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Why do concrete slabs crack?

posted by Bob Monday, April 11, 2011 at 3:05 PM

Concrete is one of the most long lasting, economical building materials ever devised by man. When placed properly and in the right application it will last a long, long time. However, (don’t you just love it when a “however” follows an opening positive statement) nothing good lasts forever and concrete is no exception. It will crack; it is just a matter of when. Take a look at the sidewalk, the driveway, even the floor in stores where the concrete is not covered with tile. They have cracks.

Some of those cracks appeared within hours of pouring. Others took many years to develop. So, if we can put a man on the moon why can we make concrete that won’t crack?

The answer to this is more complicated than I can deal with in this short blog. I also don’t actually know the complete answer so I will just blame it on being too complicated. Seriously I do know some of the causes and even what you can do to avoid them. If you already have cracks, reading this article may be entertaining but it won’t help your cracks. This blog is about avoiding cracks. Without further ado, cracks fall into one or more of several categories: 1) plastic shrinkage 2) settlement 3) drying shrinkage 4)chemical 5) corrosion and 6) overload. Let’s look at these one at a time and see what causes them. By knowing what causes cracks you can help avoid them. Keep in mind this in not a doctoral dissertation so I am going to oversimplify.

1) Plastic shrinkage cracks occur when water evaporates too quickly from the surface. This causes the top of the slab to dry more quickly than the bottom and they pull each apart. This is more likely to happen when it is hot, windy or there is low humidity. To avoid this use proper curing procedures. The key is to keep the surface moist. This can be done by placing wet burlap, mats or towels on the concrete. You can also put your sprinkler on the mist setting and let it run. There are also chemicals that can be applied that retard the water evaporation. The time to begin any of these processes is after the final trowel application and the concrete has stiffened to the point where a wet burlap bag would not leave an impression on the concrete. In very warm weather this may need to be continued for several days. It may seem silly to water new concrete the way you would new grass, but that is exactly what you need to do. Not to complicate things but it is possible on cool, overcast days that you don’t need to do any of this.

2) Settlement cracks occur when the ground under the concrete moves. This can be the result of poorly compacted soil, the wrong kind of soil (sand), water erosion or tree roots. If you have poorly compacted soil, dig it out and replace it with crushed stone before placing the concrete. If you have a situation where water drains across the sidewalk, backfill first with several inches of gravel. If I were a younger man I would go through the neighborhood and plant trees for free as long as they would let me plant them next to the sidewalk. Then when the trees grew and the roots took over I would come by and offer to replace the sidewalk-which would not be for free. If you plan to stay in your home for a while don’t plant trees next to the sidewalk.

3) Drying shrinkage occurs when a slab that is restrained is drying and shrinking. This usually does not occur on free floating slab. It is more of a problem when a slab is tied into another structure like a wall with rebar.

4) There are two ways that chemical reactions can crack concrete. The first is because the concrete itself contains aggregates or cements that simply are not compatible. This isn’t something you should be too concerned about because the manufacturer of the concrete should already know which of these is an issue in your area. The second one is very much under your control. A few years back my church built a beautiful new sanctuary. In front of the church is a large concrete apron that goes out to the street. I told them DO NOT PUT SALT on this slab for a few years. Use sand. So what did they do, they used salt. Guess what. It looks awful. They wanted to blame the contractor. I told them the church did not have a “prayer” of winning that argument.

5) Corrosion occurs when concrete that contains steel re-bar or steel wire mesh gets wet and comes in contact with oxygen. The only way this can happen is when small cracks develop in the concrete due to one of the reasons stated above and channel water into the crack. When water reaches the steel it begins to rust. Rust is expansive. As the steel rusts it pushes out and causes even more cracking. The prevention here is to make sure you treat all little cracks before they become big cracks. See my other blogs for fixing cracks.

6) Concrete is designed to take a certain load. Most sidewalks and residential driveways are designed to take the weight of a car or small truck. If you should decide to back up a loaded tandem axle dump truck or an M-60 tank (for you younger folks that’s what we had when I was in the 1st Armored Division a long time ago) on your driveway, don’t be surprised if it cracks.

It is important to know the way the pros avoid at least some types of cracks. They use control joints. Control joints are basically an acknowledgment that concrete will crack. The control joints can help to eliminate cracks or in a worse case scenario channel where the cracks will appear. Have you ever notice how your sidewalk or driveway has either dividers essentially making several slabs out of one big slab or it has cuts running through it every 3 or 4 feet? These are control joints. There is no set pattern for how often you need these. There are a lot of factors involved here. Maybe I will deal with that in a separate blog.

Finally, for those of you who this is too late for and you already have cracked pavement. I would not be doing my duty if I didn’t remind you that Sakrete has a full line of crack filling and concrete repair products that will help make your concrete cracks a problem of the past.

.

USER COMMENTS

Hi- I must replace 11 wood expansion joints in my walk that is 6' wide by 30' long.It is 30-40 years old with no cracks and each joint is 1.5" wide by 4" deep. Can I just fill this with your crack free cement or mortar cement or must it be a tedious project? Several oldtimers I talked to at Home Depot told me to just fill it with cement and forget it-that sounds too easy.What do you suggest
- Frank Golder
Friday, July 15, 2011 at 3:34 PM

Frank, in order to recommend the correct Sakrete product we need to gather some additional information on your application. Could you please contact our Technical Service Department at 1-866-SAKRETE and we would be glad to assist you.
Dean - Tech Service Team
Friday, July 15, 2011 at 4:30 PM

I'm so glad that the ientnret allows free info like this!
- Patch
Thursday, July 21, 2011 at 8:54 PM

How do address the issue when it is under tile and the grout is cracked? I don't want to keep replacing the grout and keep having the same thing happen again and again. That is what the previous owner did and now I have to deal with it :/
- Traci
Thursday, May 17, 2012 at 8:47 PM

Traci, it sounds like there is movement in the slab causing the crack to continue to reappear. By placing a rigid material such as grout it can't take movement so it will crack. You may want to look into a caulk that will remain flexible. There are caulks in the ProSpec product line, which is a sister company of Sakrete, that come in various colors that could be used in that application. For more information on the ProSpec caulks contact the technical service department at 1-800-334-0784.
Dean - Tech Service Team
Friday, May 18, 2012 at 4:50 PM

if the concrete is not smooth on the finish and does not have expansion joints will that make it crack .
- jim
Saturday, June 23, 2012 at 3:10 PM

Jim, as stated above there are many factors that determine whether a concrete slab will crack or not. For typical driveways and walkways one of the main reasons for cracking is settlement, thats why having control joints are good. Cracking that occurs will often follow the joints.
Dean - Tech Service Team
Monday, June 25, 2012 at 11:42 AM

Is it necessary to seal up the cracks along the joints if there is steel wire mash under the concrete? If so, what material should be used? Thanks!
- Samantha
Tuesday, July 10, 2012 at 12:05 PM

Samantha, it is recommended to treat cracks to prevent further damage. If the steel wire mesh gets wet and is in contact with oxygen then it can start to rust which can cause more cracking. These small cracks in the joints can be treated with either the Polyurethane Self Leveling Sealant or Non-Sag Sealant.
Dean - Tech Service Team
Monday, July 16, 2012 at 2:34 PM

I bought a new home 4 months ago and about 2 months after I started noticing that some of the tile started to crack. I contacted the builder and he decided to removed a couple of tile to see what was causing the tile to crack and we discover that the slab had vety thin cracks. The builder said that I should not be worried about it since it was normal for the slab to have those small cracks. He agreed to remove all the tile and apply someting on the slab to stop the cracking and install a strong tile. Would this resolve the problem?
- sergio
Tuesday, August 21, 2012 at 7:45 PM

Your contractor is correct concrete will crack. To correct the problem with your tiles cracking your contractor is going to install a crack isolation membrane. Crack Isolation membranes are commonly used to isolate in plane cracking of the concrete from transfering up through the tiles. As long as the cracking in the concrete is in plane and does not move up and down or become to large the crack isolation membrane should resolve the cracking tile issue.
- Mike
Wednesday, August 22, 2012 at 1:45 PM

I just done my sidewalk and put two spacers in between the slabs. I use a thin board and poured the cement now I'm thinking I do wrong cause that will let water get in under the sidewalk. What is the best way to seal the cracks to keep the water from getting below the cement sidewalk?
- Quentin
Monday, August 27, 2012 at 11:57 AM

Quentin, you may use the Sakrete Polyurethane Self Leveling Sealant to fill the joint between the two slabs. You will need to fill the joint with backer rod since the joint is deep before applying the Self Leveling Sealant. The maximum depth that the Self Leveling Sealant can be applied is 1/2".
Dean - Tech Service Team
Monday, August 27, 2012 at 4:18 PM

Thanks.
- Quentin
Monday, August 27, 2012 at 5:24 PM

Hello. I am planning to make a basement. The whole area is going to be 32 by 44 foot, devided into several spaces above it, built in wood frame. But the slab and foundation will be one whole. In order to prevent cracking, can I pour a slab that big or is it wiser to break it down a bit? And are expansion joints allowed in foundation as well or should that be one piece without exeption and it's just a fable?
- Andre
Friday, October 19, 2012 at 2:41 AM

Andre, for a slab that size it would be best to incorporate expansion joints between sections. That would be a very large slab to do one continuous pour of concrete. As far as constructing a foundation you may want to contact your local building department, they more than likely have specs that need to be met.
Dean - Tech Service Team
Friday, October 19, 2012 at 5:05 PM

I moved in a house 4 month back. I am already cracks in the driveway at at least two or three places. Should it be covered in the warranty. Thanks, Sanjeev Jain
- Sam
Sunday, November 4, 2012 at 11:39 AM

Hi Sam, which warranty are you referring to? Which Sakrete product used on your driveway?
Dean - Tech Service Team
Monday, November 5, 2012 at 5:04 PM

How long does it take for sakre to dry? And not it was section that was 3 ft long by 1 ft wide. It was a side walk repair. - Jesse
- Jesse
Wednesday, November 21, 2012 at 2:46 PM

Jesse, the dry time will depend on temperature, humidity and air flow. When placing concrete it is recommended to "cure" the concrete for a few days as needed. A typical cure time for concrete is 28 days, this is how long it will take for concrete to acieve it's designed strength.
Dean - Tech Service Team
Tuesday, November 27, 2012 at 10:21 AM

What is the reason for Concrete cracks in mid span of the slab.
- V.K.Krishna
Tuesday, February 26, 2013 at 3:52 AM

There can be many reasons for why concrete cracks. A lot of times settlement can be the cause of concrete cracking if there was not proper drainage in place when the slab was constructed. Plastic shrinkage could also contribute to the concrete cracking if it was not cured properly. We have a more detailed description above on why concrete may crack.
Dean - Tech Service Team
Tuesday, February 26, 2013 at 5:11 PM

We are putting up a small (Lowe's kit) shed in an area where we must remove a tree (about 6-8" in diameter). The shed kit has a floor. What should be done about the stumpt of the tree? Do we have to remove it or kill it?
- Linda W.
Thursday, April 18, 2013 at 7:00 PM

Linda, are you placing a concrete slab over the tree stump? If that is the case then the tree stump would have to be removed.
Dean - Tech Service Team
Friday, April 19, 2013 at 4:32 PM

Hi - Im from over here in New Zealand and wanted to find out reasons why concrete cracks and I came across your blog. I am experimenting with a new form of cement, with no lime. I made a pavement slab on the 6/4/2013 and it has been drying down in my shed.I think plastic shrinkage is my problem, because it dryed to fast on the top and still wet on the bottom causing a crack.Its not in the sun or wind but the humidity is really low to the point of cold. Charles T.
- Charles T.
Wednesday, May 1, 2013 at 4:53 PM

Charles, you are correct, if the concrete dried too quickly and was not cured to allow the surface to retain moisture then that more than likely is the cause of cracking.
Dean - Tech Service Team
Friday, May 3, 2013 at 4:14 PM

We poured a Driveway last fall and the landscapers never back filled the driveway. There is a swale for the water to run across the driveway, but since it wasn't back filled it appears the water went under the driveway, and since we are in Wisconsin, we think it froze under the driveway and cracked the concrete. Does this sound feasible?
- Dave
Thursday, June 20, 2013 at 1:24 PM

Dave, that is a possibility, also it could have been from erosion that caused settlement cracking.
Dean - Tech Service Team
Monday, June 24, 2013 at 3:08 PM

I've been told to water my slab - I live in Texas. should I water it and if so, when using a soaker hose, for how long and for how often should I water it? thanks
- Liz
Friday, July 26, 2013 at 12:02 PM

Hi Liz, it's a good idea to keep the surface of a new concrete slab moist for several days if needed in order to cure properly but I've never heard of wetting down the concrete if it has been fully cured.
Dean - Tech Service Team
Monday, July 29, 2013 at 4:02 PM

I wasn't talking about new concrete - I have a slab that has begun to crack and shift - the house is 50+/- years old - how do I help decrease more cracking by watering with a soaker hose - how often and for how long. Texas is very dry! thanx
- Liz
Monday, July 29, 2013 at 7:39 PM

Liz, unfortunately since the slab has already cracked and is fully cured watering with a soaker hose won't prevent cracking. It could be due to settlement underneath the slab which has caused it to crack. To seal those cracks you can fill them with a polyurethane or the Concrete Crack Filler.
- Dean
Tuesday, July 30, 2013 at 8:38 AM

I moved into a brand new house with a 50 foot driveway. I have a thin crack extending for 25 toward the street. Another crack near the curb is about an 1/8" wide. My builder says this is normal. I am not buying it. Control joints are almost 20 feet apart on 3 of the slabs. Did they screw up? Thanks,
- Terry
Friday, August 30, 2013 at 10:27 PM

Terry, it's difficult to say what caused this cracking of the newly poured slab. Concrete unfortunately will crack, it's just a matter of time, some slabs take longer than others. At this point what I would recommend would be to use a crack filler such as the Concrete Crack Filler, this will seal up those cracks and prevent any issues with freeze/thaw cycles.
Dean - Tech Service Team
Friday, September 6, 2013 at 2:59 PM

If I am doing a science fair experiment in which I am testing which material is the strongest; Concrete, wood, or plastic by placing weights on each one. Will the concrete crack
- E.
Saturday, November 9, 2013 at 9:40 AM

E, depending on the load that is placed on the concrete it could crack. Concrete is tested for its compressive strength by placing cylindrical concrete specimens in a machine that breaks them by compression.
Dean - Tech Service Team
Monday, November 11, 2013 at 5:07 PM

I have heard of something called a slump test for concrete before it is poured to determine its strength. Can this be easily done and what would be an appropriate strength for a driveway slab?
- John
Thursday, December 19, 2013 at 3:57 PM

John, ASTM C143 is the Standard Test Method for Slump of Hydraulic-Cement Concrete and that covers how to test the slump. As stated in the spec, the slump will give you an idea of the consistency of fresh concrete, it typically will increase proportionally with water content and can inversely be related to the concrete strength. For a driveway a general 4000psi concrete mix will be sufficient.
Dean - Tech Service Team
Friday, December 20, 2013 at 8:57 AM

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