineers.
Beck, B. F. (2005). Sinkholes and the Engineering and Envrionmental Impacts of Karst. San Antonio, Texas: The American Society of Civil Engineers.
Tony Waltham, F. B. (2005). Sinkholes and Subsidence, Karst and Cavernous Rocks in Engineering and Construction. Chichester, United Kingdom: Praxis Publishing.
Whitman D., G. T. (1999). Spatial Interrelationships Between Lake Elevations, Water Tables, and Sinkhole Occurence in Central Florida: A GIS Approach. Photogrammetric Engineering and Remote Sensing , 1169-1178.
Sinkholes in Guatemala, Guatemala City, Wikipedia - web search 06/04/2010 - http://en.wikipedia.org/wiki/Guatemala_City
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