DIY Home Improvement, Remodeling & Repair Forum > DIY Home Improvement > Bricks, Masonry and Concrete > Cracks in new concrete




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Old 08-28-2013, 06:00 PM  
HouseRepair9
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Default Cracks in new concrete

I just had 1400 sq feet of concrete patio done last month, with color concrete, stamped. The contractor did the work on a very hot day, and then when I asked him if I should spray water on the concrete, he said no.

We had 105+ days for a week right after that, and looks like the concrete has not cured well. Right away nearly half of the controlled joints had long cracks, and cracks started appearing (and getting longer) on the concrete surface (see the pictures).

It also seems like the top layer of the concrete is not very strong, as I dropped a screwdriver from 3 feet height and it chipped the concrete.

The contractor says this is normal (no surprise there :-)). Is that correct? He says he will put a quick patching, and once the concrete is sealed, the cracks won't be visible at all.

What do you guys think? Thanks for your advice.



img_2261-large-.jpg   img_2260-large-.jpg   img_2262-large-.jpg   img_2263-large-.jpg  
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Old 09-06-2013, 05:02 PM  
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I think it is crap, but mostly I am bumping the thread for other to see it.



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Old 09-06-2013, 11:21 PM  
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What was the concrete reinforced with?

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Old 09-07-2013, 09:46 PM  
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http://www.cement.org/tech/cct_curing.asp
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Old 09-08-2013, 09:20 AM  
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I have had many many long conversations with my subcontor concrete guys and the very best way to cure concrete is to wet it as much as possible for the first 3-5 days, i went out on mine every hour and sprayed it down. Also if you poured it on a hot day and you have rebar, the hot rebar can set up the concrete around it causing cracks, I have seen driveways with cracks following all the rebar.
It is common and you will almost always have cracks in the controlled crack joints, that is why they are there.
If you poured more than one truck and there was a long time delay between trucks and the first started setting up it will likely crack where the newer concrete meets the first trucks.
Concrete is concrete and it can and will crack but there are many things that could have been done to avoid it. If he was licensed himself then he would have never old you not to cure it at all.

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Old 09-08-2013, 09:24 AM  
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Also if he used a low sack mix of concrete or poured it thin it will also crack easier. I poured 30 yards (3 trucks) in my yard on July 3rd and it was a well over 100 degree day. After many conversations with a few guys I did not use rebar but I did use a 7 sack mix (I believe standard is 4-5) and poured between 4-5 inches thick for my RV pad and widening my driveway. My first truck was on time but the second two were two hours late so I thought I would have major problems but to this day not a single crack except a few in the joints.

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Old 09-09-2013, 09:04 PM  
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Thanks for all the feedback!

I called the ready-mix concrete company my contractor had used, and inquired about what exactly was ordered by my contractor. It turns out that he ordered the concrete with a slump of 5, which is marginal at best (4 or less is desirable). So the concrete was already somewhat watery. On top of that, he added 40 gallons of water on arrival (to 16 cubic yards of concrete), making the slump to 7.5 - WAY too watery. The water to cement ratio was 0.59, making both the ratio as well as the slump too high to make acceptable concrete.

It turns out that he clearly violated the ASTM C 94 standard. According to the ready-mix quality control guy, he should have requested plasticizer instead of adding so much extra water. Plasticizers are cheap ($1-3 per cubic yard), so it is really surprising that he cut corners there.

Now I also understand why he asked me not to spray water, as he know he already had put in too much water to begin with. So we ended up with too much water during pouring, and too less water during curing.

Just to gather some conclusive evidence, I have asked the concrete quality control guy to come by tomorrow and do some Schmidt hammer testing, so that I can clearly show that the slab is weak. (For those who do not know this, the test is free, and non-destructive).

Then the fun will begin when I break this to the contractor...

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Old 09-09-2013, 09:39 PM  
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Sounds like the readymix company is on your side, good luck and good info.

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Old 09-09-2013, 11:44 PM  
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Readymix gave the guy up with the quickness

Hopefully you get it resolved without much more hassle

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Old 09-10-2013, 07:00 PM  
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The readymix company is just trying to make sure they are not part of any lawsuit. This guy likely brings them lots of problems, and they should be helping with the fight.



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