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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

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.

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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?

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:

• Frost Heave/Expansive Soil Cracks in Slabs,
• Settlement Cracks vs. Frost Heaves,
• Settlement Cracks vs. Shrinkage Cracks, and
• Standards for Repair of Cracks in Floors since further repair or other actions may be needed.

How are poured concrete slab control joints made?

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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• "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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