Minimum reinforcement due to restrained shrinkage in thickenings of a slab on piles

[user277418]

Hi

Designing a slab on piles. The slab has downward thickenings under heavy/load-bearing walls and on edges. Some sort of these, but larger (the image is only an example from internet)

Approx. slab dimensions are 45 x 30 m and thickness is 250 mm. Thickenings are Height x Width = 330 x 800 mm approx. When the slab is going to shrink the piles are going to create some restrain. As to calculate directly amount of the restraint, as well as size and location of cracks due to the restraint is very complicated, minimum reinforcement must be provided. Quantity of the minimum reinforcement depends on concrete cross-section area in tension, thus in the thickenings it rises up drastically. My supervisor said he never met such quantity of reinforcement in slabs on piles, but could not find any mistake in my calculations. To recheck myself I have tried 3 methods from different codes, standards and recommendations. All of them are giving close results. Looked through few respected books about foundations and met pretty small quantities of reinforcement there. In examples restrained shrinkage mentioned in a way of a suggestion to account it if needed, but not accounted.

So I would appreciate if you could give some references on trusted literature or share your experience on the topic.

Questions on which I am looking for answers:
1. Are there any construction methods to mitigate the effect of shrinkage? I know few for silos, but have doubts that they may be applicable in the case. Besides large portion of shrinkage is going to appear long after the construction will be finished.
2. Maybe I am missing something in the calculation? My assumption is, if the restraint is enough, whole cross-section is going to be in tension. Some reduction/redistribution factors according to codes are applied, but they don't save the situation.

PS: sorry for my English, just in case

Best regards

 

[HTURKAK]

My experience is , provide minimum 0.0013 bd reinforcement . IMO, the best practice,to check the shrinkage and provide necessary reinf . or minimize the shrinkage by concreting with zones , use of shrinkage-compensating concretes ..


1- For your case, slab dimensions are 45 x 30 m .. you may divide to 15X15 m zones and staggered pour.
2- Typical shrinkage reinforcement should be positioned in the upper half of the slab. The thread is for slab on piles and the sketches are for raft foundation. In this case, bending rebar should govern..Regarding the modelling of the slab, you may choose FEM
of slab and piles . The piles could be modelled with finite element frame elements having horizontal spring supports..

You may post some details of your model , climate etc to get better responds..

 

[user277418]

My experience is , provide minimum 0.0013 bd reinforcement . IMO, the best practice,to check the shrinkage and provide necessary reinf . or minimize the shrinkage by concreting with zones , use of shrinkage-compensating concretes ..


1- For your case, slab dimensions are 45 x 30 m .. you may divide to 15X15 m zones and staggered pour.
2- Typical shrinkage reinforcement should be positioned in the upper half of the slab. The thread is for slab on piles and the sketches are for raft foundation. In this case, bending rebar should govern..Regarding the modelling of the slab, you may choose FEM
of slab and piles . The piles could be modelled with finite element frame elements having horizontal spring supports..

You may post some details of your model , climate etc to get better responds..

1st of all, thank you for the reply

0.13% is only to avoid brittle failure. Moreover, it is not going to assure required crack width 0.3 mm (that is required in my case). The min. reinforcement, for fully restrained shrinkage, is 0.564% roughly. Such a big difference is going to lead to 5-10 times wider cracks if the restraint is actually there.

Thought about the staggered pouring, but I don't see how it will fix the shrinkage problem in long term. As I understand, it is helpful mostly to avoid early age cracking.

The thickenings, I am talking to, have exactly the same shape like on the sketches, but they are going to be supported on piles (act like beams). The sketches are only to show what I am meaning if my English terms are incorrect.

I have the FEM. But due to elastic nature of FEA, it is not capable to represent cracked behaviour of concrete structures precisely.

Best regards

 

[user277418]

A good reading on the topic

Design of slabs-on-ground regarding shrinkage cracking
CHALMERS UNIVERSITY OF TECHNOLOGY
Göteborg, Sweden 2012
Master’s Thesis

https://odr.chalmers.se/bitstream/20.500.12380/165126/1/165126.pdf

 

[dik]

If I do that, I introduce a couple of sawcuts at the thin part of the thickening... it's likley going to crack there, anyway... anytime I've thickened a slab like that, my concrete slope is 1/6... I don't like abrupt changes in slab thickness.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[user277418]

Interesting to read on the topic

Beware of Structural Designs that Use Reinforcing Steel for Shrinkage and Temperature Crack Control

https://ascconline.org/Portals/0/Technical-Article-July_WebSC.pdf?ver=2016-07-12-141059-753