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

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.

InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.

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 linear 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.

Also see TEMPERATURE EFFECT on CONCRETE POUR

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?

Reply:

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 require 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 larger 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 .

 




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Reader Comments, Questions & Answers About The Article Above

Below you will find questions and answers previously posted on this page at its page bottom reader comment box.

Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

On 2022-07-21 by InspectApedia-911 (mod) - crack about 1/16" to maybe 3/32" wide is not abnormal in a control joint

@Anonymous,

In your photo we don't see signs of a bad concrete mix and crumbling concrete - just breakage from movement as we reviewed before.

I'm reluctant to reach a broad conclusion with so little data - it's like inspecting a building through the eye of a needle.

On 2022-07-21 by Anonymous

@InspectApedia-911, thank you. Should I be concerned about the crumbly concrete? I have the same crumbly concrete in my garage surrounding hairline cracks. From your reply to a different post below, it seems the concrete mixture had too much water perhaps. Is this just mainly a cosmetic concern that I should live with?

On 2022-07-21 by InspectApedia-911 (mod)

@Anonymous,

If we ignore the fracturing it looks as if your crack is about 1/16" to maybe 3/32" wide - not abnormal in a control joint.

As long as the masonry on both sides of the crack is at the same level we don't suspect settlement or other troublesome movement.

I'd leave it alone, but if for cosmetic reasons you want to fill the crack, use a color-matched but flexible sealant.

On 2022-07-21 by Anonymous

Hello sir, is the gap created by the cracking in the control joint in my 6 month old driveway ok to leave as is? I know the control joint is meant to crack, but it seems the gap created by it is rather large. I live in a new townhouse subdivision and none of my neighbors’ driveway has this issue, only normal hairline cracks in the control joints.

Not sure if it’s related, but there are a ton of ants just around this gap. I also see crumbling concrete bits surrounding the crack. If I took those bits out, which is very easily done, the gap would be even larger. I’m under builder warranty and wondering if a fix is necessary for functional reasons (water, bugs, etc) or aesthetic reasons. Thank you for your expertise.

On 2022-01-18 by Inspectapedia Com Moderator - widening of the crack could mean that there is ongoing movement in the slab

@Beth,

Unless there was actual blasting going on at the nearby construction site the causes of crack opening on your floor are likely to be one of those discussed in the Recommended Articles at the end of the article above: settlement, heaving, shrinkage.

And nearby construction would have nothing to do with the original quality of the mix or pour of your concrete slab: if the surface is soft enough to crumble that's a mix issue as I suggested earlier.

I can't see the control joint so can't say that the crack is in a control joint - or not - it looks a bit wobbly and wavy for that term to apply.

Causes of movement in masonry are given in detail in this article series: settlement, frost heaves, expansive clay, thermal movement, and others.

IF there is cyclic movement that often explains fracturing of material within a crack.

On 2022-01-18 by Beth

@Inspectapedia Com Moderator,
Thank you! I noticed the crack worsened over the last 6 months when construction of a new home began next to my home. With my home being 3 years old, would you guesstimate the crack becoming even larger?

On 2022-01-17 by Inspectapedia Com Moderator

@Beth,

Widening of the crack could mean that there is ongoing movement in the slab.

Crumbling at the edges of a concrete slab crack could mean that the pour mix was not proper and the concrete surface is soft.

On 2022-01-17 by Beth

What would cause a control crack on a slab to widen or take on a crumbling appearance in various locations in one section of the control crack?

Concrete slab crack (C) InspectApedia.com Beth

On 2021-12-08 by Inspectapedia Com Moderator (mod)

@Paul,

I'd try to push the seal back into the joint; only if most of the original control joint has closed completely might I think more action is needed.

On 2021-12-08 by Paul

If the rubber expansion joint separates from one side of the concrete joint after movement as that expansion joint failed and needs to be redone ?

On 2021-11-18 by Inspectapedia Com Moderator - cyclic heaving can show up as jamming doors or windows

@Tricia,

Ask local builders or even your neighbours if your home is built in an area of expansive clay soils - that can explain cyclic heaving that indeed can show up as jamming doors or windows.

Then use the InspectApedia on-page search box to find our article explaining EXPANANSIVE CLAY SOILS - or find that in the Index to Related Articles above on this page

On 2021-11-18 by Tricia

I live in AZ and purchased a two-story home three years ago built in 2001. About two years we started having issues opening our master bedroom door upstairs as it would still in the top left corner (opposite the hinges). A year later, it got worse. Instead of just sanding it down, I called out foundation solutions who resolved that I had very minimal slab movement and needed to have a specific crack fixed and gutters installed.

Got two opinions with both the same overall opinions. Then started seeing floor boards lifting at every connection as well.

Had tech out to fix crack only to find out it was contraction joints cut all over my slabs. I want to say these joints were cut ever 5 to 8 feet. My house is 3200 Sq ft at two stories, I just think these joints are excessive.

About half the joints have been stitched now but lifting upstairs have nor corrected and not sure yet if they will get worse. I am not sure what to do, if I can do anything else to mitigate moving or if I literally have to have someone else come secure down the boards upstairs.

On 2020-09-19 by (mod) - control joints made with a V-tool trowel are cracked open at he bottom

Crack appears in concrete control joint - just as it should (C) InspectApedia.com LRE-posting from private email:

L asked: The scored groove control joints that were made with a V-tool trowel are cracked open at he bottom if the control joint. Is that ok? Won't water get underneath the slab and raise it up?

There were no cracks coming from the scored V groove.

The V scored groove itself split apart, like opening up a book. Is this okay? Most all the scored grooves that were made with a V-tool have split down the middle. Is that okay to have a lot of the seams split like that?


Moderator reply:

The control joint is intended to make the cracks - which are pretty much unavoidable - follow the joint. That's to avoid more-random cracks at other locations in the slab;

IF there's a water problem the control joint is not the cause; rather that would be failure to keep water away from the building outside.

On this page you will see photos of other cracks or openings in concrete control joints.

On 2019-10-15 - by (mod) -

Significant water leakage under any paved surface can cause settlement or in an area of expansive clay soils, heaving and settlement cycles; if you face those concerns, these hairline cracks at control joints are not going to be the main influencing factor - after all the entire perimeter of the drive is open to the weather too.

Details of drive construction are pertinent: over fill, over clay alone, over gravel on clay, drained, compacted, subject to or protected from surface runoff?

On 2019-10-15 by Joe

Thanks again. We're in Ga. so no danger of frost damage. That said, if the cracks are not sealed could long term water seepage cause potential soil compaction and wider cracks ? Driveway is built on clay.
Appreciate your feedback

On 2019-10-14 - by (mod) -

There's no benefit to sealing these cracks if you are not seeing frost damage and in my OPINION doing so would look ugly;

IF there were a need to keep water out you'd probably need first to saw the joints open to a wider gap then use a polyurethane sealant; I'd not do it.

On 2019-10-14 by Anonymous - do I need to seal these cracks in the control joint of my concrete?

@Joe,
Appreciate your reply. I attached more pics. When are control joint cracks deemed too wide ? Should they be sealed ? Thanks again. Joe

On 2019-10-13 - by (mod) -

Joe

I suspect these are normal shrinkage cracks occurring right where they were supposed-to, at control joints cut into the concrete drive.

A few more photos would help.

On 2019-10-13 by Joe

My driveway is 18 mos. old. The crack control joints have wide gaps within them about 1/4" wide.
What is the best way to identify the cause of such wide cracks ?
Thanks.

Joe

On 2018-11-09 1 by (mod) - proper control joints protect the concrete slab

Karen

As long as the control joints are properly cut after the slab is poured, I would be stunned to hear of a report anywhere claiming that the 9 day delay in creating those control joints created itself any problem in the slab.

The concrete has only barely begun to cure in the first nine days of its life and the stress is continue to develop over a much longer period. Even if there were already stressors in the slab they're relieved by the control joint.

I did some research to see if I could find any expert expressing a different View.

Interestingly in some situations, a seven delay between placement of a new concrete slab and cutting-in of control joints is actually necessary in some cases to permit proper cuts without damaging the slab.

See this example

Research on Concrete Control Joint Preparation, Methods, Timing

On 2018-11-08 by KarenKenny

We are in the process of having a home built. The 2100 square foot basement and 1200 square foot garage were poured cut the supervisor forgot to have contraction joints cut until 9 days after the pour.

I know the concrete will crack, all concrete does with or without cuts. My questions are 1) what is your best guess of what the effects will be...how bad? 2) is there anything we can do to reduce the impending damage?


...

Continue reading at FREEZING & WATER DAMAGED SLABS or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see CONTROL JOINT CRACKS in CONCRETE FAQs - questions and answers about control joints in concrete slabs, posted originally on this page

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CONTROL JOINT CRACKS in CONCRETE at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


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