Concrete Spalling after deshuttering soffit

[Contech2011]

Hi,

We have observed spalling of concrete upon de-shuttering the soffit formwork after 25 days. The concrete slab is 2.0 meter thickness and was poured using 45 Mpa mix with 35% flyash to meet heat of hydration requirements as the thickness is 2.0 meters.

Test cylinder samples have achieved the required strength of 45 Mpa at 28 days, except some of the samples which will be crushed at 56 days. Hopefully strength will be achieved at 56 days, as the fylash will contribute to the later strength gain.

The slab is post-tensioned. This localized spalling as visible in the enclosed pictures, we presume is due to high magnitude of poast tensioning load and also due to improper compaction with the vibrator during pouring as the slab is 2.0 metres deep and the slump of concrete is around 170mm.

Kindly evaluate the reasons for such failure of concrete.

 

[Contech2011]

The second picture was missing... please check this pic of spalling

 

[hokie66]

Both pictures are at the same area, or are the same picture.

Do you mean the forms were left in place for 25 days prior to being released from the slab? Was a form release agent used?

Something long and straight has been nailed to the formwork at that point. What is it? There are dark lines inside the spalled area. Are they deformed bars or what?

As the slab is 2 metres thick, I assume it is a transfer slab for a tall building, but perhaps you need to confirm.

How much post-tensioning? Is this near the bottom of the drapes? Bonded or non-bonded?

 

[Contech2011]

Both formwork was left for 23 days before deshuttering.. No form release was used. The dark color is of deformed bars spaced at 150 mm c/c.
Yes, the structure is a transverse slab...

We think it is a cold joint which probably occured after fresh concrete was poured on a very small layer of dry concrete and both of these layers did not have proper bonding. You see, when we pour deep slabs more than 1 meters, spillage of concrete occur when using pumps etc...these dry chunks spread on the slab bottom later become very dry and loose. and after a long gap when fresh concrete is poured...there is literally no bond between the two layers. right?

 

[hokie66]

Yes, that could very well have happened. Then when you waited so long to remove the formwork, the wood to concrete bond, aided by the nails, was stronger than the concrete to concrete bond. An indication of poor construction practice all around. Wasn't there a structural engineer involved in the placement of this critical building element?

I have never heard of anyone leaving forms in place unreleased for so long. And not using form oil...why?