Cracks in day old slab

[jvcan]

HELP!!

Hi, They poured my slab on Feb 26, one day later there were several cracks 15 to 25 feet long. One was almost 1/8" wide. I know concrete can crack but like this? When they poured it it was sunny and 70, that evening it went down into the 40's. I am really concerned there is more of a problem then just shrinkage. What can be done to repaor this. The builder says "this is normal". The slab they poured the previous Friday does not have a single crack.

What should be done?

John

 

[bylar]

What is the shape, the thickness, the purpose etc?
Concrete will shrink as it dries out. Did you provide control joint and did you cure the concrete (maintain moist state)?

 

[ishvaaag]

Normally it should be drying shrinkage, given the age...no other cause is as likely at such age... As you state this, first it is necessary to investigate which are the differences that make possible so different shrinkage behaviour, specially if the slabs are similar.

 

[jvcan]

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

 

[bylar]

I would think shrinkage is the mostly likely culprit. You can expect a slab like this to crack approximately every 15’-20’ the fiber mesh helps only in the initial shrinkage and I have found it of little value.
The amount of water added to the concrete is the most critical as to shrinkage cracking and of course the final strength. If it was a windy day the drying occurs to fast and will cause cracking and surface checks. Cracks occur usually at weak spots, thin section corners or insets or anything that deduces is length. If you did not us a curing compound at the surface as soon as the toweling was completed or kept the concrete moist, a rapid drying can occur and cause the crack.

If the backfill was not properly compacted the cracks would take longer to appear unless the fill was extremely loose.

My guess is just poor finishing procedures.

 

[Ron]

See post in concrete engineering.