Why do concrete slabs crack?

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.

SAKRETE® Concrete Crack Filler – ready-to-use, for ½"-wide to ¼"-deep cracks

SAKRETE Top‘n Bond® – for applications from ½" to a featheredge

SAKRETE Concrete & Mortar Repair – caulk for applications from 1/8" to 3/8"-wide cracks

SAKRETE Fast Setting Cement Patcher – for applications from ½" to 2" thick


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Comments:

  • Lee-Technical Service

    07.05.2017
    Sandip, it would be impossible to determine what the problem is without physically looking at it. We would recommend that you hire a professional to inspect the area and access the issue.
  • Sandip joshi

    07.01.2017
    I have a top floor flat.Years earlier the apartments decided to go for enclosing the entire terrace by asbestos sheet . Now my ceiling slab has given away.what may be the probable cause ?
  • Lee-Technical Service

    06.21.2017
    Tatum, cracks occur in concrete for several different reasons. Without knowing the structural characteristics it would be hard to determine the reason for the crack. You could have a structural engineer come out and make an assessment of your slab and tell you want has happened to cause it to crack.
  • Tatum

    06.21.2017
    On March 20, 2017 I had concert poured for a garage. This is a new construction home. We gave not moved in yet and have noticed a crack that runs the length of the concert. We contacted the builder and we were told that this is a key way and that in the winter it will close back up. Is this the case or could it be something different.
  • Lee-Technical Service

    06.02.2017
    Kenn, from what you have described, we would recommend that you get a professional to come out an inspect it in person to assess the situation.
  • Kenn

    06.02.2017
    I live on the upper floor of a condo with a slab balcony.Lived here for 30 years.There is a metal pan under my slab which is the ceiling for the patio below.I'm trying to tell my association that the rust coming from that pan and the 3 long cracks in my slab are connected. The water from rain adjacent to my patio is a deluge during storms and falls directly on the cracks in the slab.I'm concerned the patio will give way and fall to the floors below.
  • Lee-Technical Service

    05.10.2017
    Steve, shrinkage cracking will typically take placed during the initial curing cycle which is within the first 28 days. Cracking after 17 years would not be from shrinkage, it will be from an environmental anomaly. Such as erosion, rotting debris, root growth, etc.
  • Steve

    05.09.2017
    I understand that "normal concrete shrinkage" cracks always have 3 lines originating from a center. Is this correct? Also, is it possible for cracks to appear from normal concrete shrinkage on a path around our house 17 years after the pour when they were not visible before then?
  • Lee-Technical Service

    12.13.2016
    Larry, water on the surface should not be the cause of this issue. Most cracks are due from the lack of expansion joints or something has compromised the integrity of the slab bedding. If there are not enough or any expansion joint and the slab is stressed enough, then it will alleviate itself by cracking. If the bed has been compromised due to erosion or the decay of organic debris, then the slab can crack. If expansion joints are incorporated in the slab then at east the slab can crack and still be aesthetically pleasing visually. As far as a leak damaging the concrete, your moisture vapor barrier beneath the slab would have stopped the moisture from compromising the bedding.
  • Larry

    12.12.2016
    I had a water leak in my house and it drained onto the cement where this is now a large crack in the cement. No other crakes to speak of. Re-cemented in 2012. Can water from this leak cause cement damage? Thank you in advance. Larry
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