Concrete Shrinkage Gaps
Identify & Evaluate Shrinkage Cracks in Concrete Floors Along Foundation Walls
SHRINKAGE CRACKS or GAPS at FOUNDATION WALLS - CONTENTS: How to identify and evaluate shrinkage cracks or gaps in concrete floors along foundation walls or at other locations in poured concrete. Do shrinkage cracks in poured concrete always need repair?
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A detailed guide to concrete shrinkage cracks:
How to recognize cracks or separation between the concrete floor slab and building foundation walls: concrete floor slab shrinkage, how to evaluate normal concrete slab shrinkage, and how to recognize when cracks along a foundation wall at the floor indicate a problem.
This forms part of our longer concrete cracking article which describes the types of cracks that occur in poured concrete 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 in Poured Concrete Slab Floors Along the Foundation Walls
Shrinkage cracks 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.
Here we discuss two types of poured (placed) concrete shrinkage cracks: meandering fine cracks that appear in the open area of the floor or perhaps a wall (shown at left) , and straight cracks or gaps that appear at the juncture of the poured concrete floor and the foundation wall against which it abuts (shown below).
What is unique about shrinkage cracks in concrete is that when they occur in the field of a poured concrete wall or floor, they usually appear to be discontinuous, as shown at left where I marked the two ends of two shrinkage cracks with my black marker.
The shrinkage crack that occurs in the open field of a poured concrete floor or slab 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.
Watch out: while usually concrete shrinkage cracks are very fine in width, 1/16" or less across, severe shrinkage cracks due to mixing or temperature/curing problems can be much larger - enough to require more extensive repairs.
And cracks can appear in a floor or wall due to a combination of factors: shrinkage, loading, settlement, some of which may be significant or may combine forces to cause actional damage or leaks.
How to Identify Slab Shrinkage Along a Building Wall
Shrinkage cracks that occur at the juncture of slab to wall (and also within control joints) appear differently than their brothers out in the middle of the floor or wall.
Look at the photo just above and again here at left. You can see a small gap, about 3/16" wide, which runs along the entire floor slab where it meets the building foundation wall.
The gap is rather constant in width, and if you look closely you can see concrete remains on the foundation wall where the floor was touching the wall at the time it was poured.
A poured concrete slab shrinks away from its perimeter - the building's foundation walls. You will see this shrinkage of even a perfect concrete floor slab with no visible cracks elsewhere in the field of its surface if the floor was poured inside of an existing
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.
The crack shown at page top is a normal condition where a poured concrete slab is installed.
Serious Floor Slab Settlement May be Indicated by Examining Floor shrinkage Cracks at the Foundation Walls
Watch out: while shrinkage crack openings between a floor slab and its foundation (shown above) wall are normal, there is at least one case in which you can detect important floor slab settlement in this location. In the photo above the slab has pulled away from the foundation wall.
But the original concrete debris left sticking to the foundation wall is at the same height as the top of the floor slab itself.
The concrete floor slab in the photo above was probably poured on top of the foundation wall footings, and the chance that the floor would have settled down in this location is slim.
The only difference in height between the original wall/slab contact point and the cured slab would be the slight shrinkage in the thickness of the slab itself.
In our concrete floor settlement photo shown at left the green arrow points to a concrete pour trace left on the building foundation wall - this is where the top of the slab was located when the floor was first placed or poured.
The red arrow in the photo above points to the present top of the same concrete floor slab - it has settled several inches!
A different concrete floor settlement example is in the photo at left. Here the garage floor has broken close to the foundation wall, and the slab has dropped more than 1/2". We suspect improper soil compaction under the slab.
Why would part of the garage floor remain attached to the foundation wall while other floor sections break away? Perhaps the slab was pinned to the foundation wall with re-bar.
So when will we see a settlement problem in a concrete slab at a foundation wall? Some concrete slabs are poured atop deep backfill inside of a foundation.
This is particularly common when an attached garage is built adjacent to a house on a sloped lot. When the rear of the garage (opposite the entry door) is over a hill that sloped down away from the garage entry, the builder had to place extra fill inside the garage foundation before pouring the slab.
Watch out: other more varigated concrete crack patterns 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.
Conditions Leading to Garage or Building Interior Floor Slab Collapse
As we can testify from our first construction job (raking fill dirt level inside of foundation walls in Dam Neck Virginia in 1963), the builder often fails to compact the fill-soil. Pouring a slab atop of soft fill can lead to serious slab settlement, settlement cracking, and in extreme cases, a dangerous collapse of the floor slab. A floor slab can collapse under these conditions:
The floor slab or portions of it were poured on significant amounts of poorly compacted fill-soil
The floor slab was poured on fill over the foundation wall footings, and the slab was not pinned to the foundation wall
The floor slab is not resting on the foundation wall footings throughout all of its perimeter
Water from roof or surface runoff has passed under the floor slab, adding to soil compaction there
The floor slab was poured without sufficient (or any) reinforcing steel
The weight of vehicles contributes to cracking of the slab
A case was reported to us of a car actually falling through the garage floor slab.
Common Signs Warning of a Floor Slab Collapse
Slab push out foundation cracking: Our photo at above left shows how a concrete garage slab has pushed over the foundation wall easily seen from outside.
This damage occurred as the floor slab broke and began to collapse; tipping slab edges actually pushed out a surrounding concrete block foundation wall.
Watch out: because combinations of forces can be at work on a foundation wall that seems "cracked" or damaged in any way, it is possible to have both old movement that occurred at the time of backfill or construction (wavy mortar that is not broken) and more recent ongoing movement and damage from any of a variety of causes. Here are some clues to watch for:
Significant downwards movement of some cracked, uneven slab sections, shown by comparing the present location of the slab upper surface with the concrete marks left along the foundation walls when the slab was originally poured.
Significantly cracked and uneven floor slabs over an area where several feet of fill would have been placed inside the foundation walls before pouring the slab
Horizontal cracks, bulging, or leaning visible along the outside of a concrete block foundation wall near the height of the floor slab (caused by lateral pressure of earth loading as the slab sinks downwards, pushing the soil out). (As in the two photographs shown above.)
Tip for detecting voids in fill under a concrete slab: Try dragging a heavy chain over the floor of a garage or in any location where you suspect the slab was poured over deep fill. The sound of the chain will change significantly if it passes over a void in the slab.
Do we need to repair shrinkage cracks in slabs along the Foundation Walls?
Shrinkage cracks between a floor slab and the foundation wall do not need to be repaired except in these instances:
Water entry: Shrinkage cracks in a concrete slab or floor wherever they occur, including along a foundation wall might need to be repaired to avoid water leakage from below
the slab. Of course you should also be taking other steps to direct water away from the building as well since sealing a floor crack is a "last-ditch" band aid effort to address water entry and it's likely to fail in the long run.
Radon entry: Shrinkage cracks in a concrete slab or floor wherever they occur, including along a foundation wall might need to be repaired to stop radon gas from entering the building.
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 for shrinkage cracks also see how we prevent shrinkage cracks
in poured concrete floors and walls by reading CONTROL JOINT CRACKS in CONCRETE.
For detailed information about foundation repair methods, including repairs to various kinds of cracks in concrete, see the articles listed below.
Continue reading at CONCRETE SLAB CRACK REPAIR or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.
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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.
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
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
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
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 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: 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
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.
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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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