Photograph of a concrete slab control joint How to Evaluate Cracks at Control Joints in Concrete Floors & Slabs

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Concrete slab cracks at control joints:

This article describes the causes, evaluation, and repair of cracks at control joints in poured concrete slabs or floors.

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 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.

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How to Use & Inspect Control Joints in Poured Concrete Slabs

Photograph of a cracked concrete slab, cracks at a control jointHere we focus on control joints needed in poured concrete floor slabs and monolithic concrete foundations. But 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 WALL THERMAL EXPANSION CRACKS

Separately at CONCRETE SLAB CRACK EVALUATION we catalog the different types of cracks that show up in poured concrete.

What is a concrete slab control joint & why do we need control joints in concrete?

A control joint controls where and how a shrinkage crack appears in poured concrete, and it allows for thermal expansion or contraction without additional damage. ]

Without a control joint, cracks in concrete floors, walls, or ceilings appear at stress points in uneven, diagonal, or other patterns in locations where they may be unsightly or may cause damage such as cracks in ceramic tile or other floor coverings, or may be traced to leaks.

How much does concrete shrink as it cures? How much does concrete move in response to temperature changes?

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.

Photograph of a cracked concrete slab, cracks at a control jointControl 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.

Description of the concrete curing process

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?

Photograph of a cracked concrete slab, cracks at a control jointA control joint is a gap, usually formed in a straight line, placed at intervals to control where and how cracks will occur in poured concrete. When you see a "crack" or joint that is formed in a straight line, dividing poured concrete into sections, most likely it's a control joint.

Concrete control joints may also be cut by a power saw if they were omitted during the original pour.

In the photo at left is a tooled control joint in a concrete floor slab.

A concrete control joint that was formed during the pour or placement of the concrete usually is tooled to round the upper edges of either side of the joint, and the joint extends some depth into the concrete, or in some cases (such as sidewalks and some floors) the control joint may extend through the full depth or thickness of the concrete. Full-depth control joints are normally filled with a flexible material.

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.

Close-up Photos Reveal Concrete Shrinkage Cracks Within the Control Joint

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.

Photograph of a cracked concrete slab, cracks at a control joint

The fine crack shown in the photo of a concrete slab control joint (just above) 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 photo below was surprisingly large.

Photograph of a cracked concrete slab, cracks at a control joint

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

Photograph of a cracked and heaved concrete slab that had 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:

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

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?

Photograph of a concrete slab control joint

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.

Photograph of a wandering shrinkage crack in concrete slab that had control jointsPerhaps 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.

- Thanks to Jay Hodgens, P.E. for technical edits.

Control Joints in Concrete: Cutting Details

Reader Question: timing & depth of concrete slab control joint cuts

(July 29, 2014) M fetter said: [paraphrases] Can you tell me when following the concrete pour or placement the control joints should be cut? And can you tell me the required control joint cut depth in a concrete slab?


M.F. Control joints are often provided-for in the placement of concrete by installing control joint materials at the time that the forms are set and before the pour. However if after a concrete pour the engineer determines that control joints need to be added in a slab, they can be cut using a concrete saw.

Those cuts are usually not attempted until the concrete has cured sufficiently to be undamaged by both the foot traffic and the operation of the concrete saw. In my own (limited) experience, cutting a poured concrete slab immediately after it can be walked-on risks breakage of the slab at the cut edges. A typical standard would requir 16 hours of set time between poured free jointed slab sections (see below).

Branz, a New Zealand construction research firm offers additional helpful details about concrete slab pouring and concrete control joints (as do many other sources).

Concrete Slab Shrinkage Time

Concrete will continue to shrink over at least the first 12 months after it has been poured, with about half of the shrinkage occurring in the first 4 months.

Spacing Requirements for Control Joints in Poured Concrete

NZS 3604:1999 and similar standards require control joints such that no individual poured concrete slab section is largter than 24m in any direction. If the slab is reinforced using #668 mesh then the maximum concrete slab section or "bay" should not be larger than 12m x 12m. - Branz (2005)

A "Free Joint" in a concrete slab is a construction joint that has no reinforcement (such as re-bar) passing through it. When a free joint is formed by making separate pours of concrete then at least 16 hours should be allowed between the pours, allowing the side of the joint in the first pour to cure or "harden" for that interval.

Timing of Cutting Add-on Concrete Slab Control Joints

Branz (2005) advises that in summer weather control joint cuts should be made within 24-hours of the initial concrete pour while in winter the control joint cuts should be made within 48 hours of the concrete pour. I presume that this timing is in part to reduce the chances of uncontrolled shrinkage cracks appearing elsewhere in the slab.

Depth of Concrete Control Joint Cuts

When cutting a retrofit control joint in an existing poured concrete slab, an approach that can speed the whole construction process (by allowing larger slab area pours), the cut depth should be half the thickness of the slab and at least deep enough thorough the slab that any steel reinforcement that would have spanned the cut is also cut through.

Control Joint Alignment Controls

Branz describes four approaches to control joint alignment management:

  1. A tongue and groove design aligns the edges of independent pours.
  2. A pre-constructed (presumably with flexible insert) control joint in slabs whose re-bar or steel reinforcement is stopped several inches from the edges of each slab bay
  3. Dowel pinning between abutting slab sections: the dowel is set in the first pour and projects into the area where the second slab bay will be poured. The exposed dowel surfaces are covered with a flexible material to provide a separator coating.
  4. A 5 mm cut is made through at least half of the thickness of a poured slab, deep enough to cut any steel reinforcement. - (Branz 2005)

Watch out: do not lay ceramic tiles across control joints as movement there is likely to cause cracking in the filed finish-floor .


Continue reading at FREEZING & WATER DAMAGED SLABS or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.


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Concrete Crack Diagnosis & Repair Articles

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