Curing Concrete -slab on grade 1

[bjm1]

Is it possible to keep a concrete slab on grade (floor 4" thick) too wet after placing it? The slab was kept moist sprinkler or wetted with a hose for 3 1/2 to 4 weeks after placememt. The slab was inside a closed building and plastic was placed over the slap untill the color was uniform after the about 5 1/2 week it was removed. Problem is it has extensive shrinkage cracks contractor is arguing the floor was kept wet longer than it should have been during the curing. However the wire mesh was not held up during the poor (it was pulled up with a hook)and the two slabs 28x30 were not saw cut untill after the cracks started to appear. Any advise would be greatly appreciated.

 

[pba]

Usual practice is to cut the joints as soon as possible. Usually the next day.

It seem strange that such an intesive curing regime has been used when the contractor hasn't followed normal construction practice in the formation of the joints.

The claim of excessive wetting is implausible but could be checked by relatively cheap sample coring.

The most probable cause of slab cracking is the lack of crack inducing saw cuts during the curing process. Also look out for curling at free edges - The slab may not be resting on the ground below.

 

[bjm1]

I have very little experiance with concrete, but I cured the floor for my private shop and wanted to be sure the floor was of good quality. I have since learned a lot due to the major cracking over 150 lf of cracks. I now feel the contractor failed to due a lot to ensure a quality floor, however I know have to prove it. He also didn't want to put wire mesh in the floor and only did at my request, he didn't saw cut it early enough, didn't do a slump test, no poly was put down to reduce friction and moisture loss. Any additional input would be very helpfull.

 

[GeoPaveTraffic]

I agree with pga, there is no way that keeping the slab wet for 3 1/2 weeks caused it to crack. Not sawing it for 3 1/2 weeks, now that WILL cause cracks.

 

[pba]

It is always difficult to prove blame. For the sake of argument lets consider your statements above:

'He also didn't want to put wire mesh in the floor and only did at my request.' - You are the designer of the anti-crack mesh. You should have specified the method of placement, support and tolerances. I guess you didn't.

'...didn't do a slump test.' - Did you ask for slump tests? What slump did you specify? Do you suspect that the supplied slump was in sume way inappropriate?

'no poly was put down to reduce friction and moisture loss.' - Again, did you specify a slip membrane?

NOW - The above might give you some insight as to how your contractor will approach this problem. If you are lucky you may have some answers to this type of response already.

Here are some axtra questions:
Did you employ the contractor with any kind of specified performance requirement on the slab?
Did the contractors fee include the design of the slab?
If it did include design at what point did YOUR design input invalidate the contractors liability? - Did they tell you this at the time?

I suspect in reality you will never reach a satisfactory conclusion as the cost involved (in both personal time and finances)will be more than you are prepared to accept. I would recommend discussing with the contractor the defects and see if you can agree on a mutually acceptible solution to fix the problem. Remember the key words are Mutually Acceptible - That mean it will be neither what you OR the contractor wants but somewhere in between.

Now 150 linear feet of cracks? - How wide are the cracks? What pattern (if any) do they form? You state 2 slabs 28x30 - 840 sqft. If that is the total area that comes out at 1 foot of crack in every 5 1/2 sqft. That's not very high. If we are talking about fine cracking the slab is probably structurally fine and the cracks are cosmetic.

Please do supply some more information!

 

[bjm1]

Thanks that give me more to think about. The law in the state were the work was done requires the contractor to provide a product of industry quality standards. He was responsable for the design and said he could add wire if I wanted it wouldn't be a problem ( I assume he accepted liability). The contractor was hire to design and install the floor (bad choice on my part I see now)so I didn't provide any specs I relied on his 30 plus years of experiance ( or lack of). Any other thoughs would be appreciated.

 

[SlideRuleEra]

bjm1 - You cannot "over-cure" a concrete slab - the wetter, the better. Some of the best concrete is placed underwater for cofferdam seals.

Based on my experiences, it sounds like your problem came from too much water in the concrete mix. Most likely this was added in the truck, on the job site, at the request of the jobsite workers. The intent being to make the mix "more workable" (some individuals incorrectly think that concrete should be "poured" into the forms - like cake mix into a pan). A slump test (after the water was added) would have picked this up.

There are mixed opinions on having poly under the slab - I prefer the school of thought that says don't use it for "outdoor" slabs. It tends to promote uneven water loss from the concrete during curing, which can contribute to cracking. In your case, this was most likely not an issue - the damage would have been done by having too much water in mix anyway.

Proving all of this, if true, will be difficult.

Best Wishes

http://www.SlideRuleEra.net

 

[Ron]

Agree with GPT and pba....wet curing didn't cause the problem and it probably lessened the problem.

The cracks were caused by improper construction controls and jointing. Plain and simple. You probably have variations in thickness of the slab (you can usually tell this by how crooked the cracks are. If randomly oriented all over, probably varied thickness and got a lot of subgrade friction restaint. If fairly straight, then thickness control was good and restraint is uniform.

Saw cuts should be done THE DAY of placement, not the day after or even longer. As GPT said "that causes cracks"!

 

[pba]

bjm1,

An experienced contractor has supplied you with a floor which you believe is defective, under a contract where the contractor was the designer and the design quality required was 'industry good practice'. This is quite a good starting point.

You now need to quantify the defects which are outside of this construction standard. Some level of cracking in inductrial ground floors is to be expected. Indeed the purpose of the mesh reinforcement is to limit crack width rather than to eliminate cracks. A mesh reinforced slab is likly to have a grater number of cracks at closer spacing than an unreinfocred slab. The cracks are however, narrower.

Any crack less than 0.3mm (sorry I work in metric-25.4mm is 1 inch) can be disregarded as these are not likely to be detrimental to the floors performance. Measuring crack widths is subjective. Get yourself one of those clear plastic scales that surveyors use if possible. If you can't get one of those you are guessing so give the contractor the benefit of the doubt.

Plot the cracks onto a plan drawing and ask the contractor for his proposals to fix them.

Cracks can be fixed by cementitious grouts, resin injection, or local breaking of the slab and reinstatement. The choice is probably down to the circumstances of the cracks and the contractors (possibly very limited)experience.

 

[GeoPaveTraffic]

At this point I would ask the contract what he plans to do to repair the existing cracks. Point out to him that the cracks were obviously not caused by the wet curing, reference conversations with several engineers. This should give you some indication of how he is going to respond.

If the response is basically, not my problem. You will need to determine how important it is to you to get the cracks fixed. Talking to another contactor about a price for fixing the cracks would give you an indication of the amount of money you are talking about. You probably need to contact an attorney at about this time. Hopefully, the first contractor will step up and take responsibility.

Also, just to reinforce that it was not the wet cure, concrete test cylinders are normally cured in 100% humidity or under water till tested.

 

[bjm1]

I have been in contact with the contractor and we are heading to court on the matter. Would any one have refrances available that would be helpful? I will be providing the responced posted.

 

[cvg]

I would go onto the Portland Cement Association website, which has numerous publications on concrete construction and quality control. You may also be able to speak with one of the engineering representatives who are generally quite willing to assist with anything related to concrete. They may provide all the documentation that you will need.

http://www.cement.org/

 

[BigH]

Do a search on concrete cracks on the net - you can get a lot of information, for instance:

http://www.concretenetwork.com/concrete/concrete_cracks/plastic_cracking.htm

 

[pba]

One thing that still has not been stated - How wide are the cracks?

This is very important to this discussion...

 

[Wasdifferent]

30 years of nailing forms together and hoeing concrete does not make one a quality control specialist or a design professional. If you allow your subs to run your job for you, you will have one disaster after another.
Someone has got to take control of this job. Hire a good engineer, and a good general contractor. You will find that it's a lot cheaper than demolition and litigation.


-Mike

http://www.sewellconstruction.org