One thing that I was taught in my years as a concrete finisher is that there are two things in life that are certain, the first is of course death and the second is concrete is going to crack. The only thing that we can do is control where it cracks with proper concrete control joint spacing and depths at the right time. In this post I want to help you understand the why’s, when’s, where’s and how to’s of concrete cracks and control joints.
Often times people think that concrete joints are put in to prevent it from cracking but truth be told, they are simply to create a weak point so the concrete can crack in a more controlled manner. As concrete goes through it’s curing process it shrinks as much as 1/2″ over 100′ which of course will cause it to pull apart resulting in hairline cracks. It is also important to know that the more water used in the placing process, the more likely it is to shrink and crack.
When Is The Best Time To Cut Control Joints In Concrete
Very simply put, the best time to cut your concrete control joints is as soon as possible. If you cut to early though you can cause your concrete to ravel on the edge of the cuts but if you cut to late you will end up with uncontrolled cracking.
There are many different factors that come in to play when it comes to timing of administering cuts such as
- Tools used to cut concrete
- Blade type
- Saw speed
- Mix design
- And aggregate size
For example, if you are finishing a sidewalk, patio or other small pad you can generally use a concrete groover which will allow you to cut shortly after the placing process is finished as you can see in this video by Bob Harris at Concrete Network Tools
On the other hand, if you are finishing a larger shop or garage floor you will need to use a concrete saw in order to get your control joints. This is where you can run into raveling when cut to early or uncontrolled cracks when cut to late. Generally you want to saw cut 6-18 hours after the concrete has been finished unless you use an early entry (soft cut) saw. A soff cut (early entry saw) can be used within a couple hours of finishing so you have less chance of uncontrolled cracking. See the video below from HusqvarnaCP that explains the Husqvarna Soff Cut system.
Where To Put Control Joints In Concrete
Aside from timing when it comes to implementing your control joints it is very important to make sure that you are putting your cuts in the right locations. If this step is not done properly then you are essentially wasting your time. You will find that your concrete is still going to crack wild in an uncontrolled manner.
To plan out where you are going to puts your cuts it is going to be important to know the depth of your concrete slab. The reason for needing to know this is because you will want to take the depth of your slab and multiply it by 24 to 36. See examples below…
- If you have a 4 inch slab and multiply it by 24 to 32 you will find that your joints should be placed every 96 to 144 inches or 8 to 12 feet
- If you have a 5 inch slab and multiply it by 24 to 32 you will find that your joints should be placed every 120 to 180 inches or 10 to 15 feet
Or another way to figure this out is to times it by 2 or 3 in feet. So for example if you take a 6 inch slab times 2 is twelve feet or times 3 is 18. I’m sure you get the picture, pretty simple right?
Another very important part about figuring out where your control joints are going to is and outside 45 degree angle (or any sharp angles for that matter). Often times this is going to be the very first point that you will run into concrete starting to crack. It is very important to get your cuts put on your 45’s either while finishing or as soon as you possibly can afterwards. Often times I have even returned early when saw cutting my joints just to cut my corners and then came back a little later to do the rest.
Watch This Video I Created Explaining Proper Spacing
If you are at confused by this process please feel free to leave a comment below and I will try to explain this process a little better for you. Remember this is a very important step to get right when planning out your control joints.
How Deep To Cut Control Joints In Concrete
The next thing that we are going to look at is the proper depth for cutting your concrete control joints. Often times people will cut their floors and not cut them deep enough which of course will result in random cracking. A few different reasons for this are of course not knowing any better but what I see more often than not is cutting too fast.
When you are cutting your floor with a concrete saw it is very important to make sure that you are not moving your saw as faster than the blade can cut. So always be sure to keep it in the back of your mind to check your blade depth regularly, if you can’t see it from standing behind it then either have someone watch it or continuously glance around at it.
So what is the proper depth to cut your concrete slab? Again we are going to want the total depth of our concrete, and once we know this then you will want to cut at 25% of that depth. For example..
- A 4″ Slab = A 1″ Cut
- A 5″ Slab = A 1.25″ Cut
- A 6″ Slab = A 1.5″ Cut
You get the point right? It’s simple math, heck you could even use a calculator if you need to.
Do all of the cuts need to be 25% of the depth?
Of course there are going to be some exceptions when it comes to your depth. The most important thing to remember is always try to cut at 25% at or really close to your proper spacing and off of your corners. Here are a few different exceptions
- If you plan to put in decorative concrete cuts the you simply just need to score the surface in between all of your regular cuts.
- Another time when you may not want to cut as deep would be if you have heating lines or other objects in the floor. I usually try to talk people out of cutting if they have in floor heat but of course some people are stubborn.
Recap On Cutting Concrete Control Joints
Hopefully this article gave you a good idea on how to cut concrete control joints. If you do have any question comment below and I will do my best to answer them. Here is a quick recap of what you need to know
- Control Joints are not intended to stop cracking but rather control cracking
- It is best to cut joints as early as possible either while finishing with a concrete groover or within 6-18 hours after finishing with a concrete saw.
- Plan out your control joint spacing 24-36 times the depth of your concrete slab
- Always cut off of your corners as soon as possible because this is usually the first place cracking starts
- The depth of your concrete joints should be 25% of the depth of your slab
Did you get value from this article “Concrete Control Joints – Proper Spacing, Depth and Timing Explained!”? Share this article and let me know your thoughts in the comments below…