Now that I have begun my new addition I noticed a few cracks in the new slab. Are they normal? The slab is about 3 months old.
Here is a pic.
Thanks
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They are normal for you apparently.
;)
Some shrinkage cracking is normal, but that seems excessive from what little I can see. could have been too much water used
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I guess there is not much I can do now. Any suggestions?
Thats normal all over here in TX. For some reason, these folks don't know about cutting the slab and making control joints. The back patio slab is 12' by 30 with no cuts or joints in it. Predictiably, there are spider cracks running all over it.
I am shocked at the low quality concrete work all over this city. You'd think a warm climate town like this would be dressing up their concrete and be proud of their artwork and craft because it will last forever. In MI, they do great concrete work but it looks like doggie doo doo after the first winter. Bob's next test date: 12/10/07
What struck me about this picture is that thecracks are wide onsidering only three months old, and they are very jagged.The stress or shrinkage racks I see here barely show for half a year or more and fllow a straighter line than that.
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My thoughts too.
If confined o the one area of the slab shown, it could be something going on under the slab, form poor prep work.
Dave they are here and there in the slab and not just one area. We have not had much rain here lately, I wonder if that has anything to do with it.
Curing slowly is very important, but it's more like a week then a month. The first couple of days being crucial.
Hard to tell from a picture but that looks like actual movement.
Did you prep it well?
Did some poor guy follow a tamper around for a while?
Hello Henley. Movement is what i'm worried about. The slab was poured over a very compacted driveway that had lots of base and blacktop over it. The person who did the slab does have lots of experience (30+ years) and has a very good reputation for his work, not that that has kept my slab from cracking. I'll probably call him out to look at it and see what he thinks.When I asked about pouring it over the driveway he said that if he dug it up and put new base and tamped it down it would still not equal what was already there as far as compaction went.
You know, I couldn't really make a better guess from
that one picture.
The only other thing could be cure time really.
Even without expansion joints, those cracks look odd.
They looked odd to me also, maybe because I have never seen cracks like that in new concrete.
The only other thing I can come up with
would be a spring. But given the location, I doubt. it.
Earthquake?
Here are a few more pics. These are at the edges.
To me, these edge cracks represent two distinct possibilities.
First is that as they do not penetrate thr slab from top to bottom at the same width, the structural reinforcement is doing its job, even if the surface is fairly badly cracked as a result of poor curing.The other is dependent upon the axis of the underlying drive relative to the slab length. You may be seeing some differential settlement. Check to see if there is any pattern of the cracking which aligns with the drive.Te radial placement of the first pic suggests general surface shrinkage.Then, as others have said, distressing, but not fatal.Better luck with the rest of the job.Lapun.
Thanks Lapun. I'll take a closer look.
You need to lay down a ruler for scale when taking the pictures. Based on the size of those chalk lines the cracks don't seem as large as they did in the earlier picture.
It is an ironic habit of human beings to run faster when we have lost our way. --Rollo May
it would still not equal what was already there as far as compaction went.BS
Well put.
Movement is what i'm worried about. The slab was poured over a very compacted driveway that had lots of base and blacktop over it
Where are the cracks in relation to the old driveway?
Soil type means a lot in compaction.
Even if the old driveway could pass a protor test for compaction (doubtfull) and the areas on either side of it weren't compacted to the same proctor you could end up with some unwanted movement in those areas.
Extremely hot and dry conditions can cause soil shrinkage.
If the slab was poured durring a wetter,( spring time) period and it has dried out a lot since then, the movement of the soil under the slab could cause the slab to settle and crack as it follows the soil.
No way of knowing any of that now. As long as the slab stays level across the cracks it is just cosmetic. When it gets uneven across the cracks, you have significant slab settleing, but that may not be an issue if there are no loads on the slab in those areas.
Pick Brownbags mind. He is the resident concrete expert.
BTW 30 years experience doesn't mean jack to me. I been around longer than that and am willing to admit I did a lot of stuff wrong over those years. It is learning and correcting ones ways that counts. I hope I'm not making the same mistakes I did 30 years ago, but I have seen guys that do.
Hello Dave.The cracks are not where the old driveway is, that area looks fine. The slab was poured during the spring months and we have not had much rain since then. I have very large cracks in the soil surrounding the house.
I am still reading the thread thru at this point but now know more and see more with the added photos.#1 - the same soil and same compactiuon should be under the entire slab. It sounds like part of the slab was on old drive and part was not, but the contractor assumed equal l;oad bearing which is not the case. This combioned with the underlyuing soils drying and shrinking, eans that the slab settled slightly in some portion towards the edges giving it the same stress as lifting the center of the slab would do. This helped cause the crack.But #2 - I still believe too much water used in the mix was the main culprit. Almost every crete guy tends towards adding more rather than less simply because it makes working it out easier physically - up to a point.
I also see the pattern of flow in those second photos where the form boards shaped it. That looks to me like wetter flow than drier.
The shape of the cracking being so irregular tells me that it tried to follow the density of the aggregate in the mix. A structural crack caused by tension tends to run more in a straighter line along the axis of the fulcrum.The point of all that is that when too much water is used in a crete mix, it will evaporate before it combines in hydration as part of the slab. That loss of the mass of the liquid water is what causes shrinkage cracks. When it evaporates faster in a hot sun and dry wind, the cracking is worse and appears first at the surface, which is what shows there.#3 most shrinkage cracks in slabs are not a structural problem as long as other elements are proper. like well compacted bearing soils and rebar placement. Without those in place, there is a greater likelihood that the shrinkage cracks can become worse and be a structural issue at some point in the future.#4 - I see the existence of some organic debris at teh edge of slab and extending in under it. Twigs or straw and maybe sawwdust or wood chips?
I cannot see how bad that is, but in theory, there should be no organic material under the slab for two reasons.
> If there is too thick of a layer, it will decompose and then contract/compact allowing the slab to settle. That is why we always scrape the topsoil and humus off and compact in mineral soil. I do not know whether to conclude that this is only a small handful of scattered junk that got missed by the rake or if there was a lot of sawdust and other organic materials left when he poured
> The other reason is that it can be a nice home for termites and carpenter ants to get started.
Do I see the start of a termite tunnel coming up from bottom towards the crack in the second photo?
if there is a lot of organic debris under that slab and a bunch of cracks for them to travel through into the house, you will need to learn to live with a lot of chemicals to keep the bugs out. In some locations there are ways to prevent that required before placing the concrete. Are any such measures required in your location?I am also skeptical of the meaning of the 30 year reputation. Some reps are earned and some fall by default to th e last man standing
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Piffin, one added comment on your point number 4:
If the organic debris, is dried wood, it will pull moisture from the concrete, and swell, causing the slab to crack directly above it. I've seen several slabs, and about a thousand feet of formed curb and gutter ruined that way.
there is a new one for me to have learnt! Thanks
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"After final review". I think you hit the nail on the head when you mentioned wood. The slab was poured with the column supports for my carport in place. I uploaded the pics for all of the slab construction where you can see where the columns were. Well when I first saw the cracks I thought they started in the middle of the slab and then ran to where the columns were before I removed them, but I think they originated there (the columns) and then spread out from there. Take a look at the pics and you can see where the cracks start at the edges of the column.http://picasaweb.google.com/blownonfuel
When I asked about pouring it over the driveway he said that if he dug it up and put new base and tamped it down it would still not equal what was already there as far as compaction went.
That's a valid point - it would be a lot of (unnecessary) work to achieve the compaction and stability of an existing driveway that has been there for years. You can do it, but why spend the time and money for no benefit?
What did he put between the old drive and the new concrete? I usually spec 2" of pea gravel. You want the new concrete to be able to move relative to the old concrete, because they will have different rates of expansion and contraction. If you just pour on top of the old concrete, you will get cracks. Hmmm.
Regarding wet curing period, concrete is rated to achieve most of it's rated strength after 30 days, but it never really "fully" cures. A week is too short, in my experience, to wet cure - the concrete is at best 50% of load rate, most of the curing has yet to happen. All it takes to disrupt the curing process is for the concrete to be too dry once - and that means not enough water to support the chemical reaction, in other words it can be wet but not wet enough.
One typical solution is to nail 2x2 to the forms after pouring the concrete and then to flood it, once the initial tooling and flashoff is set. So they keep it essentially under water for at least a week.
hello WT9, The driveway was not concrete but lots of base with blacktop that had been there for years. Nothing was put over the base or blacktop , he just poured over it.
You remove the driveway, and prep it to the same standard as the surrounding base so that you do not get differential settlement.
This slab definitely needed control joints. The rule of thumb if using 3000-psi concrete, is that you need one every 1.25 feet per inch of slab thickness.
Can't really comment on the cause of these cracks with out a lot more information.
This slab definitely needed control joints. The rule of thumb if using 3000-psi concrete, is that you need one every 1.25 feet per inch of slab thickness
Can you explain that a little better?
There is an equation in one of the Portland Cement Association (PCA) handbooks, that goes into how to estimate the approximate tensile strength of concrete based on the compressive strength. (It is a function of the square root of the compressive strength). Once you have an estimated tensile strength, there is another equation you use to determine the spacing of the control joints, based on the depth of the slab.
Years ago, I ran out the numbers for the plane Jane, 3000-psi concrete that most entities specifiy for flat work like sidewalks, driveways, slabs on grade etc. It worked out that you need a control joint just under every 1.3 feet per inch of slab thickness.
After looking at the standard drawings for things like sidewalks, which are 4-inches thick, and have a contol joint every five feet, 6-inch thick driveways with with joints at 8-feet, I decided that the 1.25 is a good conservative number to use. I don't know whether the joint spacing had been worked out from an equation, or they just got there from experience and expermentation. But, the 1.25 ft per inch of slab thickness always works.
That is 1.25 times the thickness of the slab and cut both ways?
Gabe gave a ROT in an article for JLC a few years ago. I think it was to place control joints every 100 sq. ft. of surface. I have stretched that sometimes but it still has worked pretty well for me.
I'm also glad to see you mention the organic material and dry form situations. I started oiling flatwork wood forms years ago, after having to rub out several hundred feet of curbs. Even when I was young, that was hard on my knees :)
I seldom pour 3000 lb. mix any more. 3500 or even 4000 lb. only add a few more bucks a yard and is worth evry penny of it in finish quality alone.
Thanks.
Yes it should be for both directions. Concrete doesn't have a "grain", so the stress is equal in all directions.
All in all it doesn't look like it cured to fast
anyhow, but...
Most residential slabs are rarely kept wet more then a couple of day's,
Let alone a week. I'll go along with the idea that you should protect it for seven days or so. As to this month business, maybe if your pouring a deck, but not a driveway. It's not necessary and just not practical.
More to the point, the question of compaction does sound more likely.
cracking is cause by, movement, vibration and heatmovement- if your subgrade is not completely compacted and uniform, the slab will crack, the crack will be wide and get wider till the settlement finish. it usually will be a crack completely across the shear, in a straight line. this is due to uncompacted soil. But the crack will not start till a load is applied, couple weeks.vibration- like if the rebar is too close to surface and the bar some how move before ths slab has set up. also lack of rebar on a corner of a block out. slab alway crack on a corner. that why alot of people has stop using square block out and went to round like five gallon bucket or sonic tube. any type of penetration like plumbing will crack a slab.heat_ basically if one part of slab dry quicker than the other it will crack at that point. this is what we are looking at here. it is not structual but ugly as hell. if you have two truck, different slumps, it will crack where they meet. Cold joints will crack. no vapor barrier will crack. The dirt under the slab is weeping the moisture out quicker than the top.in a perfect world you want the slab to dry across the slab in a uniform time as you place. start on one end and go across, like you would putting varnish on wood, keep a wet edge. as soon as you can walk on it without leaving a mark, cut the control joint , deep at least a inch and half. I like control joints no more than 10 feet each way.as soon as that is finsh wet down good and cover with plastic, wet freqeucy and keep covered at least seven days. I like to put a soaker hose under the plastic and turn on twice a day.also on heat, the sun will kill you on cracking, that why we place at night so finish trowel is in early morning sun
Edited 7/23/2008 9:50 pm by brownbagg
Getting rather long winded aren't you? (G)
Good post.
hey I cut it back, just dont ask mne about rebar, you know how I get.
I think that6 the movement is that it is shrinking away from itself
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Not pretty but probably not fatal. Could be too much water in the mix like Piffin says. We use water reducer and very little water. Also, possibly the slab was not wet cured. We keep sprinkling water on them for a few days after placement.
My guess is real hot weather and they did not keep water on surface as jim said.
Unless is is a large slab with no control joints.
Concrete does 2 things, gets hard and cracks. Contractor should layout and plan for cracking and help the contrete to try to crack in a planned area. Doesnt always work but that is how it should be done.
Is it a structural slab? What is is for?
If it is a high visibility aesthetic area, you can epoxy it and some companies say that the crack wont telegraph thru but I have never seen it.
If it is a basement slab, I would likely do nothing.
Thanks Jeffin, The slab in 26' wide by 43' long with no expansion joints.It is for the addition to my home so it will have loads on it.As long as it is not something fatal to the slab I won't worry about it.
Thats a big slab for no expansion joints so no surprise it cracked. Concrete shrinks when curing and if hot and it dries too with the sun baking it, it moves even more.
I assume there are footings under the slab where the walls will come down or the slab is thickened at those locations.
Likely aok
Yah Jeffin, the footings run anywhere from 24 to 36" deep.
Then the slabs are just holding the carpet and vinyl, etc. and non-load bearing interior walls.
If the final flooring is sitting on top of the slab, make sure your flooring company can manage around the cracks, otherwise, it is pretty much as it is.
My guess it should have had control joints ... 2 wide and 3 long (i.e. divid the slab into 2 x 3.
Out here on the west coast, we never place expansion joints or saw cuts in an addition slab. If it's a driveway or patio slab yes but never an addition slab. Pull up your carpet I will guarantee you will find similar cracking. Show me a new tract of home where someone is watering the slabs on a daily basis until it cures.ML
"Show me a new tract of home where someone is watering the slabs on a daily basis until it cures."
Certainly THAT'S not your quality goal to achieve, is it? To meet the lowest level acceptable?
No Pete is it not I'm just showing that there is not always someone watering the slabs so they do not dry out to quickly.ML
I believe we all understand that.
Afterall, that's why we are visiting the webite of Fine Homebuilding, not Shoddy Construction Unlimited.
Those that care, do. Those that don't, well...
I am 'out west', too. Just to be clear if I wasn't before, I'm talking control joints, not expansion joints. I've learned you always put in control joints. My slab has no evidence of anything but a superficial surface crack. I finished my slab (i.e. acid etched) ... so I worked with it a number of times very closely ... I don't recall noticing any cracks at all. I used WWM. I don't recall if the contractor used any type of special finishing or additives in the concrete. I poured late fall and the concrete stayed wet for a long time after the pour. While framing walls, we often had standing water on the slab. Right after pouring, we had to provide some freeze protection ... so that gives you an idea of the weather conditions. No severe freezing, but we did have some snow around that time.
It may depend on your finish. Like the one guy said ... not likely fatal. Acid etch and you will see it. Anything else and you may not ... may affect tile, though.
Did you pour inside a foundation wall? Did he put fiberboard around the perimeter? Any control joints?
I agree w/ the Texan ... and I've seen it here in SoCal, too ... lots of mediocre work ... no control joints on a 2000 sqft slab!!! ... go figure. When I did a finished slab in my new house, the conc contractor insisted on control joints at minimum every inside corner of the plan going both directions. Never had anything more than superficial surface cracking (nothing noticeable at all).
I don't know if I qualify to be called a Texan yet. I still haven't received my passport or visa to stay here and I don't yet speak drawl. Bob's next test date: 12/10/07
When you start speaking Spanish, then you'll be a Texan!
Si. Bob's next test date: 12/10/07
Texans don't speak Spanish, just their own version of Spanglish.
Spanglish, uses phrases like, "es sleeping bag", instead of the correct Spanish which would translate into, "the bag of blankets with a zipper for sleeping".
Edited 7/22/2008 12:34 pm ET by Jigs-n-fixtures
You do know it's pronounced "Tex-is" with the emphasis on the first syllable, right?
y'all don't speak drawl?
I'm guessing you didn't keep it wet for 30 days after it was placed?
That will cause cracking like that.
I wish the person who did the slab told me to do that.
I wish the person who did the slab told me to do that.
Unfortunately, most won't. It's impractical to do commercially, so they typically use a sealer to try to hold the water in long enough for a good cure. But for the homeowner, the best solution is to keep it wet for a month.
"Thanks Jeffin, The slab in 26' wide by 43' long with no expansion joints."
A slab that size should have been divided into 6 or 8 sections by cutting control joints (or contraction joints - not expansion joints) to a min. depth of 1/3 the slab thickness within 24 hours.
That plus hot weather plus excess water added at jobsite plus rapid drying/cure plus lower-strength concrete (if done) plus mesh too low (if done) plus shorting the slab thickness (if done) = cracks.
These days I'm specifying both fibermesh and wiremesh because of the potential for these variables as well.
Jeff
I no longer spec wire mesh. I have pulled up far too much concrete and found the mesh stomped to the bottom by the finishers.
If I want reinforcement beyond fibermesh, I spec #4 bars. I have a lot more confidence that they will end up in the mid slab where they actually do something.
I'm with you on using rebar rather than mesh, but have trouble deciding where the bar should sit.
If it is to stop cracks due to expansion and contraction, mid-span sounds right. It it is to reinforce the slab against load, to be effective it should be in the bottom third. If the substrate applies uneven loading to the underside, the top third would be best. See my problem? Any insights?
In a slab on grade, I would usually want it at mid slab. The subgrade if properly designed and prepared will bear the load. The reinforcement is mostly to help control cracking, and limit the potential for movement if cracking does occur.
Yes - makes sense - but I review the mesh hook and pull-up with the finishers. Onces it's pulled up and the aggregate supports it in the curing mix it tends to be pretty stable unless, as you said, it gets stomped down.
Jeff
that why i like fiber
Plus it keeps you regular ;o)Jeff
that why i like fiber
Two COMPLETELY DIFFERENT applications. Fiber limits microcracking caused by curing too fast, but offers essentially no structural strength.
Mesh holds the concrete together when it cracks and provides limited structural strength.
Rebar provides structural strength if sized and placed properly.
Too many people are trying to use fiber as a replacement for mesh. It doesn't work.
And the other poster's comment about the mesh is correct - it does no good if it is one the bottom of the slab. Best to have it in the middle.
I,m sorry, you can have my concrete man card
you can have my concrete man card
You said that the list time you posted comments about using fiber. You may recall that several of us provided manufacturer's information and PCA information to support our statements.
There is a REASON fiber is not approved for reinforcement by code.
It looks like curing issue to me. No sealer right? That coupled with your temperatures and no moisture during curing is not a great situation for avoiding such cracks. I can see why you texans would have such a problem. The temperature on the top and bottom of the slab are significantly different and this leads to internal stress within the slab. The cold(er I suppose in texas) surface (bottom) wants to shrink and the hot surface (top) wants to expand and that causes tensile stress at the top of the slab. The cracking tensile strength of concrete is around 5% of its compressive strength so it cracks pretty easily during curing.
Saw cuts are an attempt to control the cracks, but offer no guarantee to control the pattern of the cracks. I am sure most would admit to seeing cracks migrating away from the control joints on occassion. The reality of concrete is that it will crack thats why rebar is used. Sometimes the cracks are micro ones you cannot see or as in this case larger ones show due to poor curing procedures. There seems to be other mitigating factors in this case also as suggested. Is it a problem, I dont think so in this case given you are going to cover it up with a flexible floor covering and it is essentially non-load bearing. If you plan to tile large areas I would be careful. If you want to fill the cracks there are lots of products on the market to do so. Even some that will expand with the crack should it continue to move. Visit Sika's website or any of the other major manufacturers of specialty concrete repair products.
To avoid cracks, cast suspended slabs or add some post tensioning. Allowing concrete to creep and shrink like it wants helps reduce cracks, and the ground restrains the slab from doing so. Both options cost way more, so most just live with the risk of some cracks.
Brad
Thanks Brad. I guess ill just have to live with it and hope it does not get any worse.
Curing could be a possibility. Are the cracks at unequal heights? Water may have run towards your slab due to grading issues.
No arcflash, they are the same height. There is no step in them.
Then, I personally believe that your slab is safe to build on. If you are worried, maybe you could patch the slab (once future movement has been arrested) before you begin framing.
actually the heat on the top of slab in texas does the opposite of what you are thinking. It causes it to dry too fast and the shrinking that follows the evaporation is what causes the cracks, not tensile stresses from thermal expansion.There is tensile stress at work because of his soil prep - or lack of it but not from heat causing expansion.
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I agree. Another thing I see here after looking at the pictures is the amount of internal slab restraints in the form of footings. I understand this is a common design in these parts, but it seems things must be executed very well to avoid such problems that the OP is experiencing. A slab that is free to shrink will not necessarily crack, but if you restain it and the shrinkage stress exceeds the concrete tensile strength, it will crack. It would seem prudent in a design such as this to isolate the slab elements between footings to allow some shrinkage to occur at little more freely. I have seen slabs 100' long shrink away from a foundation wall up to 3/8" when it was allow to and not crack, yet if that same slab were rigidly tied at its ends it would crack somewhere.
Brad
"I dont think so in this case given you are going to cover it up with a flexible floor covering"
Well that's the whole point - cracks can telegraph through lots of flexible coverings - sheet vinyl, linoleum etc.
Here's an example installation spec - http://www.ardex.com/cms/assets/Products/Documentation/ard-all_substrate_prep-tech-us-e.pdf
Jeff
Personally, I am not a fan of the slab on grade concept for a house, but in my area the slab on grade is not an option. High plastic clays, and freezing temps. I agree they can telegraph through lino, but that is not necessarily a structural problem so long as there is enough bar maintain the slab integrity. His granular layer looks pretty cobbly and does not appear to be compacted very well, but without experience with those local conditions, that is pure speculation.
Brad