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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.



Me, thanks, first you will need to prep the area and remove any, dirt, debris, paint, stain, sealer, loose material, etc. to ensure that whatever material is used it has a solid bond with the existing concrete. Second I need to know whether these gutters are suspended or on grade. I also need to know width and depth of the area being repaired before I can make a product recommendation.
- Chris Technical Services
Thursday, July 10, 2014 at 1:10 PM

You are funny - just wanted to find out how to repair a gutter made with concrete. Great writing by-the-way
- Me
Wednesday, July 9, 2014 at 4:52 PM

Carl, it sounds like the best approach would be to open those cracks up more using a chisel and remove the built up crack filler. Based on the information this may also be caused from lack of expansion joints resulting in the concrete creating its own expansion joint. After you have cleaned out the cracks and removed all the loose material, dirt, debris, etc. use either the Sakrete Top ‘N Bond or the Sakrete Polyurethane Non-Sag Sealant to patch those areas. The Sakrete Top ‘N Bond is used for cracks up to ½” down to a feathered edge. However if these cracks are still moving we recommend using Sakrete Polyurethane Non-Sag Sealant which will provide a flexible bond. The joints must be a minimum of ¼” wide and ¼” deep and a maximum of ½” deep for the Sakrete Polyurethane Non-Sag Sealant.
- Chris Technical Services
Monday, June 16, 2014 at 3:12 PM

My concrete garage floor has numerous hairline cracks. I have tried to fill them by forcing the crack filler into the crack with a flat metal tool, but the crack always reappears. Now I have a buildup of crack filer on both sides of all my cracks. How can I remove the buildup and seal those cracks in my floating garage floor?
- Carl
Saturday, June 14, 2014 at 1:35 AM

Peter, when pouring a concrete driveway you need to follow the mixing instructions on the bag. Avoid a “Soupy” mix. When using that much concrete it is recommended to rent a Concrete Mixer so you can mix multiple bags at a time. Add water in stages until the recommended amount is used. Do NOT over work the concrete and make sure to include expansion joints to avoid cracking. Expansion joints should be placed no greater than every 8 to 12 feet in each direction. Driveways should be a minimum of 4” thick and you should check with your local building codes before you start the project for any discrepancies. Use a household broom to lightly brush in traction marks and finish any unsightly trowel marks. If you have any other questions regarding this project call us at 1-866-725-7383.
- Chris Technical Services
Tuesday, May 6, 2014 at 12:14 PM

What is the ratio of water added to concrete when pouring a driveway?. How much is too much per yard?. By the gallon?. Thanks.
- Peter
Tuesday, May 6, 2014 at 11:02 AM

Mohit, cracks in concrete develops from several different reasons, unfortunately I do not have enough information to answer your question. If you would please call us at 1-866-SAKRETE, and we will be able to help you with this issue. Thanks.
- Chris Germain Technical Services
Monday, April 28, 2014 at 7:49 AM

I have a concrete slab which have cracks in lower portion of slab & seepage problem in slab but on upper portion there are simply some hair cracks. I want to know that is it shrinkage cracks or settlement cracks( may be shuttering problem).
- Mohit Kumar
Sunday, April 27, 2014 at 4:41 AM

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

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



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