Crack in underside of pre-stressed pre-cast concrete slab

rok

We have a crack that has just appeared in the underside of one of our pre-stressed pre-cast concrete slabs.
I believe it was caused by the concrete company's delivery driver dropping 2 bales of blocks onto the top side of the slab. He was not very competent at all with the crane off loader. The floors are propped with acro jacks.

The crack is just under 1 metre long, and the pre-stressed coil looks to have snapped out from the end and unravelled. See pics attached.

We have not started blockwork on the pre-cast flooring yet.

I would appreciate your comments/advice before I contact my Engineer and the suppliers.
Thanks

Poor Uncle Tom

It looks as though there was not enough cover on the reinforcement anyway. You had best contact the floor slab manufacturers and suppliers and make their engineers aware of the fault.

muffler

Poor Uncle Tom wrote: »

It looks as though there was not enough cover on the reinforcement anyway. You had best contact the floor slab manufacturers and suppliers and make their engineers aware of the fault.

+1.

get on the phone - pronto

Johnniep

That slab has failed and should be rejected. Pre-stressing strand has debonded and is effectively useless. Don't let you builder screed the slab as it will be much harder to remove!

rok

Thanks very much for your replies.
I contacted the slab manufacturers and they've agreed to replace the slab.
Luckily the slabs were not screeded and no blockwork yet on the slab.

I believe it was caused by their driver dropping the bales of blocks,
but I am a bit worried now about the rest of the slabs.

Is it very un-common for a pre-stressed slab to fail like this? (slab is 150mm)

Does it look like there was not enough concrete covering the re-inforced bar?

Johnniep

Very difficult to make a call on the level of cover etc. Cover required will have a number of factors including the force in the tendon.

As for the failure, you may be right. If the slab was overloaded during delivery, this typie of failure may have resulted. Best to get your engineer to comment further and agree with the pc unit supplier.

Edit: Make sure your engineer has received calculations from the precaster justifying the 150mm unit.

tred

rok wrote: »

We have a crack that has just appeared in the underside of one of our pre-stressed pre-cast concrete slabs.
I believe it was caused by the concrete company's delivery driver dropping 2 bales of blocks onto the top side of the slab. He was not very competent at all with the crane off loader. The floors are propped with acro jacks.

The crack is just under 1 metre long, and the pre-stressed coil looks to have snapped out from the end and unravelled. See pics attached.

We have not started blockwork on the pre-cast flooring yet.

I would appreciate your comments/advice before I contact my Engineer and the suppliers.
Thanks

did the company in question supply with you a prop drawing for where to locate the acrows? Mine is just being measured today.is the bale of blocks off the slab now?

rok

tred wrote: »

is the bale of blocks off the slab now?

Yeah funny enough most of the blocks are on other slabs.
Think there is 1 bale at the other end of this slab that is spanning this cracked slab and an adjoining slab.