Why do concrete slabs crack?

posted by Bob

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.

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58 USER COMMENTS

We had a 6" floating slab poured for a garage next to house with 2 rows rebar around and fiber mesh. Ground needed to be built up about 5' high in back & on 1 side. The ground underneath is clay. The stone and fill were never compacted. Pad cracked in the months after & over the following year it cracked into 8 sections. The cracks started across the middle in both directions & worsened with back sections have lowered and dropped. Is this an excavating problem or improperly poured concrete?
- Brenda
Friday, April 22, 2016 at 7:49 AM

Brenda, based on the information given, it sounds like the issue is from the lack of compaction in the base. We would recommend that you consult an Engineer and have them evaluate your project.
- Lee-Technical Service
Thursday, April 21, 2016 at 4:31 PM

We had a 6" floating slab poured for a garage next to house with 2 rows rebar around and fiber mesh. Ground needed to be built up about 5' high in back & on 1 side. The ground underneath is clay. The stone and fill were never compacted. Pad cracked in the months after & over the following year it cracked into 8 sections. The cracks started across the middle in both directions & worsened with back sections have lowered and dropped. Is this an excavating problem or improperly poured concrete?
- Brenda
Thursday, April 21, 2016 at 4:09 PM

Pete, First you should check with their local building code department. Most code enforcement require rebar epoxied into the existing foundation/substrate to stabilize the new poured area. However the joint between the old and new will need to be honored through the overlayed wear surface as it will crack or move independently.
- Chris Technical Services
Friday, February 12, 2016 at 4:26 PM

I will be adding an extension of 12'x20' slab to my old 12'x20' kitchen slab. I would like to pour a top cap of 5 sack pea gravel, 1 3/4" thick with wire mess. I would like to know how I can achieve this without the top cap breaking where the old slab meets the new slab so I can have one solid look. I will be placing Metallic Epoxy floor coating to the entire new slab. any ideas ??
- Pete
Tuesday, February 9, 2016 at 3:44 PM

Eng Kilwa, it sounds like the beam is cracking due to an excessive point load. We suggest you have a structural engineer look at this.
- Chris Technical Services
Monday, January 25, 2016 at 3:05 PM

what is relation ship between deflection and cracking beam
- eng kilwa
Saturday, January 23, 2016 at 12:49 PM

Mike, it sounds like the cracks that your seeing are expansion joints, and are certainly needed when pouring a new garage slab. Expansion joints are cut in to provide a place the slab can crack when movement occurs. Slabs that you see that have cracks going every which way like "lightning bolts" are sometimes caused by a lack of expansion joints, tree roots, settling, etc. Your contractor should be able to explain this in more detail.
- Chris Technical Services
Monday, November 30, 2015 at 5:00 PM

Just had a garage poured on New Build. The whole length of garage north to south and east to west the whole length where form boards were positioned have a hairline crack the whole way. Perfect straight line cracks going each direction from one end of garage to the other. Is this normal or should I be asking for this to be fixed.
- Mike
Tuesday, November 24, 2015 at 12:25 PM

Susie, it’s a little unclear from the description what is actually happening; however, if you are trying to fill the cracks with a material make sure it’s flexible. Our Sakrete Self Leveling Polyurethane Sealant will help fill the cracks and avoid water from penetrating (freeze/thaw). For deep cracks more than ½” use Sakrete Backer Rod, then fill over the backer rod with sealant.
- Chris Technical Services
Wednesday, September 30, 2015 at 2:25 PM

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