New Concrete Deck Cracking - Is this Normal?

[Underwater Man]

We had our pool renovated this summer and the exposed aggregate deck was poured on July 16th. 3/8 rebar was used, and the layout looked good. It's about 4" thick. The expansion joints were tooled rather than using redwood or vinyl strips. Cracks are beginning to show... I quess you would call them hairline, all the way across five of the joints. Plus another crack not quite mid way between two joints. The deck at this location is about 8 ft. wide, and the crack runs the entire width. One of the cracked joints crosses right across the middle of a skimmer.

The week the deck was poured temps were in the mid to upper 90's. I had planned to keep it lightly misted for a few days to slow down curing, and hopefully strenghten the concrete. Well... bright and early the next morning the crew to sandblast the old plaster from the gunite shell were there. I mentioned to them that the deck was just poured, and they said it was fine. The sandblast rig was a heavy steel container that probably weighed at least 200 to 300 pounds when full of sand. It had two rear wheels and a padded steel brace on the front . As the sand would get low the helper would raise up on the rig and drop the front brace down on the deck. He did this quite often. I told him my concern, and gave him an old quilt for extra padding. Also called the pool supervisor, who said it wasn't a problem. It was a big concern to me, and now I wish I would have told him to stop. Our sidewalk which was poured in the mid 70's has tooled expansion joints, and only two joints have cracked. I'm afraid my deck is just going to get worse. Anyone out there with a similar experience, or experience in concrete work.

Thanks, Dan

 

[duraleigh]

Dan,

I can't quite envision the location of the cracks...do they parallel the expansion joints or generally run perpendicular to them?

Tell us the length in both directions of the concrete in between the joints. As a rule, concrete slabs should not exceed 15' in any direction without an expansion joint. (I tried to cheat on that on my garage pour and it cracked....just like they told me it would )

Anyway, if the rebar was properly webbed, I doubt it will get much worse but a properly prepped and installed concrete pour doesn't have to crack if it's done right.

Your right about the heat during a pour but that usually results in surface "crazing" where the tiny cracks are more web-like but don't penetrate the depth of the concrete....yours sounds like they go all the way thru.

 

[JohnT]

Cracking is inevitable, but I'm like Dave in that I'm not quite sure what you are describing. The concrete shrinks a little as it cures, and this often results in a crack in an odd place, but the thing to be concerned about is settling cracks caused by improper fill under the concrete.

Often when going around corners, the guys cutting the joints don't do a good job of spacing them and leave a large section of slab without any joints.

 

[divnkd101]

Dan,

Alot of times, pictures speak 1000 words. If you can, I would suggest posting a few of what you are describing.

 

[Craig]

I am by no means a mason; but when you had the pool renovated, was there any digging? If so, perhaps the ground was not fully settled before the concrete was poured? An additional item to think of was there enough stone laid before the concrete was poured. Again I am not a mason but these might be good questions to consider.

 

[Underwater Man]

Thanks to everyone for your responses...

Duraleigh, all of the cracks run parallel to the expansion joints, actually in them, except for the one. The space between the joints are roughly 7 to 12 feet. The crack that I am most concerned with is the one between the joints. It's in front of the storage bldg in the attached photo. They put a joint at each end of the bldg, they're 12 feet apart. In hindsight I would have had the deck a little deeper here so there would have been a clean joint to fill. There is a drop of about 3 or 4 inches, and the two slabs do not tie together well. I have not decided quite how I will fix this. The slab of the storage bldg was originally level with the old deck, and was separated by a channel drain. I wanted the stone walls, so the slab was raised, and deck drains installed. Wish I could go back in time and fine tune this. Also included a photo of the rebar, and the base material. They prepared the base before putting in the pipes for drainage and got some of the subsoil mixed in with the base material. My old deck, which held up well for about 30 years had a base of nice looking cushion sand.

John T, Thanks for your input about shrinkage, I've noticed a crack forming between the stone walls and the decks. It's about 1/8" now. I plan to seal it with deck o seal. About the void under the deck... that would be bad. I sure hope that's not the case. In the attached photo that shows the two slabs meeting you can see an open spot. That's where the crack is. It doesn't look like much of a void underneath, just a a difference where the cement flowed under the form. Does the base material look ok?

Divnkd101, Thanks, you're right about the photos. I tried to get some good shots of the problem. The cracks do appear to go all the way through the deck. At least the one that is between the joints.

Craig, Thanks for the question. Yes, the soil was disturbed most of the way around the pool, including the area in front of the storage bldg where the crack doesn't run in the expansion joint. All of the plumbing was replaced, and those guys just quick filled the trenches. We had the rainest summer in years, so the soil was heavily saturated several times before the deck was poured. It also had lots of traffic from the other crews, especially the guys doing the stone walls. I was hoping the rain and traffic did a good job of compacting the soil.

I would appreciate any additional thoughts, Dan

 

[Hpdrifter]

As they say there are only 2 types of concrete! Concrete that's cracked and concrete that's going to crack! That being said it shouldn't crack that quickly. some of those smaller cracks should be expected but I'd be concerned about some of the larger stuff. Looks like it was pretty wet before thye did the pour. Did they use a tamper and how much stone did they lay down?

 

[The Mermaid Queen]

It is my understanding that the expansion joints are there to allow "controlled cracking". Like HPD said, concrete, in any large expanse, is gong to crack.

I would, however, be concerned about cracks that do not run along the joints, mainly since this is a new deck.

 

[Riles_J]

I agree. The joints that are scored into the concrete are "contraction" joints and their sole purpose is to provide a weakened plane to control cracking. Cracking along these joints is expected. Expansion joints go all the way through the concrete and typically have fiberboard or some material to allow for differential expansion of the concrete.

The mid-panel crack in front of the storage building is likely due to the 12' spacing of the contraction joints, which was probably a little too much to control cracking in this panel. It probably needed another contraction joint there. With that being said, I don't see it being a problem as the rebar should be adequate to keep that crack tight.

Riles

 

[duraleigh]

Riles' post above is pretty much what I'm thinking as well. Although, I have always used 15' as a limit without an expansion joint and, with somewhat limited experience, have had no cracking yet.

I would have to beleive that the crack outside the control joints is a more a result of poor sub-base preparation than anything else. If it was anywhere near that wet when it was poured, it's almost impossible to get the base settled in. I think the base settled underneath the pour allowing the poor to crack. Control joints (contraction) are helpful but they simply won't cover for what appears to be inadequate prep work.

 

[Riles_J]

I would say that you have to take into account the width of the concrete being poured. 15' may be OK if the width is at least 10' or so, but the less width you have the closer that spacing would need to be. Usually the closer you can get to squares the better.

What are the dimensions of the panel with the mid-panel crack? Another piece of information that is valuable is whether or not the control joints cracked on either side of the mid-panel crack. Sometimes if have a high spot in your base and you don't score control joints enough you might not be creating the weakest plane at the joint. In other words if there is point somewhere in the slab in which the concrete is thinner that depth below the control joint, then it will want to crack where the concrete is thinner.

 

[Underwater Man]

Thanks everyone... I sincerely appreciate your help with this.

hpdrifter... Thanks for your reply . The soil was in good shape when the concrete was poured. The photo I included of standing water was somewhat misleading. I was trying to show that we may have had enough rain to help settle the soil after the plumbing went in. Unfortunately, they did not use a tamper to compact the soil. The wheel barrows used to bring in the rock for the wall left some pretty good ruts in the wet soil. They just spread out the base evenly, then put in the pipes for the deck drains, rebar and poured. There's a good chance the soil settled in the ruts, as well as around the drain pipes. I wasn't very pleased with the base material used, especially mixing in soil from where the drain pipes were laid. The photo shows the prep work prior to the rebar. The crack is slightly in front of the workers shovel. Thank goodness they did a pretty good job on the rebar. Hopefully the crack will remain small for a long time. There is about 75 feet of stone wall, 20" tall, 16" wide. Another 4" of stone wall is below deck level.

The Mermaid Queen... Thank You, hopefully the crack outside the expansion joints will remain small. I have the same sentiment as you... hate to see a crack so quickly. These guys always seem to be in so much of a hurry. I would have liked to seen a little better prep work. I'm afraid the "lack of" will cause me more problems down the road.

Riles_J... Thanks, you've eased my mind somewhat about the cracks in the joints. I appreciate you explaining the difference between contraction, and expansion joints. The cracked panel is 11ft. long, 8 1/2" at widest, narrowing to 7 1/2". They put a contraction joint at each end of the little storage bldg shown in the photo. The contraction joints on either side of the crack are not cracked. The crack falls about 4 ft. from one of the joints, so it looks as though you may have found the answer. A weak spot between the joints. Hopefully the rebar will keep it in check, and the base hasn't settled in too many other places.

duraleigh... Thanks, I wasn't very happy with the way the base was prepared. They were just in too much of a hurry. I'm not sure if the material they used was even adequate, I was expecting something more like sand. The soil was fairly dry when they poured. The photo showing the standing water was just to say that hopefully all of the rain had helped settle the soil that was disturbed for the plumbing. I trusted the contractor against my better judgement. Hopefully I don't get settling/cracks in many other places.

Thanks, Dan

 

[txborn]

Hi Dan, I have a question after looking at the photo showing the rebar. I don't see any chairs installed under the rebar to raise it up off the ground. If they did not raise the rebar above grade (around 2 inches) so that the concrete goes underneath, they may as well not even used any rebar at all. It will provide no strength or support to the poured concrete lying on the ground.

Hopefully they raised it up and the picture you took was before they added the chairs.

And no - that many cracks in a slab that is only about 3 months old is not normal.

 

[duraleigh]

Txborn's sharp eye is on the money. The rebar must be imbedded in the concrete to be effective. If they left it lying on the base when they poured, it loses a great deal of it's value.

 

[cobra46]

Yes TXborn is right. Look closely at the photo. It was taken mid pour. You can see the new concrete at the top. More importantly, you can see that the rebar is not spaced up like it should be.

If this were my house, I would certainly go back to contractor in an attempt to have it redone correctly.

Good Luck

 

[txborn]

cobra46, in the concrete business (and most all aspects of construction) there is what's called "Best Practices". There are several issues here.

Prep should include at least minimal compacting of the ground, adding at least 3 - 4" of cushion sand, wire tying the rebar intersections together, raising the rebar with chairs, and sawcutting stress joints to a depth equal to 50 - 60% of the thickness of the concrete; none of which is evident in the pics. Unfortunately not every contractor uses Best Practices

 

[Underwater Man]

Thanks again for everyones help.

txborn, we're fairly close to you. I'm in SW Fort Worth close to Hulen Mall. I asked about the rebar being raised. I was told that they pulled the rebar up as they went along. I watched and they were doing this, not sure if it stayed up in all places. I had a pool built about 13 years ago by a supposedly reputable local builder, and they did the same thing. except they used hog wire instead of rebar. Thanks for your thoughts and the "best practices" info. I will be talking with the builder again. I have photos of all aspects of the rebuild. Fortunately I could be there all the time and documented the work with many photos. Appreciate the info about cobra46 background. Wish me luck. No way did they use 2 or 3 inches of cushion sand, but they did tie the rebar together. The stress joints were just trowled in after the pour was almost dry.

duraleigh. Thanks for staying on top of this thread. I really appreciate your comments.

cobra46, great to have a professionals opinion. I have a feeling I won't get far with the builder. They wanted to shortcut everything they could, and could not understand why I asked so many questions. The onsite supervisors were useless. I asked about raising the rebar with the chairs, they said they never use them. As far as tamping, I should have known to push that from how to books I've read. How should the rebar have been spaced? I'll add two more photo of the pour. The guy with the red handle tool was pulling up the rebar. The other photo shows the great base material used. I'll let all know how the builder responds.

 

[cobra46]

Underwaterman,

I'm sorry if my post was misleading. I am NOT a construction/concrete professional, however, I have poured a fair amount of concrete myself.

Kevin

 

[Ilan]

Underwater Man, I must say that even with those cracks you do have a beautiful deck there.

Good luck sorting this all out.

 

[CaliDesigns]

txborn, I disagree with your depth of saw cut. If you cut 60% of the thickness, you would cut through the rebar if placed properly. ACI standards are 1/4 the thickness.