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 www.4greatconcrete.com 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.
Question sent from a reader: I have a large concrete driveway that has obviously been enlarged over the years and pieced up with mortar at various points. Consequently it’s a mismatch of colours. I was thinking about painting or staining the driveway with exterior paint or concrete stain, assuming I can find any here in the UK, and sealing it the same day using a concrete seal?
Would such a process be effective or would I be wasting time and money?
Here’s our expert’s answer:
It won’t be miraculous buy it will be effective.
We have a waterbased stain that will color everything including your sneakers. Very potent.
Acid stain may not be as effective with different surfaces and patched areas.
How do I level (screed) the concrete over an area that is wider than my screed board.
You can tackle this several ways. The professionals will drive grade stakes inside the forms at convenient intervals (less than the width of the screed board). The grade stakes will have the proper level marked on the stake, usually a nail in a wooden stake. You can place some concrete in the forms and using the grade stakes as a guide, begin screeding the concrete. As you get the proper grade on the concrete, you can pull the grade stakes. As you can tell, this requires a little experience to get it just right, but beginners can do it with less accuracy.
Another method is to get a probe, such as a stick or steel stake, mark it with the depth you want the concrete to be, then place the concrete in the forms. Screed the concrete and probe to ensure the proper depth. This is not as efficient as the first method, but it will get the job done with a little more effort. You can set construction joints (1×4’s, metal keyway forms, etc) inside the forms at intervals of 25-30 times the depth of the slab. You can use these as screed guides.
Finally, you can set screed guides inside the forms using 2×4’s and stakes so that the bottom of the 2×4’s are at the top of where you want the concrete to be. Nail ears on the end and top of your screed board so that when you set the screed board on the screed guides, the bottom of your screed board is even with the bottom of the screed guide, thus level with the top of where your concrete will be. Once you get your concrete down and screeded, you can pull these screed guides and fill in the stake holes and re-level by eye.
So, you want to put in a patio, driveway or sidewalk, and you are going to use concrete. A very wise choice, we can all agree. One thing to know before you put in your concrete all concrete cracks. You say, “Wait a minute, I’ve seen concrete that doesn’t have any cracks. How can you say all concrete cracks?” Concrete typically consists of cement, rock, sand and water. In the fresh, or plastic stage, concrete is fluid.
By Kenneth Wayne Meyer
As it hardens, the cement and water begin to shrink, and the stresses created by this shrinking cannot be overcome by the small amount of strength developed by the young concrete. If you place the concrete on a windy day, the top may start to harden before the bottom, which will cause the concrete to shrink unevenly (plastic shrinkage cracks.) Also, if the ground underneath the concrete is not level, there will be an unequal dragging force while the concrete shrinks, also causing stresses the new concrete cannot withstand. So, how do you get concrete with no VISIBLE cracks in it? By following a few simple steps before and after you place the concrete, you will have a very nice looking structure that will require very little maintenance, and give you years of enjoyment.
Before you place the concrete, make sure your subgrade (ground beneath the concrete) is thoroughly compacted and level. The absolute best thing to do is get a garden tiller, till the soil to a depth of 6 inches, then rent a hand operated compactor and compact the soil vigorously. This will help ensure there are no soft spots. You can apply a layer of cushion sand if you want. This will help achieve a totally level surface and allow a consistent friction to the shrinking concrete. Four inches of washed sand ought to be plenty for the cushion. If you use a wire mesh for reinforcement, use panels and not rolls. The rolled wire mesh is extremely difficult to keep in the top half of the concrete, where it HAS to be in order to do its job. You can also use reinforcement bars (rebar) tied together with steel wire, but spacing and size requirements vary based on load and soil conditions, so it is hard to recommend a standard set up for that. If you do use rebar, it is essential that you keep it in the top half of the concrete. You can use stones, broken brick or you can buy plastic chairs that the steel will sit on to keep it in the proper position when you place the concrete. You can also have the ready mix concrete company supply fibers to the mix. These fibers are usually nylon or polypropylene. They help keep the cracking of the concrete on a micro level instead of a macro level (where you can see the cracks with your naked eye.) Steel reinforcement also helps keep cracking in check, but if cracking does occur, the steel, when properly placed in the concrete, will hold the concrete together, whereas fibers will not do that.
Okay, you’ve got your subgrade ready, you have placed a plastic vapor barrier on the subgrade for slabs that will support dwellings, your steel is sitting nicely on your plastic chairs in the proper position, and you now have 14 of your closest friends on their way over to help you place the concrete you have coming. When the concrete arrives, if you don’t have a vapor barrier, wet the subgrade without puddling the water so that the water in the concrete will not be absorbed by the dry subgrade, thus causing uneven drying and the dreaded plastic shrinkage cracks. Once the concrete is placed, make sure to protect it from high winds and direct sunlight so the concrete will dry evenly from top to bottom. You are now ready to perform the most important step in preventing noticeable cracking. Contraction joints are the secret to no cracking! By placing contraction joints that are at least 1/4th the depth of the concrete and on intervals of 25 to 30 times the depth of the concrete (usually easiest with a jointing trowel or tool while the concrete is still fresh), you will almost ensure there will be no visible cracking in your concrete. If your slab is 4 inches thick, the joints must be at least 1 inch deep and placed every 100 to 120 inches. If you cannot use a jointing tool to put the joints in, you can hire a concrete sawing contractor to do this for you. Make sure he cuts the joints a minimum of 1/4th the slab depth. This jointing method helps the concrete crack at the weakest point. This is why it is so important for the joints to be deep enough. Variations in subgrade levels could cause greater stress in the concrete in an area where the joint isn’t deep enough, and the concrete will crack outside the joint. Once your joints are in place, and the concrete has cured for about two weeks, you are ready to seal the joints. This will prevent water from migrating into the subgrade and expanding and contracting, or getting into the joints and freezing, causing the water to expand and breaking out the concrete around the joints. You now have a concrete structure that will serve you well.