How to Evaluate Cracks at Control Joints in Concrete Floors & Slabs
InspectAPedia® -
Evaluating Cracks at Control Joints or Expansion Joints in Concrete Slabs & Floors
Causes of and types of floor slab cracking at poured concrete control joints
Are cracks at control joints or expansion joints in concrete a problem?
Are control joints always needed in poured concrete?
Recommended methods for sealing cracks in concrete floors & slabs
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This article describes the causes, evaluation, and repair of cracks at control joints in poured concrete slabs or floors.
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.
See SLAB CRACK EVALUATION. And although here we focus on control joints needed in poured concrete floor slabs and monolithic concrete foundations, control joints are also required in certain masonry walls, including brick walls and in some cases concrete block walls as well as poured concrete walls. See Brick Thermal Expansion Cracking.
How to Use & Inspect Control Joints in Poured Concrete Slabs
What is a concrete slab control joint & why do we need control joints in concrete?
Because concrete shrinks as it cures (about 1/16 inch for each 10 liner feet or by other sources, about .66 inches per 100 feet), and because there may also be some expansion and contraction of poured concrete in response to
temperature (about 0.25 inches per 100 feet per 25 degF temperature change, with a maximum of about 0.5" per 100 feet) and moisture changes in its environment, a large solid slab of poured concrete for a floor or slab is likely to crack.
Control joints, called "relief joints" by some builders and more loosely speaking, "expansion joints" by others, are built into a well-designed poured
concrete slab so that the occurrence of more random, ugly cracks is less likely.
Remember that concrete shrinkage itself is a normal process. If a pour and control joints are perfect, cracks caused by concrete shrinkage
will not be noticeable - they'll occur inside the control joints (as we show below), or if a slab shrinks perfectly with no internal cracks,
you'll see a gap opening around the perimeter of the slab where it abuts the foundation walls.
During the concrete curing process, a chemical process called hydration, concrete hardens, using some of the water molecules in its original content.
Concrete typically takes 28 days to reach its design strength; a considerable portion of concrete shrinkage is going to occur during this interval,
particularly during the first week or less. Even though the concrete's design strength is reached in about a month, concrete continues to harden
for days or weeks after that point too.
What do control joints or "expansion joints" look like?
The photograph at page top and the photo just above where Andy is walking away from the camera show expansion joints in a garage floor slab in Arizona. Even in a climate
where we do not anticipate freezing, control joints are needed to prevent random shrinkage cracks that would otherwise occur in a large
concrete floor slab pour like this one. Notice that we do
not see other cracks in this slab.
Control joints are likely to appear as straight lines at regular intervals across
a poured concrete slab (if they were used in the construction of the slab) such as we show in the sketch below, at the lines marked (G) at 4' intervals
or larger depending on the concrete materials and slab design used.
Shrinkage cracks that occur at control joints such as shown in the pair of close up concrete slab control joint crack photos here,
are occurring where they are supposed-to. The fine crack shown in the left-hand photo of a concrete slab control joint is normal - this crack
would have occurred in a random pattern instead of along the control joint if this floor slab (the same floor shown at the top of this page)
had been poured without any control joints.
In a different building, the width of the control joint crack in the right-hand photo above was surprisingly large. These cracks are not normally a defect in the slab but may be
a source of water or radon gas entry into the building and may need to be sealed.
Frost Damage Can Cause Damage Exceeding the Capability of Concrete Control Joints
Uneven, heaved concrete: If if concrete surface of the floor or slab or sidewalk on either side of an apparent "shrinkage crack"
in a concrete surface is at two different heights, forces other than simple concrete shrinkage are at work. In this photo the outdoor slab
has been heaved by frost, probably exacerbated by wet soils and perhaps poor drainage below the poured concrete.
Notice the steel manhole in this photo. Our first guess was that a buried sewer drain became clogged, stopped, and frozen, causing
the ground (and concrete) to heave along the path of this pipe.
We sometimes find this concrete floor failure pattern in basements of homes built in freezing climates if
the home has been left un-heated during freezing winter.
If your
concrete slab or sidewalk cracks look like this, you should review the text at the following diagnostic articles:
The mason who is pouring a slab greater than twenty feet in any direction has to prepare the site for the pour, including
the provision of control joints in the slab when its concrete forms are being placed or else during the pour itself.
An individual control joint is made by inserting a flexible
material (plastic or in the old days, jute or strips of Homasote™) which is 1/4" to 1/2' in thickness (width) and which runs the length of the control
joint. The same material may be placed around the perimeter of a floating slab where it contacts the perimeter of an existing building foundation wall.
Similar control joints are often used where a concrete sidewalk abuts a building or other structure.
Methods for providing control joints in concrete slabs
Flexible joint inserts in poured concrete floors or slabs, using 1/4" to 1/2" thick flexible control joint material
V-tool trowel can be used to score a groove in the still flexible poured concrete floor or slab before it has fully hardened, creating a pre-defined
and straight "weakened" point in the slab which invites shrinkage cracks or other cracks to occur at that location. The depth of the "vee"
cut by this trowel is much less than the thickness of the concrete slab, running from about 3/16" to 1" in depth of cut.
Sawn control joints are cut into a cured and hard poured concrete slab (and into other masonry surfaces) after the concrete (or other masonry)
has hardened. We've recommended this approach (along with other repairs) where we found destructive thermal expansion of large brick masonry walls that were
constructed without expansion joints. Sawn control joints are normally filled or partly filled with a special caulk or masonry sealer
(described below).
The page top photograph above shows an outdoor poured concrete slab that had control joints or something that looked like them. Even the best control joints
were no match for having poured this concrete over episodically wet, frost-heaving soil.
Only by providing excellent drainage would the cracking
and heaving visible in this photo have been avoided.
How deep and wide should a concrete control joint be? At what intervals should we place control joints in concrete slabs?
The width of a concrete slab control joint is the same as the control joint insert (1/4' to 1/2" in width) or of the vee-trowel (about 3/8" wide), or
of the saw blade used to make the cut after the concrete has hardened - typically about 1/8". In concrete roof slabs using lightweight concrete such as Perlite(R), control joints
may be specified at a much wider thickness of 1" around roof penetrations like stairways and skylights.
This is because a rooftop is
exposed to wider temperature swings than indoor building areas such as a basement floor slab.
The depth of a concrete slab control joint should be equal to one fourth of the thickness of the slab, or deeper. So a six inch thick poured
concrete floor would use control joints of about 1.5" in depth. You'll notice that this is deeper than the depth provided by the "vee trowel" discussed
above. A vee trowel is more commonly used to make pseudo-control joints in concrete sidewalks.
The spacing interval for control joints in a slab
varies depending on the kind of slab (monolithic slab foundation, floating slab floor inside an existing foundation, sidewalk, vehicle pavement),
the dimensions of the slab, the kind of concrete being poured (perhaps containing crack-resisting fibers), and the presence of other reinforcing
materials (steel re-bar or steel mesh).
Do Cracks Ever Occur Out of the Control Joints in Poured Concrete?
Cracks in poured concrete can indeed occur out of a control joint. Reasons for this bad behavior might include deficiencies in the concrete
mix or curing conditions that cause shrinkage forces to occur in locations between control joints and in spite of them.
An example is shown
in this photograph of a small (and insignificant) concrete shrinkage crack that occurred at the intersection of several control joints in
a floor slab.
Perhaps the worker did not cut the control joints deep enough in this location where we see the intersection of four
control joints, or other forces may have been at work.
Still, at the end of the day, you can expect far less cosmetic or other
more problematic cracks in a poured slab if control joints are installed at the proper interval and proper depth.
Are Control Joints Absolutely Necessary in Poured Concrete Slabs?
Strictly speaking, perhaps not. Some builders and masonry contractors use concrete which contains reinforcing fibers or other additives intended to reduce slab cracking, and indeed
to be fair, we've inspected some large slabs that had no control joints, and in which we did not see shrinkage cracking.
But based on having inspected
quite a few pours with and without anti-cracking-additives, our opinion remains that best practice
is to always include properly-spaced and properly-designed control joints in a slab or concrete floor concrete pour in residential buildings.
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Additional technical contributors & reference sources for this article are listed below.
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"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
Jay Hodgens,P.E., Hodgens Engineering Service, Newburgh, NY, james@hodgens.net, for assistance with links and references to regulations regarding underground storage tanks as well as comments on other topics.. Mr. Hodgens has been licensed as a professional engineer in eight states and has developed over 450 SPCC plans in compliance with reguilations in twelve states. Mr. Hodgens can be reached at 845-496-0494. His proposed amendments to US EPA 40 CFR part 112 can be read at http://www.hodgens.net/hes/10-07comments.pdf . 1/9/2009. Mr. Hodgens is a frequent contributor to InspectAPedia.com.
Repair of Foundation Cracks
For detailed information about foundation repair methods, including repairs to various kinds of cracks in concrete, see:
FOUNDATION REPAIR METHODS for our catalog of Foundation Repair Methods - Examples of Typical Foundation Repairs for various types of foundation cracks, leaks, settlement, movement, or other failures
"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
Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
Our recommended books about building & mechanical systems design, inspection, problem diagnosis, and repair, and about indoor environment and IAQ testing, diagnosis, and cleanup are at the InspectAPedia Bookstore. Also see our Book Reviews - InspectAPedia.
The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 2010, $69.00 U.S., is available from Carson Dunlop. The Home Reference Book is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. InspectAPedia.com ® editor Daniel Friedman is a contributing author. Field inspection worksheets are included at the back of the volume.
Design of Wood Structures - ASD, Donald E. Breyer, Kenneth Fridley, Kelly Cobeen, David Pollock, McGraw Hill, 2003, ISBN-10: 0071379320, ISBN-13: 978-0071379328
This book is an update of a long-established text dating from at least 1988 (DJF); Quoting: This book is gives a good grasp of seismic design for wood structures. Many of the examples especially near the end are good practice for the California PE Special Seismic Exam design questions. It gives a good grasp of how seismic forces move through a building and how to calculate those forces at various locations.THE CLASSIC TEXT ON WOOD DESIGN UPDATED TO INCLUDE THE LATEST CODES AND DATA. Reflects the most recent provisions of the 2003 International Building Code and 2001 National Design Specification for Wood Construction. Continuing the sterling standard set by earlier editions, this indispensable reference clearly explains the best wood design techniques for the safe handling of gravity and lateral loads. Carefully revised and updated to include the new 2003 International Building Code, ASCE 7-02 Minimum Design Loads for Buildings and Other Structures, the 2001 National Design Specification for Wood Construction, and the most recent Allowable Stress Design.
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.
"Avoiding Foundation Failures," Robert Marshall, Journal of Light Construction, July, 1996 (Highly recommend this article-DF)
"A Foundation for Unstable Soils," Harris Hyman, P.E., Journal of Light Construction, May 1995
"Backfilling Basics," Buck Bartley, Journal of Light Construction, October 1994
"Inspecting Block Foundations," Donald V. Cohen, P.E., ASHI Reporter, December 1998. This article in turn cites the Fine Homebuilding article noted below.
"When Block Foundations go Bad," Fine Homebuilding, June/July 1998
Masonry structures: The Masonry House, Home Inspection of a Masonry Building & Systems, Stephen Showalter (director, actor), DVD, Quoting: Movie Guide Experienced home inspectors and new home inspectors alike are sure to learn invaluable tips in this release designed to take viewers step-by-step through the home inspection process. In addition to being the former president of the National Association of Home Inspectors (NAHI), a longstanding member of the NAHI, the American Society of Home Inspectors (ASHI), and the Environmental Standard Organization (IESO), host Stephen Showalter has performed over 8000 building inspections - including environmental assessments. Now, the founder of a national home inspection school and inspection training curriculum shares his extensive experience in the inspection industry with everyday viewers looking to learn more about the process of evaluating homes. Topics covered in this release include: evaluation of masonry walls; detection of spalling from rebar failure; inspection of air conditioning systems; grounds and landscaping; electric systems and panel; plumbing supply and distribution; plumbing fixtures; electric furnaces; appliances; evaluation of electric water heaters; and safety techniques. Jason Buchanan --Jason Buchanan, All Movie Review
Masonry Structures: Behavior and Design, Robert G. Drysdale, Ahmid A. Hamid, Lawrie R. Baker, The Masonry Society; 2nd edition (1999), ISBN-10: 1929081014, ISBN-13: 978-1929081011
Straw Bale Home Design, U.S. Department of Energy provides information on strawbale home construction - original source at http://www.energysavers.gov/your_home/designing_remodeling/index.cfm/mytopic=10350
More Straw Bale Building: A Complete Guide to Designing and Building with Straw (Mother Earth News Wiser Living Series), Chris Magwood, Peter Mack, New Society Publishers (February 1, 2005), ISBN-10: 0865715181 ISBN-13: 978-0865715189 - Quoting: Straw bale houses are easy to build, affordable, super energy efficient, environmentally friendly, attractive, and can be designed to match the builder’s personal space needs, esthetics and budget. Despite mushrooming interest in the technique, however, most straw bale books focus on “selling” the dream of straw bale building, but don’t adequately address the most critical issues faced by bale house builders. Moreover, since many developments in this field are recent, few books are completely up to date with the latest techniques. More Straw Bale Building is designed to fill this gap. A completely rewritten edition of the 20,000-copy best--selling original, it leads the potential builder through the entire process of building a bale structure, tackling all the practical issues: finding and choosing bales; developing sound building plans; roofing; electrical, plumbing, and heating systems; building code compliance; and special concerns for builders in northern climates.
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.
"Geoscape Ottowa-Gatineau Landslides", Canada Department of Natural Resources, original source http://geoscape.nrcan.gc.ca/ottawa/landslides_e.php - quoting from that source:
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.
[1] Sarah Cervone, [web page] data from the APIRS database, Graphics by Ann Murray, Sara Reinhart and Vic Ramey, Vic Ramey is
the editor. DEP review by Jeff Schardt and Judy Ludlow. The web page is a
collaboration of the Center for Aquatic and Invasive Plants, University of Florida, and the Bureau of Invasive
Plant Management, Florida Department of Environmental Protection contact: varamey@nersp.nerdc.ufl.edu [A primary resource for this article
[2] 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
Sink Hole & Related Engineering References
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.
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