PDA

View Full Version : OT... cracking concrete slab



1-800miner
09-28-2012, 12:43 PM
My ten year old concrete patio slab has a crack starting. Of course it is six inches away from an expansion joint and going the other direction.
I have heard welders talk of drilling a hole at the end of a crack in steel to relieve the stress.
Think it will work on a slab?
I have a diamond core bit about two or three inches diameter.

Peter.
09-28-2012, 01:33 PM
Nope, won't work. The crack will already have formed, you're just seeing the bit that's opened up. Most likely you have a bit of soft ground under one part of the patio.

flylo
09-28-2012, 03:27 PM
Works in sheet metal, airplane wind screens, etc. I'd try it. I've seen where they've pumped or packed concrete under slabs to raise them.

toyjeep73
09-28-2012, 04:40 PM
Concrete does two things -- its gets hard and it cracks.

If you have a crack in a slab it is most likely from differential settlement. The only way to stop the crack is to stop the settlement, I doubt a hole is going to do that.

How big is the crack now, hairline or already opening up?

Boucher
09-28-2012, 05:19 PM
Concrete does two things -- its gets hard and it cracks.
Actually it cures (gets hard) and it shrinks.
Post tension slabs are the only ones that you will see that don't ultimately crack.

1-800miner
09-28-2012, 05:21 PM
Hair line, with very slight dampness showing for half inch on each side of the crack.
I need to do some surveying to see if the crack (middle) or the edges are settling.

Bob Ford
09-28-2012, 06:06 PM
800,

With out a lot of work to correct problem it will likely get bigger. Use a concrete saw and widen the crack, Then fill with flexible caulk. The good stuff that is made for concrete and cold weather. It will then look like it was intended. You could have settling or earth quake damage. If settling, the ground should have been compacted before the slab was poured and the house was built. If earth quake, it will keep cracking and maybe move up down at the same time.

Bob

kwoodhands
09-28-2012, 06:20 PM
Pretend it's not there. A hairline crack may or may not get bigger. When you can lose a pencil in it then worry about it.Hairline cracks indicate shrinkage in new concrete.Your slab is older so possibly one of two things happened.First,the soil under the cracked area has settled.Second ,temperature change can crack a slab,especially since you say it is next to the expansion joint.
Possibly extreme summer heat caused the crack, if so the crack probably will not get larger.
mike

oldtiffie
09-28-2012, 09:05 PM
Was the reinforcing mesh the correct type and size and was it put down with the correct spacing and support as well as cover?

co_farmer
09-29-2012, 12:53 PM
The place we purchased 6 years ago has a pole barn. 1/4 had a concrete slab floor and was completely enclosed shop and storage area. The concrete floor has several significant cracks. I had a concrete contractor prepare and pour concrete for the remaining 3/4 of the building. I noticed the original concrete had plastic sheeting under the concrete and asked the contractor about it. He said that was exactly the reason for the cracked floor. It kept ground moisture from migrating to the concrete and so it dried out over many years and shrank.

He said as the moisture trapped in the concrete evaporates over years and decades, it needs to be replaced by moisture from the ground. Otherwise shrinkage will cause cracks in spite of sawing the slab to prevent the cracks.

After 5 years, I see his work is also cracking. I irrigate fruit trees and garden near the building, so there must be some moisture under the slab, but probably not in the right places.

I suspect there is not much you can do except fill the cracks, as suggested.

Paul in Central Oregon

Peter.
09-29-2012, 01:35 PM
That guy was full of it. DPM is universally used under concrete slabs - it isn't the cause of cracking.

uncle pete
09-29-2012, 02:32 PM
That guy was full of it. DPM is universally used under concrete slabs - it isn't the cause of cracking.

100% Correct. Cast concrete grain or endless other dry material silos sitting there baking in the sun, heated concrete buildings, yada, yada and they don't crack. Concrete cracks because of a plastic vapor barrier? That guy must think there's a tooth fairy also. I've broken thousands of tons of concrete and loaded it out. Some of it on purpose ;) To state the obvious, as a material it's real brittle, and in the lighter sections it's super easy to break with very little force. It reacts a lot like glass in some ways. There's more than a bit of evidence that it can age harden also. Your slab cracked due to ground movement, not quite tight enough compaction before pouring the concrete, slightly incorrect mix, or maybe a little bit too thin to support itself without cracking if and when there is some ground movement, or just maybe for more than a few other reasons. There's 100s of millions of square feet of concrete that never cracks. They even use it under the worlds largest and heaviest forging presses, lathes, planers, etc. It's a little thicker than your average home slab though. 30' or more in depth isn't unheard of. If it's engineered and poured thick enough with re-bar added, it then becomes self supporting even with a bit of ground settlement. But I didn't say it was cost effective to do so. Possibly that crack won't get any larger, But I doubt anyone could say for sure. Other than the specialized concrete injection under the original slab, I don't know of any way to stop that crack from getting larger if it's going to.

Pete

SteveF
09-29-2012, 03:13 PM
He said as the moisture trapped in the concrete evaporates over years and decades, it needs to be replaced by moisture from the ground. Otherwise shrinkage will cause cracks in spite of sawing the slab to prevent the cracks.



Another agreeing with Pete and Pete, your contractor doesn't know what he is talking about. Probably related to the roofers that roofed my house who didn't have a clue how to flash a chimney. Word of advice on flashing - the chimney flashing on the uphill side of the chimney goes UNDER the roofing felt and siding flashing!!

The reason the slab cracked next to the expansion joint is because the saw cuts weren't at least 1/4 of the way through the slab like they are supposed to be.

Steve

jep24601
09-29-2012, 04:19 PM
The only way to make a crack in concrete look better is as follows:

Pressure wash the crack or pressure blow the crack as clean as possible. Wait until the crack and concrete are completely dry - this is important. Then take dry neat cement powder (comes in 94# bags unfortunately) and with a fine brush you brush the dry cement into the dry crack and when the crack takes no more you brush the surface completely clean. Leave alone for a couple of weeks and repeat the cement procedure as necessary - use no water. If the crack is not a moving crack this procedure can make the crack virtually disappear - and it is the only procedure I know of which can. Caulking the crack will make it stand out like a sore thumb.

michigan doug
09-29-2012, 05:50 PM
There are lots of reasons concrete can crack. Polyethylene sheet is not one of them.

1. Strength of the original mix not good enough, I like to specify what psi I want.

2. Not enough steel. I like 1/2" re-rod every 16" both ways. More is ok. Mesh I'm not too impressed with.

3. Not enough thickness. I rarely do small projects less than 5", and mine never cracks...My shop I did 6" or a bit more, Never cracked.

4. Not enough compaction. I like to do it twice, a week apart if possible.

5. Not enough cure time before loading. Green concrete is pretty fragile. A couple weeks without big concentrated loads helps a lot.

6. improper/not enough expansion joints

7. Not protected from freezing until ~full strength is attained.



Not much you can do about it once it's cracked until it's done moving.


Finest regards,

doug

darryl
09-29-2012, 06:43 PM
I had a concrete slab here that I tried to make crack, and I couldn't get it to. It was an 8x8 garden shed which I wanted to get rid of to make room for the newer and larger one. The best I could do with a sledge hammer is chip away at the corners and edges. As luck would have it, my neighbor had a machine in to dig his foundation, and I made a deal with the operator to pull it out and send it away in one of the dump runs. He lifted and dropped it a number of times til it broke, then dropped the resulting pieces onto each other til they broke small enough to load and haul. No reinforcing in it, just solid old concrete.

Johnh57
09-29-2012, 06:47 PM
One thing the plastic can cause is curling. Especially if the exposed surface of the concrete is not properly cured - flood cured is best or at least a cure/seal product. If just left exposed to sun and wind the top surface can dry faster than the bottom surface - this causes curling - or edges lifting. When the edges lift things want to crack because the edges aren't supported.

Mesh is fine, rebar is better, in both cases though your contractor has to use chairs etc. to support it and keep it from pushing down into the fine grade when they are placing and finishing. Fibermesh seems to do well at controlling hairline cracks - but you need steel if you do get cracks that want to shift.

I looked at a slab the other day in the basement of a house - cracks everywhere. Not a control joint in the whole basement. Between no control joints, loaded too soon with construction loads, probably poor compaction, and I suspect a poor mix design - the slab never had a chance.

Mix design is critical in controlling cracks, adding water on site, etc. can really cause problems with strength and shrinkage (which will lead to cracks) If you need the stuff soupy to pour it - use a plasticizer - not added water. To control shrinkage you need to control the water/cement ratio - but an ideal wc ratio results in a mix that is very stiff - so you add water at the site and blow the mix design.

Control joints dividing the slab into squares - or if rectangles not more than a 1.5 :1 aspect ratio. No more than 3 * slab thickness in feet (i.e. a 4" slab no more than 12' for control joints)

Your stop hole won't do anything. Either ignore the crack, or grind it out and caulk it with a good grade of concrete caulk. Sika makes a good self leveling caulk (http://usa.sika.com/en/solutions_products/02/02a008/02a008sa53.html).

saltmine
09-30-2012, 12:07 AM
Put a "FOR SALE" sign in your front yard, and the slab will disentigrate to dust.

ogre
09-30-2012, 02:01 PM
I found that for some reason it looked like someone put the grooves in the concrete for frickin looks only cause they only went 3/8" deep. I always felt they needed to be deeper in slabs ive seen. I suspect theres a depth for thickness rule but i always went alittle deeper in mine to pray for sure if it was gonna crack then i wanted it to be that joint and not anywhere else.

boslab
09-30-2012, 07:24 PM
I found this real good grout that will sort your crack out for good lol
http://www.demolitiontechnologies.com/
Bust get some! pun
mark