Prevent Concrete from Cracking

[Shirker]

Have not gotten full quotes from pool builders yet, but what should I look for in the concrete decking?

Sand or gravel base? Rebar size? Thickness?

We have very clay soil around here if that makes a difference. I'm willing to pay extra to make sure everything is done correctly. I really just don't want it to crack. Every bit of Concrete we had done while building our house (driveway, patios, garage) has cracked.

 

[DJFaninTN]

there are two types and only two types of concrete. there is concrete that has already cracked and there is concrete that is going to crack.

 

[Divin Dave]

Hi Shirker,
as DJ says, concrete is going to crack. Its a fact of life. The question of the matter is when, and where will it crack? Express your concerns to a good concrete contractor and perhaps they will recommend or spec out, the concrete mix (there are different types),

The best way to control cracks though is to make sure the installers install "Control Joints". Control Joints is a groove cut into the concrete at a specific location and the purpose of it is for the concrete to crack in that joint, so it wont be visible on the surface. They have to be put in the right place for them to work as they are supposed to, so picking a conctractor that understands this is going to be important.

Is mind boggling how many concrete people there are out there who dont understand this.

Do yourself a favor. Google concrete control joints, so similar term and learn a little about the technicalities of it. A little education will help you when speaking with contractors, so that you know what they tell you seems logical and follows the basic concrete guidelines of crack control.

 

[jtburf]

Most good cement folks put joints in to control the cracking. they know it will so they make it crack where they want it to!!!


John

 

[d_red1]

1. While a sand or gravel base is not needed it is helpful over the clay. At the very least you want to make sure the subgrade has been compacted.
2. "Typical" rebar spacing for deck/sidewalk is #3 (3/8" rebar) at 18" spacing. You also see a lot of welded wire fabric for residential sidewalks but don't settle for that in the decking. Fibermesh (small fiberglass fibers in the concrete) should just be used as a compliment to the rebar and not as a substitute.
3. I doubt they would pour anything less than a 3000psi mix but get the mix design/strength in writing
3. As stated above, concrete cracks, that what it does. The control joints (either sawed or tooled) will give the concrete a weakened point to crack. I prefer sawed joints as it is a more cleaner look
4. If you are pouring against coping or existing concrete (foundation, sidewalk, garage, etc) you will also need expansion joints. These are typically black fiber board or redwood. The expansion joints have some compression give and will allow the concrete to expand and contract against to prevent cracking and movement.
5. Last thing, when it is time to install try to have them pour before noon before it gets to hot and they have to add more water to keep the concrete workable (which could result in small hairline shrinkage cracks). If possible, check the time on the concrete ticket to see how long the truck has been on the road, more than an hour is unacceptable and grounds to reject the load. The longer it has been on the road the more water will have been added to it.

 

[Shirker]

Thanks for all the info, @d_red1. We plan to have a cantilevered coping. This needs to be two different pours? We are planning for a winter build so I will do a little research on temp and concrete curing.

 

[Divin Dave]

I agree with sawing the control joints instead of tooling them in. I sawed mine myelf because my concrete contractor and his workers were overly lazy IMO, and I told em so.

One thing about control joints is that eventually they may fill up with dirt and weeds start growing out of em.

In the worse case, they can fill with water and freeze which isn't very good either. Especially if the concrete has already cracked in the joint.

To prevent this, after I sawed all of my joints and all the concrete dust blown out of em, I filled the joints with self leveling "Sitka" caulk . it worked like a charm and added a bit of design element to my deck. Its not hard to saw the joints and caulk it, but you have to just be mindful to saw a straight line, and then be careful to not overfill with the caulk.

 

[trivetman]

I just had a stamped concrete deck done and came across this thread. I am wondering about filling the control joints and wondering if there are alternatives to sitka or other caulks. How about play sand? The kind used for paver joints (not polymeric). Or does the sand possibly pack in too tight and take away some of the flex in the joint?

 

[sktn77a]

Problem 1 is cracking. Problem 2 is lifting/sinking. Problem 1 you can't do anything about - it's going to crack! Have them put in stress joints every so often. Problem 2 can be mitigated with wire mesh or rebar.

 

[jimmythegreek]

If the concrete is properly spec'd and renar/mesh addex, the proper base is installed and compacted, the concrete will only crack at the control joints. It costs alot more to do it right

 

[trivetman]

The new concrete is poured and it cost plenty....so it must have been done right ?..

Im wondering if the joints will fill with dirt and weeds and if play sand might alleviate that. Its not so much about preventing the controlled cracking. I’d rather not deal with caulk.

 

[jimmythegreek]

You can use high performance caulking. I hate sika it never looks good unless you take hours to lay it

 

[Newdude]

Ground clear by Ortho does exactly what it claims to. Nothing will grow for a few months and it’s real easy to spray a couple of times a year depending on how long your season is. It always burns me to stuggle throughout the yard to keep the grass and plants healthy yet dandelions and weeds grow like wildfire in every last patio seam and crack.

Make sure to read the label though. They have two kinds, concentrated and already diluted. They are both very similar in price but you get 4 times as much with the concentrated one.

 

[smracing]

I have over 3000SF of dyed and stamped concrete with cut control joints. I was tired of cleaning the dirt out of them so I grouted them. Grout will still allow for the joint work properly.

 

[trivetman]

3000 SF!! A stamprete parking lot!!