Striking sofit formwork to thick slabs 2

[haideeb]

A method of calculating the required strength of concrete to strike the slab formwork as described in CIRIA 136 is by taking the ratio of the slab selfweight + construction loads to the design loads in comparison with the design concrete strength. I am however dealing with thick transfer slabs 600 to 1200mm in which I really can't get a ratio based on design loads as they are not uniform, what would be the more appropriate way to calculate the concrete strengths required to strike the formwork?

 

[SlideRuleEra]

...what would be the more appropriate way to calculate the concrete strengths required to strike the formwork?

Use proven specifications that have been developed over the past century, or so, with similar slabs... highway bridge decks. Here is an exert form the South Carolina DOT specs:

http://www.SlideRuleEra.net

 

[MotorCity]

Ideally, you would have test cylinders to break and confirm the strength of the concrete. Then compare the concrete strength to the actual loads that are applicable at the time of formwork removal.
As a general rule, we recommend that the formwork can be removed when 75% of the design strength is achieved.

 

[haideeb]

In several texts, I found the usual striking time for slab formwork (replaced with backprop) is 3 days. That's too early to get 75%. What could be the basis for this guide?

 

[SlideRuleEra]

What could be the basis for this guide?

Doubt that guide is referring to thick, long-span, flat slabs that by themselves are significant dead load... and are not practical to backprop.

http://www.SlideRuleEra.net

 

[BridgeSmith]

Generally there are 3 criteria I know of that influence how long before formwork should be removed:

1) Curing - The concrete must be protected from excessive surface evaporation. This can be mitigated if curing compound is applied when the forms are removed.

2) Adequate strength to resist stresses from loading transferred to the concrete.

3) Acceptable deformation due to creep. The creep factor drops exponentially with age, so green concrete will sag significantly more.

 

[CANPRO]

Only 3 days before stripping the forms for a slab doesn't sound right...unless your spans are very short. What text did you read this? If you're designing or working with formwork I highly recommend getting a copy of Formwork for Concrete, by M.K. Hurd.

Be aware of the timing between stripping the transfer beam and work with the adjacent slab - depending on the timing and formwork arrangement, your transfer beam shoring might see loads from the slab.

 

[slickdeals]

A 3 day form stripping is very common for post-tensioned slabs in high rise construction
Day 1 - Concrete is poured
Day 2 - Concrete gains 75% f'c and tendons are stressed
End of Day 2 or Day 3 - shores and dropped and slab is reshored

 

[BridgeSmith]

The timing of stripping forms for post-tensioned construction is somewhat different than for standard reinforced concrete. The creep/sag is accounted for and can be mitigated during post-tensioning. Sometimes, that 'excessive' deformation is desirable to produce the desired final shape in a shorter time frame. The more it sags initially, the earlier the post-tensioning can be applied to counteract that sag.

One of the issues with standard RC is that if the concrete hasn't gained enough strength or stiffness before the support/shoring is removed, it may develop unacceptable deformations that are very difficult to mitigate.

 

[Ingenuity]

The more it sags initially, the earlier the post-tensioning can be applied to counteract that sag.

Can you explain/expand on what you refer here?

Post-tensioning is 'applied' (i.e. stressing commences) based on achieving the required/specified concrete strength, not on any 'sag' criteria.

 

[BridgeSmith]

Ingenuity, the newer the concrete is, the more it creeps under load. The sooner the concrete is allowed to sag under its own weight, the more it will sag. The greener the concrete is when the post-tensioning is applied, the more it will camber. So, more initial sag = earlier application of post-tensioning force to mitigate the greater sag. There are limits, of course - too much deformation will cause unacceptable cracking.

It's similar to how we control camber in prestressed bridge girders - if we want more camber, say to more closely align with a crest vertical curve grade profile, they set the transfer of the strand force to the concrete earlier. If a flatter profile is desirable, they let the concrete cure longer and get stiffer before transfer.

 

[hokie66]

Striking of forms and backpropping of slabs, both post-tensioned and with deformed bars, is typically done after 3 days. But it has to be done correctly. As soon as a form panel is dropped, the backprops have to go in. So there is never a substantial area of slab which is unsupported by props.

For a thick transfer slab, I don't see much difference. As SRE implied, it takes a lot of backpropping. So that just means that all the shoring for the placement must remain in place for backpropping.

 

[haideeb]

Thanks for all your inputs

 

[haideeb]

In BS8110, section 6.2.6.3.2 striking period for cast in situ concrete, it is stated that formwork supporting cast in situ concrete in flexure may be struck when the strength of the concrete in the element is 10 N/mm2 or twice the stress to which it will be subjected, whichever is greater, provided that striking at this time will not result in unacceptable deflection. I know BS8110 has been replaced by EC2 but it was used for a long time. So considering the above, the slab soffit formwork can be stripped off in 1 day, perhaps applicable to usual slab thicknesses maybe up to 300mm? and back-propped?