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Cracks at the bottom of slab

Cracks at the bottom of slab

Cracks at the bottom of slab

(OP)
Hello.

Kindly give your advice on the below problem of cracks appearing in the bottom of slab,

Project: High rise building construction
Concrete Mix - C45 Flowing
Ambient temps - Min 25 to Max 45
Crack width range - 0.3 to 1.0 mm
Slab thickness - 300 mm
Time of cracks appearing - cannot be sure. Only after removal of support formwork which is usually 7 days cracks are visible

The cracks are showing-up after removal of support formwork. There are very few to negligible cracks on the top of slab surface. Good curing and protection of concrete slabs done.

Are these cracks some design issue? or plastic settlement?

RE: Cracks at the bottom of slab

The cracks are not plastic settlement. Those would show in a distinct pattern on the top of the slab. It is likely that your cracks are drying shrinkage cracks caused by the restraint of the forms. Your pump mix is susceptible to this form of cracking. Wood forms will exacerbate the issue if you are using them.

The cracks occurred early but likely widened after form removal.

RE: Cracks at the bottom of slab

(OP)
Thanks Ron.

But how to avoid these types of cracks effectively?

Also, the subgrade formwork and reinforcing steel used to be very hot before concrete placement. What will be the affect of fresh concrete with temp of 28-30 Deg C placed on a formwork/steel with temp of more than 90 deg C. Will it not create a thermal shock?

Thanks again

RE: Cracks at the bottom of slab

Yes it will cause thermal shock resulting in a "flash set" of the concrete and increasing the crack potential. Cool the forms prior to placement and make sure they are well-oiled.

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