Concrete Forum: Ask the Experts and Industry Peers

If you have any questions you can always visit the Concrete Forum and post your questions. Here’s an example of the great information shared for one person who had a question about why their concrete was cracking after only four months.

I am in the late stages of an addition to my home which includes a master bath and covered patio. The slab was poured in late November 2010 all at the same time (bath slab and patio). We had a major storm roll thru the afternoon/evening the concrete was poured but the concrete did sit for 4-6 hours before it started raining. As the addition progressed, I started noticing cracks in the concrete. The tile guy found nine hairline cracks across the bath area alone. Last month I noticed a crack all the way across the patio, which is about 16′ x 30′. I refuse to believe that this is “normal” like the contractor says. I had a detached garage and driveway extension built summer of 2009 and that concrete doesn’t have any cracks at all…..even with the recent trucks driving over it for the current addition. I believe the concrete supplier didn’t mix the concrete properly. Is there any way to test a sample of the concrete or have it inspected by an expert? What are my options if I find out the concrete is substandard?

Some of the helpful replies:

It’s extremely common for concrete to crack. it’s far more uncommon for it NOT to crack. There are several reasons for cracks. The two most likely are:
1) The concrete was poured much too wet, leading to excessive shrinkage as it cured. Many times, contractors add a lot of water to the mix to make it easier to pour it out of the truck. When the concrete cures, it loses water and shrinks a bit. Too much added water leads to a high water to cement ratio and therefore a lot of cracking, and typically within a couple of days of the pour!
2) There were not enough crack control joints, or they were improperly installed. These are the joints that are either tooled in while the concrete is wet, or sawed in as soon as possible after the concrete sets up. They are designed to control random cracking. There are specific ways they need to be installed. Probably the least likely scenario is that the concrete was defective. It’s more typically contractor error. Did the same contractor pour both of the slabs you mentioned? I wrote an article about concrete cracks. It can be found at under the artcles and publications link. It goes into much more detail as to other causes of cracking as well. Good luck.

Although you were not there when they actually filled the truck with the appropriate materials, once the truck comes to the job site, the contractor or the man in charge of the concrete usually takes a look at the concrete by pouring a little into the shute. So the driver of the concrete truck gives this blank look and looks at the contractor or the man in charge and sometimes the man in charge will have the driver add water so he can work the concrete easier. At that point the question would be, who is responcable for the mixture of the concrete? As it is not feasable to hire a state licensed testing company to be there during the pour on a small job, that would have been possibly the only way to protect yourself. So the options are tear it out or live with it, sorry to be blunt.

If concrete is of perfect mixture and of perfect placement and perfect subgrade(of course there is nothing perfect in concrete)over a period of time will crack every 15 feet. The control joints just try to get the crack to happen at the control joint. So the object is to make the control joints asthetically pleasing to the eye with the surrounding architecture. The more control joints the better chances you have of hiding the cracks. Climate – soil conditions – placement voids – where the rebar is inside the concrete and mix all will have a contributing factor in a crack.

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