What is Concrete| Foundation Cracks and Concrete Cracks

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Wed, Aug 09, 2017 at 2:25PM

What is Concrete? Why does it Crack?  

 What is concrete? Why are there foundation cracks and concrete cracks. Concrete is a mixture of three basic parts. Although those parts may be slightly different they are all required to get the best performance out of your concrete. Thousands of years have taught us to make some pretty good concrete. Concrete is composed of a paste, aggregate (usually rocks) and water. The paste which is basically Portland Cement and water coats the surface of the rocks. Then through a process called hydration the mixture hardens into solid concrete.
Concrete has literally been around in various forms for thousands of years. It's kind of like the Arnold Schwarzenegger of mudpies. Typically concrete used in foundations is 10-15% cement,60-70% rocks or aggregate and 15-20% water. The mixture is critical because the more voids you have when the concrete cures the less hold it has upon the aggregate and the easier it cracks. Water purity also has an effect on concrete's durability. Water with impurities can cause efflorescence, staining, corrosion of the metal reinforcement and reduce the concrete's durability. Believe it or not if the water is safe to drink then it will be a good standard to mix into the concrete.
Rock selection for aggregate is not as flexible. The type of aggregate depends upon the thickness of the concrete and purpose of the concrete itself. Sometimes much larger rocks may be used in the pouring of foundations for dams for example. I have helped poor foundations for large buildings but I have never seen rocks larger than 2 inches or so in a pour. However I have heard that much larger were used in the past. Maybe that was actually to save money because large rocks were dug up and either had to be used or carried off.
Soon after concrete is properly mixed it begins to harden. This is why you will occasionally here the term a hot mix or a cold mix. After concrete is poured often a vibrating device will be used to assure the concrete penetrated all of the openings (voids) in the pour. This simple invention has greatly increased the effectiveness of larger thicker pours.

Concrete Curing

Curing of concrete begins after it has been troweled and floating is finished. There are tricks to this also but this is more information than You probably wanted on concrete itself. I spent a whole summer pouring and pulling concrete. It is savage work. I literally could not eat enough food to keep my weight on. Also I once got a splinter in my finger while I was working and got lime poisoning. It is some dangerous stuff.
There have been several items through the years used to strengthen concrete. Back in the late 1980's we used what looked like 1 inch bobby pins mixed into the concrete. Now the in thing and has been for a while is called fibercon. Fibercon is actually a metal fiber. However there are fibers made of other materials. I know there was a plastic one for a while and fiberglass fibers are very common. If you have a garage and you use fiberglass fibers in your concrete you may notice an itch if you or your pets lay on the concrete. No worries this can be fixed by going over the surface with a torch and painting the concrete with a garage floor coating. You may notice fibercon in your pool deck also. It looks like small grey hairs.
Speaking of pools there is also shotcrete or gunite. You will hear the term used to describe pools. Gunite is a mixture of concrete, sand and water sprayed under pressure. It offers reduced shrinkage and permeability. Also it can use steel fibers but will not be applied with an aggregate larger than 3/8's of an inch. You may occasionally hear it called shot-crete but i rarely have heard anything other than gunite. When you see someone with a hose spraying concrete, not pumping but spraying then you can assume it is gunite. You will also notice that it may be drier than traditional concrete and workers are finishing it as soon as they can. 
Most of the time the gunite is sprayed wet. The wetter mixture allows a little more time for working the concrete and creates less dust which is better for everyone. You do not want to breathe it. Portland cement contains lime or calcium oxide, shale, chalk and etc. Remember what is in it and do not breathe it. Having a experienced operator at the nozzle helps application considerably. Gunite does need to be cured. Preferably it should be out of the rain for 4-5 hours after application and cured for 4-7 days. Seven days is better. Concrete in general is over 90% cured after 1 week and in the high 95-98% cured after 1 month. The thicker the slower and the proper mixture etc. Concrete commonly takes over 5 years to fully cure. I have heard that in much thicker pours like dams that some areas may never fully cure.

Why does concrete crack? 

Disclaimer so I do not get sued. First of all who am I to answer this. I am a Realtor. However i have been through trade school as a carpenter and a Ironworker. I have put in concrete reinforcement which is known as "working in a rod patch" as ironworkers called it. I have built commercial and residential forms for pouring concrete. I have poured 100 loads plus of concrete. I stink at finishing it. However I have pulled and spread it more than I care to admit. I am not a certified contractor. But I am right about this. Most cracks that are not due to natural causes (shifting soil and tree roots) are caused by human neglect or error. Which is normally bad engineering and workmanship and most often laziness or ignorance of the pourers. Most of the cracks you see are because of a wet pour. Too much water in the concrete. When concrete is soupy it is easier to pour and workers think they have more time to work it. Or they will say that it needs to be wetter to penetrate into the forms better. They may not even know that it is to wet. The chemicals that were mixed could have been bad also. A concrete fact is that a 100 ft long pour can shrink 1/2 of an inch. That puts stress in the concrete and it cracks. 
Slump test
A slump test is a test performed at a job site to determine the workability of concrete. The slump test can be a great indicator or not. It depends on the human factor. Just like lab results in a hospital.  


When do cracks matter?

There are two easy answers to this. Most of the time cracks matter when their appearance bothers you. The other times you may not notice them until damage results and they cost you money. Let's expound on both of those. What I am going to say is opinion and not the advice of a contractor, engineer or home inspector.

Stucco Cracks. These are very common in older homes. Especially 15-20 year old homes.

Basement cracks

Most of us in Florida do not have basements. However you may move up North or be moving from up North and this could help. Especially since so many homes are older in the great North. A word of caution if there are pipes anywhere around a crack or if there is conduit get them inspected. 

As you can see concrete is fairly easy to repair and there are no shortage of products today to repair it. Now I will chime in. When you fix old concrete with new often the old concrete will just break right next to the newer concrete. The best fix is to either do not buy that house or if possible have a professional come in that has been in business for a while and will guarantee their work. Licensed and insured. If you are buying a home with cracks offer less money for repairs. If you do not repair cracks they only cost you more later. Obviously there are times you opt for the repair.If your house is older and the ground has settled or the cause of the crack has been dealt with you may get lucky. You could repair the crack as seen above and just paint over it. In older homes up North long ago before they could be told not to do it builders and homeowners would pour concrete on frozen ground. This is one of the reasons some older homes up north have concrete in bad shape. You can not really blame them for having done it. Many of them probably needed a place to live in a hurry. Some of the older homes up North still have some pretty scary wiring too. 

Floor cracks

I am going to tell you a secret first. Before you pour a home's floor you tamp the soil to compact it. Most people never realize this but most likely if your home is more than 5 or ten years old there is a space between your concrete and the dirt. The dirt still settles. It usually makes no difference at all. Because concrete is awesome. Most of your plumbing is buried in the dirt and only comes up through the concrete. So cracks in your floor can just be repaired unless you have other issues. Contractors have been sued enough to figure out the best ways to pour concrete. Most homes are just fine.

Where to be cautious is in homes similar to I live in. I live on a lake and they most likely built up the soil around this lake so they could build on it. So the ground settles and you get little cracks everywhere. In fact a couple of the doors in this home look like they have been affected by the settling. This is about a 800k home. I don't think the owner has any idea. 

The point of this article is to reinforce your need for a home inspection. Especially if you are curious about the cracks in the concrete. If you are up North and have cracks in your stucco get a moisture inspection. If you are in Florida and think you see evidence of cracks being repaired maybe ask for a CLUE report. A CLUE report will tell you about any insurance claims. A Seller should tell you about any damages they have had repaired. Those damages may have required work permits. Some counties require inspections to look for non permitted work before closing. This is more common up North than in Florida.

Driveway cracks. 

That driveway crack repair is to only show it can be done. This is the reality of concrete repairs on driveways. They do not usually work. here are a list of the issues if you are considering having it done and a recommendation. 

The concrete should be pressure washed or cleaned and wire brushed where you are going to patch it. The patch will crack is there is a any flexibility in the driveway when someone's 3000 lb car drives over it. Often one side has actually risen because of a root or the other side has collapsed due to water. You may have a lawn sprinkler line leak under the driveway. The best fix if you can is to cut out a section and have a section re-poured. There are several choices to deal with coloration concerns. 

I am a Realtor in Orlando Florida. I Live in Windermere Florida and would love to work with you in purchasing your new home. I would love to help you make sure it is your dream home. Feel free to contact me through the number on this page or call me at 407-864-4626. I think we have answered the question what is concrete and what are foundation cracks and concrete cracks.

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User Comments

Foundation cracks
Nicely explained!! Yes if there are pipes around cracks, that means some serious problem.
Author: / Wed, Jul 17, 2019 at 8:42AM

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