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Cracks in day old slab
6

Cracks in day old slab

Cracks in day old slab

(OP)
HELP!
I really would like you professional advise.
They poured the slab for our new home on Feb 26, 02 in Jacksonville, FL. Upon inspecting it on the following morning there are several cracks which run 15 to 25 feet long across the center and in from the corners. some of the cracks are already 1/8 inch wide. One runs almost the whole width (40'). I understand some cracks are possible, but the slab they poured on Feb 22 has no cracks.  The day they poured my slab it was in the 70's and the temp dropped to the 40's over night.  What should be done? I am really concerned that with this happening so soon it will only get a lot worse.  What should I do?

RE: Cracks in day old slab

The cracking sounds pretty bad and abnormal. Can you tell us some more. Was the slab uncovered or inside? How was it cured if at all? Was it ready mix or mixed on site? Is it reinforced? What is the size of the panel? How thick is it? Does it have a plastic underlay? What is the base like? How many cracks are there? Do they run in a particular direction? The temperature swing was quite large but if it remained above freezing it should still be OK. My first reaction with that sort of cracking would be that it should be redone but first you want to know what was wrong so it doesn't happen again. It may be that there was something seriously wrong with the mix.

Carl Bauer
www.bauerconsultbotswana.com

RE: Cracks in day old slab

Okay,

The slab was outside in the Jacksonville ,FL area. I was told it was 3000lb fibre crete (It is evident there is firbe in it). They used a Power Trough to finish it.  It was premix.  There are 2 #5 re-bar around the perimeter.
The slab is approx. 40 x 55, 4" thick,yes on the plastic.
One crack comes from the center rear and runs 25' to the middle, a couple of the cracks run from the center where the kitchen pipes are stubbed out( this is where the largest crack is) and a coule in from the sides. They do not run in any certain direction. The base is sandy dirt.

Thanks

RE: Cracks in day old slab

It is easy to say this from a great distance but...

What you have is a no-good slab, and your builder should be obliged to remove it and replace it.  The time to do that is now.

Under no circumstances should you be persuaded to allow anything to be built on it.

RE: Cracks in day old slab

I would agree with austim and carlbauer and i think it is also important to understand why THIS slab cracked but the previous slab placed on Feb 22 did NOT.

1. Was the previous slab very different (area size, shape, mix etc) to this one?
2. You mention 2#5 at the perimeter - did they use mesh reinforcement for the general slab area or no rebar at all?
3. 1/8" crack is a real wide crack - and IS cause for your concern.

Call your contractor and demand removal-and-replacement. If immediate action is not forthcoming call you county building department. Hopefully it will not get to the legal stage!

RE: Cracks in day old slab

One other thought.  Don't rule out there may be problems with the subgrade.  If the backfill was poorly compacted, such as the trench for those pipes you mention, it may have settled, and a new (weak) slab wasn't able to span it. However, poor subgrade preparation likely wouldn't cause multiple cracks throughout the slab.

Carl

RE: Cracks in day old slab

Hi there,

They did not use any re-bar except for around the perimeter.
It had a monolithic footer around the edge.
The other slab was identical except for the fact they did not have to bring in any fill.  The concrete finisher told me my slab went down a lot easier then the other one.  He said they really had to work the other slab.  This makes me wonder if they had to much water in my mix.

Thanks for all the great advice. The construction super is coming out tomorrow to look at it.  They already tried the "concrete always cracks".  I told them "yes but not this bad in less then 24 hours.

I will keep you up on what happens

John

RE: Cracks in day old slab

jvcan..
You have a problem!  I am very familiar with your area and concreting in the Jacksonville area.

Sounds like an overly wet mix that got caught in a temperature problem as well as drying shrinkage.  As the temperature was falling the humidity was falling as well, and the wind kicked up to between 20 and 30 mph.  All of this combines to create significant initial drying shrinkage which is what cracked your slab.  The strength of the concrete had not gained sufficiently to resist these drying shrinkage stresses.  The reason you have few, but wide cracks is because of the polypropylene fibers in the mix.  

If you would like to provide an email address I can give you more info.

Ron

RE: Cracks in day old slab

HI, all

Ron has just mentioned something that has been mulling in my mind for an hour or two.  He has a distinct advantage over me in that (a) I have no experience anywhere in USA, (b) I have no experience with fibre reinforced concrete. Despite those difficulties, here is my extra 2 cents worth.

If the cracked slab has 'monolithic footings' round the edge, I take that to mean that the slab is thickened at all edges.  If that is so, then the shrinkage will effectively be restrained by 'abutments' at opposite edges, and will inevitably lead to cracking sometime.  (Though obviously a 1/8 inch crack in 2 days must involve some other contributing factors).  

If there is no bar reinforcement to control crack width, then the inclusion of fibre reinforcement will in fact have an adverse effect, since the additional tensile strength provided will permit restrained shrinkage stresses to develop over greater distances between cracks.  Hence fewer cracks but larger.

Is it common USA practice to rely solely on fibre reinforcement in situations where the slab has significant obstacles to free shrinkage movement at the edges?  If it is, what does the ACI say about that?

RE: Cracks in day old slab

austim,

i too have no experience with this type of "design" BUT i thought exactly the same as you, AUSTIM, when i read the post - the effect of the perimeter downturns with no reinforcement was astounding to me...but then again i tend to get involved with post-tensioned slabs-on-grade (some with very high axle loads etc) where we take the effects of restraint very seriously. Seems that this slab was not engineered in any sense.

Ron,

is this a somewhat "common" form of residential SOG construction in FLORIDA? Judging by your past posts on SOG, you have considerable experience in SOG construction, so it would be great to "pick your brains" on this. thanks

RE: Cracks in day old slab

Drying shrinkage does seem likely made worse by the restrained edges and fibre reinforcement. This is especially likely if the slab was left uncovered and it got dry and windy as Ron said.

Carl Bauer
www.bauerconsultbotswana.com

RE: Cracks in day old slab

austim, Ingenuity, Carl...
Yes, common residential construction technique in Florida.  Mostly sandy soils requiring little preparation.

Cracks in residential slabs are common due to exactly the points you have noted...edge restraint is common culprit, coupled with the tendency of residential concrete subs to take a 5-inch slump and make it 10 inches!!

Austim...your point about the adverse affect of fiber in these conditions is exactly right.  It provides enough tensile enhancement that it increases the distance between cracks, and when they occur they are much wider.

The weather conditions this week in Northeast Florida were just right to screw up concrete!  Unexpected cold snap with relatively high winds...add that to contractors who don't know a great deal about concrete technology and it's not pretty!

Ron

RE: Cracks in day old slab

   That is norm here in north Florida.  A 2500 psi monolithic 4" (actual 3 to 3 1/2") thich with a edges reinforcement 2 - #5 the bottom in the 12" thick x 20" high perimeter.  The local codes allow the 6x6 #10 wire to be replace with poly fiber, since the wire usually end up on the bottom.  There may additional effects than mentioned above.  Did they pull the forms off the first day, since it is easer (probably yes), was the slad sprayed with a curing film (prob. no) was any effort put forth to insure the Souther Building Code require 7 day continuos moist cure (prob no),  was there 45 degree dowling at interior corners (prob. no), was control cuts added?
   Typically there is ~5% increase in drying shrinkage for each 2-inch increase in slump.  Additional there is a 100 psi reduction in strenght for each gal/yd added.  I remember here was an allowable crack size but I can not find it.

RE: Cracks in day old slab

From what has been said it would appear that a poorly compacted subgrade could be the reason for the slab cracking. To check this use a string line set up on level across the slab and measure down to the slab. All of the measeurements should be within 5mm (1/5") of the average level for a good pour.

The reference to the concrete being easier to put down is a statement that the concrete has an improved workability than the previous concrete and in general may contain more water and hence have a lesser strength.

I also agree with Austim, carlbauer and ron.

I hope you haven't paid the contractor yet as he has not constructed what you ordered, a good quality slab on grade. I would have him pull up the slab and do it again. If he wont then get another contractor to do it. Oh, and being in the USA you had best talk to a solicitor with respect to suing the original contractor for the costs (including your legal fees).

regards
sc

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