Can we remove the ply from bottom of slab within 3-4 days after casting of slab?

amd007

Elite Member
Please see image for deshuttering time as laid out in IS codes.
Deshuttering time.png

Concrete gains strength with time after casting. It takes much time for concrete to gain 100% strength and the time for same is still unknown. The rate of gain of concrete compressive strength in higher during the first 28 days of casting and then it slows down. If you see the table then at 7 days concrete gains around 65% strength of fck.

Now the IS code allows us to deshutter the slab (see table) in 3 days, however, the condition being we need to keep the props in place. the code allows us to remove and then refix. However, I am of the opinion that we can remove the shuttering but by designing the shuttering in such a way that we can remove the shutters by keeping the props in place similar to what we do in case of aluminum shuttering ( generally referred to as Mivan ).

Further another critical aspect which most of the time is overlooked while deshuttering is the absence of engineers supervision while deshuttering, just as erection of shuttering is important equally critical is at deshuttering.

Engineers tend to neglect this part here in India and that's the reason for some of the collapses or deflection of slabs/beams after deshuttering. I would give similar attention while deshuttering to avoid untoward incidents.

Before deshuttering ensure the strength of concrete by testing the cubes on day you want to deshutter so take extra cubes for say day 4 and see what is the concrete strength and if it has gained the desired strength only then you can proceed with the deshuttering. However keeping props in place after deshuttering is mandatory till the concrete gains its strength as per the design.

Strength gain of concrete in days.png
 
Please see image for deshuttering time as laid out in IS codes.
View attachment 2852

Concrete gains strength with time after casting. It takes much time for concrete to gain 100% strength and the time for same is still unknown. The rate of gain of concrete compressive strength in higher during the first 28 days of casting and then it slows down. If you see the table then at 7 days concrete gains around 65% strength of fck.

Now the IS code allows us to deshutter the slab (see table) in 3 days, however, the condition being we need to keep the props in place. the code allows us to remove and then refix. However, I am of the opinion that we can remove the shuttering but by designing the shuttering in such a way that we can remove the shutters by keeping the props in place similar to what we do in case of aluminum shuttering ( generally referred to as Mivan ).

Further another critical aspect which most of the time is overlooked while deshuttering is the absence of engineers supervision while deshuttering, just as erection of shuttering is important equally critical is at deshuttering.

Engineers tend to neglect this part here in India and that's the reason for some of the collapses or deflection of slabs/beams after deshuttering. I would give similar attention while deshuttering to avoid untoward incidents.

Before deshuttering ensure the strength of concrete by testing the cubes on day you want to deshutter so take extra cubes for say day 4 and see what is the concrete strength and if it has gained the desired strength only then you can proceed with the deshuttering. However keeping props in place after deshuttering is mandatory till the concrete gains its strength as per the design.

View attachment 2853

It's interesting to see codes & requirements from different governing bodies/agencies. I find it interesting that the IS Codes provide form removal requirements in "days" as opposed to "strength." Or is the table just an excerpt of a longer specification?

The jurisdictions that I have constructed projects for in the US, self-supporting concrete structures (like a bridge deck) require the design flexural strength to be achieved AND the curing period completed before falsework or forming can be removed, but in the absence of strength tests, forms remain for 14 days (which is relatively consistent with amd007 first table.

And it's worth re-mentioning - Don't forget that just because you removed your forms that you are done - You still need to maintain your curing operations for the specified time period.
 

Dnyan Deshmukh

Staff member
Yes you can remove the ply from bottom of slab on 3rd or 4th day after casting, provided concrete achieves its 70% strength at the time of de-shuttering at least for residential projects where spans are smaller.

Before de-shuttering of slab we always do check the compressive strength of the concrete, if we get desired strength results, we proceed for de-shuttering. if we do not get required strength then we need to wait.

For Aluminium Formwork even though slab is de-shuttered on 3rd day, the span is very small as the props are not disturbed till 7 days. so the strength requirement is lower in this case.

Best way to know, when we can de-shutter slab is to ask structural consultant for at what strength of the concrete, it is safe to de-shutter.

If you wish to de-shutter all slab at 3rd or 4th day, then do revise the mix design of concrete which will achieve desired 70% or whatever specified by consultant at 3rd day. obviously this mix will give very high strength at the age of 28 days due to achieving desired strength in 3 days - this will affect the cost of concrete, cost of concrete will go on higher side but if time is more important than cost, then you can proceed for higher grade design which will suffice the design strength parameter for de-shuttering of slab.

Normally slabs are de-shutter after getting 70% strength and in case of aluminium formwork where props are not disturbed for 7 days it depends on structural consultant specifications.

So you can make your wish come true by pouring higher grade concrete in slab which will achieve 70% of your actual concrete grade strength in 3 days.